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1 DYKE SWARM AT ~ 1.88 GA AND IMPLICATIONS FOR SUPERCONTINENT RECONSTRUCTIONS By MERCEDES ELISE BELICA A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR TH E DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2012
2 2012 Mercedes Elise Belica
3 To my parents
4 ACKNOWLEDGMENTS This work was supported by a grant from the US National Science Foundation to J.G. Meert (EAR09 10888). I would like to thank my graduate advisor, Dr. Joseph Meert, for introducing me to the field of paleomagnetism and guiding my education for the past 2 years. I want to thank my c ommittee members, Dr. Ray Russo and Dr. David Foster for their helpful insight and teaching during my graduate career I would also like to thank Candler Turner and Manoj Pandit for their assistance with field work in India. I also thank the office staff including Pamela Haines, Nita Fahm, Carrie Williams, and Susan Lukowe. I also need to thank Carlos Ortega and Matthew Ce lestino for their assistance in geochronology. Finally I thank Alberto Carmenate for patiently teaching me the ways of Adobe Illustr ator an d my parents for supporting me and believing in me.
5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 6 LIST OF FIGURES ................................ ................................ ................................ .......... 7 LIST OF ABBREVIATIONS ................................ ................................ ............................. 8 ABSTRACT ................................ ................................ ................................ ..................... 9 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 11 Regional Geology ................................ ................................ ................................ ... 13 Previous Work ................................ ................................ ................................ ........ 16 ~2.37 Ga dykes ................................ ................................ ................................ 16 ~2.21 2.18 Ga dykes ................................ ................................ ........................ 17 ~1.88 Ga dykes ................................ ................................ ................................ 18 2 METHODS ................................ ................................ ................................ .............. 24 Paleomagnetism ................................ ................................ ................................ ..... 24 Rock Magnetic Experiments ................................ ................................ ................... 25 Geochronologic Methods ................................ ................................ ........................ 25 3 RESULTS ................................ ................................ ................................ ............... 28 ~2.37 Ga Dykes ................................ ................................ ................................ ...... 28 ~2.21 2.18 Ga Dykes ................................ ................................ .............................. 29 ~1.88 Ga Dykes ................................ ................................ ................................ ...... 30 Geochronology ................................ ................................ ................................ 30 Paleomagnetism ................................ ................................ ............................... 31 4 DISCUSSI ON ................................ ................................ ................................ ......... 51 ~2.37 Ga Dykes ................................ ................................ ................................ ...... 51 ~2.21 2.18 Ga Dykes ................................ ................................ .............................. 54 ~1.88 Ga Dykes ................................ ................................ ................................ ...... 56 5 CONCLUSIONS ................................ ................................ ................................ ..... 69 LIST OF REFERENCES ................................ ................................ ............................... 72 BIOGRAPHICAL SKETCH ................................ ................................ ............................ 72
6 LIST OF TABLES Table page 3 1 Paleomagnetic results for 2.37 Ga dykes. ................................ .......................... 34 3 2 Paleomagn etic results for ~2.2 2.18 Ga dykes. ................................ .................. 36 3 3 Paleomagnetic results for 1.88 Ga dykes. ................................ .......................... 37 3 4 Geochronological results. ................................ ................................ ................... 39 4 1 Ca. 2.4 Ga paleomagnetic studies .. ................................ ................................ ... 62 4 2 Ca. 2.2 Ga paleomagnetic studies. ................................ ................................ .... 63 4 3 Ca. 1.88 Ga paleomagnetic studies. ................................ ................................ ... 64
7 LIST OF FIGURES Figure page 1 1 Columbia reconstruction accord ing to Zhao et al. (2002, 200 4) ........................ 21 1 2 Generalized geo logic map of Peninsular India ................................ .................. 22 1 3 Field area for the p resent study of Dharwar dykes ................................ ............ 23 2 1 Tera Wasserburg U Pb concordia diagram for zircon data ............................... 27 3 1 Orthogonal vector plots from the 2.37, 2.21, and 2.18 Ga suite of dykes showing typical characteristic re manent magnetization directions ..................... 40 3 2 Curie temperature analysis ................................ ................................ ................. 42 3 3 Isothermal remanence acquisition curves and back field IRM.. .......................... 44 3 4 Baked contact tests ................................ ................................ ........................... 46 3 5 Orthogonal vector plots from the 1.88 Ga suite of dy kes showing typical characteristic remanent magnetization directions.. ................................ ............. 48 3 6 Galls projection of mean normal and reverse paleomagnetic poles f or the 1.88 Ga suite of dykes ................................ ................................ ....................... 50 4 1 Orthogonal projection showing the paleopositi ons of Paleoproterozoic cratons ................................ ................................ ................................ ............... 65 4 2 Mollweide projection showin g the paleopositions at ~2. 2 Ga ............................ 66 4 3 Paleogeographic reconstructions at ~ 1.9 Ga. ................................ .................... 67 4 4 APW path for the Dharwar craton utilizing the paleopoles from ~ 2.37 Ga to 1.8 8 Ga (Tables 3 1, 3 2, and 3 3) ................................ ................................ .... 68
8 LIST OF ABBREVIATIONS AF Alternating Field APWP Apparent Polar Wander Path BD2 Bastar Dykes 2 CITZ Central Indian Tectonic Zone D Declination E W East West EGGB Eastern Ghats Granulite Belt Ga Billion years I Inclination IRM Isothermal Remanent Magnetization LIP Large Igneous Province Ma Million years MAD Mean Angular Deviation MSWD Mean Square Weighted Deviation N S North South NE SW Northeast Southwest NRM Natural Rema nent Magnetization NW SE Northwest Southeast Sm Nd Samarium Neodymium Tc C Cooling Curie Temperature Tc H Heating Curie Temperature U Pb Uranium Lead VGP Virtual Geomagnetic Pole 95 cone of confidence about the mean direction
9 Abstract of Thesis Presente d to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for th e Degree of Master of Science DYKE SWARM AT ~ 1.88 GA AND IMPLICATIONS FOR SUPERCONTINENT RECONSTRUCTIONS By Mercedes Elise Belica December 2012 Chair: Joseph G. Meert Major: Geology Here we report new paleomagnetic results from the Dharwar craton at 1.88, 2.21 2.18, and 2.37 Ga. The presence of a ~85,000 km 2 radiating dyke s warm with a fanning angle of 65 has been confirmed within the I ndian subcontinent at 1.88 Ga. Paleomagnetic results from the Large Igneous Province (LIP) represent the largest data set for India at ~1.9 Ga, with a declination=287.1, an inclination=5.4 corresponding paleolatitude of 2.7N (site 34). The mean paleomagnetic pole falls at previously published sites. Our continental reconstruction for India at ~1.9 Ga conflicts with the archetypal Columbia model, suggesting that the exact configuration needs modificati on. We also report two preliminary paleomagnetic directions from NW SE S (D=252.6, I= ite 35) trending ~2.2 Ga dykes. We attribute this difference in directions to the separate magmatic pulses at 2.21 and 2.18 Ga identified by French and Heaman (2010). Our results place India at intermediate latitudes (plat=26.6N; site 571) at 2.18 Ga an d are supported by a positive baked contact test. New paleomagnetic results from E W and
10 NW SE trending 2.37 Ga dykes (14 dykes; D=85.2, I= combined with previous work in the Dharwar craton, yields a paleomagnetic pole of s (plat=70N; site 596). Finally we present a Paleoproteroz oic APWP for the Dharwar craton, and consider potential cratonic relationships for each interval.
