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India in the Proterozoic

Permanent Link: http://ufdc.ufl.edu/UFE0022255/00001

Material Information

Title: India in the Proterozoic Two Key Spatial and Temporal Constraints
Physical Description: 1 online resource (78 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: geochronology, india, neoproterozoic, paleomagnetism, rodinia
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The first paper presented in this thesis documents a paleomagnetic and geochronologic investigation was undertaken on the Majhgawan kimberlite near Panna, India. 40Ar/39Ar ages on phlogopite separates from the kimberlite yield a mean age of 1073.5 +/- 13.7 Ma (2s). Paleomagnetic samples from the brecciated kimberlite yielded a mean direction with a declination of 45.3 degrees and an inclination of -25.1 degrees (k=37, a95=9.3). When combined with directional data from an earlier study they yield a virtual geomagnetic pole at 36.8N, 212.5E (dp=9.0, dm=16.6). This VGP overlaps with a paleomagnetic pole in the overlying Bhander-Rewa Groups (41.6N, 32.3E; dp=3.8, dm=7.2). The new age for the Majhgawan kimberlite constrains the ages of Upper Vindhyan sedimentation (Bhander-Rewa) to less than ca. 1075 Ma. The second paper within this thesis presents new paleomagnetic and geochronologic data on the Malani Igenous Suite (MIS), Rajasthan, Central India, to improve the paleogeogrpahic reconstruction of the Indian subcontinent between dispersal of the Mesoproterozoic supercontinent Rodinia and Neoproterozoic assembly of Gondwana. MIS comprises a voluminous initial phase of felsic and mafic volcanism followed by granitic plutonism. A zircon U-Pb age on a rhyolitic tuff constrains the initial volcanism in the MIS at 771 +/- 5 Ma. Large (up to 5 m wide) mafic dikes mark the final phase of igneous activity. A virtual geomagnetic pole from 4 mafic dikes has a declination=358.8 degrees and inclination=63.5 degrees (with k=91.2 and a95=9.7). It overlaps with previously reported results from felsic MIS rocks. This normal direction includes a fine-grained mafic dikelet that showed a reversed direction with declination=195.3 degrees and inclination=-59.7 degrees (k=234.8 and a95=8.1) and also records an overprint of normal polarity from the larger dikes. The VGP obtained from this study on mafic dikes is combined with previous studies of the Malani suite to obtain a mean paleomagnetic pole of 67.8N, 72.5E (A95=8.8). Supported by a tentative baked contact test, we argue that this pole is primary, and permits an improved reconstruction of the Indian subcontinent at around 770 Ma. Data on the MIS, and equivalent data on Seychelles at 750 +/- 3 Ma, are compared with paleomagnetic data on the 755 +/- 3 Ma Mundine Well dikes in Australia, to indicate a latitudinal separation of nearly 25 degrees between the Indian and Australian plates. This suggest that East Gondwana was not amalgamated at ca. 750 Ma and therefore that these two cratons were assembled later into the Gondwana supercontinent, during the Pan-African ca. 550 Ma Kuunga Orogeny.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Meert, Joseph G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-11-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022255:00001

Permanent Link: http://ufdc.ufl.edu/UFE0022255/00001

Material Information

Title: India in the Proterozoic Two Key Spatial and Temporal Constraints
Physical Description: 1 online resource (78 p.)
Language: english
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2008

Subjects

Subjects / Keywords: geochronology, india, neoproterozoic, paleomagnetism, rodinia
Geological Sciences -- Dissertations, Academic -- UF
Genre: Geology thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: The first paper presented in this thesis documents a paleomagnetic and geochronologic investigation was undertaken on the Majhgawan kimberlite near Panna, India. 40Ar/39Ar ages on phlogopite separates from the kimberlite yield a mean age of 1073.5 +/- 13.7 Ma (2s). Paleomagnetic samples from the brecciated kimberlite yielded a mean direction with a declination of 45.3 degrees and an inclination of -25.1 degrees (k=37, a95=9.3). When combined with directional data from an earlier study they yield a virtual geomagnetic pole at 36.8N, 212.5E (dp=9.0, dm=16.6). This VGP overlaps with a paleomagnetic pole in the overlying Bhander-Rewa Groups (41.6N, 32.3E; dp=3.8, dm=7.2). The new age for the Majhgawan kimberlite constrains the ages of Upper Vindhyan sedimentation (Bhander-Rewa) to less than ca. 1075 Ma. The second paper within this thesis presents new paleomagnetic and geochronologic data on the Malani Igenous Suite (MIS), Rajasthan, Central India, to improve the paleogeogrpahic reconstruction of the Indian subcontinent between dispersal of the Mesoproterozoic supercontinent Rodinia and Neoproterozoic assembly of Gondwana. MIS comprises a voluminous initial phase of felsic and mafic volcanism followed by granitic plutonism. A zircon U-Pb age on a rhyolitic tuff constrains the initial volcanism in the MIS at 771 +/- 5 Ma. Large (up to 5 m wide) mafic dikes mark the final phase of igneous activity. A virtual geomagnetic pole from 4 mafic dikes has a declination=358.8 degrees and inclination=63.5 degrees (with k=91.2 and a95=9.7). It overlaps with previously reported results from felsic MIS rocks. This normal direction includes a fine-grained mafic dikelet that showed a reversed direction with declination=195.3 degrees and inclination=-59.7 degrees (k=234.8 and a95=8.1) and also records an overprint of normal polarity from the larger dikes. The VGP obtained from this study on mafic dikes is combined with previous studies of the Malani suite to obtain a mean paleomagnetic pole of 67.8N, 72.5E (A95=8.8). Supported by a tentative baked contact test, we argue that this pole is primary, and permits an improved reconstruction of the Indian subcontinent at around 770 Ma. Data on the MIS, and equivalent data on Seychelles at 750 +/- 3 Ma, are compared with paleomagnetic data on the 755 +/- 3 Ma Mundine Well dikes in Australia, to indicate a latitudinal separation of nearly 25 degrees between the Indian and Australian plates. This suggest that East Gondwana was not amalgamated at ca. 750 Ma and therefore that these two cratons were assembled later into the Gondwana supercontinent, during the Pan-African ca. 550 Ma Kuunga Orogeny.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis: Thesis (M.S.)--University of Florida, 2008.
Local: Adviser: Meert, Joseph G.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2008-11-30

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2008
System ID: UFE0022255:00001


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1 INDIA IN THE PROTEROZOIC: TWO KE Y SPATIAL AND TEMPORAL CONSTRAINTS By LAURA C. GREGORY A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2008

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2 2008 Laura C. Gregory

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3 To my Dad, who consistently encourages my enthusiasm for science and the natural world.

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4 ACKNOWLEDGMENTS I would like to thank the m emb ers of my committee, Joe Meer t, David Foster, and Neil Opdyke, for consistently guiding me through this research and offering fantastic advice and critical review. I especially wa nt to thank Joe for encouraging creative scientific thinking and teaching me so many interesting things. I tha nk my parents, Christine and William, and my sister, Amanda, for all of their help along the wa y. My dad really taught me the joy of science from day one and learning has always been a cruc ial part of my life beca use of my parents. I want to thank all of my closest friends in geologywe have really been through everything together, from the very beginning of undergraduate geology into competent scientists with the highest ambitions. Alexa van Eat on deserves special thanks for her constant encouragement and unrelenting enthusiasm. I thank Emily Grudem, Laura Ruhl, Elodie Bourbon, Braden Fitzgerald, and Jessica Yff. I also want to thank Misty Stroud for being my enthusiastic sounding board and partner in tectonic geology. There are some very crucial people that contit ributed significantly to the science involved in this researchin lab training, analyses, field work, and scientific discussion and review. They are Vimal Pradhan, Shawn Malone, James V ogl, George Kamenov, Be rnard Bingen, Trond Torsvik, Dan Gorman, Endale Tamrat, Kainia n Huang, Linda Sohl, Dhiraj Banerjee, Bob Tucker, and Alan Collins. M. Whitehouse is th anked for operating the NORDSIM laboratory and controlling quality of U-Pb data. I want to thank the Geological Sciences faculty for guiding me for the past five years and teaching me vast amounts of geology. I especia lly thank James Vogl and Ray Russo for their contribution. Finally, I thank my best friends throughout the past few years who have always lended a seemingly interested ear to my enthusiasm for science, and with whom I have had some of the greatest experiencesAmber Beu tler, Nicole Yutulis, and Annie Reynolds.

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5 Funding for this research was provided by the National Science Foundation grant EAR040901 to Joseph Meert, and the Universityof Florida University Scholars Program grant.

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS...............................................................................................................4 LIST OF TABLES................................................................................................................. ..........8 LIST OF FIGURES.........................................................................................................................9 LIST OF ABBREVIATIONS........................................................................................................ 10 ABSTRACT...................................................................................................................................12 CHAP TER 1 INTRODUCTION..................................................................................................................14 2 A PALEOMAGNETIC AND GEOCHR ONOL OGIC STUDY OF THE MAJHGAWAN KIMBERLITE, iNDIA: IM PLICATIONS FOR THE AGE OF THE UPPER VINDHYAN SUPERGROUP.................................................................................. 16 Introduction................................................................................................................... ..........16 Geology of the Kimberlite...................................................................................................... 18 Paleomagnetism................................................................................................................. .....19 Paleomagnetic Experiments............................................................................................ 19 Rock Magnetic Experiments...........................................................................................19 Geochronology.......................................................................................................................20 Results.....................................................................................................................................20 Geochronology................................................................................................................20 Paleomagnetism............................................................................................................... 21 Discussion and the Age of the Upper Vindhyan.................................................................... 22 Conclusions.............................................................................................................................25 3 PALEOMAGNETISM AND GEOCHRONOL OGY OF T HE MALANI IGNEOUS SUITE, NORTHWEST INDIA: IMPLICAT IONS FOR THE CONFIGURATION OF RODINIA AND THE ASSE MBLY OF GONDWANA........................................................ 34 Introduction................................................................................................................... ..........34 Geology and Tectonic Setting................................................................................................ 36 Previous Studies......................................................................................................................38 Paleomagnetism............................................................................................................... 38 Geochronology................................................................................................................39 Methods..................................................................................................................................40 Paleomagnetic Sampling and Experiments..................................................................... 40 Rock Magnetic Experiments...........................................................................................41 Geochronology................................................................................................................41 Results.....................................................................................................................................42

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7 Geochronologic Results...................................................................................................42 Rock Magnetic Results.................................................................................................... 42 Paleomagnetic Results..................................................................................................... 43 Discussion...............................................................................................................................44 Significance of Paleomagnetic and Geochronologic Data .............................................. 44 Implications for the Configuration of Rodinia................................................................ 48 Conclusions.............................................................................................................................50 4 CONCLUSION..................................................................................................................... ..66 LIST OF REFERENCES...............................................................................................................69 BIOGRAPHICAL SKETCH.........................................................................................................78

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8 LIST OF TABLES Table page 2-1 Age constraints on Vindhyan sedimentary sequences....................................................... 31 2-2 Summary of pale om agnetic results.................................................................................... 32 2-3 Analytical Data............................................................................................................ ......33 3-1 Summary of Paleomagnetic a nd Virtual Geom agnetic Poles............................................ 62 3-2 Summary of geochronologic results.................................................................................. 63 3-3 SIMS zircon UPb data on rhyol ite tuff from Malani igneous suite................................. 64 3-4 Paleomagnetic results...................................................................................................... ...65

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9 LIST OF FIGURES Figure page 2-1 Generalized stratigraphic colum n, regional and local geologic m ap................................. 26 2-2 Stepwise degassing spectrum for Majhgawan phlogopites............................................... 27 2-3 Zijderveld plots, stereoplots, and th er mal/AF demagnetization trends for select samples...............................................................................................................................28 2-4 Site mean directions, paleomagnetic and virtual geom agnetic poles for critical rock units.......................................................................................................................... ..........29 2-5 Magnetic characterization data.......................................................................................... 30 3-1 Typically accepted Gondwana fit fo r 560 Ma, taken from deWit et al., 1988.................. 52 3-2 Map showing Precambrian stratigraphic units of the Aravalli Mountain R egion in NW India with sampling area boxed (a dapted from GSI publications)............................. 53 3-3 Field photos............................................................................................................... .........54 3-4 Inverse concordia diagram................................................................................................. 55 3-5 Magnetic characterization data.......................................................................................... 56 3-6 Demagnetization results from sites 35 and 36................................................................... 57 3-7 Demagnetization results from site 34................................................................................ 58 3-8 Stereoplot of individual site means, ove rall m ean and reversed polarity mean with common India overprints from the Deccan Traps and Rajmahal Traps indicated............ 59 3-9 Stereoplot of VGPs from the three studies, averaged to the m ean pole for the MIS......... 60 3-10 Reconstruction at 770 Ma of pertinent eastern Gondwana components ............................ 61 4-1 APWP for India with re liable P roterozoic poles................................................................ 68

