Using Paleomagnetism, Geochronology and Numerical Methods to Create and Assess Spatiotemporal Geological Relationships Through Earth History

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SPACE AND TIME: USING PALEOMAGNETISM, GEOCHRONOLOGY AND NUMERICAL METHODS TO CREATE AND ASSESS SPATIOTEMPORAL GEOLOGICAL RELATIONSHIPS THROUGH EARTH HISTORY

ANTHONY FRANCIS PIVARUNAS

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2019

To my family, for supporting my unique co-obsession with magnets and rocks, Scott, for opening the door, and Joe, for guiding me through it

ACKNOWLEDGMENTS

I thank my parents, for supporting me the entire time. Your encouragement, advice, and love are amazing. My siblings, for talking, listening, and loving. My extended family, for asking me “What is the actual use of what you’re doing, Anthony?” enough times that I finally could answer it. I need to thank the SUNY Geneseo Geology department, in particular all the amazing teachers. Scott, Dori, Nick, Amy, Jeff, Ben, and Nancy, you helped guide me into geology. Particular thanks and blame for turning me onto paleomagnetism goes to Scott. It’s all your fault. To the Geowizards, there is no one I’d rather confound the Geneseo Physics department with. The long jaunt south was all started by Rob Van der Voo, my academic grandfather, who took the time to reply to my fumbling email and get me in touch with Florida.

Thank you, Rob. I want to thank the entire University of Florida Geological Sciences department. Special thanks to the unsung heroes in the office and workshops: Pam, Carrie,

Kristin, Diana and Dow and Ray. Thanks to all my committee members: Dave (for teaching me how to be a careful scientist), Alessandro (for teaching me how to be a creative scientist), Ray

(for teaching me how to be a useful scientist), and Jim (for teaching me to be a communicative scientist). They all did all those things, but those are the particular parts. To all the graduate and undergraduate students at UF during my time here, thanks for being great friends and colleagues.

All my paleomagical lab mates: Kara, Scott, Austin, Erin, Aubrey, Kelli, Claudia, Landon, and

Rachel.

Now I need to try to put into words my deep thanks to my advisor, Joe Meert. You have supported me, advised me, pushed me, and guided me unfailingly over the past years. You even saved me from being shot during field work! I would not be where I am without your constant example and teaching. Thanks for everything, Joe.

TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...... 4

LIST OF TABLES ...... 8

LIST OF FIGURES ...... 9

LIST OF ABBREVIATIONS ...... 12

ABSTRACT ...... 14

CHAPTER

1 INTRODUCTORY REMARKS ...... 16

1.1 Paleogeography ...... 16 1.1.1 Paleomagnetism ...... 17 1.1.2 Geochronology ...... 18 1.2 Research Objectives ...... 18 1.3 Overview ...... 20

2 PALEOMAGNETISM AND GEOCHRONOLOGY OF MAFIC DYKES FROM THE SOUTHERN GRANULITE TERRANE, INDIA: EXPANDING THE DHARWAR CRATON SOUTHWARD ...... 22

2.1 Introduction ...... 22 2.1.1 Rationale ...... 22 2.1.2 Regional Geologic Setting and Area of Study ...... 23 2.1.3 Previous Paleomagnetic Work ...... 25 2.2 Methods ...... 27 2.2.1 Paleomagnetic Analysis ...... 27 2.2.2 U–Pb Geochronology ...... 28 2.3 Results...... 29 2.3.1 Geochronology ...... 29 2.3.1.1 Site 10 geochronology ...... 29 2.3.1.2 Site 35 geochronology ...... 30 2.3.2 Paleomagnetism ...... 30 2.3.2.1 Low to moderate temperature/coercivity components ...... 30 2.3.2.2 Steeply inclined directions ...... 31 2.3.2.3 Shallowly inclined directions ...... 33 2.3.2.4 Dual polarity intermediate directions ...... 34 2.3.2.5 Other directions ...... 35 2.4 Discussion ...... 37 2.4.1 Paleomagnetism at Scale ...... 37 2.4.2 The South Indian “bar code” ...... 38

2.4.3 The Northern Block of the SGT and the Dharwar Craton: A Metamorphosed Chip off the Old Block ...... 41 2.5 Conclusions...... 44

3 PALEOMAGNETIC RESULTS FROM SINGHBHUM CRATON: REMAGNETIZATION, DEMAGNETIZATION, AND COMPLICATION ...... 68

3.1. Introduction ...... 68 3.1.1 India: A Rich and Complex Precambrian Record ...... 68 3.1.2 Geologic Setting ...... 70 3.1.3 Prior Paleomagnetic Work ...... 72 3.2. Methods ...... 74 3.3. Results...... 75 3.3.1 Magnetic Overprints ...... 75 3.3.2 NW-Shallow Reverse Magnetization ...... 75 3.3.3 Steeply-Inclined Dual Polarity Magnetic Data ...... 78 3.3.3.1 NNE-trending dykes with intermediate-steep dual polarity magnetizations ...... 79 3.3.3.2 WNW-trending dykes with intermediate-steep dual polarity magnetizations ...... 80 3.3.3.3 Steep magnetizations in host rocks and dyke margins ...... 81 3.2.3.4 A unified model for Singhbhum intermediate-steep paleomagnetism ...... 82 3.3.4 Dual Polarity NE-SW Shallow Magnetization ...... 82 3.3.5 Easterly, Intermediate Reverse Direction ...... 84 3.3.6 Northerly, Intermediate Normal Direction ...... 86 3.4. Discussion ...... 87 3.4.1 Magnetic Relationships within Singhbhum Craton ...... 87 3.4.2 Comparison with Other South Indian Block Cratons ...... 91 3.5. Conclusions...... 93

4 ASSESSING THE INTERSECTION/REMAGNETIZATION PUZZLE WITH SYNTHETIC APPARENT POLAR WANDER PATHS ...... 115

4.1 Introduction ...... 115 4.2 Random Walk Construction ...... 117 4.3 Results...... 119 4.4 Discussion ...... 121 4.5 Conclusions...... 124

5 PROTRACTED MAGMATISM AND MAGNETIZATION AROUND THE MCCLURE MOUNTAIN ALKALINE IGNEOUS COMPLEX ...... 135

5.1 Introduction ...... 135 5.2 Setting ...... 136 5.2.1 Regional Geology ...... 136 5.2.2 Previous Paleomagnetic Studies ...... 137 5.3 Methods ...... 139

5.3.1 Geochronological Methods ...... 139 5.3.2 Paleomagnetic Methods ...... 140 5.4 Results...... 140 5.4.1 Geochronologic Results ...... 140 5.4.1.1 Ordovician trachyte ...... 140 5.4.1.2 Cambrian lamprophyre ...... 141 5.4.2 Paleomagnetic Results ...... 142 5.4.2.1 Southeasterly paleomagnetic data ...... 142 5.4.2.2 Steep paleomagnetic data: recent, but how recent? ...... 145 5.5 Discussion ...... 146 5.5.1 Deformation and Magnetization ...... 146 5.5.2 Far-field Tectonic Implications ...... 148 5.6 Conclusions...... 150

6 CLOSING REMARKS ...... 158

LIST OF REFERENCES ...... 159

BIOGRAPHICAL SKETCH ...... 178

LIST OF TABLES

Table page

2-1 Geochronological results for site 10 ...... 49

2-2 Geochronological results for site 35 ...... 51

2-3 Paleomagnetic results from 2.37 Ga dykes in the Southern Granulite Terrane ...... 54

2-4 Paleomagnetic results from Southern Granulite Terrane dykes overprinted in the Ediacaran...... 57

2-5 Grouped paleomagnetic results of uncertain age from the Southern Granulite Terrane (likely 2.21 Ga to 1.88 Ga) ...... 59

2-6 Directional paleomagnetic results from a baked contact test at sites 30a,b and 31 ...... 62

2-7 Comparative Gondwanan poles from the Ediacaran-Cambrian ...... 66

3-1 Overprint directions from Singhbhum craton, most pronounced in north near Jamshedpur ...... 97

3-2 NW-shallow magnetic component in Singhbhum craton ...... 99

3-4 NE-SW shallow inclination magnetic components in Singhbhum craton ...... 107

3-5 Northerly and easterly intermediate single polarity magnetic components from Singhbhum craton ...... 113

4-1 The Van der Voo (1990) quality criteria as outlined in the original paper. Our work focuses on criterion VQ7...... 126

4-2 Numerical model results for both approaches, Euler pole approach results boxed. Model parameters as discussed in the section on construction...... 130

5-1 Combined paleomagnetic data from this study and French et al. (1977) ...... 153

5-2 Combined paleomagnetic data from this study and French et al. (1977) ...... 154

LIST OF FIGURES

Figure page

2-1 Map of India showing major cratonic nuclei and Proterozoic-aged sedimentary basins features (adapted from Meert and Pandit, 2010). Box indicates Figure 2 location...... 46

2-2 Regional map of dykes sampled in this study. Groups of directions are represented by different colored line segments...... 47

2-3 Electron backscatter imaging and concordia plot of analyzed zircons from site 10...... 48

2-4 Concordia plot from zircons recording the crystallization age of the dyke at site 35, with an age of 2363 ± 6.6 Ma...... 50

2-5 Susceptibility vs. temperature curves for selected sites from the recovered paleomagnetic directional groupings ...... 52

2-6 Day Plot (Day et al., 1977) showing magnetic domain characteristics from a representative suite of dyke samples throughout the Southern Granulite Terrane...... 53

2-7 Demagnetization data for reverse (a,b) and normal polarity (c) sites from 2370 Ma dykes in the Southern Dharwar craton and northern Southern Granulite Terrane...... 55

2-8 Map view schematic of baked contact test at site 30 (a), along with demagnetization data...... 56

2-9 Demagnetization data for “B-component” dykes with minor overprints ( ...... 57

2-10 Map view schematic of baked contact test at site 18 (a) along with demagnetization data from the dyke interior (b), transitional zone (c), and distant host rock (d)...... 58

2-11 Demagnetization data from bivectorial normal data (a) and near univectorial normal data (b) from a NW-SE intermediate paleomagnetic direction of unknown age...... 59

2-12 Demagnetization data for dykes provisionally associated with 2210 Ma emplacement ...61

2-13 Two examples of contrasting paleomagnetic data in close proximity in the Southern Dharwar craton and northern Southern Granulite Terrane...... 62

2-14 “Barcode” of igneous activity for primarily the South Indian Block. Green lines indicate both geochronological and paleomagnetic control on the igneous event...... 63

2-15 Comparison of the paleomagnetic pole calculated from our primary 2363 ± 6.6 Ma paleomagnetic direction (green) with other studies from the Southern Dharwar craton ...64

2-16 Poles calculated from the shallow paleomagnetic component seen in 12 dykes of our study. Mean pole is compared to the “B” component of Halls et al. (2007)...... 65

2-17 Ediacaran – Early Cambrian poles and VGPs from Southern India compared to paleomagnetic data of the same time interval from other Gondwana blocks (see Table 7 for detailed information and rotations parameters)...... 67

3-1 Tectonic sketch map of India, modified from Meert et al. (2010), showing major features from the Precambrian geologic history of India...... 94

3-2 Simplified geological map of Singhbhum craton and Google Earth traced dykes (modified after Kumar et al. (2017)...... 95

3-3 Paleomagnetically sampled dykes in the (a) northern Singhbhum craton and (b) southern Singhbhum craton, trends as determined via field observations and satellite imagery...... 96

3-4 General overview of overprint data from Singhbhum craton...... 98

3-5 Summary of the NW shallow inclination paleomagnetic direction recovered from the Singhbhum craton ...... 100

3-6 Baked contact test on a northerly-trending dyke cut by a 1765 Ma dyke with conflicting paleomagnetic data...... 100

3-7 Steep paleomagnetic data from Singhbhum craton...... 103

3-8 Paleomagnetic stability tests on NNE-trending dykes in Singhbhum craton...... 104

3-9 Paleomagnetic stability tests for WNW-trending dykes in Singhbhum craton...... 105

3-10 Two apparent groupings in Neoarchean paleomagnetic data for Singhbhum craton...... 106

3-11 Summary of the NE-SW, shallow inclination paleomagnetic direction recovered from the Singhbhum craton...... 108

3-12 Baked contact test at site I1637, showing respresentative Zijderveld diagrams and stereoplots of vectors ...... 109

3-13 Mean directional data for both intermediate, single polarity groupings, both northerly and easterly ...... 110

3-14 Baked contact tests on the easterly-intermediate direction, particularly at Bhima Kunda...... 111

3-15 Dense dyke emplacement at Kanjhari Reservoir, along with stereoplots of magnetic directions from the dykes here...... 114

4-1 An illustration of typical results from the segmented model...... 127

4-2 A schematic view of the flow of our synthetic APWP model...... 129

4-3 Illustrations of the random Euler pole model ...... 132

4-4 Large Phanerozoic paleomagnetic datasets provide useful real-world examples of apparent polar wander path architecture...... 133

4-5 Case studies of intersections in a real paleomagnetic data set: Laurentia...... 134

5-1 Map of sampling areas in the Wet Mountains. Simplified geologic map based on mapping of Taylor et al., (1975a,b) and Wobus et al., (1979)...... 151

5-2 Plot of 206Pb/238U zircon ages for trachyte (blue) and lamprophyre (orange). Vertical bars show 2σ error, horizontal line is at the weighted mean age calculated...... 151

5-3 Equal area stereonet of our data as compared with other studies (French et al., 1977; Lynnes and Van der Voo, 1984)...... 152

5-4 Equal area stereonet of notable Paleozoic magnetic directions from around the McClure Mountain igneous complex. Note the declination break between the two groupings...... 155

5-5 A summary of baked contact test results from around the igneous complex and environs. The geometries are discussed in more detail in the corresponding sections. ...156

5-6 Comparison of Paleozoic data from the McClure Mountain igneous complex with the reference North American apparent polar wander path, along with a schematic summary of age constraints on intrusions comprising the MMIC. L ...... 157

LIST OF ABBREVIATIONS

AF Alternating Field

APWP Apparent Polar Wander Path

CITZ Central Indian Tectonic Zone

ECD East Coast Dykes

EDD Eastern Dharwar Domain

EGMB Eastern Ghats Mobile Belt

Ga 1 billion years

GAD Geocentric Axial Dipole

IOG Iron Ore Group

LA-ICP-MS Laser Ablation Inductively Coupled Plasma Mass Spectrometer

MMIC McClure Mountain Igneous Complex

Myr 1 million years

NAM North America

NCC North China Craton

NIB North India Block

NRM Natural Remanent Magnetization

OMG Older Metamorphic Group

OMTG Older Metamorphic Tonalite Gneiss

PCSZ Palghat-Cauvery Shear Zone

PEF Present Earth’s Field

REM Random Euler Pole Method

SAMBA South America Baltica

SGT Southern Granulite Terrane

SIB South India Block

SM Segmented Model

SOA Southern Oklahoma Aulacogen

TNCO Trans-North China Orogen

TPW True Polar Wander

TRM Thermal Remanent Magnetization

VGP Virtual Geomagnetic Pole

VQ7 Van der Voo Quality Criteria 7

WDD Western Dharwar Domain

Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy

SPACE AND TIME: USING PALEOMAGNETISM, GEOCHRONOLOGY AND NUMERICAL METHODS TO CREATE AND ASSESS SPATIOTEMPORAL GEOLOGICAL RELATIONSHIPS THROUGH EARTH HISTORY

Anthony Francis Pivarunas

May 2019

Chair: Joseph Meert Major: Geology

Tracking the arrangement of continents on the Earth back through time is one of the frontiers of geologic research. However, as we extend our studies back into deep time, the rocks we have to work with are often significantly altered. Two main avenues of research are available to create and assess spatiotemporal frameworks in the past: paleomagnetism and geochronology.