11 CHAPTER 1 INTRODUCTION Recent advances in paleo punctuated by numerous supercontinent configurations (Columbia, Rodinia, Gondwana, Pangaea; Meert 2012; Li et al. 2008; Meert and Lieberman 2008; Rogers and Santosh 2002, Zhao et al. 2002, 2004; Hou et al. 2008). The general makeup of the most recent sup ercontinent, Pangaea, is well constrained from seafloor magnetic anomaly data, paleomagnetism, geology, and faunal evidence (Benton 2005), although there are still vigorous debates regarding the exact configuration (Domeir et al., 2012). Given the controv ersies surrounding the different Pangaea reconstructions, it is no surprise that establishing the makeup of earlier supercontinents is far more difficult. In part, this is due to the lack of adequate geologic, geophysical and paleontological data (Meert 2 001; Meert and Torsvik 2003; Li et al. 2008). In attempting to decipher past continental configurations, it is important to seek regions where unaltered sequences of igneous and sedimentary Precambrian rocks are preserved. Peninsular India is one such r egion, and previous work indicates a high potential for generating useful data from India that can be used in conjunction with other regions to produce paleogeographic maps for the Precambrian (see Pradhan et al. 2010; Piper 2010; Bispo Santos et al. 2008; Pesonen et al. 2003; Hou et al. 2008; French et al. 2008; French and Heaman 2010; Zhao et al. 2002, 2004; Condie 2002a,b; Rogers and Santosh 2002; Buchan et al. 2000, 2009; Pisarevsky and Sokolov 1999; Elming et al. 2001; Salminen et al. 2009; Piispa et al. 2010). In attempts to reconstruct previous Proterozoic supercontinents, geologists used geologic similarities and the alignment of features such as orogenic belts in order to
12 establis h contiguity (Zhao et al. 2002, 2004); however, paleomagnetic techniq ues remain the only quantitative test of such reconstructions (Meert 2002). As an example, the geologic record present in Precambrian terranes suggests a major global rifting event from 2.2 to 2.0 Ga followed 300 million years later by global widespread o rogenesis from 1.9 1.7 Ga (Zhao et al. 2002, 2004). The orogenic belts that formed during this amalgamation were used to generate a plausible supercontinental assemblage coined Columbia ( Figure 1 1 ; Zhao et al. 2004; Rogers and Santosh 2002; Meert 2012). In spite of the fact that high quality paleomagnetic data are being generated more rapidly in recent years, there is no current consensus on the exact make up and geometry of Columbia (E rnst and Srivastava 2008; Meert 2012). In addition to paleomagnetic da ta, the Large Igneous Province (LIP) record is also used for constraining continental reconstructions (Ernst and Srivastava 2008). LIPs are large volume and geologically brief magmatic events that typically occur in an intraplate setting and commonly acco mpany the rifting or assembling of supercontinents (Ernst and Srivastava 2008). A LIP typically has a focal point (plume source) that can be identified by the convergence of the associated radiating dyke swarms. Coeval radiating dykes can be used as pier cing points between different continental nuclei, where the focus of each swarm overlaps in the correct reconstruction (Ernst and Srivastava 2008). Igneous dykes are ideal for paleomagnetic studies because they cool rapidly and therefore provide an accura techniques were developed recently to separate Uranium bearing minerals from mafic
13 dykes. These minerals (prima rily zircon and baddeleyite) are used to establish the crystallization ages of the dykes (French and Heaman 2010; French et al. 2008; Pradhan et al. 2012; Pradhan et al. 2010; Halls et al. 2007). These favorable characteristics of mafic dykes have spurred a number of recent paleomagnetic studies that attempt to establish a well dated emplacement age with a stable paleomagnetic direction in order to constrain better paleomagnetic poles for the Precambrian (Meert et al. 2011; Pradhan et al. 2008, 2010; Halls et al. 2007; French et al. 2008; French and Heaman 2010; Lubnina et al. 2010; Piispa et al., 2010; Pradhan et al. 2012). Here, we present new paleomagnetic data from previously identified and hypothesized LIPs within the Indian subcontinent at ~1.88, ~2. 21 2.18, and ~2.37 Ga, and discuss the implications for the Dharwar craton in Paleoproterozoic reconstructions. Regional Geology The Indian subcontinent consists of four distinct cratonic nuclei: the Aravalli Bundelkhand craton in the northwest and central regions, the Bastar craton in the south central region, the Singhbhum craton in the eastern region, and the Dharwar craton in the southern peninsular region ( Figure 1 2 ; Naqvi et al. 1974; Naganjaneyulu and Santosh 2010; Bandari et al. 2010). The Indian subcontinent was assembled by the amalgamation of these individual cratonic blocks along the Central Indian Tectonic Zone (CITZ) or the Satpura Belt, but the exact timing of the event is still debated. A number of Proterozoic granitoids including the Clos epet Granite (2.51 Ga) in the Dharwar craton, and the Berach Granite in the Aravalli craton (2.56 2.44 Ga), intrude the older basement gneisses and supracrustals within the Indian subcontinent (Meert et al. 2010; Wiedenbeck et al. 1996; Jayananda et al. 20 00). Meert et al. (2010) suggested that the
14 and that the ~2.45 2.5 Ga intrusive granitoids mark a major stabilization phase for peninsular India. Others contend that this stabilization phase did not occur until 1.6 Ga (Pisarevsky et al. 2012 ; Yedekar et al. 1990; Roy and Prasad 2003; Roy et al. 2006; Bandari et al. 2010); however, younger (1.1 1.0 Ga) ages reported for this collision likely represent reactivation alon g the preexisting zones of weakness during collisional events in the Eastern Ghats region to the east or those to the west in the Aravalli region (Bho wmik et al. 2011; Singh et al. 2010). The Dharwar craton is bordered by the Deccan Traps to the north, th e Eastern Ghats and the Godavari Rift to the east, the Arabian Sea to the west, and the Southern Granulite Terrane to the south (Rogers 1986; Naqvi and Rogers 1987). The Dharwar protocontinent consists of the Dharwar, Bastar, and Singhbhum cratons, and is separated from the northern Aravalli Bundelkhand protocontinent by the CITZ (French and Heaman 2010). The 2.55 2.51 Ga Closepet granite divides the Dharwar craton into eastern and western portions (Friend and Nutman 1991; Ramakrishnan and Vaidyanadhan 20 08; Naqvi and Rogers 1987). The Eastern Dharwar Craton (EDC) is floored by 3.0 2.55 Ga granites and gneisses, and the Western Dharwar Craton (WDC) basement contains 3.4 2.7 Ga tonalite trondhjemite gneisses (Jayananda et al. 2006; Balakrishnan et al. 1990 ; Vasudev et al. 2000; Chadwick et al. 2000; Ch ardon et al. 2002; Meert et al. 2010). The southern peninsular region of India contains several intracratonic basins (Purana basins) that developed during the Paleo Neoproterozoic (French et al. 2008). The la rgest of these is the crescent shaped Cuddapah basin locat ed in the EDC ( Figure 1 3 ). The basin spans an area of ~44,500 km 2 with a minimum total stratigraphic thickness of ~12 km (French et al. 2008; Nagaraja Rao et al. 1987; Ramam and Murty
15 1997; Singh and Mishra 2002). It is composed of four sub basins: the Papaghni, the Kurnool, the Srisailam, and the Palnad (Nagaraja Rao et al. 1987). The Papaghni sub basin is located in the western portion of the Cuddapah basin ( Figure 1 3 ) and produced several rob ust ages that help constrain basin sedimentation and magmatism (Bhaskar Rao et al. 1995; Anand et al. 2003). The Gulcheru Formation quartzite is the lowest unit within the basin; it rests nonconformably on the underlying basement rocks of the Dharwar crat on (French et al. 2008; Nagaraja Rao et al. 1987; Murty et al. 1987). The age of the underlying peninsular gneiss is constrained from dating of the Closepet Granite ( 2310 2350 Ma; Friend and Nutman 1991; Jayananda et al. 1995). Above the Gulcheru Quartz ite lies the Vempalle Formation that contains stromatolitic dolomite, chert and shale, with interbedded mafic sills and basaltic lava flows in the upper portion (Saha and Tripathy 2012). An unconformity rests between the Vempalle Formation and the Pullive ndla quartzite which lies at the base of the Tadpatri Formation. The Tadpatri Formation overlies the Vempalle and contains numerous dolerite, basaltic, and picrite sills within the quartzites, shales, stromatolitic dolomites, and cherts (Saha and Tripathy 2012). A sill at the base of the Tadpatri formation (Pullivendla sill) has a well constrained U Pb age of 1885 3.1 Ma (French et al. 2008). This age provides a constraint for the underlying sedimentary layers (Papaghni Group >1900 Ma; Saha and Tripath y 2012). An elliptical positive gravity anomaly is present in the western portion of the Cuddapah basin that parallels the NW SE trending Papaghni sub basin (Bhattacharji and Singh 1984). The southwestern portion of the basin also contains the densest co ncentration of mafic sills and flows, indicating the likely presence of a lower crustal lensoid mafic body (Bhattacharji and Singh 1984; Saha and Tripathy 2012). This
16 body is most likely an expression of the 1890 Ma magmatism identified by French et al. ( 2008). Widespread coeval magmatic events in the Bastar and Dharwar cratons as well as the Cuddapah basin lend evidence for a plume or mantle upwelling at ~1.9 Ga that may be the precursor to early extension within the Papaghni sub basin, and possibly a si te of continental breakup (see discussion; Saha and Tripathy 2012). Previous W ork The Indian cratons are cross cut by numerous Precambrian mafic dyke swarms as well as sills and mafic ultramafic intrusions (Ernst and Srivastava 2008). The Dharwar craton contains the densest concentration of these dykes ( Figure 1 3 ) and is central to numerous supercontinent reconstructions, so obtaining accurate emplacement ages for the dykes is essential for any paleomagnetic reconstruction (French and Heaman 2010). The dykes crosscut Archean granites and gneisses and range in orientation from E W, WNW NW, NE ENE, and N S (Rao et al. 1995; French and Heaman 2008, 2010; Halls et al. 2007; Pradhan et al. 2010). Sections 3.1 3.3 of this manuscript review the characteristics and geochronology of the Paleoproterozoic dyke swarms. A combined list of previously published (and unpublished) paleomagnetic direction s and statistics are listed in Tables 3 1, 3 2, and 3 3 ~2.37 Ga dykes The E W trending Bangalore dyke swarm has been previously identified within the Dharwar craton, and contains several precise U Pb ages of 2367 1 Ma (Yeragumballi diabase dyke, baddeleyite; Halls et al. 2007), 2365.4 1.0, 2365.9 1.5 and 2368.6 1.3 Ma (Harohalli, Penukonda, and Chennekottapalle dykes, baddeleyite; French and Heaman 2010), as well as 2368.5 2.6 Ma and 2367.1 3.1 Ma (Karimnagar dykes, baddeleyite; Kumar et al. 2012 a ). The predominately E W trending
17 dyke swarm is at least 300 km wide and 350 km long, and consists of iron rich t holeiites (Ernst and Srivastava 2008). Paleomagnetic directions from the dykes show a steep remanence that was argued to be primary due to a positive baked contact test (Halls et al. 2007). The directions place India at high latitudes at ~2.4 Ga. Halls et al. (2007) connected the Bangalore dyke swarm to the Widgiemooltha dykes (2410 2418 Ma) of the Yilgarn craton, Australia, and provided a reconstruction placing the two cratons at high latitudes. Kumar et al. (2012 a ) recently obtained six equivalent U P b ages for the Dharwar giant dyke swarm, indicating emplacement within 5 Myr. Kumar et al. (2012 a ) report a NE trend for the northern dykes and an E W trend for the southern dykes. They speculate that the difference in trends could be due to the presence of a giant radiating dyke swarm converging at a point approximately 300 km west of the Dharwar craton boundary. Another possibility is that the dykes are part of a linear E W trending swarm with some tectonic complications at the northern end. The NW SE trending Mesoproterozoic (1600 1500 Ma; Chaudhuri and Deb 2004) Godavari rift is located just north of the NE trending dykes. It is possible that the formation of this rift caused a rotation of the E W dykes into a more NE orientation, leaving the southe rn dykes unaffected (see discussion). The rift extends from the Eastern Ghats Granulite Belt (EGGB) in the southeast to the CITZ in the northwest, and marks the boundary between the Bastar and Dharwar cratons (Chaudhuri and Deb 2004). ~2.21 2.18 Ga dykes Two previously unidentified Paleoproterozoic magmatic events were identified in the Dharwar craton by French and Heaman (2010). U Pb ages of 2221 2209 and 2181 2177 Ma indicate the presence of two large (100,000 km 2 ) dyke swarms at approximately 2.18 and 2.21 Ga with a possible convergence point west of the Deccan