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10 LIST OF ABBREVIATIONS 2 two sigma units (expression of error) 95 circle of 95% confid ence about the mean AF alternating field 40Ar/39Ar ratio of argon isotopes 40 and 39 C degrees Celcius ca. circa ChRc characteristic remanent magnetization CL cathodoluminescence EAO East African Orogen Ga Giga annum (Latin: billion years) IGRF90 1990 International Ge omagnetic Reference Field IMSLEK Collection of cratons: India, nor theastern Madagascar, Sri Lanka, East Antarctica, and the Kalahari craton IRM Isothermal Remanence Magnetization k kappa precision parameter Ma Mega annum (Latin: million years) MIS Malani Igneous Suite m micro-meter mT millitesla MSWD mean squared weighted deviates NRM natural remanent magnetization Pb-Pb lead-lead isotope geochronology Rb-Sr rubidium-strontium isotope geochronology SIMS secondary ion mass spectrometry TcC Curie temperature on cooling

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11 TcH Curie temperature on heating TRM Thermal Remanent Magnetization VGP virtual geomagnetic pole

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12 Abstract of Thesis Presen ted to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science INDIA IN THE PROTEROZOIC: TWO KE Y SPATIAL AND TEMPORAL CONSTRAINTS By Laura C. Gregory May 2008 Chair: Joseph G. Meert Major: Geology The first paper presented in this thesis documents a pale omagnetic and geochronologic investigation undertaken on the Majhgawan kimberlite near Pa nna, India. 40Ar/39Ar ages on phlogopite separates from the kimberlite yield a mean age of 1073.5 13.7 Ma (2s). Paleomagnetic samples from the brecciated kimberlite yielded a mean direction with a declination of 45.3 and an in clination of -25.1 (k=37, a95= 9.3). When combined with directional data from an earli er study they yield a virtual geomagnetic pole at 36.8N, 212.5E (dp=9.0, dm=16.6). This VGP overlaps with a paleomagnetic pole in the overlying BhanderRewa Groups (41.6 N, 32.3 E; dp=3.8, dm=7.2). The new age for the Majhgawan kimberlite constrains the ages of Upper Vindhyan sediment ation (Bhander-Rewa) to less than ca. 1075 Ma. The second paper within this th esis presents new paleomagnetic and geochronologic data on the Malani Igenous Suite (MIS), Rajasthan, Cent ral India, to improve the paleogeogrpahic reconstruction of the Indian subcontinent between dispersal of the Mesoproterozoic supercontinent Rodinia and Neoproterozoic assembly of Gondwana. MIS comprises a voluminous initial phase of felsic and mafic volcanism followed by granitic plutonism. A zircon U-Pb age on a rhyolitic tuff constrains the initia l volcanism in the MIS at 771 Ma. Large (up to 5 m wide) mafic dikes mark th e final phase of igneous activ ity. A virtual geomagnetic pole

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13 from 4 mafic dikes has a declination=358.8 and inclination=63.5 (with k=91.2 and 95=9.7). It overlaps with previously reported results from felsic MIS rocks. This normal direction includes a fine-grained mafic dikelet that showed a re versed direction with declination=195.3 and inclination=-59.7 (k=234.8 and 95=8.1) and also records an overp rint of normal polarity from the larger dikes. The VGP obtained from this st udy on mafic dikes is combined with previous studies of the Malani suite to obta in a mean paleomagnetic pole of 67.8 N, 72.5 E (A95=8.8 ). Supported by a tentative baked contact test, we ar gue that this pole is primary, and permits an improved reconstruction of the Indian subcon tinent at around 770 Ma. Data on the MIS, and equivalent data on Seychelles at 750 Ma, are compared with paleomagnetic data on the 755 Ma Mundine Well dikes in Australia, to indicate a latitudinal separation of nearly 25 between the Indian and Australian plates. This suggest that East Gondwana was not amalgamated at ca. 750 Ma and therefore that th ese two cratons were assembled later into the Gondwana supercontinent, during the Pan-African ca. 550 Ma Kuunga Orogeny.

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14 CHAPTER 1 INTRODUCTION The Proterozoic eon quadruples the span of tim e represented in the Phanerozoic, yet scientific knowledge of Proter ozoic geology is a small fraction compared with our understanding of the past 542 million years. A consistent pr oblem in understanding Precambrian geology is the dearth of accurate, well-tested data. Incomplete or deformed rock records often impede accurate interpretations, and may lead instead to broad speculation for Precambrian tectonics and environments. However, well-constrained data provide crucial anchor points for generating viable scenarios that can be te sted and improved. Important tools for Proterozoic geology include paleomagnetism, geochronology, detrital zircon analysis, geochemistry, stratigraphy, and structural geologyeach technique with its own advantages and susceptibility to complications. Paleomagnetic data are especially useful for an cient continent reconstructions, but only when paired with precise age determinations. Paleoloca tions of continents prov ide the building blocks for understanding the dynamics of tectonic regi mes, extreme environments and biological evolution during the Precambrian. The notion of a midto lateProterozoic supe rcontinent partially arose when similar aged paleomagnetic poles followed coinciding appare nt polar wander paths (Piper, 1976; Bond et al., 1984). This supercontinent, Rodinia, amalgamate d around 1.1 Ga and the subsequent breakup of major constituents was likely initiated around 750 Ma. This gene ral concept of supercontinent assembly and breakup is oversimplified, and a mo re comprehensive approach reveals a complex history of plate motions associat ed with Rodinia constituents. Paleolocations of the Indian subcontinent can be especially problematic. Geochronologic resolution for India in the extant literature can be greater than 500 million years for one unit, even for an intrusion that is typically emplaced within a few days (i.e. the Majhgawan kimberlite,

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15 Chapter 2). A reliable paleomagnetic pole is render ed useless when no relative age is available. The credibility of paleomagnetic data is often questioned, and every additional positive test for primary magnetization is imperitive. A unique solution for the assembly and disper sal of supercontinent constituents can be determined only with the combination of various types of high quality, relia ble data. This thesis presents two projects that add small, but si gnificant, segments to the complex puzzle of Proterozoic tectonics. Both manuscripts included are either published or submitted to Precambrian Research.

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16 CHAPTER 2 A PALEOMAGNETIC AND GEOCHRONOLOGI C ST UDY OF THE MAJHGAWAN KIMBERLITE, INDIA: IMPLICATIONS FOR THE AGE OF THE UPPER VINDHYAN SUPERGROUP1 Introduction The age of sedim entation with in the Vindhyanchal basin is a controversy that has been disputed for over one hundred years (Venkatach ala, 1996). While the age of the Lower Vindhyan Group is now well constrained, the Upper Vindhya n section has yet to be dated with any certainty. The onset of sedimentation in the Vindhyanchal basin comme nced sometime after 1850 Ma based on U-Pb ages from underlying vo lcanic rocks (Deb et al., 2002). Geochronologic data from the Lower Vindhyan sequence (Figure 2-1a ; Table 2-1) are derived from U-Pb ages on porcellanites that yield 1628 Ma, 1631.2 5.4 Ma and 1630.7 0.8 Ma (Rasmussen et al., 2002; Ray et al., 2002). Ages from the Rampur ash beds, below the Rhotas limestone, yield SHRIMP Pb-Pb ages of 1592 12 Ma and 1602 10 Ma. Less precise Pb-Pb from carbonate yielded ages of 1599 48 Ma (Sarangi et al., 200 4) and 1601 130 Ma (Ray et al., 2003) for the Rhotas limestone. In comparison, the Upper Vindhyan sedimentary rock s lack reliable age constraints. Ray et al. (2003) reported a very poorly defined Pb-Pb age on the Bhander Limestone of 750 Ma that they considered unreliable. Ray et al. (2003) also used Sr-isotopic data on the limestones to estimate a ca. 750 or 650 Ma age for the Bhander limestones; however, the Sr-isotopic data are also consistent with older and younger sections of the global curve. De (2003) reported Ediacaran-like organisms with very poor pres ervational characteristics from the Bhander limestone that are consistent with the estim ate by McElhinny et al. ( 1978) of an Ediacaran1 Reprinted with permission from Gregory, L.C., Meert, J.G., Pradhan, V.R., Pandit, M.K., Tamrat, E., Malone, S.J., 2006. A paleomagnetic and geochronologic study of the Majhgawan kimberlite, India: Implications for the age of the Upper Vindhyan Supergroup. Precambrian Research 149, 65-75.

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17 Cambrian age for the Bhander-Rewa Groups. McElhi nny et al. (1978) compared directions in the Bhander-Rewa rocks to known Cambrian rocks from the Salt range in Pakistan. A more recent paper by De (2006) yields Ediacaran fossils wi th much better preser vation in the Lhakeri Limestone (Bhander Group). If the fossil find is confirmed by future work, then the Bhander Group is Ediacaran aged (ca. 635-542 Ma; Condon et al., 2005; Jiang et al., 2003; Zhang et al., 2005). The Majhgawan Kimberlite, that intrudes Upper Vindhyan rocks (Figure 2-1b,c), is important for obtaining a more reliable age fo r the basin due to its stratigraphic location (intruding the Kaimur sandstones). The reported ag e of Majhgawan from previous studies range from 1630 353 Ma to 947 30 (see Table 1; also Auden, 1933). The kimberlite was dated by Crawford and Compston (1970) with mica to yi eld an age of 1116 12 Ma. Two K-Ar whole rock and one mica determination yielded ages of 947 30, 1170 46 (whole rock, Paul et al., 1975) and 1056 (mica, Crawford and Compst on, 1970). Two recent ages of 1044 22 Ma and 1067 31 Ma were reported using Rb-Sr (Smith, 1992; Kumar and Gopalan, 1992 respectively). The Majhgawan Kimberlite is located in Madhya Prad esh, India (Figure 2-1). It is one of several Proterozoic-age kimberlites/lamproites intruding th e peninsular Indian crust. Other major bodies thought to be broadly consanguineous with th e Majhgawan kimberlites are the Lattavaram, Wajrakur, Narayanpet and Mulligiripalle kimberlites of the Dharwar craton (Miller and Hargaves, 1994; Haggerty and Birkett, 2004) and th e Hinota pipe (Aravalli craton). Not all of these kimberlites have reliable age constraints. The Wajrakur kimberlite has ages ranging from 840-1350 Ma (Paul, 1979; Crawford and Compst on, 1973; Paul et al., 1975). The Lattavaram kimberlite is dated between 933-1505 Ma (Paul, 1979; Paul et al., 1975). A U-Pb (perosvkite) age of 1079 was cited in Miller and Hargraves (1994) for the Muligirip alle kimberlite. The

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18 Lattavaram, Wajrakur and Mulligiripalle pipes have also been studied paleomagnetically by Miller and Hargraves (1994). The Wajrakur samples were unstable and yielded no useful data whereas the Lattavaram and Mull igiripalle pipes yielded sim ilar paleomagnetic data, although the site statistics showed poor grouping and larg e errors. Miller and Hargraves (1994) conducted a paleomagnetic study of Majhgawan and reported a paleomagnetic pole at 38.9 N, 216.5 E. Geology of the Kimberlite The Majhgawan kim berlite intrudes Baghain sandstone in the upper Kaimur Group, which lies below the Bhander-Rewa sequence in the Upper Vindhyan basin (Figure 2-1). The body is pear shaped with steeply dipping walls (80) a nd its surface originally outcropped over an area of 500 x 320 m (Chatterjee and Rao, 1995). Deformation of the Baghain sandstone is severe in the vicinity of the pipe. The Kaimur rocks have been shattered and tilted inward toward the pipe, but elsewhere in the region the Baghain sandstone and overlying rocks of the Bhander-Rewa Groups have very low dips (< 10) or are undeformed. The pipe has a va riable lithology and is composed of concentrically arranged units. The classification of the pipe is debated as it has characteristics of both a kimberlitic intrusion and an olivine la mporitic tuff. Chatterjee and Rao (1995) classify it as an intermediate between the two. The kimber lite is observed to ha ve three intervals of intrusion: (1) deep-green brecciated kimberlite; (2 ) botryoidal zone of kimberlite contaminated by shale xenoliths; and (3) basaltic kimberlite. The pipe is mined for diamonds and also contains megacrysts of olivine, phlogopite, ilmenite, pyr ope and enstatite with phologopite mica as a constituent of a serpentine and calcite matrix (Mukherjee et al., 1997). Phlogopite megacrysts can be placed into two distinct groups. Megacrysts are either rounded and commonly weathered or hexagonal and fresh (M ukherjee et al., 1997). Intraformational conglomerates within the Rewa Group near Panna are diamond-bearing suggesting that the deposition of the Rewa Group post-dates intrusion of the kimberlite (Mathur,