The former tracks the position of rocks by measuring their magnetic memory, while the latter can provide the ages of the rocks in question. This dissertation describes coupled paleomagnetic and geochronologic data from separate Proterozoic cratons of India, numerical answers to long- standing paleomagnetic data evaluation controversy, and the tectonic setting of Cambrian rocks from Colorado.

My research in southern India provided better constraints on the early Paleoproterozoic connection between Dharwar craton and its metamorphosed equivalent to the south. To the northeast, I evaluated the magnetic history of Singhbhum craton, and evaluated its connection with the other South Indian Block cratons. Numerical simulations of apparent polar wander allowed me to resolve a long-standing debate among paleomagnetists about the significance of

data resemblance in deep time. In Colorado, a longer magmatic history for an intrusive complex allowed me to critique a regional-scale connection with rifting activity far to the east.

CHAPTER 1 INTRODUCTORY REMARKS

1.1 Paleogeography

Paleogeography is concerned with assessing the positioning of continental landmasses throughout the vast expanse of geologic time. The well-developed theory of plate tectonics holds that the outer shell of the Earth is fragmented into moving tectonics plates (Le Pichon, 1967).

Therefore, throughout time, continents have shifted vast distances, shuffling and reshuffling themselves, coalescing and cleaving (Nance et al., 2014). But why do we care? What relevance does the ancient orientation of continents on the Earth surface hold?

There are many important questions that paleogeographical reconstructions can answer.

When did plate tectonics begin, and how far back did it operate? Maybe it has changed in its style through time – perhaps plates moved faster, or stayed still longer. Rewinding Earth’s bumper car continents will tell us the answer. Since life on Earth is profoundly influenced by the configuration of landmasses, a concept known as biogeography, how exactly do these configurations affect life? How did continental configurations affect climate? And conversely, can we track back past latitudinally-dependent climatic zones? Resources, such as precious metals and oils, develop in specific circumstances that are heavily controlled by the style of continental interactions. All these, and many more questions can be evaluated with paleogeographical methodologies. So what can we do to assess paleogeography in the past?

There are many tools that can and have been used to track the movement of continents; this dissertation focuses on the application of several well-developed methodologies: paleomagnetism and geochronology. Paleomagnetism provides the only truly quantitative constraint on position of continents. However, it is incomplete without a framework of time, which is precisely what geochronology can provide. The combination of these methods, along

with a strong emphasis on field and numerical techniques, is the most robust way to provide data concerning the ancient movement and interaction of continents.

1.1.1 Paleomagnetism

The magnetic field of the earth is a vector field that can be described in terms of three quantities: declination (angle of bearing with respect to a north-south axis), inclination (angle with respect to a horizontal plane), and intensity (magnitude of the vector). The magnetic field is recorded by rocks as they form, resulting in a measurable remanent magnetization. The principal form of remanent magnetization studied in this dissertation is thermal remanent magnetization, or TRM. TRM forms as rocks cool from a higher temperature to a lower temperature. During this process, magnetic minerals in the rock acquire a remanent magnetization that reflects the ambient magnetic field at the time.

In practice, there are multiple complications involved in paleomagnetism, specifically concerning how rocks get and stay magnetized. The magnetic field of the Earth is a matter of concern. We assume it approximates a geocentric axial dipole (GAD), but at any given time this assumption is patently untrue. Over time periods of ~10 – 50 kyr, the average magnetic field does indeed conform to GAD expectations (McElhinney et al., 1996). Thus, we need to sample rocks from a sufficient time interval to average out irregular geomagnetic field behavior.

Various secondary effects can change the magnetic signature of a rock post-acquisition: heat, fluid flow, lightning, deformation, magnetic effects. To assess these effects, we need to carefully demagnetize samples and ascertain their magnetic carriers. Aditionally, field tests of magnetic stability such as baked contact tests are critically important to ensure that the magnetization is primary; that it represents the first-acquired magnetization of the rock in question. Finally, the magnetic vector information recovered from the rock needs to be numerically converted into a site-independent paleolatitude or a site-dependent virtual geomagnetic pole (VGP) in order to

facilitate comparison with paleomagnetic results from other locations. These comparisons form the quantitative grounds for assessing the likelihood of continental locations in paleogeographic reconstructions.

1.1.2 Geochronology

The radioactive decay of isotopes provides an important constraint on the age of rocks.

The U-Pb system – the well-characterized decay of different isotopes of uranium and thorium into lead over geologic timescales – is the most utilized dating method in the Precambrian.

Zircon and baddeleyite in rocks provide a crystallization age for the unit in question. A pitfall often noted in mafic dykes is the inheritance of zircons from the surrounding rocks (Black et al.,

1991). These xenocrystic zircons do not reflect the true age of the unit they are found in, but can provide information about the host rocks. Particularly in intrusive rocks (such as mafic dykes), a framework of absolute and relative ages can be ascertained by taking into account both isotopic ages and cross-cutting relationships.

1.2 Research Objectives

As we track the position of continents back through time, we run into a data problem.

The number of suitably unaltered rocks begins to shrink the farther back we examine. ‘Full- plate’ models are growing popular for Phanerozoic time (Domeier and Torsvik, 2014) and even

Neoproterozoic time (Merdith et al., 2017). These continuous kinematic models have an advantage in that they present a unified, evolving geologic framework rather than discrete time slices (Li et al., 2008). However, extending these models deeper into the Precambrian (and even into the Precambrian at all) it becomes necessary to interpolate through a fundamentally underdetermined system. Robust determinations of space and time are needed to construct truly feasible models. For example, for the southern cratonic nuclei of India (Dharwar, Bastar, and

Singhbhum) throughout the Proterozoic, we have roughly a dozen key paleomagnetic poles.

These poles are not even all on the same cratonic nuclei – the majority are on the Dharwar!

Simple math tells us that each pole is shouldering the burden of well over 100 Myr of space and time. This example shows the essential issue to be solved before we incorporate many of the

Archean cratonic puzzle pieces (Bleeker, 2003) into kinematic reconstructions. Some of the issues are mitigated when cratonic nuclei come together such as the ‘United Plates of America’

(Hoffman et al., 1988), since this allows paleomagnetic results post-dating the adjoinment from all members of the union to apply to the whole. Again, we can take India as an example. It contains five large cratons, three in the south (Dharwar, Bastar, and Singhbhum) and two in the north (Aravalli-BGC and Bundelkhand). Dharwar craton has the most robust paleomagnetic record in the Paleoproterozoic, yet has less for the Mesoproterozoic. Because of coeval paleomagnetic data (Meert et al., 2011; Belica et al., 2014), we know that Dharwar and Bastar cratons can be treated together post-1885 Ma. This allows results from 1465 Ma dykes in Bastar craton to at least provisionally locate both Darwar and Bastar cratons, at at time when we lack paleomagnetic data for Dharwar craton. We now can see the importance of constraining the positions of cratons in both a regional and global sense. Furthermore, this gives a flavor of the vast stretches of time involved, and the accompanying complications. Precise age data is a critical component of this work.

The main goals of the research comprising this dissertation all focused on various aspects of paleogeography and were as follows:

1. To investigate the paleomagnetic and geochronologic record of the northern Southern Granulite Terrane (SGT) and determine its relationship with Dharwar Craton to the north (Chapter 2)

2. To unravel the complex magnetic history of Singhbhum Craton in northeast India, test the primary nature of paleomagnetic data therein, and establish its relationship with the other South India Block (SIB) cratons (Chapter 3)

3. To assess the usefulness of apparent polar wander path (APWP) coincidence as a ‘quality’ assessment in the Precambrian as proposed by Van der Voo (1990) (Chapter 4)

4. To revisit the McClure Mountain igneous complex (MMIC), confirm previous paleomagnetic studies, and provide new geochronological data for intrusive units (Chapter 5)

1.3 Overview

This dissertation covers four studies and focuses on acquiring spatiotemporal data (i.e. paleomagnetic and geochronologic) and testing ideological and tectonic models. Chapters 2 and

3 focus on the spatial relationships among the Archean southern Indian cratons. Chapter 4 quantitavely examines an ideological sticking point of Precambrian paleomagnetism. Chapter 5 moves upward in time to provide a fresh interpretation of Paleozoic paleomagnetism in

Colorado.

Chapter 2 is dedicated to Southern India, in particular Dharwar craton and its southern extension into more metamorphosed terranes. This research showed that the Southern Granulite

Terrane (just below Dharwar craton) contains a full paleomagnetic record from India, despite being variably altered in the late Neoproterozoic. Key results were the confirmation of ca. 2370

Ma connection between the Dharwar and northern SGT, as well as geochronological and paleomagnetic evidence for Cambrian magmatism in southern India. This research has been published in Tectonophysics.

Chapter 3 also examines the tectonic history of India, but in the northeasterly Singhbhum craton. Recent paleomagnetic work posited primary paleomagnetic data from both the

Neoarchean (~2762 Ma; Kumar et al., 2017) and Paleoproterozoic (1765 Ma; Shankar et al.,

2017). Our research showed that the Neoarchean dykes may not preserve a primary magnetization. Paleoproterozoic magnetizations, however, are preserved and constrain

Singhbhum and Dharwar consolidation. This research is in prep for submission to Precambrian

Research.

Chapter 4 quantitatively examined one of the foundational quality criteria for paleomagnetic data. By constructing numerical simulations of apparent polar wander paths, we showed that over long stretches of geologic time, it is inevitable for APWPs to intersect each other. Therefore, viewing paleomagnetic pole intersections as problematic (i.e. suggestive of remagnetization) is not feasible in the absence of other evidence. This research has been published in Geophysical Journal International.

Chapter 5 revisited legacy paleomagnetic investigations of an intrusive complex in southern Colorado. We found that structural and hydrothermal disturbances render the paleomagnetic data unsuitable for tectonic inferences. A key date was recovered from a late dyke near the complex, however. This age pushes the record of magmatism in the area forward in time, which enables re-interpretation of the MMIC’s relationship with far-field tectonic structures such as the Southern Oklahoma Aulacogen. This work is currently in review at

Lithosphere.

Chapter 6 summarizes key general findings from the other chapters.

CHAPTER 2 PALEOMAGNETISM AND GEOCHRONOLOGY OF MAFIC DYKES FROM THE SOUTHERN GRANULITE TERRANE, INDIA: EXPANDING THE DHARWAR CRATON SOUTHWARD*

2.1 Introduction

2.1.1 Rationale

Crustal elements of the Indian subcontinent have a place in every supercontinent reconstruction, from Ur to Pangea (Rogers and Santosh, 2003; Meert, 2012; Meert, 2014).

Suitable target rocks for paleomagnetic study throughout peninsular India are available to aid in the reconstruction of India within the Proterozoic supercontinents of Kenorland, Columbia

(Nuna) and Rodinia. The relationship of major tectonic elements of the Indian shield has been a matter of debate vis-a-vis the timing of final assembly of the five major Indian cratons (Aravalli,

Bundelkhand, Singhbhum, Bastar, and Dharwar). The ENE-WSW trending Central Indian

Tectonic Zone (CITZ) separates the North Indian Block (NIB comprised of Aravalli and

Bundelkhand cratons) and South Indian Block (SIB - Singhbhum, Bastar, and Dharwar cratons).

Proterozoic tectonothermal events in the CITZ can be resolved into three pulses of tectonothermal activity at approximately 2.5 Ga (Stein et al., 2004, 2014), between 1.8-1.5 Ga and again from 1.0-0.9 Ga (Bhandari et al., 2011; Bhowmik et al., 2012a; Bhowmik et al.,

2012b; Meert et al., 2010; Meert and Pandit, 2015). As even a brief review of the conflicting arguments on the CITZ is beyond the scope of this paper, we merely note that deep-seated disagreement exists regarding the timing of final assembly of the NIB and the SIB. This is precisely where paleomagnetism is useful as a test of the null-hypothesis. While the lack of concordant, contemporaneous paleomagnetic data linking the various pieces of the puzzle do not

* This chapter is reprinted from an article published in Tectonophysics with permission from A.F.P., and can be cited as: Pivarunas, A.F., Meert, J.G., Pandit, M.K., Sinha, A., 2018. Paleomagnetism and geochronology of mafic dykes from the Southern Granulite Terrain, India: Expanding the Dharwar craton southward, Tectonophysics.

intrinsically discount their mutual linkages, discordant, contemporaneous paleomagnetic data certainly do. Alternatively, and more positively, coeval and concordant paleomagnetic poles between different blocks provide strong evidence for their juxtaposition. Fortunately, the

Precambrian shield regions of peninsular India are cross-cut by extensive Proterozoic-age mafic dyke swarms suitable for paleomagnetic studies.

The Southern Granulite Terrane (SGT) located to the south of the Dharwar Craton and north of the Palghat-Cauvery Shear Zone (Figure 2-1), hosts numerous phases of mafic dykes; however, paleomagnetic data from those dykes are less well-resolved than elsewhere in India

(Dash et al., 2013; Radhakrishna et al., 2013b). In part, this is due to the fact that some of the dykes show clear evidence of alteration. In keeping with our “India first” approach to

Precambrian paleogeographic reconstructions, we examine connections between the Salem

(Northern) Block of the SGT with the Dharwar Craton to the north. Current consensus, based on isotopic and geochemical studies, is that the Salem Block and other northern granulites are the metamorphosed equivalents of the Dharwar Craton (Collins et al., 2014). Recent paleomagnetic work supports this contention, as a steep paleomagnetic direction from mafic dykes within the

SGT was dated to 2318 ± 60 Ma with a Sm-Nd whole-rock isochron age (Dash et al., 2013) that agrees, within error, with the 2370 ± 1 Ma U-Pb baddeleyite age from the “Bangalore swarm” of dykes in the Dharwar Craton (Halls et al., 2007). We have extensively sampled mafic dykes from the northern SGT to test their correlation with the greater Dharwar craton and report these new paleomagnetic and geochronological data in this paper.

2.1.2 Regional Geologic Setting and Area of Study

The geological history of Peninsular India revolves around five major cratonic nuclei – the Aravalli, Bundelkhand, Singhbhum, Bastar, and Dharwar cratons (Figure 2-1). A collage of crustal blocks of various ages collectively make up the Southern Granulite Terrane (SGT) to the

south of Dharwar Craton (Meert et al., 2010, Santosh et al., 2017). The older northern blocks of the SGT are separated from the southern (Neoproterozoic) blocks by the Palghat-Cauvery Shear

Zone (PCSZ). This shear zone is thought to represent an Ediacaran-Cambrian suture formed during the final assembly of Gondwana (Collins et al., 2007).

Southern India can be divided into two main geologic terranes: the Dharwar Craton (DC) in the north and the Southern Granulite Terrane in the south (see Figure 2-1). The Dharwar

Craton is further subdivided into Eastern Dharwar Domain (EDD) and Western Dharwar Domain

(WDD) separated by the Closepet granite (Ramakrishnan and Vaidyanadhan, 2008; Meert and

Pandit, 2015). Large intracratonic basins are present throughout the Dharwar Craton, typified by the massive, crescent-shaped Cuddapah Basin in the EDD. The metamorphic grade increases southward in the Dharwar Craton and reaches its highest grade in the Southern Granulite Terrane south of the Fermor Line (i.e. the charnockite/non-charnockite boundary; Fermor, 1936).