18 Flood basalt province (French and Heaman 2010). A stationary and long lived mantle plume from 2.21 2.18 Ga may explain this protracted period of magmatism (35 Ma), and can be linked with the b reakup of an Archean Paleoproterozoic continent (French and Heaman 2010). The 2181 2177 Ma igneous pulse was previously classified as the Mahbubnagar swarm (Ernst and Srivastava 2008). More recently Kumar e t al. (2012b ) obtained two whole rock mineral Sm Nd isochron ages of 2173 43 Ma and 2190 51 Ma for N S trending mafic dykes within the Dharwar craton. The dykes are part of a 450 km long N S trending swarm shown to be fairly chemically homogenous ( Kumar et al. 2012b ). The NW SE, N S, and NE SW dyk es have a range of textures including gabbro, dolerite, and metapyroxenite, and geochemically they are quartz or olivine normative, tholeiitic and sub alkalic in composition (Ernst and Srivastava 2008; Pandey et al. 1997). Paleomagnetic analysis of the 3 dykes, including a 350 km long outcrop, yield a mean paleomagnetic pole at 32S and 302E (dp=8, dm=10). Srivastava et al. (2011) provide further age constraints for the N S trending dykes and report two U Pb baddeleyite ages of 2215.2 2.0 and 2211.7 0.9 Ma, corresponding to the ~2.21 magmatic pulse identified by French and Heaman (2010). Preliminary paleomagnetism of NW and E W trending dykes belonging to the ~2.18 magmatic pulse will be presented here (see results and discussion). ~1.88 Ga dykes Re cent work within the Dharwar and Bastar cratons has hinted at the presence of a remnant Large Igneous Province (LIP) at ~1.88 Ga. French et al. (2008) obtained high precision U Pb dates of 1891.1 0.9 Ma (baddeleyite) and 1883.0 1.4 Ma (baddeleyite and zircon) for two NW SE trending mafic dykes from the BD2 dyke swarm in the Southern Bastar craton, as well as an age of 1885 3.1 Ma for the Pullivendla
19 mafic sill within the Cuddapah basin. These ages indicate that magmatism spanned at least 10 million years (French et al. 2008). French et al. (2008) informally named this magmatic event the Southern Bastar Cuddapah large igneous province, and speculated that this event could have transected a wide area of cratonic India. The presence of 1890 Ma magmati sm on both the Bastar and Dharwar cratons indicates that the two blocks formed a coherent entity at this time. The dykes trend NW SE and E W and consist of sub alkaline basalts, ranging from quartz normative tholeiites, with subordinate olivine and nephe line normative tholeiites (French et al. 2008; Ramchandra et al. 1995). Ernst and Srivastava (2008) linked ~1.88 Ga NW SE trending dykes in the Bastar craton (French et al. 2008) with an E W trending dyke west of the Cuddapah basin (Halls et al. 2007) and hypothesized that a major radiating dyke swarm could be present within the Dharwar craton, with a convergence point east of the craton boundary. This focal point may mark the position of an 1890 Ma mantle plume (see discussion; Ernst and Srivastava 2008). Ernst and Srivastava (2008) noted that ~1.88 Ga mafic magmatism is common on other Precambrian cratons worldwide, including the Superior craton, Slave, Kaapvaal, Siberian, and possibly East European cratons (French et al 2008). The global distribution of ~1.88 Ga intracratonic mafic magmatism likely indicates a period of either anomalous mantle plume activity or a large scale mantle 2008). This magmatism may have been accompanied by the development of several intracratonic basins in the Dharwar protocontinent, including the Abujhmar and Cuddapah basins (French et al. 2008).
20 Meert et al. (2011) presented a preliminary paleomagnetic analysis of five ~1.88 Ga Bastar mafi c dykes, and found a dual polarity magnetization with a NW SE declination and shallow inclination. The paleomagnetism from the newly sampled Keskal dyke swarm is in agreement with previous studies on the Cuddapah Traps Volcanics (Clark 1982) and an E W dy ke adjacent to the Cuddapah basin (Kumar and Bhalla 1983), indicating that they may be part of the same ~1.88 Ga giant radiating dyke swarm (French et al., 2008; Meert et al. 2011). This result places India at equatorial latitudes at ~1.88 Ga, and shows l ittle resemblance to the archetypal position of India within the proposed supercontinent Columbia (Zhao et al. 2006). Here we present new paleomagnetic and supplementary geochronologic data from Dharwar mafic dykes and the Pullivendla sill in southeaste rn peninsular India. Sample areas included swarms located near Hassan, Tiptur and Kunigal (west of Bangalore; Fig ure 1 3 ) as well as a dense concentration of dykes in the Tirupati Chitoor region (E NE of Bangalore; Figure 1 3 ). Our new paleomagnetic resu lts help refine the Apparent Polar Wander Path (APWP) for the Dharwar craton during the Paleoproterozoic, from about ~2.37 Ga to ~1.88 Ga. Implications for reconstructions during this interval will be discussed, and proposed supercontinent configurations will be evaluated using recent well dated paleomagnetic poles and coeval magmatic events on other continents.
21 Figure 1 1 Columbia reconstruction according to Zhao et al. (2002, 2004). Dark shaded cratons (green) have paleomagnetic data availab le at 1.9 Ga and lighter shaded cratons hav e no paleomagnetic data (Table 4 3 ). Legend Kp=Kaapvaal craton;Zm=Zimbabwe craton;NCB=North China Block; SCB=South China Block;M=Madagascar; SAM=SouthAmerica blocks (Amazonia, Rio de la Plata); WAfr=West Africa; 2 .1 1.8 Orogens (blue; smaller italics); TNC=Trans North China; CITZ=Central Indian Tectonic Zone; TA=Transantarctic; C=Capricorn; L = Limpopo; CA= Central Aldan; Ak = Akitkan; W=Wopmay; TT= Taltson Thelon; TH= Trans Hudson; Pe = Penokean; F = Foxe; U = Ung ava; NQ = Nugssugtoquidian; K = Ketilidian; Sf=Svecofennian; V=Volhyn; P=Pachelma; KK=Kola Karelian; E=Eburnean; Taz=TransAmazonian.
22 Figure 1 2. Generalized geologic map of Peninsular India Displayed are the major cratons and various dyke swarms intruding each craton The Dharwar craton (focus of this study) is located in southern peninsular India. The Pullivendla sill is represented by the yellow star
23 Figure 1 3. Field area for the present study of Dharwar dykes.
24 CHAPTER 2 METHODS P aleomagnetism We sampled 87 sites, averaging about 10 cores per site, with a total of ~870 core specimens from the mafic dykes intruding the Dharwar craton. Forty four sites (dykes) yielded consistent results and are discussed in this paper ( Figure 1 3 ; T ables 3 1, 3 2, and 3 3). Samples were drilled in the field using a portable gasoline powered hand drill. The samples were oriented using a Brunton magnetic compass as well as a solar compass to correct for any magnetic interference and in order to deter mine local magnetic declination. The location, size, orientation, and quality of each outcrop were recorded, and where the geology allowed, baked contact samples were collected from the regional basement gneisses or granites. Typically we drilled several cores within the baked zone (~half width of the dyke), several cores from the hybrid zone, and where needed, several from the unbaked host rock. Samples were returned to the University of Florida, where they were cut into standard sized cylindrical speci mens of relatively equal volume, and natural remanent magnetization (NRM) was measured on either a Molspin spinner magnetometer or a 2 G cryogenic magnetometer. Preliminary pilot samples (2 partner cores from each site) were stepwise treated using therma l or alternating field demagnetization and the proper demagnetization method and steps were chosen for each site based on the preliminary evaluation of these samples. Alternating field demagnetization was carried out using a home built AF demagnetizer wit h fields up to 150 mT, while thermal demagnetization was conducted using an ASC Scientific TD 48 thermal demagnetizer up to temperatures of 600C. Linear segments of the resulting demagnetization paths were analyzed through principal component
25 analysis (K irschvink 1980) and great circle paths the using Super IAPD software (Torsvik et al. 2000). Rock Magnetic Experiments Curie temperature experiments were conducted on one powdered sample from each site in order to identify the magnetic carriers present in the dykes. Experiments were conducted with a KLY 3S susceptibility bridge adapted with a CS 3 heating unit, and susceptibility was measured incrementally during the heating and cooling of the samples. Susceptibility vs. temperature was plotted and heatin g and cooling Curie temperatures were calculated using the Cureval 8 software ( M. Chadima & V. Jelinek ). Isothermal remanent magnetization (IRM) studies were carried out on an ASC Scientific Model IM 10 30 impulse magnetizer for select samples in order to further characterize the magnetic carriers. Backfield IRM was also performed on previously AF demagnetized cores. Geochronologic Methods Samples from the NW SE and E W trending dykes were processed for geochronology ( Figure 2 1 ). Each sample was pulver ized and the zircons were isolated using conventional methods of mineral extraction and standard gravity and magnetic separation techniques at the University of Florida. The sample was crushed, disk milled, and sieved to a < 250 m grain size fraction. H eavy liquid mineral separation with multiple agitation periods was used to isolate grains in the higher density fractions. Samples were then repeatedly passed through a Frantz Isodynamic Magnetic Separator up to a current of 1.2 A (10 tilt). Two euhedr al zircon grains were handpicked from the remaining sample using an optical microscope, and were mounted in resin and polished to expose medial sections. The plugs were further cleaned in 5% nitric acid (HNO3) to
26 remove common Pb surface contamination. U Pb isotopic analyses were conducted at the Department of Geological Sciences (University of Florida) on a Nu Plasma multicollector plasma source mass spectrometer equipped with three ion counters and 12 Faraday detectors. The LA ICPMC MS is equipped with a specially designed collector block for simultaneous acquisition of 204 Pb ( 204 Hg), 206 Pb and 207 Pb signals on the ion counting detectors and 235 U and 238 U on the Fa raday detectors (Mueller et al. 2008). Mounted zircon grains were laser ablated using a Ne w Wave 213 nm ultraviolet laser beam. During U Pb analyses, the sample was decrepitated in a He stream and then mixed with Ar gas for induction into the mass spectrometer. Background measurements were performed before each analysis for blank correction and contributions from 204 Hg. Each sample was ablated for ~30 s in an effort to minimize pit depth and fractionation. Data calibration and drift corrections were conducted using the FC 1 Duluth Gabbro zircon standard, and long term reproducibility was 2% for 206 Pb/ 238 U (2 ) and 1% for 207 Pb/ 206 Pb (2 ) ages (Mueller et al. 2008). Data reduction and correction were conducted using a combination of in hou se software and Isoplot (Ludwig 1999).