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19 1962, 1981). Rau and Soni (2003) examined th e provenance of the diamondiferous conglomerates in the Rewa Group and concluded that the source for the diamonds in the Rewa Group may be something other than the nearby Majhgawan or Hinota pipes. No significant erosional surfaces are pres ent in between the Kaimur, Rewa and Bhander Groups. Paleomagnetism Paleomagnetic Experiments Sa mples were drilled in the field using a gasoline powered drill and oriented using magnetic and sun compasses. Sun compass read ings were used to correct for the local declination and any rock magnetic interference. Limited outcrop a nd security concerns allowed us to collect only 10 samples from two sites in the brecciated kimberlites and basaltic kimberlites. Samples were cut into standard sp ecimens and stored in a magnetically shielded room at the University of Florida paleomagnetic laboratory. Preliminary samples were stepwise treated using thermal or alternating field demagnetization and after evaluation, a series of demagnetization steps was chosen. Alternati ng field demagnetization was conducted using a home-built AF-demagnetizer and with fields up to 100 mT. Thermal demagnetization was conducted up to temperatures of 600 C with an ASC-Scientific thermal demagnetizer and all samples were measured in a ScT Cryogenic magnetometer. Principle component analysis (Kirschvink, 1980) was used to determine the best fit lines for each sample. Rock Magnetic Experiments The susceptibility of each sam ple was measured before treatment on an Agico SI-3B bridge. In order to further characterize magnetic carriers, Curie temperature runs were conducted on several powdered samples using a KLY-3S susceptibility bridge adapted with a CS-3 heating unit. For this experiment, susceptibility is measured incrementally during heating and cooling of the samples. Isothermal remanence acquisition studies (IRM) were also conduc ted on select samples.

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20 Geochronology Sa mples of micaceous brecciated kimberlite we re crushed, sieved and phlogopite separates were hand picked. The individual phlogopite samples were then ultrasonical ly cleaned and rinsed several times. The grains were examined again under microscope and any grains with weathered margins or impurities were discarded. The grains were then wrapped in aluminum foil and sent to Oregon State Universitys irradiation facili ty. Gas from the samples was stepwise treated using CO2 irradiation laser and measured on a MAP215-50 mass spectrometer. The flux monitor was GA1550 biotite (age 98.8 .5 Ma; Renne et al., 1998) and analyses were performed by James Vogl. Results Geochronology The phlogopite m icas contain roughly 10% K2O (Mukherjee et al., 1997). Standard mineral separation and 40Ar/39Ar techniques were followed (see methods) on two splits of large grains of unweathered phlogopite s. Results for stepwise degassi ng of the samples are shown in Figure 2-2a (analytical results are given in Table 2-3, errors reported are 2s). The first split yields a well-defined plateau at 1061.7 9.7 Ma (66% of the gas, MSWD= 1.02) and a total fusion age of 1068.2 9.4 Ma. The isochron age for split #1 is 1068.3 14.0 Ma with a low MSWD of 0.46 (Figure 2-2b). The second split yields a concordant plateau age of 1078.4 11.4 Ma (82% of the gas, MSWD=3.31) and a total fusion age of 1080 10 Ma (Figure 2-2c). The isochron age for split #2 is 1072.2 21 Ma with an MSWD of 2 .70. Our best estimate for the age of the phlogopite micas from Majhgawan is given by a weighted mean of the ages within the plateau segments of both samples, which is 1073.5 13.7 Ma. Kimberlites and lamproites are both thought to be emplaced into the shallow crust on time scales ranging from several hou rs to several days (Skinner a nd Marsh, 2004). Thus, the age of

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21 the phlogopites should closely date the time of upper crustal emplacement a nd also the age of the magnetization in the rocks. Paleomagnetism A total of 10 samples were collected from the brecciated kimberlite and basaltic kimberlite (eight samples exhibited stable demagnetization behavior). Typical dema gnetization behavior is shown in Figure 2-3 using both alternating fi eld (Figure 2-3a) and thermal (Figure 2-3b) treatments. Alternating field tr eatments up to 60 mT removed over 90% of the natural remanent magnetization (NRM). Thermal demagnetization wa s applied up to temperatures of 600C and a loss of 90% of initial NRM stre ngth occurred at about 550C with a discrete unblocking temperature range between 500 and 550C. The mean direction from our samples is D=45.1, I=25.1 (k=37 and a95=9.3; Figure 2-4) Paleomagnetic data from Majhgawan, includi ng the data from Miller and Hargraves (1994) yield a mean direction with a declination of 37.5 and an inclination of -26.5 (k=66 and a95=15.3; Figure 2-4) with a virtual geom agnetic pole (VGP) of 36.8N, 212.5E (dp=9.0, dm=16.6; Figure 2-4). The VGP translates to a si te latitude of 14 +/-8.3/10. Rock magnetic and demagnetization behavior (Fig ures 2-3 and 2-5) indicate th e primary carrier is magnetite. Curie temperature runs (Figure 2-5) show a sharp drop in sus ceptibility at 591.3C and 605.1C, which are both slightly above the Curie temp erature for magnetite. Isothermal remanence magnetization tests show a rapid rise in intensity and near saturation at 0.4 to 0.7 Telsa. The rock magnetic tests and demagnetization behavior are both characteristic of magnetite with perhaps a small contribution from hematite. Petrographic observations were described in Miller and Hargraves (1994) who noted the presence of fine ma gnetite grains in the matrix and within mica flakes which in some cases had been altered to hematite. These observations are consistent with the rock magnetic data collected during our study (Table 2).

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22 Discussion and the Age of the Upper Vindhyan The age of the Upper Vindhyan sequence is cont entious. Previous estim ates on the age of Kaimur-Rewa and Bhander Groups unrealistically span over 1 billion years, from 1650 Ma to as young as Ordovician (Ray, 2006; Auden, 1933). The Bhander-Rewa Groups are at the center of this argument because they are generally considered to be Neoproterozoic in age (750-650 Ma; Ray et al., 2003; Kumar et al., 2002). The age estim ates of Ray et al. ( 2003) and Kumar et al. (2002) are based primarily on the comparison to global curves of 87Sr/86Sr ratios and d13C dates from the Bhander and Lhakeri limestones. These ratios were used to argue for an age of Bhander sedimentation between 750 and 650 Ma. A 750 Ma age for the Bhander limestones is precluded by the assumption of a primary magn etization carried in the Bhander-Rewa rocks (McElhinny et al., 1978). India has a well-defined paleomagnetic pole at 750 Ma derived from the Malani Igneous Suite (Torsvik et al., 2001) that is distinct from the published Bhander-Rewa directions (Figure 2-3). A late Neoproterozoic or Cambrian age for the Bhander-Rewa was supported by a comparison between paleomagnetic directions from Late Proterozoic-Cambrian age sedimentary rocks in the Salt Ranges of Pa kistan (Ref, McElhinny et al., 1978; Meert, 2003). Although the paleomagnetic poles from the Salt Ra nge rocks are similar to the Bhander-Rewa directions, Klootwijk et al. (1986) demonstrated significant vertical axis rotation of units in the Salt Ranges (up to 45 ) during the Tertiary negating the vali dity of comparisons between these units. Additional evidence forwarded in support of a Neoproterozoic age is generally poor. The previously noted fossils from De (2003) do not pr ovide a well-constrained date. Identification of the impressions as Ediacaran is problematic; however, more recently documented fossils (De, 2006) from the same region show much better pr eservational characteristics and would suggest that the Lhakeri limestone is younger than ca 635 Ma. Chakrabarti (1990) also documents

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23 possible trace fossils in the Bhander Group. These are noted as burrow zones with large diameter burrows (0.5-4.5 cm) and micro-burrows (less than 1.5 mm) that are lined with a thin layer of clay. Yet it is mentioned that these traces may be dubiofossils produced by either sand collapse or fluid escape. Nevertheless, Chakrabarti (1990) suggested a Riph ean age (1400-800 Ma) for the Bhander-Rewa Groups. Both Chuaria and Tawu ia fossils have been found in the BhanderRewa (Rai et al., 1997; Kumar a nd Srivastava, 2003), but the age range of Chuaria may span as far back as the Paleoproterozoic (Steiner, 1997) and thus they are not useful as index fossils for the Neoproterozoic. Chuaria and Tawuia are repo rted in the Suket shal es of the Lower Vindhyan Semri Group (Kumar, 2001). Steiner (1997) gives a preferred range for the Chuaria-Tawuia assemblage from 1000-700 Ma. Lastly, although the exact numbe r of glaciations and the exte nt of those glaciations are debated, the Neoproterozoic is known for the pres ence of glaciogenic sequences on nearly all the continents (Evans, 2002). The Upper Vindhyan sequence shows only cr yptic evidence of glaciogenic sediments despite a nearly continuou s record of sedimentation (Prasad, 1984; Kumar et al., 2002). Kumar et al. (2002) noticed a large negative shift in 13C values in the Lhakeri limestone (Bhander Group, Rajasthan) and argued that it might represent a peninsular India equivalent of a cap-carbonate. Kumar et al. (2002) also menti oned the occurrence of tilloid rocks beneath the Lhakeri limestone in Rajastha n as potential representa tives of the Snowball Earth glaciation. In contrast, the Lhakeri limes tone in the Son Valley section shows no negative d13C excursion (Ray et al., 2003) sugge sting either that (a) previous correlations of these units is incorrect or (b) one of the carbon studies is incorrect or (c) carbon isotopic rations show lateral variations across the basin (see also Kaufman et al., 2006). Our geochronologic data from the

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24 Majhgawan kimberlite suggests that the Bha nder-Rewa Groups are both younger than ca. 1075 Ma. Paleomagnetic data from our study along with previous studies of the Majhgawan kimberlite (Miller and Hargraves, 1994) and the Bhander-Rewa Groups (McElhinny et al., 1978) present an intriguing alternat ive interpretation for the age of the Upper Vindhyan. Prior paleomagnetic studies have been report ed on the Bhander-Rewa Groups overlying the Majhgawan kimberlite (McElhi nny et al., 1978, Klootwijk et al., 1973; Athavale, 1963). The paleomagnetic directions from the Bhander-Rew a yield a mean declination of 34.2 and an inclination of -20.3 (k=35, a95=7). This compares favorably with the Majhgawan data reported in this study and that by Miller and Hargraves (1994; Figure 2-3). There are several explanations for the similari ty in directional data from the Majhgawan kimberlite and the Bhander-Rewa Groups. One option is that they are both remagnetizations of Neoproterozoic or Cambrian age. Other possibilitie s are (a) that the Mes oproterozoic directions in the Majhgawan kimberlite are fortuitous ly identical to the much younger Bhander-Rewa Group (b) the kimberlite was remagnetized during Bhander-Rewa time or (c) unrecognized tilting of the Majhgawan kimberlite might result in the incorrect use of the in-situ directions. A final possibility is that the Bhander-Rewa Groups are only slightly younger than the Majhgawan kimberlite. At present, we feel co mfortable rejecting the possibility of unrecognized tilting of the kimberlite. Tilting of the Kaimur sandstone away from the intrusion is minor (<10 ) and therefore any post-intrusion tilting is likely to be minor as well. The other explanations require a detailed paleomagnetic study of the Bhander and Rewa Groups. We are currently in the process of completing such a study.

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25 Conclusions Our geochronologic study of Majhgawan yiel ds a well constrained 40Ar/39Ar age of 1073.5 13.7 Ma and a virtual geom agnetic pole at 36.8N, 212.5E (dp=9.0, dm=16.6) that is similar to previously published paleomagnetic poles in the overlying Bhander-Rewa Groups (41.6 N, 212.3 E; dp=3.8, dm=7.2). Our age help s constrain the age of sedimentation in the Upper Vindhyan Bhander and Rewa Groups to less than 1075 Ma. Acknowledgements: The authors would like to acknowledge support from the National Science Foundation EAR04-09101 (to JGM), Jim V ogl for his assistance in acquiring and analyzing the argon data in this paper, for Linda Sohl, Dhiraj Ba nerjee and Bob Tucker for their assistance in the field.