The SGT consists of three late Archaean to Neoproterozoic age, high-grade metamorphic blocks (Figure 2-1); including the Salem (Northern) Block, the Madurai Block, and the

Trivandrum Block (Drury et al., 1984). The SGT crustal blocks are separated by several large- scale shear zones (Meert et al. 2010; Collins et al., 2014; Pandit et al., 2016). The northernmost unit of the SGT, the Salem Block, is bounded to the south and east by the Moyar–Bhavani and

Attur Shear Zones (MBSZ and ASZ), respectively. The Madurai and Trivandrum blocks occur south of the Palghat-Cauvery Shear Zone. The main lithologies present in the Salem Block are orthogneisses and corresponding charnockites, with subordinate mafic granulites and metasedimentary rocks (Collins et al., 2014). Major metamorphic intervals range in age from the late Archean to the earliest Proterozoic (Peucat et al., 1993, Ghosh et al., 2004; Clark et al.,

2009, Anderson et al., 2012; Collins et al., 2014). Charnockitic rocks in the Salem block have

been dated to 2538 ± 6 Ma and 2529 ± 7 Ma via SHRIMP Pb-Pb dating (Clark et al., 2009).

Distinct, younger zircon rim ages of 2473 ± 8 Ma and 2482 ± 15 Ma were interpreted by Clark et al. (2009) to reflect timing of accretion of the Salem block to the Dharwar Craton.

Neoproterozoic metamorphic U-Pb zircon and monazite ages of ca. 880 Ma and 730 Ma (Ghosh et al., 2004; Bhutani et al., 2007) indicate tectonothermal activity at these times, although causes are “cryptic” (Collins et al., 2014). Later metamorphism occurred during the Cambrian (Ghosh et al., 2004); however, the spatiotemporal distribution of this metamorphism is poorly constrained

(Collins et al., 2014).

The Salem Block, like the adjoining Dharwar Craton to the north, is infested by a number of mafic dyke swarms. The dykes are concentrated in the Krishnagiri-Hosur area, near

Dharmapuri, and around Tiruvannamalai (Figure 2-2). Trends of these dykes vary but the majority trend E-W to NW-SE. The K-Ar ages for the Dharmapuri and Tiruvannamalai

“swarms” range from ~2300 to 1350 Ma and ~2300 to 1650 Ma, respectively (Radhakrishna et al., 1986; 1999). A Sm-Nd whole-rock age from a dyke near Tiruvannamalai yielded an age of

2318 ± 60 Ma (Dash et al., 2013; Figure 2-2). Dykes sampled in this study intrude both granitic gneiss and charnockite hosts. Some dykes in the northern regions of the SGT can be traced over long distances in the field or in satellite images. The dykes range in size from a few meters to tens of meters and often show sharp contacts with the host rocks.

2.1.3 Previous Paleomagnetic Work

Paleomagnetic data from mafic dykes in the SGT are limited (Venkatesh et al., 1987;

Radhakrishna and Joseph, 1996; Dash et al., 2013). The pioneering paleomagnetic study of SGT mafic dykes sampled dykes from Kunnam, Tiruvannamalai and Attur (Venkatesh et al., 1987).

In that study, remanences were subdivided into four groups with the following declination (D) and inclination (I) values: (a) D=86, I=-35 (α95=5°), (b) D=33, I=-48 (α95=18°), (c) D=133,

I=-75 (α95=6°), and (d) D=294, I=-52 (α95=23°). The (c) group results are in general agreement with the well-established steeply inclined remanence of the giant “Bangalore swarm”, dated at 2370 Ma by Halls et al. (2007). This steeply inclined direction is ubiquitous throughout the Dharwar Craton (Radhakrishna et al., 2013; Belica et al., 2014). The (d) group of directions is similar to those found in the 2.21 Ga dykes from the Dharwar Craton (Kumar et al., 2012a).

Radhakrishna and Joseph (1996) argued for multiple dyke swarms in the Dharmapuri and

Tiruvannamalai regions of the SGT. The analytical process followed by Radhakrishna and

Joseph (1996) was to divide the dykes by arbitrary geographical regions without regard to other field information (trends/composition). That approach is questionable given that a dyke from

Dharmapuri (D6) which exhibited a paleomagnetic signature characteristic of most dykes from

Tiruvannamalai (steep negative inclination) was discarded under this scheme. Dyke trends noted in their paper were also relatively uniform for each area, which contrasts with our observations from the same regions. Their restrictive analysis resulted in 4 ‘distinct’ groupings which they named the Agali-Anaikatti dykes (D=13.9°, I=+27°, α95=26°, based on 2/9 sites); the

Dharmapuri dykes (D=360°, I=+9°, α95=11°, based on 5/10 sites); the Tiruvannamalai dykes

(D=107°, I=-78°, α95=9°, based on 7/14 sites); and the North Kerala dykes (D=217°, I=-50°,

α95=25°, based on 2/5 sites).

Dash et al. (2013) focused on two concentrations of mafic dykes in the Southern

Granulite Terrane. These are known as the East Coast Dykes near Chennai and the

Tiruvannamalai “swarm”. The latter area has been sampled in four separate paleomagnetic investigations – including three studies with radiometric ages (Radhakrishna and Joseph, 1996;

Dash et al., 2013; this study). Dash et al. (2013) obtained a direction from NW-SE trending

Tiruvannamalai dykes and combined them with data from group (c) of Venkatesh et al. (1987)

and Radhakrishna and Joseph (1996) to obtain a group mean (15 dykes) of D=125, I=-74

(α95=8°) for the Tiruvannamalai dykes. Dash et al. (2013) report a whole-rock Sm-Nd isochron age of 2318 ± 60 Ma which they correlated with the 2367 Ma “Bangalore” swarm of Halls et al.

(2007) in contrast to the previously reported K-Ar age of 1650 ± 10 Ma (Radhakrishna et al.,

1999). The primary nature of the steep paleomagnetic direction in Tiruvannamalai dykes was confirmed by a positive baked contact test (Dash et al., 2013). The East Coast dykes yielded a group mean direction (10 sites, 5 dykes) of D=89, I=+34 (α95=7°) from both NW-SE and E-W trending dykes combined with group (a) of Venkatesh et al. (1987) and some “anomalous” directions of Radhakrishna and Joseph (1996).

2.2 Methods

2.2.1 Paleomagnetic Analysis

A total of 51 sites were collected from dykes throughout the Southern Dharwar and northern Southern Granulite Terrane, primarily from quarries, roadcuts or areas where there was no ambiguity about the in-situ nature of the outcrop (Figure 2-2). Samples were also collected from baked and unbaked host rocks at sites suitable for baked contact tests. Paleomagnetic samples were collected in the field with a water-cooled, gasoline-powered drill and oriented with magnetic and sun compass with the exception of three sites where drilling was not feasible. All samples were returned to the University of Florida where they were trimmed to a standard size.

Natural remanent magnetization (NRM) directions were measured using either a Molspin spinner magnetometer or 2G-77R cryogenic magnetometer. Pilot samples from all sites were demagnetized by thermal and alternating methods using either an ASC TD-48 thermal demagnetizer or DTech 2000 AF demagnetizer. Subsequent demagnetization treatment was

optimized based on the results from the pilot samples. Paleomagnetic vector directions were determined via principal component analysis (Kirschvink et al., 1980) using IADP software

(Torsvik et al., 2016). Powdered material from several samples at each site was analyzed with a

KLY-3S Kappabridge with a CS-3 furnace attachment and a vibrating sample magnetometer

(VSM) in order to ascertain magnetic carriers and magnetic domain characteristics.

Approximately 2-3 kg of samples were collected from the coarse-grained centers of two dykes for geochronological investigation (sites I1510 and I1535). Standard mineral separation techniques (crushing, sieving, heavy liquids and magnetic methods) were used to isolate zircons from the dyke samples.

2.2.2 U–Pb Geochronology

Zircon grains were hand-picked under a binocular microscope then transferred to double- sided tape in preparation for mounting. Site I1510 (SW Tiruvannamalai) yielded thirteen grains suitable for analysis. Site I1535 (near Krishnagiri) yielded 8 zircons for analysis. The grains were set in epoxy then polished to expose interiors. Zircons were examined on an SEM with backscatter electron (BSE) and cathode luminescence (CL) imaging. The epoxy plug was then washed and sonicated in an HNO3 solution prior to LA-ICP-MS analysis.

The U-Pb isotopic analyses were conducted on the Nu-Plasma multi-collector plasma source mass spectrometer at the Department of Geological Sciences at the University of Florida.

This LA–ICPM–MS is equipped with a custom-designed collector block for simultaneous acquisition of 204Pb (204Hg), 206Pb and207Pb signals on the ion detectors and 235U plus 238U on the

Faraday detectors (Mueller et al., 2008). Mounted zircon grains were laser ablated using a New-

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

204Hg. Each sample was ablated for ∼30 s in an effort to minimize pit depth and fractionation following standard practice for zircon analyses at the University of Florida. Data calibration and drift corrections were conducted using the FC-1 Duluth Gabbro zircon standard. Data reduction and correction were conducted using a combination of in-house software and Isoplot (Ludwig,

1999).

2.3 Results

2.3.1 Geochronology

2.3.1.1 Site 10 geochronology

Table 2-1 presents U-Pb isotopic data on zircons recovered from a mafic dyke sample from Site 10, just southwest of the town of Tiruvannamalai. The sample yielded two concordant zircon ages at 2753 ± 10 Ma (Figure 2-3a), consistent with basement ages in the area (Ghosh et al., 2004; Collins et al., 2014). Eleven zircon grains define an age of 527.5 ± 2.6 Ma while a single discordant grain falls at 584 ± 15 Ma (Figure 3b). Clark et al. (2009) suggest the 525-540

Ma period for collision of the Madurai Block (Azania) with the Salem Block that is consistent with these U-Pb ages (see also Santosh et al., 2017). Site 10 is an abandoned quarry, one of three paleomagnetic sampling locations on a NW-SE trending dyke in the Tiruvannamalai area. The characteristic remanence of this dyke (D=115, I=-70; k=98, α 95=3) was consistently overprinted by a northerly and shallow component (D=16, I=-17; k=10, α 95=16). This dyke is cut by a number of subparallel NE-SW trending dykes, including dyke T9 of Radhakrishna and Joseph (1996). One of these NE-SW cross-cutting dykes was observed in the abandoned quarry at site 10 but was unreachable due to high water levels. Given the discrepancy between the paleomagnetic data at site 10 and the geochronologic data, it is likely that the 527 ± 2.6 Ma age dates the crystallization age of the cross-cutting NE-SW dykes. This age therefore delineates limited Cambrian magmatism in the Tiruvannamalai area (see discussion for details).

2.3.1.2 Site 35 geochronology

Table 2-2 lists U-Pb isotopic data for zircon separates from the Site 35 dyke near

Panchalli. Samples from this site yielded six concordant analyses from 3 inherited zircon grains

(2755 ± 15 Ma) that correspond with basement rocks in the region (Ghosh et al., 2004; Collins et al., 2014). In addition, they yield 3 concordant analyses from 3 zircon grains that define an age of 2363 ± 6.6 Ma that we attribute to the crystallization of the dyke (Figure 2-4). Paleomagnetic data from this NW-SE trending dyke yield a direction of D=61, I=-84; k=169, α95=4.

2.3.2 Paleomagnetism

Paleomagnetic results from the dykes of the Southern Granulite Terrane fall into two main groups (accounting for 50 percent of sites in this study) consistent with previously reported directions (Belica et al., 2014; Dash et al., 2013; Halls et al., 2007; Kumar et al., 2012a,b; Meert et al., 2011; Pisarevsky et al., 2013; Radhakrishna et al., 1996; Venkatesh et al., 1987). In the following discussion, we refer to positive inclinations as ‘normal’ polarity and negative inclinations as ‘reverse’ polarity.

2.3.2.1 Low to moderate temperature/coercivity components

Low/intermediate temperature and coercivity components were removed by 500 °C or

15-60 mT. The majority of the low-intermediate temperature/coercivity directions yielded shallow northerly directions with D=353°, I=-5°; k=8, α95=16° (12 sites; Figure 2-7e). This overprint is similar to the “B” component of Halls et al. (2007). However, some well-grouped secondary components were distinctly different from this more common direction. Sites 22 and

31 (each on a separate dyke) share the same overprint direction of D=102°, I=+1°; α95=50°, k=27. This overprint is close to a high temperature component seen in the basal Marwar

Supergroup (of late Ediacaran age) with D=89°, I=-1.0°; α95=9° (Davis et al., 2014). It is also

comparable to the paleomagnetic direction recovered at site T9 (Radhakrishna and Joseph,

1996).

2.3.2.2 Steeply inclined directions

Following the removal of lower temperature and coercivity components, the most prevalent paleomagnetic direction characteristic of 17 dykes (20 sites) across our study area was a relatively steep dual-polarity remanence isolated at high temperatures or high applied fields

(Table 2-3; Figure 2-7). Some sites showed univectorial decay (Figure 2-7c). The remanence is consistent with prior results on dykes dated to 2370 Ma in the SGT/Dharwar craton west of our sampling area (Halls et al., 2007) and north of our sampling area in the Dharwar craton “proper”

(Belica et al., 2014) and 2318 Ma in the Southern Granulite Terrane (Radhakrishna et al., 2013;

Dash et al., 2013). The 17 reversed sites (14 dykes) had a mean D=28°, I=-84° (k=31; α95=7°) while the 3 normal sites (3 dykes) are less well grouped at D=277°, I=+72° (k=21; α95=28°;

Figure 2-7d). A reversal test (McFadden and McElhinny, 1990) between the two directions (with normal polarity dykes P69, GR, and GT from Belica et al. (2014) added to the normal polarity data) passed the test with a classification of Rc (γc=15.3° and a γobs=4.3°).

A baked contact test was conducted at Sites 30a,b and 31 (Figure 2-2; Table 2-6; Figure

2-8i). These samples were collected about 1 kilometer apart on the same ~E-W trending dyke

(Figure 2-2). The main dyke at site 30a is intruded by a smaller, fine-grained, subparallel mafic dyke that is not present at site 31 (Figure 2-8a). Directional data from site 31 show two components of magnetization. The first, a low-coercivity component (< 10 mT), is somewhat scattered but forms a reasonable grouping to the SE-shallow (D=110°, I=+15° (k=14, α95=11°) and a higher coercivity direction with a mean D=342°, I=-68° (k=146, α95=4°; Figure 2-8b).

The magnetic signature at site 30a,b is more complex due to the presence of the smaller dyke. Samples from the smaller dyke at site 30b show a consistent WNW shallow to

intermediate upwardly directed magnetization with a mean D=284°, I=-25° (k=375, α95=5°;

Figure 2-8d) that is nearly antipodal to the overprint at Site 31. Samples from the interior portion of the larger E-W trending dyke at site 30a reveal a multicomponent magnetization. A shallow to intermediate upwardly directed magnetization is unblocked at lower applied fields/temperatures with a mean at D=333°, I=-33° (k=33, α95=10°; Figure 2-8c). A direction comparable to site 31 is identified at higher temperatures and coercivities with a mean D=360°,

I=-67° (k=94, α95=6°; Figure 2-8c).

A gneissic sample adjacent to the smaller dyke at site 30b shows a direction indistinguishable from the smaller dyke (Figure 2-8e). Outside of the baked zone from the smaller dyke (>>0.5 dyke width, 30b), but still within the baked zone of the main dyke at site

30a (<<0.5 dyke width); samples show steeply inclined upwardly directed magnetizations identical to those at site 30a and 31 (Figure 2-8f,g). Gneissic samples distant from the contacts of both dykes show intermediate NE-downwardly directed magnetizations (Figure 2-8h). These directions are identical to the stable mean direction obtained at basement reference site 46 (Table

2-6).