27 Figure 2 1 Tera Wa sserburg U Pb concordia diagram for zircon dat a from dyke I10 19
28 CHAPTER 3 RESULTS ~2.37 Ga D ykes Fourteen E W, NW SE, and NE SW trending dykes (Table 3 1) have paleomagnetic directions with NRM intensiti es ranging from 0.12 to 9 A/m. Representative demagnetization be havior is displayed in Figures 3 1 a and b. Thermal demagnetization revealed unblocking temperatures between 550 and 570C (Figures 3 1 a and b), and alternating field treatments show median destructive fiel ds of 40 to 70 mT. Unblocking temperatures in this range are consistent with t hat of magnetite (Butler 2004). Samples from sites 14, 39, and 45 show a sharp drop in intensity (<50%) near 320C upon heating, indicating the likely presence of pyrrhotite as a magnetic carrier in those samples ( Fig ure 3 1 a). Representative results of thermomagnetic analysis are shown in Figure 3 2 a. Curie temperature experiments (susceptibility vs. temperatures) reveal nearly reversible heating and cooling curves with a sin gle magnetic phase. Sites 14, 39, and 45 reveal two magnetic phases, with a sharp decrease in susceptibility at ~320C (pyrrhotite), and a larger drop by ~565C (magnetite; Butler 2004). The heating Curie temperature Tc H from Site 62 is 563.8C and the c ooling Curie temperature Tc C is 557.7C ( Figure 3 2 a). IRM curves reveal magnetic saturation between 0.2 and 0.3 T and backfield coercivity of remanence ranged from 0.08 to 0.12 mT. These values are consistent with magnetite as the magnetic carrier ( Chikazumi 1997 ). Figure 3 3 a displays IRM curves for sites 16 and 62, with magnetic saturation values of 0.1 and 0.15 T, and backfield coercivity remanence values of 0.1 and 0.12 mT. Ten dykes revealed a stable uni vectorial demagnetization trend during both treatments, while four dykes yielded multicomponent directions. The main direction is
29 carried by the highest coercivity and unblocking temperature. The direction carries a steep inclination previously recognized and precisely dated (Halls et al. 2007; French and Heaman 2010; Piispa et al. 2010; Kumar et al. 2012 a ; Figure s 3 1 a and b). These dykes are part of the ~2.37 Ga E W trending Dharwar giant dyke swarm and have a declination=85.2, and an inclination= ry remanence is supported by a positive baked contact test ( Figure 3 4 a). At site 14, twelve samples were collected from the gneissic host rock at the contact, and three additional samples were collected from unbaked gneisses within the swarm. Dyke sampl es yielded a steep negative inclination (D=44.5, I= 77.7, MAD=2.9), the hybrid gneiss samples yielded similarly steep inclinations (D=161.9, I= whereas the unbaked gneiss yielded an intermediate and positive inclination ( Figure 3 4 a). ~2.21 2.18 Ga D ykes Six N S, NW SE, NE SW and E W dykes (Table 3 2) revealed paleomagnetic directions with NRM intensities ranging from 0.1 to 4.6 A/m. Representative demagnetization be havior is displayed in Figures 3 1 c and d. Unblocking temperatures we re between 560 and 570C for thermal treatments. Curie experiments show reversible heating and cooling curves with one magnetic phase. The heating curie temperature Tc H for site 17 is 555.2C, and the cooling curie temperature Tc C is 515C ( Figure 3 2 b) IRM curves reveal magnetic saturation between 0.2 and 0.25 T, along with a backfield coercivity of remanence value of 0.08 mT ( Figure 3 3 b).
30 Dykes reveal both stable uni vectorial demagnetization trends ( Figure 3 1 c) as wel l as multicomponent direction s Secondary components are removed by ~400C. Three N S, NE SW and NW SE trending dykes yielded a west southwest declination and a fairly steep negative inclination (D=252.6, I= Figure 3 1 c). The direction is similar to results recently obtained from N S trending dykes in the Dharwar craton (Kumar et al. 2012b ) that have been identified as part of th e ~2.2 Ga LIP The three dykes yielded a mean virtual geomagnetic pole (VGP) at 20S and ions from Kumar et al. (2012b ) combined with our three dykes yield an average paleomagn event. Three NW SE and E W trending dykes have a slightly different direction from the previous pole with shallower positive inclinations and northerly declinations (D=3, F igure 3 1 d). These dykes were sampled from the 2.18 Ga Mahbubnagar swarm (U Pb; French et al. 2004; Ernst and Srivastava 2008). A mean with a corresponding paleolatitude of 26.6N (site 571). A baked contact test for a dyke in the Mahbubnagar swarm (site 571) lends evidence for a primary magnetization ( Fig ure 3 4 b). The mean dyke direction has a northerly declination and positive baked hybrid gneiss has a northeast declination inclination (D=339, I= ~1.88 Ga D ykes Geochronology U Pb ages from zircons were determined for the NW SE trending dyke sample 1019 from the Kunigal region. The dyke sample yielded several zircons suitable for U
31 Pb isotopic analysis; however only 2 of the zircons yielded useful data and the remainder were highly (>50%) discordant. Three laser analyses on the two diff erent zircons yielded three discordant points Two of these zircons yielded 207 Pb/ 206 Pb ages of 1847 6 Ma an d 1839 8 Ma ( Fig ure 2 1 ; Table 3 4). These represent minimum ages for the dyke and we note that p aleomagnetic dir ections from this site and other well dated 1.9 Ga dykes are in agreement, so these minimum age s are broadly consistent with recent geochronolo gic results reported for the NW striking Pullivendla sill (1885 3 .1 Ma; French et al. 2008) and NW SE trending Bastar dykes (1891.1 0.9 Ma and 1883.0 1.4 Ma; French et al. 2008). Paleomagnetism Twenty fi ve NE SW, E W and NW SE dykes (T able 3 3 ) and the Pullivendla sill have directions with NRM intensities ranging from 0.76 to 49 A/m. The dykes record a dual polarity magnetization, and representative demagnetization behavior for both polarities is shown in Figures 3 5 a c. Thermal demagnetization revealed unblocking temperatures between 540 and 570C indicative of magnetite (Figures 3 5 b and c), and alternating field treatments show median destructive fields of 40 to 70 mT ( Fig ure 3 5 a). Representative results of thermomagnetic analysis are shown in Figure 3 2 c. Curie temperature experiments reveal two magnetic ph ases in 8 of the dykes. The firs t phase (associated with pyrrho tite) shows a sharp decrease in magnetic susceptibility near 320C, and the second phase shows a much larger drop (associated with magnetite) at 545 563C. Figure 3 2 c (site 67) has a heatin g curie temperature Tc H of 555.8C and a cooling curie temperature Tc C of 567.5C. The bulge in the heating curve around 300C indicates the presence of pyrrhotite. IRM curves reveal magnetic saturation values between 0.25 and
32 0.3 T, and backfield coerci vity of remanence values between 0.05 and 0.1 mT ( Figure 3 3 c). The majority of the dykes revealed a stable uni vectorial demagnetization trend during thermal treatments, and five of the dykes reveal multicomponent directions. Secondary components are rem oved by 350C (site 32) for thermal demagnetization and by 40 mT (site 40) for alternating field demagnetization. The main direction is carried by the highest coercivity and unblocking temperature, with either a northwest or southeast declination, and a Fig ure 3 5 ), with a corresponding paleolatitude of 2.7N (site 34). This direction matches preliminary data from ~1.88 Ga Bastar dykes (Meert et al. 2011), the Cuddapah Traps volcanics (Clark 1982) several dykes near the Cuddapah basin (Hargraves and Bhalla 1980; Kumar and Bhalla 1983) and near Tiptur (Bhalla et al. 1980), as well as Cuddapah basin sediments (Prasad et al. 1984). The dykes have a dual polarity magnetization with a mean normal pol e at 30.1N and 327.7E ( Fig ure 3 6 ; Rb observed positive baked contact test at site 87 lends support for a primary remanence ( Fig ure 3 4 c). Four contact gneiss and one unbaked gneiss were sampled at this site in addition to the dyke. The mean direction for the dyke is nearly antipodal to the unbaked gneiss. the hybrid direction is southeast and shallow (D=128, I= and the unbaked gneiss is southeast with a medium inclination ( Figure 3 4 c). The likely primary n ature of
33 this direction, three well constrained and consistent U Pb ages, and a large, statistically pole for the Dharwar craton at ~1.9 Ga.