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26 Figure 2-1. Generalized stratig raphic column, regional and local geologic map. (a) Generalized stratigraphic column of the Vindhyan Supe rgroup shown with radiometric age constraints. The Majhgawan kimberlite intr udes the Kaimur sandstone of the Upper Vindhyan. (b) Geologic map of the region in cluding the Vindhyanchal basin and the study area near Panna, India. (c) geologic map of the Majhgawan kimberlite (after Chatterjee and Rao, 1995). Sampling locations for paleomagnetism were taken from the basaltic kimberlite and the brecciated kimberlite. Samples from the Miller and Hargraves (1994) study were solely from the brecciated kimberlite.

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27 Figure 2-2. Stepwise degassing spectrum for Ma jhgawan phlogopites (2 splits). Both show welldefined plateaus that overla p at the 2s level. The average age obtained from a weighted mean of plateau steps for splits 1 and 2 is 1073.5 13.7 Ma

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28 Figure 2-3. Zijderveld plots, stereoplots, and thermal/AF dema gnetization trends for select samples. (a) Thermal demagnetization Zijderveld plots for a brecciated kimberlite sample showing univectorial behavior (b) Alternating field demagnetization Zijderveld plot for a brecciated kimberlite sample showing univectorial behavior. (c) Intensity decay plot for ther mally demagnetized sample and (d) Intensity decay plot for an alternating field demagnetized sample.

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29 Figure 2-4. Site mean directions, paleomagnetic and virtual geomagnetic poles for critical rock units. (a) Site mean directions (tilt-corrected) from 14 sites in the Bhander and Rewa Groups obtained in an ongoing study by the au thors (reported in Malone et al., 2005) and are identical to those reported pr eviously by McElhinny et al., 1978 for the Bhander and Rewa Groups (b) Site means fo r three sites in the Majhgawan kimberlite collected in this study and al so the results of Miller a nd Hargraves (1994). We also show the combined mean pole for the Malani igneous province including results from late-stage Malani dykes reported in Gre gory et al. (2005). (c) Paleomagnetic and virtual geomagnetic poles for the Bh ander-Rewa Groups, the Malani Igneous province and the Majhgawan kimberlites.

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30 Figure 2-5. Magnetic characteriza tion data. (a) Temperature-Suscep tibility graph for a sample of the Majhgawan kimberlite showing a heat ing and cooling Curie temperature of 591.3 C. (b) Temperature-Susceptibility graph for a sample of the Majhgawan kimberlite showing a heating Curie temp erature of 605.1 C and a cooling Curie temperature of 592.6 C. (c) Isothermal re manence acquisition curve for a sample of Majhgawan kimberlite showi ng saturation at 0.4 Tesla.

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31 Table 2-1. Age constraints on Vindhyan sedimentary sequences. Stratigraphic Layer Method Age Ma Reference Upper Vindhyan: Bhavpura Shale (Bhander) K-Ar 550 Crawford and Compston 1970 Bhander Limestone Pb-Pb 650 Ray et al., 2003 Bhander Limestone Sr-isotopes 750 Ray et al., 2003 Bhander Limestone fossils Ediacaran-CambrianDe, 2003 Kaimur Conglomerate K-Ar 940 Vinogradov et al., 1964 Kaimur Conglomerate K-Ar 1071.6 169.3 Srivastava and Rajagopalan, 1988 F-T 1070.5 160.4 Srivastava and Rajagopalan, 1988 Lower Vindhyan: Rotasgarh Limestone K-Ar 1400 70 Vinogradov et al., 1964 Rotasgarh Limestone Pb-Pb 1601 130 Ray et al., 2003 Rohtas formation Pb-Pb 1599 Sarangi et al., 2004 Rampur formation K-Ar 1110 60 Vinogradov et al., 1964 Rampur formation K-Ar 1124.5 157.8 Srivastava and Rajagopalan, 1988 Rampur ash beds Pb-Pb 1592 12 1602 10 Porcellanites U-Pb 1628 Rasmussen et al., 2002 1631.2 5.4 Rasmussen et al., 2002 1630.7 0.8 Ray et al., 2002 Kimberlites: Majhgawan Kimberlite Ar-Ar 1073 13.7 This study Majhgawan Kimberlite K-Ar 1116 12 Crawford and Compston, 1970 Majhgawan Kimberlite K-Ar 947 30 Paul et al., 1975 Majhgawan Kimberlite K-Ar 1170 Paul et al., 1975 Majhgawan Kimberlite K-Ar 1056 Crawford and Compston, 1970 Majhgawan Kimberlite Rb-Sr 1044 Smith, 1992 Majhgawan Kimberlite Rb-Sr 1067 Kumar et al., 1993 Wajrajur kimberlite Rb-Sr 1350 Paul, 1979 Wajrajur kimberlite Rb-Sr 1116 Crawford and Compston, 1973 Lattavaram kimberlite Rb-Sr 1505 Paul, 1979 Lattavaram kimberlite K-Ar 933 Paul et al., 1975 Muligiripalle kimberlite U-Pb 1079 Miller and Hargraves, 1994

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32 Table 2-2. Summary of paleomagnetic results. Study Ns(#sam ples) Dec Inc K 95 Pole Lat Pole Long dp dm Majhgawan This paper 8 45.325.137 9.3 33.5N 203.3E Miller and Hargraves site 11 6 40.2 -32.3 28 12.832.4 N 213.2 E Miller and Hargraves site 21 6 27.4 -21.6 54 9.2 45.3 N 220.1 E Mean of 3 sites 22 37.526.566 15.336.8N 212.5E 9.0 16.6 Malani Mean2 4 Studies 359.162 73.1 7 7.9 72.7 N 70.5 E 9.5 12.3 Bhander-Rewa Mean3 18 Sites 34.220.321.27 41.6N 212.3E 3.8 7.2 Ns= number of sites or #samples; Dec=declination; Inc=inclination; 95= circle of 95% confidence; Pole lat=latitude of the paleomagnetic pole; Pole long=longitude of paleomagnetic pole; (dp,dm) cone of 95% confidence about the paleomagnetic pole in th e co-latitude direction (dp) and at a right angle to the co-latit ude direction (dm), 1 Miller and Hargraves (1994), 2 Mean reported from Klootwijk et al ( 1975), Athavale et al. (1963), Torsvik et al. (2001) and Gregory et al. (2005), 3 Mean reported from McElhinny et al. (1978).

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33Table 2-3. Analytical Data Power 36Ar 37Ar 39Ar 40Ar 40Ar* 40Ar*/39Ar Cumulative Calculated ageError in age K/Ca (watts) Volts Volts Volts Volts % 39Ar (%) (Ma) ( 2 s.d.) Majh01 biotite; J = .00384 0.11 (2.1) 0.00067 0.00024 0.00126 0 0 0.00000 0.11 0 0.00 2.26 0.21(2.3%) 0.00013 0.00061 0. 00134 0.03697 49.84 27.68094 0.12 182.24 109.22 0.949 0.32 (2.6%) 0.001 0.00792 0.00224 0 0 0.00000 0.2 0 0.00 0.122 0.42 (3%) 0.00012 0.00147 0. 00153 0.19068 84.56 124.67805 0.14 705.77 79.65 0.448 0.74 (3.8%) 0.00026 0.00218 0. 03306 7.07956 98.91 214.17018 2.94 1082.74 12.57 6.526 0.80 (5%) 0.00004 0.00018 0. 04522 9.68697 99.87 214.23648 4.02 1082.99 8.40 110.226 0.85 (4.0%) 0.00006 0.00036 0.052 48 11.18651 99.82 213.15344 4.66 1078.87 9.84 62.358 0.95 (4.2%) 0.00022 0.00294 0.172 32 36.33149 99.81 210.83660 15.32 1070.02 14.83 25.176 1.0 (4.4%) 0.00013 0.0018 0.090 53 18.76306 99.79 207.25576 8.05 1056.26 20.53 21.625 1.06 (4.6%) 0 0.00382 0.10145 21.34383 99.99 210.38313 9.02 1068.28 14.41 11.425 1.16 (4.9%) 0.00036 0.00368 0.138 38 28.65372 99.61 207.05847 12.3 1055.5 8.87 16.163 1.27 (5.2%) 0.00002 0.00088 0.125 29 26.35047 99.97 210.30834 11.14 1068 14.83 61.311 1.38 (5.4%) 0.00003 0.00114 0.112 19 23.37715 99.95 208.38027 9.97 1060.59 12.97 42.283 fused 0 0.00019 0.24779 53.66914 99.99 216.59434 22.02 1091.93 12.90 563.464 Majh2 biotite; J = 0.00384 0.42 (3.1%) 0.00240 0.00161 0.00655 0.05114 6.72 7.80610 0.84 53.28 62.08 1.748 0.74 (3.8%) 0.00120 0.00000 0.02612 5.57510 94.03 213.44767 3.36 1079.99 11.96 0.000 0.85 (4.1%) 0.00081 0.00000 0.02899 6.52504 96.44 225.05125 3.73 1123.63 11.78 0.000 0.95 (4.3%) 0.00046 0.00000 0.03769 8.40122 98.40 222.90544 4.85 1115.64 9.06 0.000 1.06 (4.5%) 0.00030 0.00000 0.04095 8.91638 99.01 217.75048 5.27 1096.30 12.24 0.000 1.16 (4.7%) 0.00077 0.00334 0.09027 19.11802 98.81 211.79462 11.61 1073.69 8.44 11.637 1.16 (4.8%) 0.00113 0.00139 0.06515 14. 16368 97.68 217.38796 8.38 1094.93 16.83 20.122 1.16 (4.9%) 0.00054 0.00287 0.06728 14. 54292 98.90 216.14813 8.65 1090.24 9.15 10.094 1.16 (5.1%) 0.00047 0.00300 0.06300 13.28969 98.96 210.96016 8.10 1070.49 11.77 9.016 1.38 (5.4%) 0.00033 0.00186 0.09505 20.39615 99.51 214.57669 12.22 1084.28 24.72 21.950 1.48 (5.6%) 0.00015 0.00000 0.03697 7.76653 99.40 210.09280 4.75 1067.17 10.00 0.139 fused 0.00184 0.00134 0.21955 47.235 23 98.85 215.15006 28.23 1086.46 16.11 70.453 (1) Atmospheric argon, (2) Calcium interfer ence, (3) Radiogenic argon, (4) Percent of argon gas releas ed, (5) J parameter is th e standard flux monitor used to normalize the am ount of K converted to Ar during irradiation

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34 CHAPTER 3 PALEOMAGNETISM AND GEOCHRONOLOGY OF T HE MALANI IGNEOUS SUITE, NORTHWEST INDIA: IMPLICATIONS FOR THE CONFIGURATION OF RODINIA AND THE ASSEMBLY OF GONDWANA2 Introduction The notion of a Mesoto Neoproterozoic s upercontinent form ed in the aftermath of Grenvillian orogenesis began to develop in the 1970s (Piper, 1976; Bond et al., 1984). The name of Rodinia was proposed in the ear ly 1990s (McMenamin and McMenamin, 1990; Dalziel, 1991; Moores, 1991; Hoffman, 1991). There are myriad configurations proposed for the Rodinia supercontinent and the exact paleolocati ons of its constituents are unresolved (Dalziel, 1991; Moores, 1991; Hoffman, 1991, Meert and Torsvik, 2003; Li et al., 2008). The archetypal model for Rodinia outlines that the supercontinent began to form at about 1300 Ma and reached maximum size at about 1000 Ma. Fragmentation and breakup of R odinia was initiated sometime between 800-700 Ma along a rift between western (present-day c oordinates) Laurentia and East Antarctica-Australia (Bond et al., 1984; Dalzie l 1991; Hoffman, 1991; Powell et al., 1993). It is hypothesized that this rifting heralded a period of intense global cooling, sparking the development of multi-cellular life on Eart h (Hoffman, 1998;Meert and Lieberman, 2008). Knowledge of the distribution and geotectonic evolution of continents related to Rodinia breakup is critical for an improved understanding of the context and causes of extreme climatic changes and accelerated biologic evoluti on at the enigmatic boundary be tween the Neoproterozoic and the Paleozoic. The assembly of the supercontinent Gondwan a followed the fragmentation of Rodinia. Eastern Gondwana comprised crat onic blocks that are currently within India, Madagascar, Sri 2 Reprinted with permission from Gregory, L.C., Meert, J.G., Bingen, B., Pandit, M.K., Torsvik, T.H., 2008. Paleomagnetism and Geochronology of the Malani Igneous Suite, Northwest India: Implications for the configuration of Rodinia and the assembly of Gondwana. Precambrian Research, submitted.