These data suggest that dyke 30b (the smaller dyke) emplaced along the margin of dyke

30a and baked the gneissic samples along the margin of dyke 3b and overprinted samples at site

30a. Furthermore, the larger dyke at site 30a baked gneissic samples located >>0.5 dyke width from dyke 30b. Gneissic samples outside the expected baked zone of both dykes show a unique and stable direction. We consider this as a positive baked contact test for dykes 30a and 31. This unique geometry also provides an inverse baked contact test for site 30b on dyke 30a. Finally, a positive baked contact test for the smaller dyke 30b is evident by the complete overprinting of the adjacent gneissic samples and the partial overprinting of the larger dyke.

Data from Curie temperature experiments show reversible heating/cooling curves indicating relatively unaltered magnetite as the primary magnetic carrier in all dykes of this group (Figure 2-5a). All dykes showed either PSD or a SD/PSD mixed behavior on a Day Plot

(Figure 2-6).

The mean paleomagnetic pole for the steep directions across the Southern Granulite

Terrane (17 dykes) falls at 2 °N, 73 °E (A95=14°). Our results in the Southern Granulite Terrane come from more geographically dispersed dykes than those reported in Dash et al. (2013). That would indicate that dykes of the 2.37 Ga swarm cover a wide area within the SGT. We note that our findings are consistent with the directional bias towards reverse polarity seen in previous studies wherein 83% of our results show ‘reverse’ polarity (Halls et al., 2007; Dash et al., 2013;

Belica et al., 2014; this study).

2.3.2.3 Shallowly inclined directions

We also identify a north-south and shallowly inclined dual polarity magnetization (Table

2-4; Figure 2-9c). This magnetization was interpreted by Halls et al. (2007) as an Ediacaran-

Cambrian overprint. While this direction was most commonly observed as a low-temperature overprint in dykes from the Dharwar craton, it is the only well-resolved component from 12 dykes in our study. Multiple dykes showed only this direction (Figure 2-9a) while others (Figure

2-9b) showed less-well resolved overprints. Our results yield a mean ‘normal’ polarity with

D=353°, I=+5° (k=16, α95=13°; N = 10) and a mean ‘reverse’ polarity group with D=199°, I=-

16° (k=12, α95=38°; N=3).

A baked contact test was attempted at site 18 near Attur, Tamil Nadu (Figure 2-10).

Samples from the interior and margin of the dyke yielded a mean D=11°, I=+15° (k=82; α95=4°;

Figure 10b). This shallow, northerly direction suggests a complete remagnetization of this dyke.

Three samples were taken from within 45 cm of the dyke delineating a scattered mean D=352°,

I=-21° (α95=40°; Fig 2-10c) while samples taken 5 meters from the dyke-host contact showed an independent group of directions at D=117°, I=-42° (α95=17°; Figure 2-10d). Given the size of this dyke (~30 m), the group at 5 m should still be well within the partial bake zone; however, the paleomagnetic directions of the host rock do not correspond with that of the dyke (Figure 2-

10a,e). This means that the baked contact test fails (as is expected given the remagnetization of the dyke), but the host rock is not remagnetized in the same manner. The transitional nature of the basement directions with distance, roughly falling along a great circle path towards steeper directions (Figure 2-10e), may reflect the cryptic survival of the original “baked” magnetization in the host rock, now entirely overlapped by the overprinted magnetization. Since site 18 is our southernmost dyke, and south of the Salem-Attur Shear Zone, a complete overprinting of its directions is not surprising.

Curie temperature curves for these dykes generally showed lower susceptibility than primary dykes, and the heating/cooling curve disparity indicates alteration, although magnetite is still the unambiguous magnetic signal carrier (Figure 2-5d).

The overall mean pole position for the thirteen shallow inclination directions from these dykes falls at 82° N, 267° E (A95=11°). Halls et al. (2007) reported a mean pole position of 79°

N, 257° E (A95=6°) that was argued to represent an Ediacaran overprint. This paleomagnetic direction was also used to calculate the paleomagnetic pole of the Dharmapuri “swarm” by

Radhakrishna and Joseph (1996), with a pole falling at 80° N, 259° E although they assigned an age of 1800-1750 Ma to this swarm based on their prior work (Radhakrishna et al., 1999).

2.3.2.4 Dual polarity intermediate directions

A consistent dual-polarity paleomagnetic signature with a northwest/southeast declination and intermediate positive/negative inclinations (Table 2-5c; Figure 2-11c) was observed in 8

dykes (8 sites) throughout the study area (Table 2-5b). At site 1, this direction was recovered from the dyke as well as baked gneiss host, although another baked contact test at site 21 was inconclusive. Stepwise demagnetization showed the remanence as bivectorial with mostly present-day field overprints at lower temperatures/applied fields (Figure 2-11a,b). The mean direction from the 3 ‘reverse’ polarity sites is D=140°, I=-54° (k=34; α95=22°). The normal polarity mean direction is D=332°, I=+53° (k=37; α95=13°). This direction (normal polarity) was also seen in Lakhna dyke D4 in the Bastar Craton by Pisarevsky et al. (2013), but excluded from their 1.47 Ga mean. Curie temperature experiments show magnetite to be the primary magnetic carrier, with reversible heating/cooling curves showing little alteration (Figure 2-5c). Dykes carrying this remanence trend N-S or ~NW-SE in the SGT and E-W in the Dharwar Craton.

Five dykes of this grouping were of normal polarity and three dykes were of reverse polarity. The mean VGP for this direction is at 53° N and 32° E (A95=11°). A reversals test

(McFadden and McElhinny, 1990) between the two directions is classified as Rc (γc=18.1°,

γobs=11.7°). This test included one dyke of reverse polarity direction that was excluded from the

Tiruvannamalai mean by Radhakrishna and Joseph (1996) to bolster the reverse polarity dataset.

This direction is less prevalent in the Southern Granulite Terrane as compared to the 2.37 Ga direction and the shallow overprint direction.

2.3.2.5 Other directions

Two dykes from this study carried antipodal paleomagnetic directions (Table 2-5a;

Figure 2-12c), consistent with the large 2.21 Ga swarm dated by Kumar et al. (2012). These dykes trended NE-SW or E-W, which is different than the typical trend (N-S) for this swarm in the northern Dharwar craton. Both dykes showed relatively univectorial decay during stepwise thermal demagnetization (Figure 2-12a,b). The mean paleomagnetic directions for each site were roughly antipodal (Figure 2-12c) with a mean ‘normal’ polarity D=57°, I=+41° (k=45; α95=6°)

and a mean ‘reverse’ polarity of D=238°, I=-64° (k=264, α95=3°). Curie temperature runs on these dykes confirmed magnetite as the primary carrier, consistent with observations during demagnetization of the specimens (Figure 2-5b). The heating curve near 320 °C shows the presence of minor pyrrhotite in site 50 (Figure 2-5b). The mean VGP for these dykes was at 34°

S and 316° E which is similar to both the Kumar et al. (2012) pole at 32° S and 302° E (A95=9°) and the Belica et al. (2014) pole at 30° S and 297° E (A95=22°). Dykes of this age are uncommon in the Southern Granulite Terrane. Our results seem to indicate that the 2.21 Ga swarm is present throughout the entire Dharwar Craton, but not as densely emplaced as the 2.37

Ga swarm.

Three sites in this study carry paleomagnetic signatures consistent with the reverse polarity of the widespread 1.88 Ga large igneous province (Table 2-5b; Figure 2-12e). Two of these dykes are from sites 1 and 30b which exhibit multicomponent magnetizations (Figures 2-7,

2-8, 2-13). At site 31, a scattered overprint (D=110°, I=+15° (k=14, α95=11°) falls roughly antipodal to this paleomagnetic direction. Such directions at the margin of larger, older dykes imply that emplacement of the 1.88 Ga swarm in the Southern Granulite Terrane was along the same zones of weakness as the older swarms. Stepwise demagnetization revealed bivectorial demagnetization paths, with lower temperature components being removed by 500 °C (Figure 2-

12d). Curie temperature analysis confirms magnetite to be the primary signal carrier (Figure 2-

5e). The mean direction of these sites is D=291°, I=-17° (k=30; α95=23°).

After inverting the ‘reverse’ polarity sites, the mean VGP falls at 18° N and 334° E

(A95=21°). This pole would indicate a higher latitude position for India than calculated from the

Meert et al. (2011) pole at 31° N and 330° E (A95=12°) or the Belica et al. (2014) pole at 37° N and 334° E (A95=6°). However, given the limited nature of 1.88 dyke outcroppings in this area,

we believe this is due to inadequate averaging of paleosecular variation in our VGP. Indeed, our

3 dyke directions and the VGPs calculated from them are consistent with individual VGPs from both Meert et al. (2011) and Belica et al. (2014).

2.4 Discussion

2.4.1 Paleomagnetism at Scale

It is apparent from the preponderance of several dyke systems in peninsular India that assigning ages to dykes based on trends, while demanded from a logistical standpoint, may not always be accurate. It is nearly impossible to visit every single dyke in a given area – and not all are suitable for paleomagnetism and geochronology. A zone of weakness (structural grain/fractures, etc.) exploited by an early generation of dykes can remain as an efficient propagation locale for a subsequent generation of dykes. Indeed, our work in the southern

Dharwar/Southern Granulite Terrane presents multiple examples of disparate data in close proximity. The site 30 double-baked contact tests discussed earlier were an excellent example, but other sites showed similarly complex overprints and primary directions.

Site 1, near Pileru, displays at least two generations of dykes. The site was on the second level of a small roadside quarry. Samples were taken from fine-grained dyke at the contact with granitic gneiss host near the contact and from small (ca. 2 cm wide) dykes hosted by the granite away from the contact. These small dykes in the granite from the mm to cm scale are quite abundant at this site. Samples were taken from west to east down the outcrop, with samples 1-9 in the west and 10-12 ~50-70 m to the east. The more westerly samples (10 specimens) defined a northwesterly intermediate direction D=320°, I=+41° (α95=13°) while the easterly samples (4 specimens) defined a direction D =283°, I=-7° (α95=13°; Figure 2-13a). Satellite imagery of the area shows both E-W and NNE-SSW dykes cross-cutting in the vicinity of site 1.

Site 27 samples were taken from small “dykelets” that branch far from the main dyke at site 28. While the main dyke at site 28 has a steep negative inclination (D=50°, I=-84°; α95=5°), site 27 dykelets show a shallow northerly inclination (D=359°, I=+18°; α95=16°). These two directions are comparable, respectively, to the canonical 2.37 Ga direction of the giant

“Bangalore” swarm (Halls et al., 2007; Belica et al., 2014) and the B directions of Halls et al.

(2007), the latter inferred to be an Ediacaran overprint. It seems likely that the interior of the dyke managed to retain the primary remanence while the dykelets were completely overprinted.

This interpretation is supported by the fact that the low coercivity (to 30 mT) remanence from site 28 is identical to the overprinted site 27 direction (Figure 2-13b).

2.4.2 The South Indian “bar code”

Refinement in age control through more precise age data has changed the paradigm for

Paleoproterozoic dyke emplacement in the South Indian Block from relatively few episodes to several (Figure 2-14). The giant 2370 Ma “Bangalore” dyke swarm has been unequivocally dated throughout the Dharwar Craton by multiple workers (Halls et al., 2007; French et al., 2008;

Figure 2-4, this study). Consistent paleomagnetic data from this dyke swarm (Halls et al., 2007;

Belica et al., 2014, Dash et al., 2013; Radhakrishna et al., 1996, 2013a, 2013b; Venkatesh et al.,

1987, this study) are similarly widespread, from the Dharwar Craton into the Salem Block. These poles are more scattered in the Salem Block than noted elsewhere, perhaps reflecting greater tectonic reworking of this block as compared to the more stable Dharwar Craton. A comparison of poles presented in multiple paleomagnetic studies throughout Southern India (Figure 2-15) shows relatively good agreement between paleomagnetic poles from this dyke swarm. As noted before, most of these data show a reverse polarity, but normal polarity dykes belonging to this swarm have also been identified (Belica et al., 2014, this study). Dash et al. (2013) also noted a

normal polarity dyke, as mentioned earlier, yet chose to discard it. Since there are several more

‘normal’ polarity results that pass a reversal test (Belica et al., 2014, this study) we see no reason to either exclude them from the 2370 Ma paleomagnetic compilation for the Dharwar craton or to regard this swarm as containing only a single polarity. We note that there is a slight discrepancy among the poles presented (Figure 2-15). Data primarily from the Southern Dharwar show good agreement, while Southern Granulite Terrane results may indicate some rotation relative to more stable areas of the Dharwar Craton. The Salem Block is dissected by several crustal-scale shear zones (Collins et al., 2014; Figure 2-2) which may facilitate small-scale rotations of paleomagnetic data from areas of the SGT. The Dash et al. (2013) results from the

2370 Ma swarm are all from the Tiruvannamalai area, which lies east of one of these shear zones, which may provide a mechanism for the rotations. The notion of rotations in the SGT is admittedly speculative, but worth considering in further evaluation of paleomagnetic data from this area.

Demirer (2012) reported U-Pb baddeleyite ages of 2255 ± 5 Ma from four dykes near

Ippaguda in Telegana State (NE Dharwar craton). Recently, Srivastava et al. (2016) also published a 2252 ± 2 Ma Pb-Pb baddeleyite age for a NE-SW dyke from the Singhbhum Craton, underlining a widespread nature of this apparent “swarm”. French and Heaman (2008) and

Kumar et al. (2012) identified a ca. 2210 ± 10 Ma Ma dyke swarm, having a relatively clear N-S trend in the northern Dharwar Craton but apparently fanning out in the south. Paleomagnetic data from Kumar et al. (2102) and from Belica et al. (2014) showed the extent of this swarm across the Dharwar Craton. Our data shows a possible continuity of this swarm in the Southern

Granulite Terrane, although the data are limited. Moreover, French and Heaman (2010) also defined a 2180 Ma swarm in the Dharwar Craton, based on geochronological evidence.

Paleomagnetic signatures for the dykes of this age were presented in Piispa et al. (2011) and

Belica et al. (2014). Our intermediate dual-polarity directions most closely resemble these data – but the paleomagnetic poles disagree; the Belica et al. (2014) pole falls at 68° N, 85° E (A95 =

18°) versus 53° N, 32° E (A95 = 11°) for our pole. As mentioned earlier in discussion of older data, relative local rotations may help explain this discrepancy, but a direct comparison awaits geochronological supporting data from dykes of this paleomagnetic direction in the SGT.

Regardless, the 70 Ma stretch between three distinct episodes of dyke emplacement with good quality paleomagnetic data recovered for two of the episodes shows that for certain periods of

Paleoproterozoic India movement can be precisely constrained.

Kumar et al. (2015) recognized a 2082 Ma dyke swarm emplaced around the Cuddapah

Basin. Data from multiple workers and areas (Piispa et al., 2011; Belica et al., 2014; Kumar et al., 2015) provide a reasonable paleomagnetic pole for this dyke swarm.