34 T able 3 1. Paleomagnetic r esults for 2.37 Ga dykes Site/Sample Slat (N) Slong (E) N D () I () k Plat (N) Plong (E) Trend Swarm Reference 2 12.010 77.020 15 150.2 84 3.1 150 22.2 70.7 290 2.4 1 3+4 11.980 77.030 17 50.8 75.6 7.1 26 5.6 56.2 3 00 2.4 1 12 12.040 78.520 8 125.8 71.3 7.6 55 29.6 47 290 2.4 1 16 12.580 77.980 10 39.4 80.9 6.1 63 1.3 66.8 276 2.4 1 17 12.630 78.070 5 186.4 84.8 13 36 22.9 79.3 270 2.4 1 18 13.490 76.580 8 105.4 73.7 11.2 25 19.4 45.5 255 2.4 1 20 14.310 76 .630 5 72.8 76.4 9.5 66 5.6 51.9 270 2.4 1 12 12.660 77.500 6 135.1 84.7 6.4 110 20 69.6 270 2.4 2 15 12.650 77.420 6 72.6 78.1 7.4 84 5.1 55.6 295 2.4 2 16 12.660 77.420 5 60.3 81.2 7.4 109 3.8 62.5 265 2.4 2 C 12.110 79.080 3 105.2 74.5 7.4 280 17.9 49.6 300 2.4 2,3 E 12.110 79.070 6 115 75 7 75 22.3 51.5 30 2.4 3 F 12.210 79.080 4 125 75 9 58 26.8 53.4 300 2.4 3 T3 12.090 78.920 7 129.7 73.5 2.8 481 29.9 52 300 2.4 4 D7 12.080 77.890 6 105.8 75.5 5.2 168 18 50.2 295 2.4 4 T4 12.060 79.0 10 7 114.4 67.9 9.6 92 24.6 39.8 30 2.4 4 i=A=B 14.190 77.640 5 56.5 69.5 7 53 7.1 47.4 255 2.4 5 ii 14.180 77.760 3 71.9 72.7 7.5 271 2.8 47.5 250 2.4 5 1 12.900 78.200 7 170 80 5 113 32 74.3 2.4 6 2 12.900 78.200 6 88 81 6.3 82 11.7 60.3 2.4 6 3 12.900 78.200 9 127 77 4.2 124 26.7 56.2 2.4 6 BANG 12.900 78.200 3 129.4 80.9 10.8 131 23.7 63.3 2.4 6 10 12.730 77.520 4 141 75 10 50 33.5 56.6 280 2.4 7 Hol  12.790 76.230 4 62.2 78.4 3 916 1.8 56.6 310 2.4 8 Hol  12.790 76.230 4 54 .7 79.2 6.2 218 0.3 59.3 65 2.4 8 Dyke 3 8 68.9 65.2 11.2 19.6 0.26 39.9 NE SW 2.4 9 Dyke 2 15 102.8 66.6 6.8 28.3 21.4 35.5 NE SW 2.4 9 2 13.290 76.463 4 21.8 75.8 14.1 43.4 11.7 66.6 250 70 2.4 This study 10 13.050 76.800 3 48.2 78.5 GC GC 2 7 95.2 345 165 2.4 This study 14 13.105 76.753 5 44.5 77.7 GC GC 4 60.4 270 90 2.4 This study
35 Table 3 1. Continued Site/Sample Slat (N) Slong (E) N D () I () k Plat (N) Plong (E) Trend Swarm Reference hybrid 12 161.9 84.4 10 19.7 Thi s study unbaked 3 224.2 46.8 73.9 3.9 This study 16 13.183 77.041 8 339 81 6.3 77.7 3.3 83.3 260 80 2.4 This study 28 13.334 79.405 6 256.1 69.5 6.9 95.3 2.6 43.8 E W 2.4 This study 39 13.541 79.011 4 26.7 72.4 GC GC 15.5 64.5 E W 2.4 This st udy 41 13.540 79.005 6 117.4 78.9 5.1 175.8 22.4 58.5 E W 2.4 This study hyrbrid 4 2 35.2 18.4 25.8 This study unbaked 3 9.3 49.9 20.4 37.7 This study 45 13.533 79.016 5 331 78.3 6.7 130.9 32.8 66.3 E W 2.4 This study 62 14.156 78.151 7 7 4 85 6.7 80 11.2 68.4 240 60 2.4 This study 71 14.196 77.810 6 75 72.3 4 325 4.1 46.4 E W 2.4 This study 590 14.474 77.626 4 17 76 9.7 90 10.9 70 230 50 2.4 This study 592 14.313 77.637 6 234 86 8.6 42 9.6 71.1 310 130 2.4 This study 596 14.192 77. 796 7 85.2 80.4 5 182 11.9 58.8 250 70 2.4 This study 5118 13.255 76.449 7 12 81 16 17 4 72.8 E W 2.4 This study Mean 14 9.9 17.2 5.2 65.9 2.4 This study Combined 41 5.2 19.6 12 59 2.4 Notes: Slat=site latitude, Slong=site longitude, N=nu confidence about the mean direction, k=kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = pole longitude, Reference is the number of the reference: 1: Halls et al. (200 7); 2: Dawson and Hargraves (1994); 3: Venkatesh et al. (1987); 4: Radhakrishna and Joseph (1996); 5: Kumar and Bhalla (1983); 6: Bhalla et al. (1980); 7: Hasnain and Qureshy (1971); 8: Sites from canal cutting at Holenarsipur (A. Kumar, unpublished data, 1985); 9: Kumar et al. (2012 a ).
36 Table 3 2 Paleomagnetic results for ~2.2 2.18 Ga dykes Site/Sample Slat (N) Slong (E) N D () I () k Plat (N) Plong (E) Trend Swarm Reference AKLD 13.941 76.977 9 228 61 5 40 304 N S 2.21 10 dyke ii 12.962 77.376 2 245 56 28 313 N S 2.21 10 dyke iii 16.357 77.725 4 273 72 9 12 292 N S 2.21 10 17 13.183 77.041 4 218.1 69 5.9 242.7 40.4 106.6 NW SE 2.21 This study 20 13.061 77.037 8 281.9 46.9 8.9 39.4 4.1 136.9 N S 2.21 This study 35 13.547 78.921 8 252.6 61.6 4.9 126.9 21.9 127.9 215 35 2.21 This study 2.21 Mean 3 9.6 42 19.9 125.3 2.21 This study Combined 6 16 17 23.5 303.9 2.21 64 14.184 78.163 4 347.2 50.1 13.8 45.4 69.6 45.1 E W 2.18 This study 568 16.928 77.863 6 9 60 7.8 76 64.8 94 E W 2.18 This study 571 16.928 77.705 10 3 45 3.7 171 80 93.3 290 110 2.18 This study hybrid 4 23 50.2 10.2 8 3 This study unbaked 3 339 42 7 This study 2.18 Mean 3 18.1 47.3 72.9 76.3 2.18 This study confidence about the mean direction, k=kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = pole longitude, Reference is the number of the refer ence: 10: Kumar et al. (2012b ).
37 Table 3 3. Paleomagnetic results for 1.88 Ga dykes Site/Sample Slat (N) Slong (E) N D () I () k Plat (N) Plong (E) Trend Swarm Reference 532 19.600 81.600 5 142 25 7 122 40 313 NW SE 1.9 11 543 18.900 81.500 14 297 24 8 26 18 331 N S 1.9 11 524 20.100 81.600 14 120 5.4 8 27 27 338 NW SE 1.9 11 527 19.800 81.600 9 132.1 10 7 56 37 329 NW SE 1.9 11 531 19.700 81.600 8 292 0.5 8 54 27 341 NW SE 1.9 11 Cuddapah Traps 20.000 78.200 15 299 6 16 18 27 337 1.9 12 Cuddapah dyke 14.400 77.700 9 317 32 25 97 37 312 NE 1.9 5 Cuddapah Basin seds 14.600 78.600 76 303 5.8 14.4 41.9 29.3 152.9 1.9 13 Cuddapah dyke 13.600 79.300 10 296.6 26.3 50 7 21.5 328.2 E W 1.9 14 Tiptur dyke 13.400 76.000 35 287 21 12 21 13.6 331.1 1.9 6 1 13.299 76.459 8 344 15 GC GC 73.3 328.2 275 95 1.9 This study 18 13.068 77.009 11 119.6 7.1 6.1 56.4 27.8 155.8 330 150 1.9 This study 19 13.063 77.008 7 337.1 13.9 GC GC 59.6 126.8 330 150 1.9 This study 26 13.279 79.229 6 283.3 26.5 8.6 61.0 9.3 152.3 E W 1.9 This study 29 13.334 79.405 6 289.7 9.3 6 125.3 20.2 349.5 E W 1.9 This study 31 13.379 79.410 7 120.4 46.2 6.1 97.4 19.2 133.5 255 75 1.9 This study 32 13.417 79.413 4 141.8 22 GC GC 44.7 137.9 310 130 1.9 This study 34 13.488 78.831 8 287.1 5.4 4.6 147.5 17.3 347.5 E W 1.9 This study 40 13.541 79.011 5 286.3 22.3 GC GC 12.7 153.6 E W 1.9 This study 43 13.250 79.100 5 295 3 13 33 24 341 E W 1.9 This study 66 14.106 78.127 4 307.5 14.9 GC GC 33.6 148.9 310 130 1.9 This study 67 14.138 77.935 3 286.2 8.8 9.8 159.3 16.8 348.3 240 60 1.9 This study 74 (Pullivendla Sill) 14.770 78.172 5 314.4 8 12 42 43.8 339.4 290 110 1.9 This study 86 15.340 77.810 16 332 3 7 27 57 316 260 80 1.9 This study 87 16.640 77.850 7 301.2 5.1 11.9 28 30.6 340.8 310 30 1.9 This study hybrid 128 6 22 unbaked 123 32
38 Table 3 3. Continued Site /Sample Slat (N) Slong (E) N D () I () k Plat (N) Plong (E) Trend Swarm Reference 539 18.990 81.610 4 330 14 6.4 206 51 313 300 120 1.9 This study 574 16.600 77.900 6 308 10 16 20 34 330 NW SE 1.9 This study 575 16.640 77.850 4 286 12 10.6 7 6 13 337 320 140 1.9 This study 586 15.400 77.800 8 306 22 6.4 76 30.2 144.4 330 150 1.9 This study 587 15.400 77.800 3 321 4.2 29 19 48 327 330 150 1.9 This study 588 15.300 77.800 5 315 3.1 8.7 79 43.5 335 320 140 1.9 This study 597 14.200 77.810 6 320 16 10.9 39 44.5 320.9 330 150 1.9 This study 5115 13.310 76.460 7 296 12 13 22 24 333 310 130 1.9 This study KD site 13.520 78.800 6 315 1 6 139 43 335 290 110 1.9 This study Mean 24 7.1 18.3 34.6 332 1.9 This study Combined 35 5.4 21 .12 31.9 331.5 1.9 confidence about the mean direction, k=kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = p ole longitude, Reference is the number of the reference: 5: Kumar and Bhalla 1983; 6: Bhalla et al. 1980; 11: Meert et al. 2011; 12: Clark 1982; 13: Prasad et al. 1984; 14: Hargraves and Bhalla 1980.