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35 Lanka, East Antarctica, Australi a and the Seychelles. The paleoge ography of these cratons prior to the formation and after breakup of Rodinia is not well constrained. Some (Windley et al., 1994; Piper, 2000; Yoshida and Upreti, 2006; Squire et al., 2006; Paulsen et al., 2007) argue that these cratons came together in a single collisional ev ent around or even ea rlier than 1300 Ma, were fused in that same configuration within Rodinia, and remained so until the breakup of Gondwana in the Mesozoic. More consistent with available geologic, paleomagnetic and geochronologic data is the alternative formati on of eastern Gondwana as a polyphase assembly of cratonic nuclei that were se vered and separately dispersed from the Rodinia supercontinent (Meert et al., 1995; Meert and Torsvik, 2003; Meer t, 2003; Boger et al., 2002; Fitzsimons, 2000; Pisarevsky et al., 2003; Collins and Pisarevsky, 2005) This dispute may ultimately be resolved through the acquisition of high-quality paleom agnetic data coupled to high-resolution geochronologic ages from the various cratons that comprise Gondwana. Unfortunately, many extant studies are incomplete in that they do not incorporate an age with paleoposition and thus do not place strong spatial-temporal constraints on ancient con tinent localities. The location of India within Gondwana is critical for evaluating the various tectonic models related both to the assembly of greater Gondwana and models of Rodinia. Greater India is placed alongside East Antarctica in the trad itional Gondwana fit at 560 Ma (Figure 3-1; deWit et al., 1988), and some extrapol ate the India-Antarctica-Australia connection to exist within Rodinia and even earlier supercont inents (Dalziel, 1991; Li et al ., 1996; Weil et al., 1998; Owada et al., 2003). However, due to paleomagnetic a nd geologic correlations (o r lack thereof), many workers have since suggested that India maintained a significant latitudinal offset from the archetypal Gondwana fit with Antarctica (Fit zsimons, 2000; Torsvik et al., 2001a; Powell and Pisarevsky, 2002).

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36 The Malani Igneous Suite (MIS) in northw est India provides potentially critical paleomagnetic and geochronologic data for th e Indian subcontine nt during the late Neoproterozoic. Outcropping near Rajasthan (Figure 3-2), the MIS is estimated to be one of the largest felsic igneous suites in the wo rld (51,000 km2; Pareek, 1981; Bhushan, 2000). Paleomagnetic studies from the MIS are thought to define the key paleomagnetic pole for the Indian subcontinent at ca. 750 Ma. In this study, we augment previous work via the addition of paleomagnetic data from late-stage mafic dikes along with precise U-Pb ages from the earlier erupted rhyolitic tuffs. Combined, these data co ntribute a key paleopole for the Indian plate during the Neoproterozoic, and lead to a discussion on the drift of India between the diffusion of Rodinia and the formation of Gondwana Geology and Tectonic Setting Magm atism in the MIS occurred in three phases. Activity commenced with an initial volcanic phase made up of basaltic then felsic flows. The second phase is characterized by the intrusion of granitic plutons. Predominately fels ic and minor mafic dike swarms form the third and final phase of the igneous cycle. Malani felsic rocks are un-metamorphosed, but slightly tilted and folded. Late stage mafic dikes are all vertical to sub-vertical (Figure 3-3). The MIS unconformably overlies Paleoto Mesoproterozoic metasediments, and basement granite gneisses and granodiorites of an unknown age (Pa ndit et al., 1999). The suite is unconformably overlain by the flat-lying late Neoproterozoic to Cambrian Ma rwar Supergroup, made up of redbed and evaporite sedimentary sequences (Pandit et al., 2001). A volcaniclastic conglomerate lies at the base of MIS (Bhushan, 2000) and basal rhyolitic tuffs denote the initiation of basaltic and largely felsic flows of the first stage of the suite. This felsic extrusive episode was followed by the empl acement of granitic plutons and felsic dikes. Vertical to sub-vertical doler ite dikes crosscut all of the ot her components and thus mark the

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37 termination of magmatism. These mafic dikes intr ude the Jalore Granite plutons south of Jodhpur and can be wide, up to 15 meters in extent (Figure 3-3). The ma fic dike sequence near Jalore contains a relatively dense concentration of dike s with a general N-S tren d. Many of the larger dikes form conspicuous ridges only when enclosed in a granite host and weather out as bouldery traces (Figure 3-3a). Fresh in-p lace outcrop is difficult to find but we sampled 4 dikes exposed only in a granite quarry, most of which trend NS. Sampling included a very thin N-S trending dikelet that is cut by a wider E-W trending dike th at was also sampled. This dikelet is less than 2 cm wide, aphanitic and dark grey-black in co lor with obvious chilled margins (Figure 3-3b). There was no clear generative connection betwee n this small dikelet a nd larger (nearby) N-S dikes; however, we cannot eliminate the possibility th at it is rooted in a la rger dike that was not exposed at this particul ar level in the quarry. India magmatism can be compared with re lated Neoproterozoic i gneous provinces on nearby cratons. Paleomagnetic da ta juxtapose the Seychelles al ongside India, and northeastern Madagascar is also placed along the India marg in based on temporal and geological similarities (Torsvik et al., 2001b; Ashwal et al., 2002). The sequence of rocks on Seychelles is distinctly similar to the Malani suite, and it is postulated that the geochemical signatures in the Seychelles are sourced from the Archean Banded Gneiss Complex near Rajasthan (Ashwal et al., 2002). The majority of Neoproterozoic granito id and doleritic activity in th e Seychelles falls within 755-748 Ma (Ashwal et al., 2002), with a span of relia ble ages ranging from 808-703 Ma (Stephens et al., 1997). If the Seychelles suite of Neoproterozoic rocks is analogous to the Malani province, the dolerite dikes sampled in this study can be cons idered as equivalents to dolerite dikes of Seychelles. The Seychelles dikes are geochemically related to Seychelles granitoids (Ashwal et al., 2002), and a U-Pb zircon age at 750.2.5 Ma from one of those dikes (Takamaka dike)

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38 indicate they are nearly coeval with the felsic magmatic suite (Torsvik et al., 2001b). Slightly younger, but overlapping, ages (715-754 Ma) from the Daraina sequence in northern Madagascar are also correlated to the igneous activity in the Seychelles and India (Tucker et al., 1999). Among the multitude of tectonic settings proposed for Malani magmatism, it is suggested (see Bhushan, 2000) that the first stage of associated basaltic and felsic flows is generated by a hot spot source or lithospheric thinning and melting at the base of the crust. However, both Madagascar and Seychelles have igneous activity from this time that is attributed to subduction of the Mozambique Ocean (Figure 3-1 inset; Ha ndke et al., 1999; Torsvik et al., 2001b; Tucker et al., 2001; Ashwal et al., 2002). The MIS is ofte n described as anorogenic magmatism related either to crustal melting during extension or to an active hot spot (Bhushan, 2000; Sharma, 2004). Alternatively MIS magmatism can be interpre ted in the context of an Andean-type active margin (Torsvik et al., 2001a; Torsvik et al., 200 1b; Ashwal et al., 2002), closely related to the nearby, and coeval, arc activity observed in the Se ychelles islands and northeastern Madagascar. The duration of magmatism and the source of igneous activity in the MIS, Seychelles and northeaster Madagascar are st ill questionable (Collins and Windley, 2002; Collins, 2006). Previous Studies Paleomagnetism Num erous paleomagnetic studies have been performed on the felsic members of the MIS to determine the paleoposition of India at ca. 750 Ma (Table 1). Athavale et al. (1963) were the first to apply paleomagnetic tests to rhyolitic flows, and their results were similar to those obtained by Klootwijk (1975), but both studies lacked any detailed stability tests. Torsvik et al. (2001a) found a statistically positive fold test on felsic rocks fr om Malani. Folding of the Malani rocks occurred after eruption and prior to deposition of th e flat-lying Cambrian Marwar supergroup, which constrains the age of the Mala ni pole to older than Cambrian. Late stage

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39 mafic dikes had not been subjected to paleoma gnetic study prior to our work. No reversals or other field tests were found to further docum ent the exact age of magnetism. The lack of additional positive field tests to fully constrai n the age of magnetic acquisition has invoked some doubt in the primary nature of the Mala ni pole (Yoshida and Upreti, 2006). Geochronology Previous geochronolog ic results from Malani felsic volcanics span about 100 million years (Table 2). Crawford and Compston (1970) repor ted a pioneering Rb-Sr age of 730 Ma for rhyolites (re-calculated with a decay constant of 1.42 x10-11; Steiger and Jager, 1977). Later, Dhar et al. (1996) and Rathore et al. (1999) reported whole-ro ck Rb-Sr isochron ages ranging from 779 to 681 Ma for felsic volcanic rocks and granite plutons, emplaced during the first two stages of activity in the MIS (first and second stag es, respectively). This wide distribution of dates is partiall y a result of studies of the so-called ultrapotassic rhyolites found near our sampling locality. The youngest Rb-S r isochron age of 681 Ma (Rathore et al., 1999) comes from a solitary occurrence of th e ultrapotassic rhyol ite without any rock description that would ascertain whether high potassium is a primary igneous character or later alteration effect. Much younger a pparent ages of 548 to 515 Ma were obtained from whole-rock 40Ar/39Ar data on Jalore granites. These apparent ages are interpreted as evidence for a thermal disturbance by Rat hore et al. (1999) that may be related to the Kuungan or the Malagasy orogenies (Meert, 2003; Collins and Pisarevsky, 2005); however, the metamorphic grade of the rocks is incompatible with any significant thermal reset ting (Ashwal et al., 2002). Torsvik et al. (2001a) cited precise U-Pb ages of 771 and 751 Ma for rhyolite magmatism in the MIS (Tucker, unpublished), but without analytical details and sample descriptions.

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40 Methods Paleomagnetic Sampling and Experiments Sa mples were obtained in the field using a ga soline powered hand dri ll and oriented using magnetic and sun compasses. Readings from the sun compass were used to correct for the local declination and any magnetic in terference from the outcrop. Twelve samples from Jalore Granite and about 50 samples from four mafic dikes were taken at three sites. Three of the granite samples include a small (width less than 2 cm) fi ned grained N-S trending dikelet (Figure 3-3b), which is crosscut by a 4-meter wide E-W trendi ng mafic dike. The dikelet was sampled at 1.8 meters away from the larger dike, within a half -dike width distance. It was possible to drill only three cores, as boulders obstruc ted the remaining dikelet outcrop. Samples were cut into standard sized specime ns and stored in a magnetically shielded space in the University of Florida paleomagne tic laboratory. A few pre liminary samples were stepwise treated thermally or with an alternating field to determine the best method of demagnetization. After analyzing th e behavior of preliminary samp les, a series of steps were chosen for either alternating field or thermal demagnetization. Alternating field demagnetization was applied in steps using a home-built AF-demagne tizer at fields up to 140 mT. Samples were also treated thermally in a stepwise manner, up to temperatures of 600 C for ~60 minutes using an ASC-Scientific oven. Between each treatment, strong samples (generally mafic dikes) were measured on a Molspin Magnetometer and weak er samples (dikelets and granites) were measured on an ScT cryogenic magnetometer. Characteristic remanence components (ChRc) were calculated with least-squa re regression analysis implemented in the Super IAPD program ( http://www.ngu.no/geophysics ).

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41 Rock Magnetic Experiments The m agnetic susceptibility of each sample was measured on an Agico SI-3B bridge before treatment. Curie temperature experiments were run on select powdered samples using a KLY-3S susceptibility bridge with a CS-3 heating unit. For this experiment, the sus ceptibility of a crushed sample is measured at increments during he ating and cooling. The character of magnetic minerals in the sample can then be determined in detail based on the change in susceptibility with temperature. Isothermal Remanence Acquis ition (IRM) studies were also performed using an ASC-IM30 impulse magnetizer to further characterize magnetic mineralogy. Geochronology Zircon was purified from one sam ple of rhyoli tic tuff using a water table, heavy liquids and a magnetic separator. Available crysta ls were mounted in epoxy and polished to approximately half thickness. Cathodoluminescen ce (CL) images were obtained with a scanning electron microscope (Figure 3-4). U-Pb analyses were performed by secondary ion mass spectrometry (SIMS) using the CAMECA IMS 1270 instrument at the NORDSIM laboratory, Swedish Museum of Natural History, Stockhol m (Table 3). The analytical method, data reduction, error propagation and assessment of th e results are outlined in Whitehouse et al. (1999). The analyses were conducted with a sp ot size of ca. 20 m, calibrating to the Geostandard of 91500 reference zircon with an age of 1065 Ma (Wiedenbeck et al., 1995). The error on the U-Pb ratio includes propagation of the error on the day-to-d ay calibration curve obtained by regular analysis of the reference zircon. A common Pb correction was applied using the 204Pb concentration and present-day isotopi c composition (Stacey and Kramers, 1975). The ISOPLOT program (Ludwig, 1995) was used to regress and present the SIMS U-Pb data.