Precise baddeleyite/zircon ages for the Pullivendla sill at 1885 ± 3.1 Ma (French et al.,

2008) and NW-SE trending Bastar dykes at 1891.1 ± 0.9 Ma and 1883.0 ± 1.4 Ma (French et al.,

2008). The paleomagnetic pole for the 1.88 Ga dykes in the Dharwar Craton falls at 37° N, 334°

E (A95=7°; Belica et al., 2014). A limited study on the 1.88 Ga age NW-SE dykes from the

Bastar Craton also showed a pole similar to the Dharwar Craton (31° N and 330° E; A95 = 12°;

Meert et al., 2011), The extension of this dyke swarm through several cratons of the South India

Block (SIB) and possibly into the Southern Granulite Terrane (this study) presents compelling evidence for their unification at this time. The potential for India movement delineation in this time period grows when a 1785 ± 14 Ma baddeleyite age from a dyke near Pebbair, north of the

Cuddapah, is also considered (Demirer, 2012). Newer dolerites from the Singhbhum Craton are dated at 1765 ± 1 Ma (Shankar et al., 2014) which could mean widespread and protracted dyke

emplacement in India at this time – at least from the Dharwar and Singhbhum Cratons. Our limited geochronological work in the SGT did not reveal any ~1765 Ma dykes. The mounting evidence for SIB unification into the Paleoproterozoic begs the question: is there any evidence for North India Block proximity at this time? As mentioned in the introduction, NIB-SIB unification remains an area of active research. Future combined paleomagnetic and geochronologic work focused on the North Indian Block will aid in any comparisons with the well-populated SIB “barcode”.

2.4.3 The Northern Block of the SGT and the Dharwar Craton: A Metamorphosed Chip off the Old Block

Given that this study area is a part of the Southern Granulite Terrane, delineating a metamorphic component to the paleomagnetic data was perhaps inevitable. Tectonothermal activity in the SGT shows distinct peaks at the Archean-Proterozoic transition (Anderson et al.,

2012; Collins et al., 2014), mid-Cryogenian (Ghosh et al., 2004), and early Cambrian from 540-

525 Ma (Clark et al., 2009). The earliest major metamorphic event, at the beginning

Paleoproterozoic (Anderson et al., 2012) pre-dates emplacement of the oldest Southern Indian dykes. Therefore, a Neoproterozoic overprint age is more likely within the Southern Granulite

Terrane. We have identified several potential overprints in our study. The “B” component, yields a pole at 82° N, 267° E (A95=11°, Figure 2-16). Combining this component with other directional data from the SGT (Halls et al., 2007; Radhakrishna et al., 1996) a grand mean paleomagnetic pole is calculated at 81° N, 259° E (A95=6°). The “B” component pole, however, lacks direct geochronological supporting data. This widespread overprint was interpreted by both

Halls et al. (20070 and Belica et al. (2014) as Ediacaran in age. An Ediacaran overprint age is consistent with other poles in the Gondwana database (Figure 2-17; Table 2-7; Meert, 2001;

Torsvik et al., 2012; Trindade et al., 2006; Rapalini et al., 2015; Robert et al., 2017).

During the 540 – 525 Ma interval, the Madurai Block collided with the Salem Block followed by orogenic collapse (Clark et al., 2009). The cryptic geochronological results from site

10 implied at least one dyke in the Tiruvannamalai area has a crystallization age at 527 ± 2.6 Ma, which implies magmatic activity at the end of the Kuunga (later called the Malagasy) Orogeny

(Meert, 2003; Clark et al., 2009). An orogenic collapse would lead to extension, and dyke activity is consistent with an extensional environment. Do the paleomagnetic data from around

Tiruvannamalai and elsewhere in the Southern Granulite Terrane support this scenario? Dyke

T9, one of several NE-SW trending dykes cutting the large 2370 Ma dyke at sites 10, 11, and 12 has a VGP that falls at 7° N, 347 °E (A95=5°). The East Coast Dykes (ECD) sampled by Dash et al. (2013) yielded a paleomagnetic pole at 2° S, 8° E (A95=6°). Additionally, two “anomalous” overprints from the Southern Granulite Terrane yield a VGP at 7° N, 352° E (A95=12°). It seems likely that the ECD and at least several dykes from the Tiruvannamalai area preserve coeval paleomagnetic data at ca. 527 Ma. Paleomagnetic poles from stratoid granites (7° S, 353° E;

A95=14°; 521.4 ± 11.9 Ma; Meert et al., 2003) and the Carion granite (7° S, 001° E; A95=15°;

508.5 ± 11.5 Ma; Meert et al., 2001) in Madagascar show excellent agreement with the

Gondwana APWP when rotated into an African reference frame (Figure 2-17) and are roughly coeval in time with our proposed magmatism in the SGT. However, when rotated into an African reference frame, it is clear that our proposed 527 Ma Indian pole does not agree with the

Gondwana APWP (Figure 17). Since the ECD and Tiruvannamalai data are all from the same block of the SGT east of the Mettur shear zone (Figure 2-2), our earlier discussion of possible clockwise rotation of this block may hold for these data as well. However, as mentioned earlier, late Ediacaran paleomagnetic data from the Marwar Supergroup in NW India is roughly coeval with the SGT data (Davis et al., 2009; Figure 2-17), which makes significant rotation at this time

less likely. We note an interesting resemblance between the angular shift between our Ediacaran

“B” component pole with the proposed early Cambrian Indian paleomagnetic results and the large-scale apparent polar wander curve shifts around the Ediacaran-Cambrian discussed at length in Robert et al. (2017). However, the geochronological constraints on our data provide a longer time window for this possible shift (mid-Ediacaran – 527 Ma).

Early attempts at defining geologic provinces in India (Fermor, 1936; Drury, 1984) considered the SGT to extend to the roughly-defined orthopyroxene-in isograd, or the Fermor

Line. General structural continuity between the Northern Block and the greater Dharwar craton provides evidence of their contiguity (Meissner et al., 2002). Crustal thickness and seismic studies (Rao et al., 2006) suggest that the Northern Block collided with the Dharwar Craton in the late Archean (Meert et al., 2010). The 2538 ± 6 Ma and 2529 ± 7 Ma SHRIMP Pb-Pb ages of zircons from charnockitic rocks (Clark et al., 2009) with younger ages obtained from the zircon rims (ca. 2480 Ma) neatly bracket the general stabilization age of the Dharwar Craton. This either shows that the Northern Block was contiguous with the Dharwar Craton earlier and stabilized at the same time, or that their amalgamation in the Late Archean was an important part of the stabilization process. Regardless of which interpretation is correct, we concur with Dash et al. (2013) in viewing the early Proterozoic dyke swarms present in both the Northern Block of the SGT and the Dharwar as unequivocal evidence supporting their contiguity at the time of emplacement.

The well-confirmed high-latitude position for the Dharwar Craton (now shown to include the SGT down to the PCSZ – e.g. the Northern Block) allows for the testing of Paleoproterozoic cratonic reconstructions. Halls et al. (2007) tentatively reconstructed the Yilgarn and Dharwar cratons based on paleomagnetic and geochronologic data from the Widgiemooltha swarm and

the Bangalore swarm, respectively. Both the swarms are tholeiitic in composition and are indistinguishable in a Ti-Zr-Y discrimination plot which may indicate a genetic link between them given the identical concentrations of the three elements (Halls et al., 2007). The mean

2.37 Ga Dharwar pole from this study is at 2° N and 73° E (A95=14°) and the Yilgarn pole from the Widgiemooltha dykes is at 10° N, 339° E (A95=8°; Evans, 1968; Smirnov et al., 2013).

Reconstructing the Dharwar and Yilgarn according to these poles (given hemispheric ambiguity and longitudinal uncertainty) allows the two cratons to be placed together. Although an argument was forwarded to correlate these two intrusive events by Halls et al. (2007), we believe the geochronological data are problematic for that correlation. Age data from the Bangalore swarm

(Halls et al., 2007; Kumar 2012b; this study) points to rapid emplacement, with all ages falling between 2360 and 2370 Ma. The Widgiemooltha swarm, on the other hand, was emplaced around 2410-2418 Ma (Nemchin and Pidgeon, 1998; French et al., 2002). A study on the 2401 ±

1 Ma Erayinia dykes from the Yilgarn Craton produced a paleomagnetic pole at 23° N, 331° E

(A95=11°; Pisarevsky et al., 2015). Pisarevsky et al. (2015) calculated an angular velocity of

~1°/Ma for the Yilgarn craton. Therefore, the 2.40-2.41 Ga correlations shown in Pisarevsky et al. (2015) are not necessarily applicable to the Dharwar Craton since the dykes there are ca. 30

Myr older than the dykes of the Yilgarn.

2.5 Conclusions

Dykes in the northern part of the Southern Granulite Terrane reflect multiple intrusive pulses ranging in age from the earliest Paleoproterozoic into the Cambrian. Although this area was subjected to a higher degree of metamorphism than other areas in India, it still contains a remarkably complete Proterozoic paleomagnetic record for Southern India.

Our study provides ample evidence to indicate juxtaposition of SGT with Dharwar

Craton from at least 2370 Ma, also mentioned earlier by Dash et al. (2013). Our U-Pb zircon age of 2363 ± 6.6 Ma precisely ties steep paleomagnetic directions from E-W and NW-SE mafic dykes in the SGT to steep paleomagnetic directions from E-W mafic dykes in the Dharwar

Craton. Furthermore, we report more limited paleomagnetic results on dykes from other phases of magmatic activity in Proterozoic India, namely, at 2210 Ma, 2180 Ma, and 1880 Ma.

The Dharwar craton and its extension into the northern SGT was positioned at high latitudes in the early Paleoproterozoic (ca. 2370 Ma).

We have placed a tentative date of 527 ± 2.6 Ma on the East Coast Dykes (Dash et al.,

2013). The geographic extent of these dykes is still largely unconstrained in the SGT, but occurs at least as far west as Tiruvannamalai. Direct dating of the East Coast dykes and other NE-SW dykes in Tiruvannamalai would be a simple and effective test of this correlation.

Figure 2-1. Map of India showing major cratonic nuclei and Proterozoic-aged sedimentary basins features (adapted from Meert and Pandit, 2010). Box indicates Figure 2 location.

Figure 2-2. Regional map of dykes sampled in this study. Groups of directions are represented by different colored line segments. Dyke trends shown were determined from field observations and satellite imagery. Sites of baked contact tests and geochronological sampling are shown.

Figure 2-3. Electron backscatter imaging and concordia plot of analyzed zircons from site 10. Inherited basement zircon BSE imaging and concordia shown in (a). Inherited ages reflect a phase of Peninsular Gneiss emplacement (2755 ± 15 Ma) and zircons reflecting the crystallization age of dyke T9 (b) also shown, with an age of 527 ± 2.6 Ma. This age represents a relatively late magmatic event as compared to the Proterozoic dykes of the Southern Granulite Terrane. Red circles indicate laser ablation spot size.

Table 2-1. Geochronological results for site 10

Grain name 207Pb/235U 2σ 206Pb/238U 2σ 207Pb/206U 2σ 206Pb/238U 2σ 207Pb/235U 2σ 207Pb/206U 2σ (Age) Ma (Age) Ma (Age) Ma

I1510_1_core 0.66004 0.01316 0.08237 0.00154 0.05812 0.00040 510.2 9.2 514.6 8.0 534.0 14.9 I1510_2_core 13.93067 0.33208 0.52872 0.01218 0.19109 0.00116 2736.0 51.3 2744.9 22.5 2751.5 10.0 I1510_4_core 0.68337 0.01456 0.08505 0.00172 0.05828 0.00039 526.2 10.2 528.8 8.8 540.0 14.7 I1510_5_core 0.67895 0.01416 0.08463 0.00163 0.05818 0.00046 523.7 9.7 526.1 8.5 536.5 17.2 I1510_5_rim 0.67337 0.01325 0.08458 0.00154 0.05774 0.00042 523.4 9.2 522.7 8.0 519.5 16.1 I1510_6_core 0.68059 0.01478 0.08449 0.00171 0.05842 0.00046 522.9 10.2 527.1 8.9 545.5 17.2 I1510_7_rim 0.68067 0.01589 0.08484 0.00188 0.05819 0.00043 524.9 11.2 527.2 9.6 536.5 16.0 I1510_8_core 14.20742 0.29954 0.53718 0.01088 0.19182 0.00112 2771.5 45.5 2763.5 19.9 2757.5 9.6 I1510_8_core2 13.48831 0.27306 0.51318 0.00994 0.19063 0.00113 2670.1 42.3 2714.3 19.0 2747.5 9.7 I1510_9_core 0.68266 0.01421 0.08526 0.00168 0.05807 0.00040 527.4 10.0 528.4 8.6 532.0 15.0 I1510_10_core 0.66020 0.01378 0.08307 0.00164 0.05764 0.00039 514.4 9.8 514.7 8.4 516.0 14.8 I1510_11_rim 0.71942 0.01573 0.08805 0.00182 0.05926 0.00043 544.0 10.8 550.3 9.3 576.5 15.8 I1510_12_core 0.69517 0.01735 0.08603 0.00183 0.05861 0.00077 532.0 10.8 535.9 10.4 552.0 28.7 I1510_13_core 0.77821 0.02613 0.09130 0.00221 0.06182 0.00144 563.2 13.1 584.4 14.9 667.5 49.8 I1510_13_rim 0.68087 0.01449 0.08505 0.00171 0.05806 0.00041 526.2 10.1 527.3 8.7 532.0 15.5

Figure 2-4. Concordia plot from zircons recording the crystallization age of the dyke at site 35, with an age of 2363 ± 6.6 Ma. This age is a precise match with the giant Dharwar dyke swarm dated by Halls et al. (2007) at 2367 ± 1 Ma.

Table 2-2. Geochronological results for site 35

Grain name 207Pb/235U 2σ 206Pb/238U 2σ 207Pb/206U 2σ 206Pb/238U 2σ 207Pb/235U 2σ 207Pb/206U 2σ (Age) Ma (Age) Ma (Age) Ma I1535_1_core 12.16985 0.25440 0.49974 0.00994 0.17662 0.00114 2612.6 42.6 2617.5 19.5 2621.0 10.7 I1535_1_rim 11.90738 0.25137 0.48776 0.00987 0.17706 0.00106 2560.9 42.7 2597.0 19.7 2625.0 10.0 I1535_2_core 11.25309 0.23223 0.47690 0.00935 0.17114 0.00110 2513.6 40.8 2544.2 19.2 2568.5 10.7 I1535_2_rim 11.15138 0.23583 0.47794 0.00948 0.16922 0.00124 2518.2 41.3 2535.7 19.6 2549.5 12.3 I1535_3_core 11.47819 0.23160 0.48937 0.00945 0.17011 0.00100 2567.8 40.8 2562.7 18.8 2558.5 9.8 I1535_3_core2 11.45672 0.25291 0.48889 0.01041 0.16996 0.00099 2565.8 45.0 2560.9 20.5 2557.0 9.7 I1535_5_core 11.68306 0.29429 0.49492 0.01208 0.01712 0.00108 2591.8 52.0 2579.2 23.4 2569.0 10.5 I1535_6_core 9.34457 0.13934 0.44735 0.00614 0.15150 0.00089 2383.4 27.3 2372.3 13.6 2362.5 10.0 I1535_7_core 9.46768 0.17983 0.44419 0.00677 0.15459 0.00175 2369.3 30.2 2384.3 17.4 2397.0 19.3 I1535_8_core 9.16364 0.13302 0.44125 0.00589 0.15062 0.00086 2356.2 26.3 2354.4 13.2 2352.5 9.7

Figure 2-5. Susceptibility vs. temperature curves for selected sites from the recovered paleomagnetic directional groupings: (a) from the 2.37 Ga direction exhibiting a reversible heating cooling curve, this dyke was also dated (b) possibly from the 2.21 Ga direction, prominent pyrrhotite bump at 320 °C (c) from an undated direction, reversible heating cooling curve (d) dyke overprinted in the Neoproterozoic, changes upon heating indicating alteration (e) dyke associated with the 1.88 Ga direction, abrupt peak leading up to magnetite Curie temperature.