39 Table 3 4. Geochronological results G rain Name 207 Pb/ 235 U 2 206 Pb/ 238 U 2 (error corr.) 207 Pb/ 206 Pb 2 206 Pb/ 238 U (Age) Ma *2 207 Pb/ 235 U (Age) Ma *2 207Pb/ 206Pb (Age) Ma *2 I10 19_1 (core) 4.90861 3.4 0.31526 3.4 0.99 0.11293 0.32945938 1768 52 1803 28 1847 6.0 I10 19 _2 (core) 4.65324 3.9 0.30015 3.8 0.99 0.11244 0.46116253 1693 57 1759 32 1839 8.3 I10 19_3 (rim) 1.50094 5.5 0.14266 5.5 0.98 0.07631 0.74018802 860 44 931 33 1103 15
40 Figure 3 1 Orthogonal vector plots from the 2.37, 2.21, an d 2.18 Ga suite of dykes showing typical characteristic remanent magnetization directions. A) Thermal demagnetization behavior of sample 1045 3a from the ~2.4 Ga suite of dykes. The sharp drop in intensity (<50%) at 320C indicates pyrrhotite as a magneti c carrier. B) Thermal demagnetization behavior of sample 1014 7a from the ~2.4 Ga suite of dykes. C) Thermal demagnetization behavior of sample 1035 2a from the 2.21 Ga suite of dykes. D) Thermal demagne tization behavior of sample I571 8 fr om the 2.18 Ga s uite of dykes Solid circles represent projections on the horizontal plane; open circles represent projection onto a vertical plane.
42 Figure 3 2 Curie temperature analysis. A) Sample 1062 5d from the ~2.4 Ga suite of dykes shows a heating Curie tempe rature (Tc H ) of 563.8C and cooling Curie temperature (Tc C ) of 557.7C with nearly reversible Curie temperature runs. B) Sample 1017 3c from the ~2.2 Ga suite of dykes shows a heating Curie temperature (Tc H ) of 555.2C and cooling Curie temperature (Tc C ) o f 515.1C. C) Sample 1067 2b from the ~1.9 Ga suite of dykes shows a heating Curie temperature (Tc H ) of 555.8C and cooling Curie temperature (Tc C ) of 567.5C.
44 Figure 3 3 Isothermal remanence acquisition curves and bac k field IRM. A) Samples 1016 8b and 1062 8a are from the ~2.4 Ga suite of dykes. All samples saturate at about 0.1 0.15 T and coercivity of remanence values ranged from 0.1 to 0.12 T. B) Sample 1017 6b is from the 2.21 Ga suite of dykes and sample 1064 8b is from the 2.18 Ga suite of dykes. All samples saturate at about 0.2 0.25 T and coercivity of remanence values were 0.08 T. C) Samples 1018 2b and 1019 5b are from the 1.88 Ga suite of dykes. All samples saturate at about 0.25 0.3 T and coercivity of rema nence values ranged from 0.05 to 0.1 T.
46 Figure 3 4 Baked contact tests. A) Positive baked contact test from the 2.37 Ga suite of dykes (site 14). B) Positive baked contact test from the 2.18 Ga suite of dykes (site 571). C) Positive baked contact test from the 1.88 Ga suite of dykes (site 87). Baked hosts are sampled within one half width of the dyke, and unbaked hosts are distant samples.
48 Figure 3 5 Orthogonal vector plots from the 1.88 Ga suite of dykes showing typical characteristic remanent magnetization directions. A) Alternating field demagnetization behavior of sample 1074 8b from the Pullivendla sill. B) Thermal demagnetization behavior of sample 1018 5a. C) Thermal demagnetization behavior of sample 1019 2a that has a minimum discordant age of 1839 8.3 Ma (this study). Solid circles represent projections on the horizontal plane; open circles represent projection onto a vertical plane.
50 Figure 3 6 Galls projection of mean normal and reverse paleomagnetic poles for the 1.88 Ga suite of dykes. Blue squares represent normal poles and red squares represent reversed poles. Ovals represent the cone of 95% confidence about the mea n direction. Black ovals represent the mean 95.
51 CHAPTER 4 DISCUSSION ~2.37 Ga D ykes New paleomagnetic results generated from this study confirm the large geographic extent of the ~2.37 Ga Dharwar giant dyke swarm (previously identified as the Bangalore swarm) within southern peninsular India (Halls et al. 2007; Kumar et al. 2012 a ). The combined dataset represents a well dated and robust paleomagnetic pole for the Dharwar craton during the earliest Paleoproterozoic. It has been suggested that the dykes associated with this magmatic even t are part of a radiating swarm, with a focal point west of the present day craton boundary (Kumar et al. 2012 a ). The hypothesis is based on a variance in dyke trend where the majority of dykes to the south trend E W, and dykes to the north, just south of the Godavari rift, trend roughly NE. The NW SE Prahnita Godavari Basin records a major period of northeast crustal extension within the Indian subcontinent and exploits a pre existing zone of weakness along the Precambrian suture between the Dharwar and Bastar cratons (Biswas 2003). The main period of crustal extension began in the early Permian during pre breakup of Gondwana, followed by episodic rifting in the late Permian through Cretaceous (Biswas 2003). The extension associated with this rift may h ave caused a rotation of Paleoproterozoic dykes in the vicinity. Differential extension along the rift would rotate E W dykes into a NE orientation if extension was greatest along the nor thwestern portion of the rift; however, more structural research in the area is needed to support this hypothesis. Widespread paleomagnetic data indicate the existence of three large continental landmasses during the Proterozoic: Kenorland (Neoarchean), Columbia (Paleo
52 Mesoproterozoic) and Rodinia (Neoproterozoic; Pesonen et al. 2003). Existing paleomagnetic data constrains the Kenorland assemblage to include at least Baltica, Laurentia, Australia, and the Kalahari craton (Pesonen et al. 2003). The presence of mafic dykes and rift basins on several continents during the period from 2.45 2.10 Ga likely reflects a period of protracted continental breakup. The robust pole from the Dharwar craton at ~2.37 Ga can be combined with other well dated poles from other conti nents in order to evaluate the p aleogeograph y in this time interval (Table 4 1 ). Our reconstruction at ~2.4 Ga is shown in Figure 4 1 and the corresponding paleomagn etic poles are listed in Table 4 1 Several poles are available for comparison with the Dharwar at ~ 2.4 Ga, including the Karelian dykes from Balt ica (Mertanen et al. 1999), the Matachewan dykes from the Superior craton of Laurentia (Bates and Halls 1990), and the Widgiemooltha dykes of the Yilgarn craton in northern Australia (Evans 1968; Smirnov and Evans 2007). The Widgiemooltha dyke swarm has an emplacement age of 2418 3 Ma (Nemchin and Pidgeon 1998) and trends mostly E W. The Widgiemooltha dykes are mostly tholeiites and are similar in composition to the Dharwar dykes. Smirnov and Evans (2007) recently reported new paleomagnetic results fo r the swarm using modern demagnetization techniques, and found that the data was in good agreement with the previous study (Evans 1968). Halls et al. (2007) proposed a potential link between the Yilgarn and Dharwar cratons based on the patterns of dyke sw arms, and suggested that both may be the product of a single plume. Our reconstruction places the two cratons at high latitudes with about 15 of separation. The continents can be moved longitudinally so that a parallel alignment of the two swarms is pos sible; however, the
53 Dharwar dykes were emplaced over a very short time (5 Ma; Kumar et al. 2012 a ) and the error in ages leaves a significant gap (31 Ma minimum) between the two swarms, making it unlikely they evolved from the same plume. The Karelian dyke s located in the Fennoscandian shield (Baltica) have a wide geographic extent and trend mainly NW SE and E W (Mertanen et al. 1999). The dykes are mostly unaltered gabbronorites and have a U Pb age of 2450 2 Ma (zircon; Heaman 1988). Although the dyke s fail to pass a baked contact test, the magnetization of the dykes is argued to be primary due to evidence for regional reheating and remagnetization of the Archean basement at ca. 2.44 Ga (Mertanen et al. 1999). The Matachewan dykes of the Superior crat on have a U Pb age of 2446 5 Ma (Vuollo et al., 1995, 1996, 1999a,b) and trend mostly north and northwest (Bates and Halls 1990). The dykes characterize a fan angle that widens to the north, and may radiate from a southern source associated with failed rifting (Fahrig 1987). Primary magnetization is supported from several positive baked contact tests (Schutts and Dunlop 1981; Buchan et al. 1989). Our reconstruction places the Superior craton and Baltica at near equatorial latitudes, with about 10 of s eparation. The Matachewan and Karelian dykes are parallel in this configuration, providing evidence for coeval emplacement. Heaman (1997) attributed the parallel tend of the dykes to a mantle plume at 2.45 Ga that may have marked the onset of rifting fro m the Kenorland assembly. The mean paleomagnetic poles for the Matachewan and Karelian dykes give the position of both the Superior craton and Baltica with respect to the Dharwar craton, and provide a glimpse into the paleogeography during the early Paleo proterozoic ( Figure 4 1 ). Our reconstruction at ~2.4 Ga places Baltica at equatorial latitudes with about 15 degrees
54 of separation from the Superior craton, and nearly 70 of latitudinal separation from the Dharwar craton. ~2.21 2.18 Ga D ykes Magmatism w ithin the Dharwar craton at ~2.2 Ga likely represents a widespread thermal event (French and Heaman 2010). An alternative model to the unified Kenorland assembly is the supercraton solution (Bleeker 2003). Instead of a unified supercontinent, the model e mploys several supercratons as the precursors to the present cratonic nuclei (Bleeker 2003). A robust paleomagnetic pole at ~2.2 Ga for the Dharwar craton may help uncover the geometries of hypothesized supercratons such as Sclavia (Dharwar Slave connecti on; French and Heama n 2010). Kumar et al. (2012b ) reject a possible Dharwar Slave connection at ~2.2 Ga based on their preliminary paleomagnetic results as well as dissimilar Archean geology. French and Heaman (2010) identified two separate suites of dy kes in the Dharwar craton at 2.21 and 2.18 Ga. The NW SE trending Somala dyke and NNW SSE Kandlamadugu dyke have respective ages of 2209.3 2.8 Ma (baddeleyite) and 2220.5 4.9 Ma (baddeleyite), and the E W trending Bandapalem dyke and NW SE trending Da ndeli dyke have ages of 2176.5 3.7 Ma (baddeleyite and zircon) and 2180.8 0.9 Ma (badde leyite). Kumar et al. (2012b ) report paleomagnetic directions for ~2.2 Ga N S trending dykes within the Dharwar craton with southwest declinations and steep negativ e inclinations. Hamilton (2011) used precise U Pb dating to constrain the age of the dykes s ampled by Kumar et al. (2012b ), and reported two ages of 2211.7 0.9 Ma and 2215 2.0 Ma, indicating these dykes are part of the 2.21 Ga suite identified by Fren ch and Heaman (2010).