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42 Results Geochronologic Results Zircon U-Pb geochronology was conducted on a sam ple of unfoliated rhyolitic tuff representing the first stage of volcanism in the MI S. The sample, Mis5/04, was collected close to Jodhpur (26.963-72.357) at site 3 of Torsvik et al. (2001a). The sample shows ca. 5 mm automorphic phenocrysts of quartz, plagioclase an d K-feldspar in a microcrystalline devitrified groundmass of rose color. The sample contains few large (ca. 200 m) prismatic zircon crystals. They show well-terminated pyramid tips and os cillatory zoning and contain common fluid and mineral inclusions. Their habit is typical for zircon formed in a volcanic/subvolcanic magmatic environment (Hoskin and Schaltegger, 2003). Sixteen analyses were made on 10 zircon crystals. Fourteen of them are concordant and define a concordia age of 771 5 Ma (MSWD = 1.5; Figure 3-4). This age is interpre ted as the timing of magmatic crystallization and deposition of the rhyolite tuff. Rock Magnetic Results Curie tem perature runs on the mafic dike samp les show a curve that is characteristic of magnetite, but with some alteration upon cooling (F igure 3-5b). Susceptibility is higher during heating than cooling, but Curie temperatures are similar and in the typical range of magnetite. The heating Curie temperature TcH is equal to 589.7 C and the cooling Curie temperature TcC is equal to 588.3 C. When subjected to temperatures up to 700 C, mafic dikelets displayed substantial alteration and comparatively high sus ceptibility while cooling (Figure 3-5c). During demagnetization, samples are only heated to about 600 C, so Curie temperature tests on the samples from the same dike let were run up to only 600 C and Curie curves showed far less alteration (Figure 3-5d). Thus a lteration at high temperatures (>600 C) is not an issue during demagnetization, and any alteration observed at lower temperatures is probably due to exsolution

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43 and conversion from Ti-magnetite to pure magnetite. Mafic dikes also have an Isothermal Remanance Magnetization (IRM) plot that is indicative of magnetite (Figure 3-5a). Samples saturate at ~0.3 tesla and their intensity remain s constant at higher fields, up to the highest applied field of 2 tesla. Sample I434-28 is a mafic dikelet, and has an IRM curve also characteristic of magnetite, but with a lowe r absolute J value at saturation. Thermal demagnetization curves show unblocking at the ch aracteristic magnetite temperature range of 550 to 570C (Figure 3-6a). Paleomagnetic Results Table 4 lists paleomagnetic results from each site in this study. The mean direction resolved from four mafic dikes has a declination=358.8 and inclination=63.5 (k=91.2 and 95=9.7; Figure 3-8), after inver ting one reverse polarity dike. The Virtual Geomagnetic Pole (VGP) calculated from the average di rection of each dike falls at 70.2 N, 70.1 E (dp=12.1 dm=15.4 ). Figure 3-6 shows the typical demagnetizati on plots from two mafic dike sites. Most samples show a stable demagnetization trend, dependent on the treatment applied. Thermally treated samples unblock between 550 and 570 C and quickly lose over 50 percent of their intensity at this temperature range (Figure 3-6a). Samples treated with an alternating field lose intensity at a more gradual rate and do not ge nerally unblock past greate r than 80-85 percent of the original strength (Figure 3-6b). Most samples have a low temperature or low coercivity overprint that has no consistent direction, but is quickly remove d. Jalore granite samples were taken with the intent to perform a baked contac t test at site 34, but samples are dominated by multi-domain grains that have a strong, but unsta ble remanence, even with the application of low-temperature liquid nitrogen demagnetization. No stable dire ctions were obtained from any granite samples, and intensity data also have no detectable trend with increasing distance from the large dike (up to 20 meters away).

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44 Three samples were taken from the less than 2 cm wide dikelet pictured in figure 3-3b. The 4 meter wide dike at site 34 crosscuts this apha nitic dikelet at 1.8 meters away from the sampling location. Dikelet cores were taken at close to half-dike width aw ay from the larger dike to observe the effects of dike emplacement on su rrounding rocks. Unfortunately, due to limited outcrop, it was not possible to take a larger collection of dikelet sa mples. When treated with both alternating field and thermal demagnetization, samples displa y a high-temperature component that is antipodal to the three larger Malani di kes (Figure 3-7). Demagne tization trends of the dikelets include two distinct components and samples are weaker in intensity th an the larger dikes. They show an increase in inte nsity at temperatures up to about 490 C or fields to 40 mT (Figure 3-7). The low temperature and low coer civity component is identical to the mean direction from the normal polarity dikes with a declination=2.5, inclination=+57.5 (k=17.1 and 95=30.8), which is much steeper than the present-earth field in th e region (inclination=43.4 ). The high temperature and high coercivity component has a reverse polarity with declination=195.3 and inclination=-59.7 (k=234.8 and 95=8.1). Discussion Significance of Paleomagnetic and Geochronologic Data When results from Malani mafic dikes (this st udy, 4 sites) are combin ed with the trachyte and rhyolitic volcanics from Torsvik et al. (2001a; 9 sites) and Kl ootwijk (1975; 6 sites, Table 4), a mean direction is ob tained with declination=001 inclination=63.0 (k=32.9, 95=5.9) and paleolatitude of 44.5 From this mean direction, a paleomagnetic pole for the MIS is placed at 67.8 N, 72.5 E (A95=8.8 ) after averaging VGPs from the 19 s ites. The angular dispersion (S) of measured VGPs can be compared to the latitudinal variation in VGP angular dispersion as determined by Merrill et al. (1996) from IGRF 90 (1990 International Geomagnetic Reference Field). VPGs from the Malani suite at paleolatitude 44.5 have an angular variance of 20.7

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45 about the mean pole, calculated from the best estimate of angular variance for VGPs (equation 6.4.2 in McElhinny and McFadden, 1996). This value lies within th e average VGP scatter that represents the time-averaged field about the eart hs spin axis. The mean paleomagnetic pole for the MIS thus sufficiently averages secular variati on, and such a scatter is generally inconsistent with a blanket remagnetization of the area. The focus of this study is to reinforce th e mean pole for India at ca. 770 Ma with paleomagnetic data from the last stage of MI S magmatism, and pair this with a robust age determination that was previously cited as a personal communication with unpublished and unavailable analytical data. The fortuitous sa mpling of a small dikelet with unique magnetic behavior provides even further, albeit tentativ e, support for the primary nature of the Malani pole. No reverse polarity direction or baked contac t test was determined in previous work on the Malani suite. There are three possible interpreta tions for the magnetism observed in the dikelet: (1) the dikelet was emplaced in the same swarm as larger mafic dikes and experienced a true self-reversal, (2) the result is s purious and an unstable anomaly, or (3) the dikelet was emplaced aftere a field reversal and baked by the intrusion of subsequent mafic dikes, some thousands of years later and during a normal polarity field. Opti on (1) is very unlikely based on descriptions of observed natural self-reversing behavior. True self-reversal very rarely occurs in exsolved titanomagnetite compositions of basalt flows. The high Curie temperature phase (magnetite) aligns itself with the external field and influences the low temperature phase to the point of reversal (Merrill and McElhinny, 1983). This oc curs in coarse, multi-domain grains during a slower cooling than that associated with aphanitic dikelet emplacement (Petherbridge, 1977; Merrill and McElhinny, 1983). The Malani dikelet has a high temperatur e direction that is reversed from the rest of the suite, which most likely formed parallel to the external, also

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46 reversed, field. This high temperat ure direction carries the reverse di rection, thus even if this is a self-reversal, the reverse polarity direction in th e dike is primary and only the low-temperature normal polarity could be an artifact of a true self-reversal. While sampling density is not sufficient to pass statistical reversal tests (i .e. McFadden and McElhinny, 1990, reversal test), all three samples from the dikelet demonstrated a normal overprint and an tipodal directions upon heating. In our opinion, option (3) best fits the results of the dikelet when co mpared to the larger dike intrusion. The dikelet was sampled at just within half dike-width away and thus still susceptible to partial baking by the large dike. Ma gnetic intensity increases in all samples as the normal polarity direction is removed (Figure 3-7b), which is typical behavior for the demagnetization of magnetic moments that ar e antipodal to the primary high temperature direction. We suggest that the dikelet was em placed during a reversed polarity field and thousands of years later baked by a larger intrusion, resulting in a normal polarity overprint and a reverse primary direction. It is not uncommon fo r multiple dike intrusions to occur over enough time to include a field reversal. The Harohalli dike swarm in India (Pradhan et al., in press) and the Matachewan dikes in southern Canada (I rving and Naldrett, 1977; Halls, 1991; Halls and Zhang, 1998; Symons et al., 1994) are examples of dike swarms that were emplaced in multiple phases and include dual-polar ity paleomagnetic results. The Malani pole is cited as a representative pole for India during the late Neoproterozoic, yet some authors conclude that the lack of a decisive reversal or fi eld test deems Malani paleomagnetic data untrustworthy (see Yoshid a and Upreti, 2006 for example). However the results of our study not only add to the existing MIS paleomagnetic data set, but also provide additional evidence for a primary magnetization. Th e fold test provided by Torsvik et al. (2001a)

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47 is now augmented by evidence favoring a primary reverse TRM in the sequence overprinted by a normal-polarity magnetization in the mafic dikelet. Although this does not constitute a classic backed contact test, the result is most easily expl ained as baking of the smaller dike. In addition, the Malani results do not overlap with younger po les from the Indian subcontinent nor is it consistent with a recent field overprint. The positive fold test determined by Torsvik et al. (2001) constraints the age of the pole to older than Ca mbrian and there are no paleomagnetic poles from the India craton within 750 to 560 Ma that have similar directions w ith the MIS that could represent a regional overprint. Th e Malani pole is also distinct from the more common overprints observed in Indian rocks in this region (Deccan and Rajmahal Traps) and from CarboniferousCretaceous paleomagnetic results observed in th e Gondwana Supergroup and an analysis of postCretaceous paleomagnetic poles from India (Mallik et al., 1999; Acton, 1999). This even further attests the quality of this pole (Figure 3-8). The directions from Malani are also disparate from Carboniferous-Cretaceous paleom agnetic results observed in th e Gondwana Supergroup and an analysis of post-Cretaceous paleomagnetic pole s from India (Mallik et al., 1999; Acton, 1999). The new zircon extrusion age of 771 Ma (Fig 6) for a rhyolitic tuff places a robust age on the timing of the first stage of magmatism in the MIS. This age is consistent with the oldest available Rb-Sr isochron age of 779 Ma (R athore et al., 1999), based on felsic volcanic rocks from widely spaced sampling sites. It is also consistent with the first of two personal communication zircon dates at 771 and 751 Ma quoted by Torsvik et al. (2001a) for rhyolite magmatism in the first stage of MIS ma gmatism. These ages were determined with TIMS analysis at Washington University, St. Loui s, on samples from sites 3 and 4 of Torsvik et al. (2001a), but analytical detail s were not given in the publica tion. Available Rb-Sr whole-rock geochronology (Crawford and Comp ston, 1970; Dhar et al., 1996; Rath ore et al., 1999) defines a