Figure 2-6. Day Plot (Day et al., 1977) showing magnetic domain characteristics from a representative suite of dyke samples throughout the Southern Granulite Terrane. They primarily exhibit pseudo-single domain magnetite behavior. Hysteresis loops from selected samples are also shown, also consistent with magnetite.

Table 2-3. Paleomagnetic results from 2.37 Ga dykes in the Southern Granulite Terrane

Slong D α95 Plat Plong Site Slat (°) (°) Trend B/N (°) I (°) (°) k (°N) (°E) A95 Refs. 2 13.6928 78.9235 20 9 266 -65 2 630 13 124 this study 4 13.6737 79.1513 70 7 94 58 3 160 6 130 this study 7 13.0867 78.9748 70 9 190 -83 3 209 25 81 this study 10+11+12 12.1773 78.9301 120 3/22 115 -70 3 96 24 43 this study 15 12.1585 79.1736 130 7 278 -78 4 145 8 103 this study 22 12.0767 77.9324 90 7 89 -81 13 20 11 60 this study 28 12.0761 77.8397 165 5 50 -83 5 159 3 67 this study 29 12.0620 77.8343 130 6 13 -75 7 87 -15 72 this study 30i+31 12.0505 77.8224 140 2/17 350 -68 4 103 -26 85 this study 32 12.0637 77.8072 135 7 114 -80 1 1573 19 60 this study 33 12.4546 78.3539 80 7 51 -76 8 90 -5 57 this study 34* 12.4523 78.3972 130 8 16 -74 5 125 -28 64 this study 35 12.4764 77.9656 150 7 61 -84 4 169 7 68 this study 37 12.4716 77.8552 135 7 179 81 4 166 -5 79 this study 38 12.4692 77.8570 130 7 19 -70 5 121 -21 70 this study 39 12.4599 77.8429 100 7 42 -76 3 176 -8 60 this study 40 12.4839 77.8152 90 7 285 68 3 228 19 39 this study

Mean 17/152 49 -83 7 27 2 74 14 this study

NW- Tiruv. SE 10 125 -74 8 22 28 52 13 1 NW- Tiruv. SE 7 107 -78 9 42 19 55 --- 2 C 4 133 -75 6 134 31 54 11 3 - 2.37 18 65 81.7 8.3 19 6.6 63.1 8.3 4 - "A" E-W 7 94.9 80.9 7.3 69 13.6 59.6 13.9 5 Slat = site latitude, Slong = site longitude, Trend = trend of dyke, B = number of sites, N = number of specimens, D = declination, I = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = pole longitude, A95 = radius of the 95% confidence circle about the mean pole, References: 1 = Dash et al. (2013); 2 = Radhakrishna et al. (1996); 3 = Venkatesh et al. (1987); 4 = Belica et al. (2014); 5 = Halls et al. (2007)

Figure 2-7. Demagnetization data for reverse (a,b) and normal polarity (c) sites from 2370 Ma dykes in the Southern Dharwar craton and northern Southern Granulite Terrane. Note that (a,b) show overprinting consistent with the “B-component” discussed in Section 3.2.3 and shown in Figure 9. Stereoplots show characteristic site mean directions (d) and (e) consistently grouped low-temperature/coercivity components.

Figure 2-8. Map view schematic of baked contact test at site 30 (a), along with demagnetization data. Demagnetization data shown is from a distal site on the same dyke (site 31, note overprint; b), coarse-grained large dyke (site 30a; c), fine-grained marginal dyke (site 30b; c), and the baked zone to unbaked zone transition (e-h). Stereoplot with directional data also shown (i). These sites provide evidence for two primary remanences: the 2.37 Ga direction and the 1.88 Ga direction. This rather unusual situation underscores the complexities of terranes with multiple phases of dyke emplacement.

Figure 2-9. Demagnetization data for “B-component” dykes with minor overprints (a,b) in the southern Dharwar and Southern Granulite Terrane interpreted to be products of Ediacaran remagnetization. Shallow paleomagnetic directions indicative of this grouping (c), also corresponding with consistent overprints on older paleomagnetic directions.

Table 2-4. Paleomagnetic results from Southern Granulite Terrane dykes overprinted in the Ediacaran

Slong I α95 Plat Plong Site Slat (°) (°) Trend B/N D (°) (°) (°) k (°N) (°E) A95 Refs.

3 13.6979 78.9215 80 7 333 9 8 34 62 333 this study - 6 13.0883 78.9849 0 7 342 11 7 51 65 305 this study 8 13.0557 79.0384 90 7 340 0 4 138 62 320 this study 18i+m 11.6576 78.4911 30 12 11 15 4 83 81 174 this study - 19 12.1513 78.2554 130 9 190 21 5 83 87 145 this study 20 12.1306 78.3189 140 8 218 -4 7 46 76 276 this study - 24 12.1461 78.2666 60 9 183 26 2 236 87 275 this study 25 12.0708 78.3834 140 9 356 -4 3 106 67 345 this study 27 12.0778 77.8369 165 3 359 18 10 89 78 196 this study 42 12.4647 77.7673 160 8 336 21 3 170 68 226 this study 43 12.1873 77.7391 60 12 11 13 6 46 79 190 this study - 44 12.1229 77.8405 40 7 12 13 4 102 51 180 this study

Table. 2-4 Continued Slong I α95 Plat Plong Site Slat (°) (°) Trend B/N D (°) (°) (°) k (°N) (°E) A95 Refs. Mean 12/99 359 9 13 13 82 267 11 this study Grand Mean 24 81 259 6 this study Ag- Anaik. 2 14 27 26 94 76 153 1 Dharm. 5 359.8 9.2 10.8 51 79.7 281.1 1

"B" 7 00.1 1.9 8.8 48 257.3 257.3 6.0 2 Slat = site latitude, Slong = site longitude, Trend = trend of dyke, B = number of sites, N = number of specimens, D = declination, I = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = pole longitude, A95 = radius of the 95% confidence circle about the mean pole, References: 1 = Radhakrishna et al. (1996); 2 = Halls et al. (2007)

Figure 2-10. Map view schematic of baked contact test at site 18 (a) along with demagnetization data from the dyke interior (b), transitional zone (c), and distant host rock (d). Summary stereoplot of direction shown in (e). This contact test is discussed in section 3.2.3.

Figure 2-11. Demagnetization data from bivectorial normal data (a) and near univectorial normal data (b) from a NW-SE intermediate paleomagnetic direction of unknown age. Site mean directions are shown on the stereoplot (c).

Table 2-5. Grouped paleomagnetic results of uncertain age from the Southern Granulite Terrane (likely 2.21 Ga to 1.88 Ga)

Site Slat (°) Slong Trend B/N D (°) I (°) α95 k Plat Plong A95 Refs. (°) (°) (°N) (°E)

5a: 2.21 Ga dykes? 5 13.0863 79.1369 70 9 57 41 6 45 35 150 this study 50 12.6458 77.8420 100 7 238 -63 3 264 32 124 this study

2.21 N-S 6/39 236 -67 20 12 32 117 12 1

N-S dykes N-S 3 239 -64 6 43 32 122 9 2

5b: 1.88 Ga dykes? 1b 13.6526 78.9659 80 2 283 -7 13 54 -12 163 this study

Table 2-5. Continued Site Slat (°) Slong Trend B/N D (°) I (°) α95 k Plat Plong A95 Refs. (°) (°) (°N) (°E) 17 12.2599 78.9639 60 5 306 -21 12 44 -32 148 this study 30m 12.0495 77.8276 30 4 284 -25 5 377 -11 153 this study

Mean 3/10 291 -17 23 30 18 334 21 this study

1.88 29 129 9 7 19 36 331 7 1

Bastar 7 126 15 12 27 31 330 12 3

5c: NW-SE intermediate directions 1a 13.6526 78.9659 80 5 320 41 13 16 53 11 this study 9 12.0676 79.0072 70 8 142 -68 2 260 40 49 this study 14 12.2812 78.8901 105 9 131 -49 5 87 41 18 this study 21 11.9190 78.3873 130 7 341 67 6 43 49 56 this study 23 12.1171 77.8400 60 6 147 -44 4 95 57 17 this study 36 12.4067 77.9614 170 7 322 55 3 162 48 29 this study 45 12.6059 78.1158 20 7 326 56 5 113 51 32 this study 47 12.6178 77.7131 135 5 354 44 13 38 76 55 this study

Mean 8/54 327.7 53.9 9.3 36.5 52.9 32.2 10.6 this study Slat = site latitude, Slong = site longitude, Trend = trend of dyke, B = number of sites, N = number of specimens, D = declination, I = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), Plat = pole latitude, Plong = pole longitude, A95 = radius of the 95% confidence circle about the mean pole, References: 1 = Belica et al. (2014); 2 = Kumar et al. (2012); 3 = Meert et al. (2011)

Figure 2-12. Demagnetization data for dykes provisionally associated with 2210 Ma emplacement (a,b,c) and 1880 Ma emplacement (d,e).. Both dykes of the former show near univectorial demagnetization paths (a, b). The latter exhibits more of an overprint (d). Site means are shown on the labelled stereoplots (c,e).

Figure 2-13. Two examples of contrasting paleomagnetic data in close proximity in the Southern Dharwar craton and northern Southern Granulite Terrane. Site 1 (a) near Tirupati is at an area of cross-cutting dykes where the older dyke appears to have baked the surrounding gneiss, while a younger dyke intruded at the same outcrop, similar to the dykes at site 30a,b. Site 28 and 27 (b) are from a dyke and the associated dykelets respectively. The dykelets are completely overprinted, while the larger dyke preserves a 2.37 Ga direction after the removal of overprints.

Table 2-6. Directional paleomagnetic results from a baked contact test at sites 30a,b and 31

Site Slat (°) Slong (°) Trend B/N D (°) I (°) α95 (°) k 30a 12.0495 77.82763 110 10 360 -67 6 94 31 12.05145 77.81717 110 7 342 -68 4 146 baked 12.0495 77.82763 110 2 348 -78 42 10 gneiss 30b 12.0495 77.82763 110 4 284 -25 5 375 baked 12.0495 77.82763 110 1 283 -22 N/A N/A gneiss unbaked 12.0495 77.82763 110 4 44.3 39 15 40 gneiss 46 12.64368 77.74318 N/A 10 24.1 35 9 30

Figure 2-14. “Barcode” of igneous activity for primarily the South Indian Block. Green lines indicate both geochronological and paleomagnetic control on the igneous event. Highly studied events also have a star. Orange indicates only geochronological control.

Figure 2-15. Comparison of the paleomagnetic pole calculated from our primary 2363 ± 6.6 Ma paleomagnetic direction (green) with other studies from the Southern Dharwar craton (violet): Halls et al. (2007), Dash et al. (2013), and Belica et al. (2014). Our pole is lower latitude and more easterly, but overlaps with the Belica et al. (2014) pole within confidence limits. Note that purely SGT results (Dash et al., 2013; this study) may be rotated with respect to results from farther northward in the Dharwar Craton.

Figure 2-16. Poles calculated from the shallow paleomagnetic component seen in 12 dykes of our study. Mean pole is compared to the “B” component of Halls et al. (2007).

Table 2-7. Comparative Gondwanan poles from the Ediacaran-Cambrian Unit Name abbreviation region Age (Ma) Plat Plong A95 Weight Ref. (°N) (°E) Brachina BFm Australia-S 603 -33 328 16 B McWilliams Formation and McElhinney (1980) Baganpalli BQ India 589 -74 54 4 B Goutham et quartzite al., (2006) "shallow" SO South India Ediacaran 82 267 11 B this study overprint Sierra de las SdlA Rio de la 578 -12 259 14 B Rapalini et Animas Plata al., (2015) complex C Nola NMD Congo 571 -62 305 8 B Moloto-A- metadolerite Kenguemba et al. (2008) Adrar-n- A-n-T West 570 -57 296 16 B Robert et Takoucht Africa al., (2017) Lower LA Australia-N 561 -44 342 10 A Kirschvink Arumbera et al., Formation (1978) Tadoughast Tad West 561 22 31 16 B Robert et Africa al., (2017) Sinyai SMD Congo 547 -29 319 4 A Meert and metadolerite Van der Voo, (1996) Marwar Marwar NW India late -1 344 7 B Davis et al., Supergroup Ediacaran (2009) Dichol- D-B West 536 23 27 15 B Robert et Boho Africa al., (2017) Dyke T9* T9* South India 527? 7 347 5 C this study East Coast ECD South India 527? 2 8 6 B Dash et al., Dykes (2013) SGT SGTO* South India 527? 7 352 12 C this study overprints* Stratoid SG Madagascar 520 -7 353 14 A Meert et al., Granite (2003) Carion CG Madagascar 508 -7 1 15 A Meert et al., Granite (2001) Abbreviations are those used in Figure 17. Ages are nominal averages of uncertainty. Poles are not rotated in this compilation, but were rotated into an African reference frame for comparative purposes (see Figure 17) using the following Euler rotations. MAD-AFR: -3.41, -81.7, 19.73; IND-AFR: 27.9, 43.64, -64.4; AUS-AFR: 24.5, 112.3, - 56.3; SA-AFR: 45.5, -32.2, 58.2. Weight is a measure of confidence in the result, with “A” being highest confidence.

Figure 2-17. Ediacaran – Early Cambrian poles and VGPs from Southern India compared to paleomagnetic data of the same time interval from other Gondwana blocks (see Table 7 for detailed information and rotations parameters). Indian poles are shown in green, Australian in baby blue, African in orange, and Madagascar in violet. The running mean Gondwana APWP of Torsvik et al. (2012) is shown by the dashed blue line, with age squares until 390 Ma provided. Outlines of Gondwana blocks shown in light gray. Note that TD and the overprint direction represent VGPs, and not paleomagnetic poles proper.

CHAPTER 3 PALEOMAGNETIC RESULTS FROM SINGHBHUM CRATON: REMAGNETIZATION, DEMAGNETIZATION, AND COMPLICATION

3.1. Introduction

3.1.1 India: A Rich and Complex Precambrian Record

Untangling the paleomagnetic record of geologic terranes with complex tectonothermal histories is a difficult endeavor. Peninsular India – the Dharwar, Bastar, Singhbhum,

Bundelkhand, and Aravalli cratons along with the Banded Gneiss Complex (BGC) – has a substantial history of growth and deformation beginning in the Eoarchean (Meert et al., 2015;

Miller et al., 2018). As is typical for Archean cratons (Halls, 2008), these five Indian nuclei are cross-cut by numerous mafic dykes. A concerted effort to precisely date the many generations of mafic dykes has yielded a wealth of robust geochronologic data in the Dharwar (Halls et al.,

2007; French et al., 2008; Pradhan et al., 2008; Pradhan et al., 2010; French and Heaman, 2010;

Demirer, 2012; Kumar et al., 2012a; Kumar et al., 2012b; Belica et al., 2014; Kumar et al., 2015;

Nagaraju et al., 2018a; Pivarunas et al., 2018; Nagaraju et al., 2018b; Soderlund et al., 2018),

Bastar (French et al., 2008; Pisarevsky et al., 2013; Shellnutt et al., 2018), Singhbhum (Kumar et al., 2017; Shankar et al., 2017; Srivastava et al., 2018), and Bundelkhand (Pradhan et al., 2012) nuclei. Such a protracted intrusive history, necessarily restricted within a finite amount of space, creates unusual complexities in the evaluation of paleomagnetic data from the dykes (Pivarunas et al., 2018). Constraints on the age of magnetizations are of paramount importance to paleomagnetic studies. If paleomagnetic data lack temporal constraints, then correlations between continental blocks or inferences of movement are premature at best and spurious at worst. A variety of field tests can be used to assess magnetization ages (Butler, 1992), but the

“gold standard” for mafic dykes is the baked contact test (Everitt and Clegg, 1962).