55 We also sampled ~2.2 Ga dykes dated by French et al. (2004) from the NE Dharwar craton (E W trending dolerite dyke; 2180 Ma; U Pb baddeleyite and zircon) within the Mahbubnagar swarm (Ernst and Srivastava 2008). Our ~2.2 Ga direction s differ slightly to those of Kumar et al. (2012b ), with different declinations and shallower inclinations. The positive baked contact test from the Mahbubnagar dyke ( Figure 3 4 b; this study) likely confirms the primary nature of this direction. The prim ary remanence of the 2.21 Ga suite of dykes is unconfirmed; however, four of the dykes sampled in this study are in agreement with the directions re ported by Kumar et al. (2012b ), so we report a tentative average VGP for 2.21 Ga here. French and Heaman (2 010) attribute the protracted period of magmatism at ~2.2 Ga to a possible long lived and stationary plume beneath the Dharwar craton. Due to the overlapping nature of the 2.21 and 2.18 Ga dykes, the rate of plate movement over the hypothesized plume is irresolvable; however, it is possible that the difference in paleomagnetic directions between 2.21 and 2.18 Ga ( Figure 4 2) is due to the rotation of the Dharwar craton during this interval. The Malley dykes of the Slave craton and the Senetterre dykes o f the Superior craton are used in conjunction with both the 2.21 Ga (combined) and 2.18 Ga paleomagnetic poles from the Dharwar craton to construct a paleogeographic map at ~2.2 Ga ( Figure 4 2; Table 4 2 ). The NE trending Seneterre dykes of the Superior c raton have a well constrained U Pb age of 2214.3 12.4 Ma (baddeleyite; Buchan et al. 1993). A primary magnetization is inferred from several positive baked contact tests of the Biscotasing dykes that carry t he same paleomagnetic direction (Buchan et al. 1993). The NE trending Malley dyke swarm of the Slave craton has a precise U Pb age of 2231 2 Ma (baddeleyite; Buchan et al. 2012) and extends from the central Slave
56 craton to near the Kilohigok basin. A primary remanence has not yet been confirmed; h owever, a positive baked contact test at the intersection between the Malley and younger Lac de Gras dyke (2.03 Ga), and no evidence for regional overprinting, lends support for a primary direction (Buchan et al. 2012). Our reconstruction at ~2.2 Ga places the Dharwar craton at intermediate latitudes. The individual cratons are located within ~30 of each other ( Figure 4 2). A north polar projection was used in an attempt to correlate the N S trending Dharwar dykes with the NE trending dykes in the Slave craton to evaluate the possibility of the supercraton Sclavia that rifted during this interval. French and Heaman (2010) hypothesized that the present day western margin of the Dharwar craton may have been connected to the western margin of the Slave crat on ba sed on the pattern of similarly aged radiating dyke swarms. To test this configuration, we plotted the two cratons at their respective latitudes and moved them longitudinally in position for a best fit scenario. Preliminary paleomagnetic data from ~ 2.2 Ga Dharwar dykes leaves about of 15 of separation between the two cratons. It is possible that the d yke swarms may have been coeval and that the l atitudinal separation is due to paleomagnetic error 16 ). An expanded dataset at ~2.2 Ga may help confirm the existence of Sclavia ~1.88 Ga D ykes Twenty five dykes from the current study, combined with the Cuddapah Traps volcanics (Clark 1982), Bastar dykes (Meert 2011), Dharwar a nd Cuddapah dykes (Hargraves and Bhalla 1980; Kumar and Bhalla 1983; Bhalla et al. 1980 ), and Cuddapah basin sediments provide a new precise paleomagnetic pole for the Dharwar craton at ~1.9 Ga. Well constrained U Pb ages from the Pullivendla sill (French et al. 2008), Cuddapah basin sediments, and a NW SE Kunigal dyke (site 19; this study)
57 provide a precise age for the characteristic remanence. The dual polarity magnetization present in both Bastar and Dharwar dykes as well as a positive baked contact te st (this study) support a primary magnetization. Our new paleomagnetic pole positions India at equatorial latitudes at ~1.9 Ga. Our paleomagnetic results combined with those of the Bastar craton (Meert et al. 2011) and available geochronologic data (Fren ch et al. 2008), support a connection between the Dharwar, Singhbhum, and Bastar cratons at ~1.9 Ga. The 1.88 Ga Bastar Cuddapah LIP event identified by French et al. (2008) is confirmed here by the presence of a large (~85,000 km 2 ) radiating dyke swarm wi thin the Dharwar craton. Dykes to the north within the Bastar and Dharwar cratons trend mainly NW SE to almost N S. The Pullivendla sill, located in the southwestern portion of the Cuddapah basin, trends roughly 290, while dykes south of the basin have an E W trend. A fanning angle of 65 defines the radiating swarm, with a focal point east of the Cuddapah basin ( Fig ure 1 3 ). Extension from the Godavari rift may have rotated the northern dykes counterclockwise from their original trend; however, these dykes trend mostly NW SE, so a restorative rotation would place the dykes in a more N S orientation, providing an even larger fan ning angle for the swarm. The focal point of the swarm may denote the position of a ~1.88 Ga mantle plume. The NW trending po sitive gravity anomaly (interpreted as a mafic lensoid body) beneath the southwestern section of the Cuddapah basin could be linked to the associated plume magmatism. The Gulcheru Formation (lowest stratigraphic member) of the Cuddapah basin has a paleoma gnetic direction equivalent to the ~1.9 Ga Dharwar pole, indicating that extension began in the basin at least before 1.9 Ga. This extension was most likely the
58 result of plume related magmatism. A northwest trending Fe rich tholeiite dyke with a U Pb ag e of 1832 72 Ma (zircon; Lanyon et al. 1993) is also present within the Vestfold Hills, East Antarctica. If we align the eastern border of the Dharwar craton against the Vestfold Hills, the dykes have a radiating pattern. Currently there is no paleomag netic data from the Vestfold Hills dykes, and most reconstructions place the collision between the Dharwar and East Antarctic blocks at 1 Ga during Rodinia assembly (Li et al 2008; Zhao et al. 2002), so a possible connection between the two cratons at this time is speculative. A number of well constrained paleomagnetic poles a re available at 1.88 Ga (Table 4 3 ), and allow us to test one of the possible configurations of the supercontinent Columbia (Zhao et al. 2004). Our paleomagnetically based reconstru ction at 1.88 Ga is shown in Figure 4 3a. To test the Columbia model, continents were plotted at their respective latitudes from the paleomagnetic data ( Table 4 3 ) and were moved longitudinally in position for a best fit with the Columbia configuration (Z hao et al. 2004). Poles from individual continents were selected based on the reliability of the paleomagnetic and geochronologic data, and span no more than 60 Ma apart. Our placement of Baltica comes from the thorough Paleoproterozoic compilation of P esonen et al. (2003), who presented a mean paleomagnetic pole for Baltica at 1.87 1.89 Ga (mean of the Vittangi, Kiuruvesi, Pohjanmaa, and Jalo koski gabbros and diorites). The paleomagnetic pole selected for Siberia comes from the well dated 1878 4 Ma A kitan group in southern Siberia (Didenko et al. 2009). A positive fold test and intraformational conglomerate test support a p rimary remanence for the pole. The tentative 1.83 Ga paleomagnetic pole from the Plum Tree Volcanics of Northern
59 Australia (Idnu rm and Giddings 1988; Idnurm 2004) is used in our reconstruction. The pole may be representative of western and southern Australia as well if the arguments by Korsch et al. (2011) are correct. The Zimbabwe craton is host to the well dated Mashonaland sil ls (Soderlund et al. 2010) in the northeastern section of the craton. Here we use the recalculated paleopole (Letts et al. 2011) from Evans et al. (2002) that combines dual polarity results from McElhinny and Opdyke (1964) with results from Bates and Jone s (1996). The paleomagnetic pole selected for Laurentia is the 1.87 Ga Molson B dykes pole of the western Superior craton (Halls and Heaman 2000). The Superior, Rae, and Wyoming cratons may have been in close proximity at ~1.9 Ga, so we show the Superior craton with an outline of greater Laurentia in our reconstruction ( Fig ure 4 3a ). Paleomagnetic poles from the Kaapvaal craton come from the 1.87 1.88 Ga Black Hills and post Waterberg dykes in northern South Africa (Hanson et al. 2004; de Kock 2007; Lubi na et al. 2010). The Kaapvaal and Zimbabwe cratons collided during the interval from 1.90 to 2.06 Ga (Lubina et al 2010), and our reconstruction places the two in close proximity at 1.9 Ga. Similarities between our paleomagnetically based reconstruction a nd that of Zhao et al. (2004) include the relationship between Baltica and Laurentia (Figures 4 3 a and b). Our reconstruction places Baltica at equatorial to mid latitudes and Laurentia at mid high latitudes. The position of India at 1.9 Ga is near the e quator, in contrast to the archetypal Columbia model that places India at higher latitudes adjacent to the North China craton along with the Australian and South African nuclei. Here the Australian and South African blocks occupy mid latitudinal positions however; the proposed relationship between the blocks is consistent with the geologic model (Figures 4 3a and
60 b; Zhao et al. 2004). In the archetypal Columbia fit, Siberia is located just north of the Laurentian margin at high latitudes. Paleomagnetic data from Didenko et al. (2009) used in our reconstruction places Siberia at more equatorial latitudes, and is in sharp contrast to the continental relationships proposed by Zhao et al. (2004). Hoffman (1988; 1989ab; 1997) proposed a close relationship be tween Laurentia, Baltica, and Siberia within the Columbia (Nuna) supercontinent based on the similarities between the Archean Nain and Karelia cratons, the Ketilidian and Svecofennian orogens, the Labrador and Gothian Orogens, and extensions of the Slave C hurchill collision zone (Thelon Orogen) across the Arctic. Our reconstruction shows a 70 latitudinal spread of the three continents, and does not support a close relationship at ~1.9 Ga. Hou et al. (2008) proposed a configuration for the supercontinent at 1.85 Ga based on the alignment of orogenic zones and patterns of radiating dyke swarms ( Figure 4 3c). Key differences between our paleomagnetically based reconst ruction and the former include the relative positions of India and Siberia within the super continent. Hou et al. (2008) place Siberia at intermediate latitudes 20 north of Baltica, while our reconstruction positions both Siberia and Baltica near the equator. The Indian subcontinent is positioned at mid latitudes and linked with the Canadian S hield in the 1.85 Ga reconstruction; however, our paleomagnetic pole places India at the equator and gives at least 15 of separation between Indi a and Laurentia (Figures 4 3a and c). The addition of our well constrained paleomagnetic poles from 2.37 1.8 8 Ga allows us to construct an APWP for the Dharwar craton during this interval ( Figure 4 4). Paleolatitudes were calculated from each direction using central site locations in the Dharwar craton and using only north poles. At 2.37 Ga, a steep inclinatio n corresponds
61 to a paleolatitude of 70N (site 596), at 2.21 and 2.18 Ga intermediate inclinations correspond to paleolatitudes of 43N and 31N (sites 35 and 64), and at 1.88 Ga a shallow inclination corresponds to a paleolatitude of 4N (site 67). True plate velocity is calculated from the combination of latitude, longitude, and rotation; however, longitude is unconstrained here so we calculate the minimum rates for latitude and rotation along one line of longitude. An average latitudinal rate is about 2.3 cm/yr and average rotational rate is about 5.1 cm/yr during the Paleoproterozoic.