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48 time span of nearly 100 m.y. (779 to 681 Ma), so one cannot rule out that the second and third stages of the magmatism are significantly younger than 771 Ma. Nevertheless, the consistency of paleomagnetic data for the different stages of magmatism argues for a comparatively short duration of activity. Paleomagne tic data from Malani dikes overlap with the ones from the 750.2 .5 Ma Takamaka mafic dikes in Seychelles, if the two plates are juxtaposed together (Torsvik et al., 2001b). This provides further support for their correlation. In Figure 3-9, published site VGPs determined from th e early stage of rhyolitic magmatism in the MIS (Klootwijk, 1975; Torsvik et al., 2001a) are co mpared to the VGPs derived from the third stage of magmatism of the suite (mafic dikes, this study). The mean direction from all VGPs is indicated (starred, Figure 3-9). We suggest that this indicates a relatively short eruptive history for the Malani suite, contradicting the over 100 m illion year span of apparent ages derived from Rb-Sr data (Rathore et al., 1999). Younger ages fr om Rb/Sr data can be accounted for by local disturbances or element mobility during minor episodes of metasomatism. Considering the nature and timing of magmatism in the Seychelles and India, the bulk of gr anitic and subsequent mafic magmatism in those regi ons was constrained to the in terval from ca. 771 to 751 Ma. Implications for the Configuration of Rodinia It is postulated (Powell et al., 1993; W indley et al., 1994; Dalziel, 1997; Yoshida and Upreti, 2006) that a coherent East Gondwana existed from the Mesoproterozoic through the bulk of the Precambrian and until the Mesozoic break up of Gondwana. This conclusion is largely based on paleomagnetic and detrital zircon data w ith high flexibility of interpretation and poor age control, as well as the alleged lack of ev idence for appropriatel y aged oceanic sutures between eastern Gondwana cratons. Yoshida an d Upreti (2006) discuss evidence for the Neoproterozoic juxtaposition of India and Austra lia-East Antarctica based on similarities in cratonic and orogenic detrital zircon and neodymium isotopic si gnatures. Yet the notion of a

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49 united East Gondwana through the Proterozoic and Cambrian is contradicted by high quality paleomagnetic data (Meert and Van der Voo, 1997; Meert, 2001; Torsvik et al., 2001a; Collins and Pisarevsky, 2005). Fitzsimons (2000) and Meert (2003) also review the evidence for appropriately aged mobile belts separating dis tinct segments of eastern Gondwana elements, which accounts for a later (Cambrian) ocean closure. In their discussion of the proximity of India and Australia-East Antarctica, Yoshida and Upreti (2006) argue that the paleomagnetic data used to constrain the possible separation of these con tinents do not include a well-constrained age and have been reset by later Pan-Afri can events (ca. 530-510 Ma). We emphasize that this is not a valid argument because both the Malani pole report ed in this paper and the highly reliable pole from the 755 Ma Mundine Well dike swar m in Australia (Wingate and Giddings, 2000) include necessary field and contact tests to argue against any resetting, and both are well-dated. The paleolatitude of the 755 Ma Mundine Well dikes is 20.2 and this can be compared to the paleolatitude of the Ma lani dikes from our study (44.5 ), indicating a lat itudinal separation of nearly 25 (Figure 3-10). It we use the Mund ine pole as representative for East Gondwana at 750 Ma, it is necessary for India to be located along the paleoequato r adjacent to East Antarctica according to its placement in the typically accepte d Gondwana fit (deWit et al., 1988). Thus, the misfit between the latitude requi red by the Malani pole and India s traditional position is more than 45 It is possible that the southeast margin of India was located along the northwestern margin of Australia, but no geol ogic evidence such as oceanic sutures or similar-aged orogenic belts have been found to support this orie ntation. Younger-aged sutures between Gondwana components indicate a more complex Gondwana amalgamation as a series of distinct PanAfrican orogenies that occurred between ca. 700 and 500 Ma (Fitzsimons, 2000; Meert, 2003; Collins and Pisarevsky, 2005). The East African Orogen (EAO) is the ca. 700-650 Ma result of

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50 collision between Madagascar, Somalia, Ethiopi a and Arabian Nubian shield (collectively Azania block) and the Congo, Tanzania and Bangweulu block, first developed by Stern (1994) and has since been modified (Collins and Pi sarevsky, 2005). The later Kuunga Orogen (Meert et al., 1995) places the final Gondwana assembly at about 550 Ma with the amalgamation of Australia-Antarctica with IMSLEK (India, northea stern Madagascar, Sri La nka, East Antarctica, and the Kalahari craton) group. The Malagasy oroge ny is also suggested to occur simultaneously with the Kuunga orogeny as the EAO constituents collided with southeastern India (Collins and Pisarevsky, 2005). These major Pan-African orogen ies are congruent with a complex Gondwana assembly, and placing India alongsi de East Antarctica and Australia at 770 Ma fails to account for the existence of the c onsiderably younger sutures. Conclusions The MIS provides the best paleom agnetic pole for the Indian subcontinent at approximately 771 Ma, with a combined pole of 67.8 N, 72.5 E (A95=8.8 ). Our study strengthens the case for primary magnetization of the MIS based on the primary reversed direction overprinted by a bake d normal-polarity magnetization in a mafic dikelet. The now documented U-Pb zircon age of 771 Ma provide s a more accurate and concordant lower age limit for Malani volcanism. When combined with geochronologic da ta from mafic dykes in the Seychelles (750.2.5, Torsvik et al., 2001b), our ag e determination also hints at a shorter duration of magmatic activity in the MIS than previously stated. East Gondwana is considered by some authors to be a stable configuration from about 1.1 Ga until the Mesozoic breakup of Gondwan a (Yoshida and Upreti, 2006). However, paleomagnetic data (Torsvik et al., 2001b, this st udy) place India and th e Seychelles at much higher latitudes than coeval pol es from Australia (Mundine di kes, Wingate and Giddings, 2000). Three robust paleomagnetic result s (Mundine dykes, Malani Igneous Suite and Takamaka Dikes)

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51 argue strongly against an amalgamated East Gondwana at 750 Ma a nd therefore the younger Pan-African belts between these cratons are indi cative of a Neoproterozoic-Cambrian suturing of eastern Gondwana. Thus, we argue that if paleomagnetism is to make any contribution to Neoproterozoic plate tectonic models, the Malani pole must be seriously considered in any geodynamic explanation for the assembly of Gondwana. Acknowledgements: This work was supported by a grant (to JGM) from the National Science Foundation (EAR04-09101). The authors thank Jim Vogl and George Kamenov for help with relentless (unsuccessful) attempts to date the Malani dikes and we also thank Alan Collins and two anonymous reviewers for comments th at greatly improved this manuscript. M. Whitehouse is thanked for operating the NORDSIM laboratory and controlling quality of U-Pb data.

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52 Figure 3-1. Typically accepted Gondwana fit for 560 Ma, taken from deWit et al., 1988 Reconstructions that use a G ondwana fit come from this model. Inset highlights the paleoposition of the Seychelles (Sey) and Ma dagascar (Mad) relative to India at 750 Ma, reconstructed using the Malani pole (T orsvik et al., 2001a) and the Seychelles euler pole of rotation (Torsvik et al., 2001b).

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53 Figure 3-2. Map showing Precambrian stratigraph ic units of the Aravalli Mountain Region in NW India with sampling area boxed (adapted from GSI publications).

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54 Figure 3-3. Field photos. (a) Photo of a large E-W trending dike at site I434 (b) Photo of 1 cm wide N-S trending mafic dikelet from site I434

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55 Figure 3-4. Inverse conc ordia diagram. Diagram shows U-Pb analyses of zircon and CL image of one zircon crystal from a rhyolitic tuff representing the first stage of magmatism in the Malani Igneous Suite. The concordi a age of 771 Ma reflects magmatic crystallization of the rock.

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56 Figure 3-5. Magnetic characteri zation data. (a) Isothermal Remanence Magnetization (IRM) plots from four Malani samples. Sample I434-28 is a mafic dikelet. All samples saturate at about 0.3 tesla. (b) Curie temper ature test of typical mafic dike sample from site I434. TcH indicates Curie temp erature during heati ng, and TcC indicates Curie temperature during cooli ng, (c) Curie temperature test of a mafic dikelet. Tests were run up to 600 C because of alteration at high temperatures.

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57 Figure 3-6. Demagnetization result s from sites 35 and 36. (a) Thermal and (b) alternating field (AF) demagnetization results of mafic samp les from sites 35 and 36. In stereoplots, closed circles represent positive inclinations. In Zijderveld diagrams closed (open) circles represent the horizontal (verti cal) plane. NRM= Natural Remanent Magnetization. Thermal measurements are in C and AF measurements are in millitesla (mT).

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58 Figure 3-7. Demagnetization result s from site 34. (a) Normal pol arity sample subjected to AF demagnetization. (b) Reversed polarity dike let sample with arrows pointing in direction from NRM to origin of both the overprint and reverse polarity vector. In stereoplots, closed circles represent positive inclinations. In Zijderveld diagrams closed (open) circles represent the horizontal (vertical) plane. NRM= Natural Remanent Magnetization. Thermal measurements are in C and AF measurements are in millitesla (mT).

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59 Figure 3-8. Stereoplot of individual site means, overall mean and reversed pol arity mean with common India overprints from the Deccan Traps and Rajmahal Traps indicated

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60 Figure 3-9. Stereoplot of VGPs from the three studies, averaged to the mean pole for the MIS. Circles are from mafic dikes (this study) ; triangles and squares are VGPs from each site of rhyolite and trachyite volcanics. Closed symbols represent positive inclinations.

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61 Figure 3-10. Reconstruction at 770 Ma of per tinent eastern Gondwana co mponents. Grey India outline is plotted from the new mean Malani paleomagnetic pole, with the Seychelles euler rotation fit from Torsvik et al., (2001b) and Madaga scar is placed according to the Gondwana fit. Australia is plotted according to the Mundine Wells dikes VGP, and Antarctica and India are placed in thei r Gondwana fit locations, in the Australia reference frame. There is >20 of latit udinal displacement between the Malani and Mundine Wells study sites.

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62Table 3-1. Summary of Paleomagne tic and Virtual Geomagnetic Poles Pole Name Age (Ma) Pole latitude Pole longitudeA95 or decb incb cdReference dp/dma India Malani, aplite dike 750 74.6 N 49.8 E 16.2 352.5 60 16.2 18.6Rao et al., 2003 Malani, rhyolite 745 80.5 N 43.5 E 8/11.5 354.5 53.5 8 Klootwijk, 1975 Malani, felsic volcanics 751, 77174.5 N 71.2 E 7.4/9.7 359.5 60.4 6.4 29.9Torsvik et al., 2001 Malani, rhyolite 740 78.0 N 45.0 E 11.0/15.0 353 56 10 Athavale et al., 1963 Malani, mafic dikes, felsic volcanics771* 70.2 N**70.1 E 12.1/15.4 358.8 63.5 9.7 91.2this study Seychelles Mahe dikesIND 750.2.5 79.8 N 78.6 E 9.9/14.9 1.4 49.7 11.2 Torsvik et al., 2001 Australia Mundine Well dikesIND 755 41.47 N 130.92 E4.1/4.1 14.8 31.1 5 Wingate and Giddings, 2000 a: A95= cone of 95% confidence about the mean pole; dp/dm cone of 95% confiden ce about the paleomagnetic pole in the colatitude direction (dp) and at a right angle to the co-latitude direction (dm), b: dec/ inc= mean declination/ inclination, c: a 95= circle of 95% confidence about the mean, d: k= kappa precision parameter, *U-Pb Age data repo rted in this study, samples are from site 3 of Torsvik et al., 2001, **Virtua l Geomagnetic Pole (VGP) from four dikes

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63 Table 3-2. Summary of geochronologic results Site Study Method Age (Ma) Malani rhyolites Crawford and Compston (1970) Rb/Sr 730 rhyolites Klootwijk (1975) Rb/Sr 745 felsic volcanics Rathore et al. (1996) Rb/Sr isochron 779 ultrapotassic rhyolites Rathore et al. (1999) Rb/Sr isochron 681 Jalore granites Rathore et al. (1999) Rb/Sr isochron 727 peralkaline volcanics Rathore et al. (1999) Rb/Sr isochron 693 rhyolite this study, site 3 of Torsvik et al., 2001a U/Pb 771