The five major nuclei that comprise Peninsular India can be divided into a North Indian

Block (NIB) (Aravalli-Banded Gneiss Complex, Bundelkhand) and a South Indian Block (SIB)

(Dharwar, Bastar, Singhbhum). They are separated by the roughly east-west trending Central

Indian Tectonic Zone (CITZ). Major tectonothermal events within the CITZ have been identified at approximately 2.5 Ga (Stein et al., 2004, 2014), between 1.8 and 1.5 Ga and again from 1.0–

0.9 Ga (Bhandari et al., 2011; Bhowmik et al., 2012a; Bhowmik et al., 2012b; Meert et al., 2010,

2011; Meert and Pandit, 2015). The Stenian-Tonian aged tectonothermal event is the consensus estimate for the union of the North Indian Block and South Indian Block (Bhowmik et al.,

2012a). This is supported by concordant paleomagnetic results from the NIB (Miller and

Hargraves, 1996; Gregory et al., 2006; Pradhan et al., 2010) and SIB (Venkateshwarlu and

Chalapathi-Rao, 2013).

The history of the nuclei prior to the unification of the NIB/SIB is equally important.

Paleomagnetic data from both the Dharwar craton and the northern block of the Southern

Granulite Terrane (SGT) demonstrate that they were contiguous since at least 2367 Ma (Halls et al., 2007; Belica et al., 2014; Dash et al., 2013; Pivarunas et al., 2018). Geochronologic considerations (Meissner et al., 2002; Clark et al., 2009) suggest these areas were conjoined as early as ca. 2500 Ma (see also Meert et al., 2010). Coeval paleomagnetic and geochronologic results from the Dharwar and Bastar cratons guarantee their status as near neighbors – if not unity – by 1880 Ma (French et al., 2008; Meert et al., 2011; Belica et al., 2014; Radhakrishna et al., 2013), although an earlier amalgamation (Rajesham et al., 1993; Santosh et al., 2004) and a later quasi-separation (Santosh et al., 2004) were proposed.

The timing of Singhbhum unification with the Bastar/Dharwar/SGT triumvirate is less clear. With respect to the immediately adjacent Bastar craton, Mishra et al. (2011) proposed a

Neoarchean connection between the two nuclei. That age is also tentatively supported by a semi- continuous gneissic fabric (Chetty, 2014), an approximate age match of dyke swarms (Sm-Nd age from the Bastar and Pb-Pb age from the Singhbhum; Srivastava et al., 2018) and fiat (Meert et al., 2011; Basu and Bickford, 2015). From the perspective of the Dharwar craton, the

Singhbhum/Dharwar synchronously hosted dyke intrusion in the Paleoproterozoic at 2250 Ma

(Demirer, 2012; Srivastava et al., 2016), with a later 1790-1760 Ma intrusive phase also represented in both cratons (Demirer, 2012; Shankar et al., 2017; Soderlund et al., 2018). While similar ‘bar-code’ (Bleeker and Ernst, 2006) patterns of intrusions is considered as a strong indicator of unity, geochronological data sans paleomagnetic confirmation of proximity results in some ambiguity when considering larger-scale reconstructions.

3.1.2 Geologic Setting

The 40,000 km2 Singhbhum craton is the most northerly of the South Indian Block nuclei

(Figure 3-1). It is bordered on the north by the Chhotanagpur Granite-Gneiss Complex, an easterly section of the CITZ, and on the east by Himalayan orogeny alluvium. The southern border of the Singhbhum craton is composed of Paleozoic metasedimentary rocks of the

Mahanadi Rift separating it from the Bastar craton to the southwest, and older Neoproterozoic

Rengali Province granodiorite gneisses and supracrustal rocks of the Eastern Ghats orogenic belt to the southeast (Sharma, 2009; Dey et al., 2017; Mukherjee et al., 2017).

The four major Singhbhum craton units: the Older Metamorphic Group (OMG), Older

Metamorphic Tonalite Gneisses (OMTG), Iron Ore Group (IOG), and Singhbhum Granite complex (SG) yield overlapping Paleoarchean ages (Miller et al., 2018, and references therein).

The oldest rocks of the Singhbhum craton are orthoquartzites of the OMG with ages at 3628 ± 72

Ma (Goswami et al., 1995). The oldest igneous rocks occur as a dacitic lava interbedded in the

Southern IOG with an age of 3506.8 ± 2.3 Ma (Mukkopaday et al., 2014). The metasedimentary

rocks and amphibolites of the OMG and tonalite-trondhjemite gneisses of the OMTG occur as small enclaves within the Singhbhum Granite, which makes up the major portion of the

Singhbhum craton nucleus. The Iron Ore Group exists as three volcano-sedimentary basins surrounding the east, south, and west sides of the Singhbhum Granite complex. Secondary units such as Dhanjori volcanics, Simlipal Basin, and Dalma volcanics were added to the craton from

Neoarchean to Proterozoic time (Mahadevan, 2002; Misra and Johnson, 2005; Bhattacharya et al., 2015).

The Archean lithologies of the Singhbhum are all cut by a dense array of dykes known as the ‘Newer Dolerites’, shown in green on Figure 3-2. The Newer Dolerites fall into at least 4 pulses with well-determined ages: 2800 Ma, ~2760 Ma (Kumar et al., 2017), ~2260 Ma

(Srivastava et al., 2018), and ~1770 Ma (Shankar et al., 2017). Until these recent data, geochronological control on the emplacement ages for the Newer Dolerites was poor, with the

K-Ar system yielding (roughly grouped) ages ranging between 2200-900 Ma (Naqvi and Rogers,

1987; Srivastava et al, 2000; Mukhopadhyay, 2001; Bose, 2008). Srivastava et al. (2018) separates the Newer Dolerites may into as many as 7 swarms, with the latest dyke activity occurring in the late Paleoproterozoic or later. The overall framework given in the latest work is based on the 4 precise ages, as well as cross-cutting relationships inferred from satellite imagery.

Inherent to this approach is an assumption that dyke trends are diagnostic for age. This is a good first-order approximation given the difficulties of sampling every single dyke in a craton for geochronology, however, we do emphasize that dykes from the same intrusive event may have different trends than ‘expected’.

The Newer Dolerite dyke swarm intrudes the Singhbhum basement rocks at a remarkably high concentration, and vary in modal composition from mafic to intermediate. They vary

between tholeiitic and alkali dolerites (Srivastava, 2000; Bose, 2008; Mir et al., 2011). The size of individual dykes range from 5-50 m, with small dykelets regularly off shooting the main body

(Mir et al., 2011; Katusin, 2017). Larger dykes fall into two main trend groupings, crosscutting, at N-S to NNE-SSW and NW-SE to WNW-ESE (Meert et al., 2010; Mir et al., 2011). Field observations show that the roughly northerly-trending dykes are generally the oldest.

3.1.3 Prior Paleomagnetic Work

The paleomagnetic database for the Singhbhum craton is sparse, with only three studies

(Verma and Prasad, 1974; Kumar and Bhalla, 1984; Das et al., 1996) exploring this area until recently (Kumar et al., 2017; Shankar et al., 2017).

The earliest study of Verma and Prasad (1974) examined dykes of the Newer Dolerite dyke swarm. Rudimentary low-temperature thermal magnetic cleaning on their specimens exposed three groupings of directional paleomagnetic data (Verma and Prasad, 1974). Age constraints on the dykes, however, were poor to non-existent at the time of their study.

Other early paleomagnetic studies were done on rocks of the Iron Ore Group by Kumar and Bhalla (1984) and Das et al. (1996) with mixed success. However, recent paleomagnetic work on the Singhbhum focused on precisely dated mafic dykes (Shankar et al., 2017; Kumar et al., 2017).

Shankar et al. (2017) combined Pb-Pb baddeleyite dating with paleomagnetic work on

WNW-ESE trending members of the Newer Dolerite dyke swarm. Ages recovered on two

WNW-trending dykes defines a remarkably precise age of 1765.3 ± 1.0 Ma (Shankar et al.,

2014). Stepwise demagnetization with primarily AF treatment revealed a consistent northwesterly-directed negative shallow remanence with a mean direction of D=329°, I=-23°

(α95=9°). The paleomagnetic data of Shankar et al. (2017) were broadly consistent with a grouping recognized in the low-temperature results of Verma and Prasad (1974). Secondary

components of magnetization often representing a significant fraction of total sample magnetization were removed (see Figure 3 of Shankar et al., 2017) but details as to the paleomagnetic directions of these overprints were not reported. The mean direction yields a paleomagnetic pole at 45° N, 311° E (A95 = 7°). These data are supported by a positive baked contact test and are considered by the authors to represent the first “key” paleopole for the 1765

Ma dyke swarm (Shankar et al., 2017).

Kumar et al. (2017) focused on NNE-SSW trending dykes in the Singhbhum craton, which field relationships suggest to be the eldest dykes in the craton. Pb-Pb baddeleyite dating confirmed this suspicion, with an exact age of 2762.4 ± 2.0 Ma (Kumar et al., 2017). Distinctly older and younger ages were also recovered in this study (~2800 Ma and ~2852 Ma), although the younger age may simply represent protracted dyke emplacement, as is seen in the Dharwar craton and Southern Granulite Terrane (Halls et al., 2007; Pivarunas et al., 2018; Soderlund et al., 2018). Their stepwise demagnetization revealed a steep, dual polarity remanence which was consistent with the group N.D. 2 results of Verma and Prasad (1974). A reversals test on the data was reportedly positive with an “Rc” classification (McFadden and McElhinny, 1990), however, a correct recalculation of their reversals test with yields an “indeterminate” classification. A modified Bayesian reversals test (Heslop and Roberts, 2018) provides “positive support” for a common mean between the antipodal directions. The mean direction reported – with the 3 reverse polarity sites inverted – has D=226°, I=84° (α95=6°), when combined with the

N.D. 2 data of Verma and Prasad (1974). The paleomagnetic pole calculated from this mean direction falls at 14° N, 78° E (A95 =11°), and is considered by the authors to be primary for

Neoarchean dyke emplacement in the Singhbhum (Kumar et al., 2017). However, a primary,

Neoarchean origin of this steep magnetization has not been proven. Therefore, field tests are crucial.

Here, we present new, geographically widespread paleomagnetic data and detailed stability tests from a wide array of dykes from Singhbhum craton.

3.2. Methods

A total of 98 sites (800+ samples) were collected from 85 dykes throughout the northern and southern Singhbhum craton over the course of three field seasons (Figure 3-3). Sampling was targeted on unambiguously in-situ dyke outcrops, which tended to be in rivers due to deep tropical weathering in the area. Samples were collected from host rocks – typically granites and gneisses – at sites suitable for baked contact tests. Paleomagnetic samples were collected in the field with a water-cooled, gasoline-powered drill and oriented with magnetic and sun compass.

Oriented hand samples were also taken where drilling was unfeasible. All samples were returned to the University of Florida where they were trimmed to a standard size (after drilling, in the case of oriented hand samples). Natural remanent magnetization (NRM) directions were measured using either a Molspin spinner magnetometer or 2G-77R cryogenic magnetometer. Matching pilot specimens (i.e. from the same sample) from all sites were demagnetized by thermal and alternating field (AF) methods using either an ASC TD-48 thermal demagnetizer, homebuilt AF demagnetization apparatus, or DTech 2000 AF demagnetizer. Subsequent demagnetization schemes for remaining samples from sites were optimized based on results from the pilot samples. The demagnetization technique employed was extremely important, given the complicated thermochemical history of Singhbhum craton (see Results and Discussion). After complete demagnetization, paleomagnetic vector directions were recovered with principal component analysis (Kirschvink, 1980) using IAPD software (Torsvik et al., 2016). Within-site analysis and statistical analysis of directions was done using both IAPD and the PmagPy

software package. Rock magnetic data were collected from samples at selected sites with a KLY-

3S Kappabridge with a CS-3 furnace attachment and/or a vibrating sample magnetometer (VSM) in order to more fully characterize magnetic signal carriers in the samples.

3.3. Results

A total of 75 sites (62 individual cooling units) recorded stable paleomagnetic directional data. Paleomagnetic sites from this study can be geographically subdivided into northern (Figure

3-3a; near Jamshedpur) and southern (Figure 3-3b; near Keonjhar) areas of the Singhbhum craton. Stable paleomagnetic data was recovered from dykes of various trends, and paleomagnetic directions were not perfectly correlated with dyke trend.

3.3.1 Magnetic Overprints

Paleomagnetic data from the Singhbhum craton often contained multiple components of magnetization. Some of these overprint directions also displayed reasonable grouping (Table 3-

1; Figure 3-4), although others did not. The most notable low-temperature component in

Singhbhum craton was a northwest-directed, shallowly inclined direction (Figure 3-4). When found in combination with other directions, this component was typically removed by 350° C.

Curie temperature analysis also confirmed a sharp drop at ~330° C pointing to pyrrhotite as the magnetic carrier. (Figure 3-4). Separation of magnetic components during demagnetization was best accomplished by thermal demagnetization. Overprinting of dykes was most focused in the northern part of Singhbhum craton, evidenced by dykes where only low-temperature and/or low coercivity characteristic remanent directions were present.

3.3.2 NW-Shallow Reverse Magnetization

We found a northwest-declination, shallow up-inclination magnetization in 6 dykes (6 sites) within Singhbhum craton (Table 3-1). This is consistent with the paleomagnetic results of

Shankar et al. (2017), who considered this a primary paleomagnetic pole for 1765 Ma. We

caution that the early results were from a limited subset of individual dykes, with multiple sites from the same dykes treated as separate. Northwesterly, shallow magnetic direction are prevalent in Singhbhum craton as pyrrhotite-associated overprints (Section 3.1), however, this magnetic signature also occurs as a characteristic remanence in dykes. When this magnetization occurred as the primary component within dykes, Curie temperature analysis showed it was carried by magnetite (Figure 3-5). This remanence was isolated in these dyes after the removal of typically scattered) low-medium temperature/low coercivity components (Figure 3-5). However, two of the three WNW-trending dykes this direction occurred in carried a completely different surviving magnetization in their fine-grained margins. Great-circle analysis was required in one case to determine a mean direction (I1729). This dyke, which was NNE-trending, was next to another dyke that had a strong overprint WNW-shallow inclination overprint.

A baked contact test was carried out at a small northerly-trending dyke (I172) near the

Khanjhari Reservoir in close proximity to a large, WNW-trending 1765 Ma dyke (I1637+I171)

(Figure 3-6a). Veins of pyrrhotite are visible within core samples of this dyke. After the removal of un-grouped components, a consistent paleomagnetic remanence was found within the dyke at D=330°, I=-18° (k=174, α95=3.5°). Two samples from the granite beside the dyke overlapped this direction with D=327°, I=-27°. Away from the dyke, unbaked granite had a distinctly different direction at D=358°, I=20°, rendering this a classical, positive baked contact test. Due to the proximity of the much larger WNW-trending dyke (I1637+I171) – of known

1765 Ma age – this baked contact test is crucial for determining the true age of the NW-shallow remanence. Given the intrusive relationships visible in the field, the NW-shallow up-inclination remanence from the northerly-trending dyke I172 resulted from the emplacement of the large

WNW-trending dyke and subsequent thermal disturbance. Additionally, samples from WNW-

trending dyke at the contact with the northerly-trending dyke show directions travelling away from their NE-shallow up-inclination direction (Section 3.3) and toward the NW-shallow up- inclination remanence of the northerly-trending dyke I172 (Figure 3-6d). Other samples from site I1637 also moved to the same NW-shallow direction after a combination of both alternating field and thermal demagnetization. Thus, these parallel lines of evidence suggest that the NW- shallow paleomagnetic direction is primary, and the NE-shallow magnetic component is from a later remagnetization event.