62 Table 4 1. Ca. 2.4 Ga paleomagnetic studies Pole name Continent Plat Plong Age Reference Karelian dykes Baltica 10 256 2.45 Ga Mertanen et al. ( 1999 ) Matachewan dykes Laurentia 42 238 3 2.45 Ga Bates and Halss ( 1990 ) Widgiemooltha Australia 9 157 8 2.42 Ga Evans ( 1968 ) ; Nemchin and Pidgeon ( 1998 ) Dharwar dykes India 1 2 59 5.2 2.37 Ga This study
63 Table 4 2. Ca. 2.2 Ga paleomagnetic studies Pole name Craton Plat Plong Age Reference Malley dykes Slave 51 310 6,8 2.23 Ga Buchan et al. ( 2012 ) Dharwar dykes (2.21) Dharwar 23.5 123.9 17 2.21 Ga T his study Dharwar dykes (2.18) Dharwar 72.9 76.3 18 2.18 Ga This study Tulemalu Rae 1 122 6,10 ~2.19 Ga Fahrig et al. ( 1984 ) Seneterre Superior 15 104 4,7 2.22 Ga? Buchan et al. ( 1993 )
64 Table 4 3. Ca. 1.88 Ga paleomagnetic stu dies Pole name Continent Plat Plong Age Reference Mean Baltica Baltica 41 233 5 1.88 Ga Pesonen et al. (2003) Akitan Group Siberia 31 99 1.87 Ga Didenko et al. (2009) Mashonaland Sills Zimbabwe 7.6 338.2 5.1 1.88 Ga Letts et al. ( 2011 ) Molson dykes B Superior 27 219 4 1.87 Ga Hall s and Heaman (2000) Ghost dykes Slave 0 190 1.88 Ga Buckam (p. comms.) Post Waterberg Kaapvaal 9 15 14 1.87 Ga Hanson et al. (2004); de Kock (2007) Black Hills Kaapvaal 9 352 5 1.88 Ga Lubina et al. ( 2010 ) Dharwar dykes India 31.9 331.5 5.4 1.88 Ga Th is Study Plum Tree volcanics Australia 29 195 14 1.82 Ga Idnurm and Giddings (1988)
65 Figure 4 1. Orthogonal projection showing the paleopositions of Paleoproterozoic cratons. The positions of the Dhawar (blue), Yilgarn (blue), and Superior (red ) cratons as well as the Fennoscandian shield (red) at ~2.4 Ga are based on the paleomagnetic poles given in Table 4 1 Bolded black lines represent the trends of the dykes us ed for paleomagnetic analysis. White lines represent the outline of present day continents.
66 Figure 4 2. Mollweide projection showing the paleopositions at ~2.2 Ga. T he positions of the Slave (yellow), Superior (red), and Dharwar (purple) cratons at ~2.2 Ga are based on the pale omagnetic poles given in Table 4 2 The Dharwar cr aton is plotted at both 2.21 Ga and 2.18 Ga for comparison. Bolded black, red, and pink lines represent the trends of the dykes used for paleomagnetic analysis. Outlines (dotted fill) of present day continents are shown for reference.
67 Figur e 4 3. Paleogeographic reconstructions at ~1.9 Ga. A) Paleogeographic reconstruction at ~1.9 Ga based on the pale omagnetic poles given in Table 4 3 Select orogens are included for comparative purposes to Fig ure 1 1 In=India (purple), Na=Northern Austr alia (pink), Ba=Baltica (dark blue), Si=Siberia (red), La=Laurentia (green), Zm=Zimbabwe (orange), Kp=Kaapvaal (light blue), Ea=East Antarctica (dotted fill). The present day continental outline for Australia is shown for reference. Bolded red lines repr esent the trends of 1.88 Ga Dharwar and Vestfold Hills dykes. East Antarctica is only plotted to show the relationship between dyke trends, and not as an argument for contiguity. B) Columbia reconstruction according to Zhao et al. (2002, 2004). Note: The reconstruction has been rotated 90 in order to compare relative latitudes from the reference point (red star). C) Reconstruction according to Hou et al. (2008).
68 Figure 4 4. APW path for the Dharwar craton utilizing the paleopoles from ~ 2.37 Ga to 1.88 Ga (Tables 3 1, 3 2, and 3 3). The Dharwar craton is shown in purple and the Indian subcontinent is shown in pink. Colored bolded lines within the Dharwar craton represent the trends of the dykes used for paleomagnetic analysis. Blue squares represe nt the poles and red ovals represent the cone of 95% confidence about the mean direction. The red oval represents a likely plume center at 1.88 Ga.
69 CHAPTER 5 CONCLUSIONS Paleomagnetic evidence for multiple episodes of continental assembly and breakup i While there is no current consensus on the exact make up and geometry of the supercontinent Columbia, the addition of new paleomagnetic poles and precise U Pb ages will help clar reconstruction at 1.88 Ga demonstrates that the history of continental assembly and dispersal is complex and that the traditional geologic models need some reevaluation in spite of new robust paleomagnetic data. Below we list the main conclusions of this study. 1. Paleomagnetism of 14 dykes from the present study adds to the combined data set for the Dharwar craton at ~2.37 Ga. The dykes are part of the E W trending Dharwar giant dyke swarm previously identified and confirmed by Halls et al. (2007) and Kumar et al. (2012 a ). While the majority of dykes trend E W, we cannot reject the hypothesis of a radiating swarm from Godavari related tectonics. The combined paleomagnetic pole places India at polar latitudes during the early Paleoproterozoic. 2. We present two paleomagnetic poles for the Dharwar craton at ~2.2 Ga representing the separate magmatic suites identified by French and Heaman at 2.21 and 2.18 Ga. Recent paleomagnetic res ult s from Kumar et al. (2012b ) are most likely from the 2.21 suite of dykes (Srivastava et al. 2011); however, a primary remanence is still unconfirmed. We report paleomagnetic results from the well dated 2.18 Ga Mahbubnagar swarm (French et al. 2004; Ernst and Srivastava 2008) and provide a positive baked contact test for the dykes. Using existing well dated paleomagnetic
70 poles from the Slave and Superior cratons we provide a reconstruction at ~2.2 Ga, and show a 30 latitudinal separation between the three blocks. 3. We confirm the Southern Bastar Cuddapah LIP event (French et al. 2008; Ernst and Srivastava 2008) by the presence of a large (~85,000 km 2 ) radiating dyke swarm within the Dharwar craton at 1.88 Ga. The swarm has a fanning angle of 65, with NN W SSE trending dykes in the Bastar craton and north of the Cuddapah basin, the NW SE (290) trending Pullivendla mafic sill, and E W trending dykes west of the basin. The dykes converge at a focal point east of the Cuddapah basin, marking the position of an ancient plume. Extension within the Papaghni sub basin of the Cuddapah most likely initiated as a result of this plume related magmatism. Further evidence comes from a gravity imaged mafic lensoid body beneath the southwestern Cuddapah basin (Bhattach arji and Singh 1984) and the associated intrusive Cuddapah volcanics. 4. The paleomagnetic dataset reported here yields a new, precise paleomagnetic pole for the Dharwar craton (and possibly greater India) at ~1.9 Ga. The well constrained ages from the Pu llivendla mafic sill, Bastar dykes, and a Kunigal dyke (this study) provide a robust geochronologic age for the pole. Using other well dated and robust paleomagnetic poles from other continents at 1.88 Ga, we tested a possible configuration for the Columb ia supercontinent. Well accepted models for the supercontinent propose continental breakup at 2.2 2.0 and assembly at 1.9 1.7 Ga. The paleomagnetic based reconstruction at 1.88 Ga indicates that if the Columbia supercontinent was assembled at this time, the proposed models need modification (Zhao et al. 2002, 2004; Hou et al. 2008; Rogers and Santosh 2002; Hoffman 1988,
71 1989ab, 1997), and that many of the linked geologic similarities are inconsistent with the most reliable poles.
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81 BIOGRAPHICAL SKETCH Mercedes Elise Belica was born in Chicago, IL, and spent the majority of her life in the Pacific Northwest. She received her Bachelor of Scien ce in Geology from Western Washington University in June 2010, and completed her m at the University of Florida in December 2012 Her future goals are to pursue a PHD in geology and geophysics, and to continue her study of paleomagnetic mode ls for supercontinent reconstructions.