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64Table 3-3: SIMS zircon UPb data on rhyolite tuff from Ma lani igneous suite ID U Th Pb 206Pb a 207Pb 207Pb 206Pb R b 206Pb c 206Pb d Disc.e 204Pb 206Pb 235U 238U 238U 238U 2 lim. (ppm) (ppm) (ppm) (%) (%) (%) (Ma) (Ma) (%) MIS5/04: ryholite tuff f n1808-01a g 222 120 31 10257 0.06439 0.9 1.024 1.4 0.1154 1.0 0.75704 7 703 7 n1808-03a 414 202 64 17681 0.06449 0.8 1.127 1.3 0.1267 1.0 0.79769 8 770 8 n1808-03b 400 175 62 22494 0.06444 0.6 1.149 1.2 0.1294 1.0 0.87784 8 785 8 n1808-04a 85 63 14 5821 0.06372 1.5 1.126 1.8 0.1281 1.0 0.57777 8 779 8 n1808-05a 183 106 29 28736 0.06445 0.9 1.137 1.4 0.1279 1.0 0.77776 8 777 8 n1808-05b 196 149 33 6602 0.06430 1.4 1.148 1.8 0.1295 1.1 0.62785 8 786 9 n1808-05c 55 31 9 3629 0.06296 2.2 1.112 2.4 0.1281 1.0 0.43777 8 779 8 n1808-06a 149 113 24 6027 0.06385 1.1 1.104 1.5 0.1254 1.1 0.68762 8 762 8 n1808-06b 134 99 22 8903 0.06380 1.4 1.115 1.7 0.1268 1.0 0.60769 8 771 8 n1808-07a 401 316 67 13951 0.06426 0.6 1.134 1.2 0.1280 1.0 0.86777 8 777 8 n1816-01a 143 101 23 6033 0.06493 1.2 1.139 2.3 0.1272 1.9 0.85772 14772 15 n1816-01b 197 154 32 14111 0.06377 1.0 1.119 2.2 0.1273 2.0 0.89772 14773 15 n1816-02a g 46 24 7 3182 0.06076 2.4 1.062 3.0 0.1268 1.9 0.62770 14774 140.5 n1816-03a 88 57 14 13350 0.06288 1.4 1.099 2.3 0.1268 1.8 0.78769 13771 13 n1816-03b 200 164 35 15016 0.06472 0.8 1.196 2.0 0.1340 1.8 0.91811 14812 14 n1816-06a 138 54 20 10092 0.06440 1.1 1.101 2.0 0.1240 1.7 0.84754 12754 12 a: Measured 206Pb/204Pb ratio, b: R correla tion coefficient of errors in isotopi c ratios, c: 204Pb corrected aged: 207Pb corrected age, c: age discordance at the closest approach of 2 s error ellipse to Concordia, f: coordinates of the sample: 26 7.963'72.357', g: analysis not selected for calculation of concordia age.

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65Table 3-4: Paleomagnetic results Site name Lat/Long n/Na Declination Inclination Kappa ( )b 95c VGP latituded VGP longituded dp, dme This study I434 (normal) 25.342 N, 72.601 E 23/27 351.4 72.6 129.55 2.7 56.9N 64.3E 4.3, 4.8 I434 (reverse) 25.342 N, 72.601 E 3/3 195.3 -59.7 234.79 8.1 70.2N 108.8E 9.2, 12.2 I435 25.341 N, 72.601 E 6/8 349.8 61.8 244.78 4.3 70.5N 49.8E 5.1, 6.7 I436 25.341 N, 72.616 E 9/9 355.4 58.2 256.88 3.8 75.9N 57.7E 4.1, 5.6 Combined mean 4 dikes 358.8 63.5 91.2 9.7 70.2N 70.1E 12.1, 15.4 Torsvik et al., 2001 1 26.0 N, 73.0 E 5 038.6 70.6 188.9 5.6 49.7N 106.8E 8.4, 9.7 3 26.3 N, 73.0 E 13 017.2 51.8 678.2 1.6 73.8N 136.7E 1.5, 2.2 4 26.3 N, 72.6 E 13 312.8 72.3 51.7 5.8 44.5N 39.0E 9.1, 10.3 5 26.2 N, 72.5 E 6 356.2 64.1 511.1 3.0 70.1N 64.7E 3.8, 4.8 6 26.4 N, 72.5 E 11 024.7 46.7 178.9 3.4 68.0N 152.8E 2.8, 4.4 8 25.7 N, 72.4 E 16 354.0 59..4 322.0 2.1 74.6N 54.9E 2.4, 3.2 10 25.2 N, 72.6 E 4 339.9 64.0 51.6 12.9 63.9N 39.5E 16.4, 20.5 13 25.6 N, 72.5 E 5 057.5 74.0 109.0 7.3 38.0N 104.7E 11.9 13.2 14 25.7 N, 72.4 E 7 012.7 62.1 200.5 4.3 69.5N 99.6E 5.2, 6.7 Klootwijk et al., 1975 RI-2 26.3 N, 73.02 E 6 003.9 62.5 133.0 6.5 72.2N 82.2E 7.9, 10.2 RI-3 26.3 N, 73.02 E 4 023.5 82.5 297.5 5.3 39.6N 80.6E 10.1, 10.3 RI-7 25.8 N, 72.167 E 4 340.0 45.0 55.0 12.5 72.1N 349.0E 10.0, 15.8 RI-10 25.8 N, 72.167 E 5 346.5 52.0 162.0 6.0 76.4N 15.4E 5.6, 8.2 RI-12 25.8 N, 72.167 E 3 337.0 54.5 316.0 7.0 68.2N 12.7E 7.0, 9.9 RI-13 25.67 N, 73.15 E 4 345.0 81.0 490.5 4.0 42.5N 67.1E 7.5, 7.7 Com bined mean paleomagnetic pole 19 sites 001.0 63.0 32.9 5.9 67.8N 72.5.E A95=8.8 a: n= samples used; N= samples collected, b: k= kappa precision parameter, c: a95= circle of 95% c onfidence about the mean, d: VGP latitude/longitude = virtual geomagnetic pole, e: dp,dm= cone of confidence along site latit ude (dp) and orthogonal to site latitude (dm)

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66 CHAPTER 4 CONCLUSION The work included in this thesis pres ents glimpses into the Precambrian history of the Indian subcontinent. There are no reliable pale omagnetic poles for India between ca. 1050 and 770 Ma. Without further information, one ca nnot envision a convinc ing APWP for India between the low latitude Majhgawan pole and th e mid-latitude Malani pole over the extended time span of 300 million years (Figure 4-1). Paleomagnetic studies of the Vindhyanchal ba sin resolve similar poles to the Majhgawan kimberlite (Miller and Hargraves, 1994), but th e age of the Upper Vindhyan sequence is very poorly constrained in the extant literature (see discussion in Chap ter 2). The highly precise age of the Majhgawan kimberlite along with the similar paleomagnetic directions to the Vindhyan basin poles provided a catalyst for our group to conduct a detailed investigation on the age of sedimentation in the Upper Vindhyanchal basin (Malone et al., in revision). Previous constraints placed the upper age limit on Vindhyan sedimentati on as Cambrian. Malone et al. resolved a paleomagnetic pole of 44N, 214E (A95= 4.3), and the similarities in poles for these two units in India (Majhgawan and the Vindhyan) are consiste nt with detrital zirc on analyses from the same study and are indicative of a much older lim it for sedimentation in the basin (ca. 1050 Ma). Thus the Vindhyan pole can be added to the Indi a APWP for the Mesoproterozoic, along with the 1192 Ma Harohalli dikes paleomagnetic pole s (24.9 S, 258 E; A95=15; Pradhan et al., 2008). Younger, pervasive overprints determined in Harohalli are tentatively assigned as Ediacaran on the basis of discordant zircon ages and coincidence of the directions with Gondwana and the ca. 600 Ma Dokhan Volcanics paleomagnetic pole from the Arabian-Nubien shield (Halls et al., 2007; Pradhan et al., 2008).

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67 Chapter 3 focuses on the Neoproterozoic Malani Igneous Suite (MIS) and its implications for India relative to other eastern Gondwana co mponents. The revised Malani pole places cratonic India at paleolatitude 44.5, about 25 of latitudinal offset from the 755 Ma Mundine dikes pole in Australia. India is consequently offset 45 from its traditional Gondwana fit with Australia-Antarctica. Multiple paleogeographic scenarios are posited for the various Gondwana components. Some suggest that East Gondwana amalgamated in the Mesoproterozoic and existed in the same configuration until Gondwana breakup in the Me sozoic (Windley et al., 1994; Piper, 2000; Yoshida and Upreti, 2006; Squire et al., 2006; Paulsen et al., 2007). This view is simplistic, and available data are more congruent with a comp licated assembly of eastern Gondwana elements as a series of collisions occurring at th e Precambrian-Cambrian boundary. The revised paleomagnetic pole for the MIS is in agreement w ith a much more complex assembly of eastern Gondwana.

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68 Figure 4-1: APWP for India with reliable Pr oterozoic poles. Poles are from Harohalli Dikes (Halls et al., 2007; Pradhan et al., 2008), the Majhgawan kimberl ite (this study), the Vindhyanchal basin (Malone et al., in revision), and the Ma lani Igenous Suite (this study). The long gaps in the paleomagnetic record are highlig hted, and the APWP between poles is dotted where data are nonexistent.

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69 LIST OF REFERENCES Acton, G.D., 1999. Apparent po lar wander of India since the Cretaceous with implications for regional tectonics and True Polar Wander, Mem. Geological Societ y of India, 44, 129175. Ashwal, L.D., Demaiffe, D., Torsvik, T.H., 200 2. Petrogenesis ofNeoproterozoic granitoids and related rocks from the Seychelles: evidence for the case of anAndean-type arc origin. Journal of Petrology 43, 45-83. Athavale, R.N., Radhakrishnamurthy, C., Saha srabudhe, P.W., 1963. Paleomagnetism of some Indian rocks, Geophysical Journal of th e Royal Astronomical Society Soc., 7, 304-311. Auden, J.B., 1933. Vindhyan sedimentation in So n Valley. Geolological Survey of India Memoirs 62, 141-250. Boger, S.D., Carson, C.J., Fanning, C.M., Herg t, J.M., Wilson, C.J.L., Woodhead, J.D., 2002. Pan-African intraplate deformation in th e northern Prince Charles Mountains, east Antarctica. Earth and Planet ary Science Letters 195, 195-210. Bond, G.C., Nickeson, P.A., Kominz, M.A., 1984. Br eakup of a supercontinent between 625 and 555 Ma: new evidence and implications for c ontinental histories. Earth and Planetary Science Letters 70, 325-345. Bhushan, S.K., 2000. Malani RhyoliesA Review. Gondwana Research 3, 65-77. Chakrabarti, A., 1990. Traces and dubiotraces: exam ples from the so-called Late Proterozoic siliclastic rocks of the Vindhyan Superg roup around Maihar, India, Precambrian Research, 47, 141-153. Chatterjee, A.K. and Rao, K.S., 1995. Majhgawa n diamondiferous pipe, Madhya, Pradesh, India A review, Journal of the Geologi cal Society of India, 45, 175-189. Collins, A.S., Windley, B.F., 2002. The tectonic e volution of central a nd northern Madagascar and its place in the final assembly of Gondwana. Journal of Geology 110, 325-340. Collins, A.S., Pisarevsky, S.A., 2005. Amalgama ting eastern Gondwana: The evolution of the Circum-Indian Orogens. Eart h Science Reviews 71, 229-270. Collins, A.S., 2006. Madagascar and the amal gamation of Central Gondwana. Gondwana Research (GR Focus) 9, 3-16. Condon, D., Xhu, M.Y., Bowring, S., Wang, W., Ya ng, A.H., Jin, Y.G., 2005. U-Pb ages from the Neoproterozoic Doushantuo Formation, China. Science 308, 95-98. Crawford, A.R., Compston, W., 1970. The age of the Vindhyan system of peninsular India. Quarterly. Journal of the Geol ogical Society of London 125, 351-372.

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78 BIOGRAPHICAL SKETCH Less than a month ago, I was in remote Mongo lia, teetering precarious ly on an outcrop of volcanic rocks for med over 500 million years ago, us ing a hand-held diamond bit drill to extract samples that will elucidate the complex details of Central Asias geologic history. I am an explorer, but not in the traditional sense. I delve into the earths elusive past, where oceans have closed and opened, mountains have uplifted and eroded away, and continents have met in vast landmasses only to break up and drift apart. My father sparked my interest in science. He is a biologist with a doctorate in entomology and I was always enamored with his careful descri ptions of the natural world and his enthusiasm for learning. A class titled The Biology of Firef lies ultimately swayed me into the pursuit of science. The professor of the course, Dr. Lloyd, spent the past thirty years vigorously investigating the atypical subject of fireflies. Dr Lloyd demonstrated that with scientific scrutiny even the seemingly insignificant firefly become s magnificently intricate. I sought to find a subject equally interest ing and fortuitously happened upon eart h sciences. Every single day of my first class in geology challenged my intellect, and I started working on research by the end of the semester. Undergraduate research allowed me to comple te a combined bachelors and masters degree, focusing on the ancient locations of continen ts using paleomagnetism and geochronology. I developed a passion for tectonics and structural geology and deci ded to apply for a doctorate degree. I will continue my graduate carr eer at Oxford University next Fall, 2008.