At a WNW-trending dyke (I176) previously investigated paleomagnetically, a positive baked contact test was reported (K10; Shankar et al., 2017). This dyke is a few kilometers to the northeast of site I172 and the Khanjhari Reservoir. We resampled this dyke to try to collect a detailed profile of magnetic directions from dyke interior, to margin, and out into the host granite. Samples from the coarse-grained interior of the dyke contained a southwesterly steep, down-inclination direction (3.2.4), D=235°, I=+63° (k=27, α95=8°) that was removed at moderately-high temperatures and coercivities. After removal of this component, a stable NW- shallow up-inclination component was recovered with D=348°, I=-20° (k=18, α95=14°). A well- exposed contact allowed for the collection of marginal dyke samples as well as granite directly at the intrusive contact. These samples displayed a completely different magnetization from the dyke interior, with a paleomagnetic direction at D=193°, I=+80° (k=17, α95=15°). One sample at the contact behaved differently with different demagnetization treatments, showing a univectorial thermal decay and a steep (D/I: 113°/+89°) direction, while a shallow component

(D/I: 338°/-9°) was recovered with alternating field demagnetization after the removal of a low- coercivity steep component. Unbaked samples of granite behaved fine individually upon magnetic cleaning, but decayed to random directions. Therefore, we see that the granite in this

area is not magnetically stable. Considering the inverted magnetic behavior at the contact and unstable granite, we cannot confirm the earlier baked contact test here. Other WNW-trending dykes (I1720 and I1734) previously sampled (Shankar et al., 2017) exhibited a high-coercivity

NW-shallow component, but behaved quite differently at their magnetically-stable margins, and under high-temperature demagnetization. These tradeoffs between steep-inclination magnetizations and the NW-shallow magnetization shows that steep magnetizations, in many cases, post-date the 1765 Ma NW-shallow paleomagnetic signature.

The mean direction from the dykes we sampled (results combined with previous data) is well-grouped and falls at D=329°, I=-12° (k=42, α95=14°). When combined with the other data

(Table 3) from Shankar et al. (2017), the mean paleomagnetic direction is D=331°, I=-21° (k=31,

α95=9°). This yields a paleomagnetic pole at 47° N, 309° E (k=45, A95=7°). Although our re- sampling could not replicate the baked contact test previously reported (I176 + K12), we did recover a positive baked contact test in a different location (I172 + K10?). Another interesting feature of this paleomagnetic direction is the lack of reversals. If we use the ‘A95 envelope’ scheme of Deenen et al. (2011) to test the averaging of secular variation, we find that our pole does fall inside the envelope (7°–21°), which indicates that it has averaged secular variation regardless.

3.3.3 Steeply-Inclined Dual Polarity Magnetic Data

We saw in the previous section that the NW-shallow component has partially remagnetized dykes that have a steep paleomagnetic component. This is the most ubiquitous stable characteristic magnetic direction in Singhbhum – a steep to moderately-inclined dual- polarity magnetization. Notably, three of the dykes we sampled (4 sites) were reported on by

Kumar et al. (2017) with two directly dated as Neoarchean (Table 3-2; Group 1). 36 dykes (42 sites) exhibited paleomagnetic directions consistent with this general grouping (Table 3-2). This

paleomagnetic direction occurs not only in north-northeast-trending dykes, as found by Kumar et al. (2017), but also in west-northwest-trending dykes. We therefore immediately see that the age-trend relationship claimed in previous studies breaks down on closer examination. This component was typically isolated after the removal of lower temperature/coercivity components

(Section 3.1; Figure 3-7). This remanence is carried by magnetite (Figure 3-7), although different dykes with this magnetization exhibit quite different rock magnetic properties. Many dykes had non-reversible susceptibility-temperature curves which indicates alteration of the specimens (Figure 3-7f). A number of sites, however, did show more uncomplicated demagnetization (quasi-univectorial) as well as reversible heating/cooling curves. This component was isolated at a wide range of temperatures and coercivities.

3.3.3.1 NNE-trending dykes with intermediate-steep dual polarity magnetizations

There were 21 NNE-trending dykes (22 sites) throughout Singhbhum craton that yielded intermediate-steep, dual-polarity data (Table 3-2; Group 2). These data closely resemble those of Kumar et al. (2012), who also sampled north-northeasterly trending dykes. The majority have north or south declinations and have steep dual-polarity inclinations. However, there is more directional variability within these NNE-trending dykes than previously reported. Since the emplacement ages of these NNE-trending dykes is well-established, the key need is to assess the relationship of the magnetization age with the emplacement age.

To test this, we carried out a baked contact test on a 9-meter wide northerly-trending dyke (site I178) parallel to a dyke (site K7) paleomagnetically sampled by Kumar et al. (2017).

Although we were able to sample site K7 for standard paleomagnetic analysis (our site I177), exposures of the country rock did not permit a baked contact test to be attempted. Instead, we sampled a parallel dyke just 100 meters to the east, with a more favorable dolerite-granite contact exposed (Figure 3-8b). The intrusive contact of the dyke with the granite was quite sharp.

Samples from the interior and exterior were consistent under both thermal and alternating field demagnetization, with a well-defined mean direction D=188°, I=+68° (k=20, α95=9°). This direction is nearly exactly antipodal to that of the mean direction from the northerly-trending dyke just to the west (K7+I177: D=011°, I=-66°; k = 233, α95=16°). The granites were very weakly magnetized, yielding less well-defined paleomagnetic directions. Samples from the baked and unbaked granite had sample directions similar to that of the dyke (Figure 3-8b), with a mean direction at D=205°, I=+63° (k=16, α95=24°). The overlapping directions from the dyke, baked granite, and unbaked granite indicate a negative baked contact test. Other baked contact tests (Figure 8a,c) also fail to show this paleomagnetic direction is primary.

3.3.3.2 WNW-trending dykes with intermediate-steep dual polarity magnetizations

There were 8 WNW-trending dykes (11 sites) across Singhbhum craton that yielded intermediate-steep, dual-polarity data (Table 3-2; Group 3). As seen in the stereoplot of the mean dyke directions (Figure 3-7), many of these data have less steep magnetic inclinations than many of the north-northeasterly trending dykes, and also tend to have easterly declination down- inclination results. However, results do overlap between dykes of NNE and WNW trend.

A baked contact test at site I1427 on a small (5 m wide) WNW-trending dyke in the southern Singhbhum craton provided a field test on its magnetic stability (Figure 3-9a). Samples from the dyke showed a consistent univectorial direction at D=66°, I=+78° (α95=8°). Samples from the contact itself, including some mixed dyke/granite specimens had a mean direction of

D=255°, I=+87° (α95=15°). Samples from out of the ‘baked’ zone, i.e. more than 5 meters away from the 5 meter dyke continued to move away in declination from the dyke samples with a mean ‘unbaked’ direction at D=288°, I=+79° (α95=8°). This baked contact test is hard to interpret due to the steep remanence, however, the dyke and unbaked samples do not overlap, which may indicate that this baked contact test is positive. Bulk magnetic susceptibility values

decrease from the dyke interior to the unbaked samples, which is also suggestive of a positive baked contact test. Another baked contact test (Figure 3-9b), however, shows no evidence of a baked contact.

3.3.3.3 Steep magnetizations in host rocks and dyke margins

Since our sampling focused on providing field tests for magnetic stability in the form of contact tests, we often sampled in the granite and gneiss host rocks along dyke margins. At several of these locations, the dyke themselves were not magnetically stable, but the country rock yielded consistent, stable steep, down-inclination paleomagnetic directions. At other locations, the interior and exterior of dykes had contrasting paleomagnetic behavior, with steep directions corresponding with altered margins. We highlight one of these situations in detail to show the complexity of magnetic behavior in Singhbhum craton, and the odd interplay between magnetic directions (Table 3-2; Group 4).

We paleomagnetically sampled a WNW-trending dyke (I176) in the southern Singhbhum craton with a well-preserved contact with different phases of granite (phaneritic and megacrystic). A positive baked contact test was reported by previous workers (K10; Shankar et al., 2017) on this dyke. Interior dyke samples showed a roughly similar paleomagnetic direction as previously reported from this dyke, which will be discussed in more detail below. However, samples from the dyke margin and immediately adjacent granite (<5 cm away) carried a steep magnetization with D=173°, I=+79° (k = 20; α95=16°). Interior dyke samples had a medium- temperature/coercivity component with D=231°, I=+63° (k = 27; α95=8°) that was removed before the shallow component. The interplay between the shallow and steep directions at this dyke is complex. However, the chilled margins of dykes is generally argued to carry a more stable magnetization (Halls, 2008), and the behavior of the granite is consistent with a partial

baked contact test. This seems to indicate that the steep magnetization pre-dates the NW- shallow magnetization.

3.3.3.4 A unified model for Singhbhum intermediate-steep paleomagnetism

The data discussed above can be separated into two major groupings (Figure 3-10), shown in blue and orange. Both include precisely dated dykes; the east-west declination, intermediate-inclination data contains a 2800 Ma dyke, while the north-south declination, generally steeper-inclination data contains several dykes from the 2762 Ma swarm (Kumar et al.,

2017).

Both datasets pass a reversals test with a “C” classification (McFadden and McElhinny,

1990). Calculating a pole from the E-W intermediate results gives 24° N, 134° E (k=22,

A95=9°). A positive baked contact test at I1427 lends support to this pole being primary. Rock magnetic results, however, indicate alteration of the magnetic mineralogy of the dykes.

Furthermore, the majority of results with this magnetic signature are from WNW-trending dykes.

From the N-S steep-intermediate results, we get a pole at 17° N, 081° E (k=10, A95=8°). This includes paleomagnetic data from previous workers (Verma and Prasad, 1974; Kumar et al.,

2017). The baked contact test at site I178 is negative, which indicates this direction is likely not primary.

While these paleomagnetic data are roughly correlative with the trend of the dykes, they do not follow it as a rule. This either indicates that these magnetizations either encompass dykes of different ages, or that the trends of these dykes are not diagnostic – possibly both.

3.3.4 Dual Polarity NE-SW Shallow Magnetization

A dual-polarity, northeast-southwest, shallow-inclination paleomagnetic direction (Table

3-4) was recovered from 11 dykes (12 sites) throughout both north and south Singhbhum craton.

Demagnetization removed minor low temperature components (Figure 3-11a,b) before decay to

the origin. Magnetizations of samples with this component were removed at variable temperatures and coercivities. Rock magnetic tests indicate unaltered magnetite as the typical carrier of this remanence in high temperature/coercivity magnetic component sites (Figure 3-11), although some sites (e.g. I1449) were more altered. These were typically recognized in the directional data analysis by demagnetizations that were completed by 350° C – more typical of the Curie temperature of pyrrhotite (Dekkers, 1991). These sites were all in the northern part of

Singhbhum craton. The reversal test of McFadden and McElhinny (1990) on the two groups results in an ‘indeterminate’ classification. This is likely most likely due to the limited number of dykes displaying this paleomagnetic direction. A Bayesian test for a common mean (Roberts and Heslop, 2018) does indicate weak support for a common mean direction. The mean direction with the southeast-directed directions inverted falls at D=51°, I=-5° (k=15, α95=13°).

A baked contact test was performed at a large WNW-ESE trending dyke (site I1637), one of a pair of prominent parallel dykes in the southern Singhbhum craton dated at ~1765 Ma (Pb-

Pb baddeleyite; Shankar et al., 2014). Notably, this site corresponds with site K11 of Shankar et al. (2017). Our samples from the dyke interior and margins have a high magnetic intensity and show consistent magnetic behavior, with scattered low-temperature/coercivity components removed by 125° C (Figure 3-12). The mean high-temperature direction for this dyke

(combination of 3 sites) is northeast and shallow at D=50°, I=-11° (k=76; α95=5°), consistent with our other results from around the Singhbhum craton. A mixed granite/dyke sample from the exact intrusive contact was included in this mean; granitic samples moving away from the dyke move progressively away from this mean: D/I = 26°/-16.1° at 9 cm away and 27°/+11° at 6 m away (Figure 3-12). Unbaked granites from a different baked contact test 10s of meters away retain a stable magnetization (Figure 3-11). The paleomagnetic signals from the dyke, baked

zone granites, and unbaked granites, along with decreasing bulk magnetic susceptibility profile from the interior of the dyke through the host rock (Figure 3-12c), seem to delineate a positive baked contact test on this direction.

Due to the disparity of our early results with those of Shankar et al. (2017), we sampled the same area again (site I171). Our results were nearly identical to our previous sampling when using thermal demagnetization (Figure 3-12). Notably, however, we saw different magnetic behavior based on which demagnetization technique was used: AF or thermal. Alternating field demagnetizations travelled along great-circles toward a NW-shallow up-inclination direction

(Figure 3-10). This difference was particularly pronounced at samples taken directly at the contact of the large WNW-trending dyke (I1637+I171) with a northerly-trending dyke (I172). A baked contact test at the northerly-trending dyke, as presented above, showed a clearly positive baked contact test on the NW-shallow direction resulting from thermal disturbance during the emplacement of the large 1765 Ma WNW-trending dyke. Because of this, we performed alternating field demagnetizations on the I1637 thermal data, which also moved NW-shallow.

Thus, the NE-shallow direction is demonstrably a magnetic overprint dating from after 1765 Ma.

The paleomagnetic pole calculated from the 10 dykes (11 sites; 4 inverted) with this direction falls at 34° N, 196° E (k=20, A95=11°). This pole likely averages secular variation as evidenced by the presence of reversals, along with its fit within the “A95 envelope” (7° – 19°) in this instance of Deenen et al. (2012).

3.3.5 Easterly, Intermediate Reverse Direction

8 sites (5 dykes) display an easterly-declination, intermediate inclination dual-polarity paleomagnetic direction (Table 3-5a). We combine a number of sites at Bhima Kunda (I1435,

I1642, I1644, and I1714) in reporting this mean direction due to their relationship as a single cooling unit. Demagnetization isolated this direction after the removal of sometimes significant

low-medium temperature/low coercivity components (Figure 3-13). This magnetic direction was sometimes univectorial – at site I1635 for instance. Demagnetization and rock magnetic experiments indicate this remanence is carried by magnetite (Figure 3-13f), although some alteration does occur on heating. Typically, the best-resolved samples which had this magnetic component also had good rock magnetic behavior, while more overprinted samples (such as much of the data from Bhima Kunda) had less ideal rock magnetic behavior. The mean direction from these dykes falls at D=90°, I=-47° (k=24, α95=16°)

Details as the relative age of this magnetization, as well as a baked contact test opportunity, were provided by the area around a large (10 meter) WNW-trending dyke (sites

I1642, I1714, and I1715) at the Bhima Kunda river section (Figure 3-14). The gneiss around the dyke was cut by cm-scale dykelets, with a larger apophysis from the dyke itself (I1644) also paleomagnetically sampled. These sampling areas carry a substantial magnetic overprint, well- defined at D=356°, I=+12° (k=24, α95=10°), isolated at temperatures up to 400 °C, which is identical to the magnetization preserved in the unbaked gneiss farther from the dyke. Although some samples from these sites only preserved random low-coercivity directions, the majority of samples from the dyke itself (I1642; I1714; I1715), baked country rock (I1435-d), and a dykelet apophysis (I1644) showed a consistent easterly, intermediate reverse polarity direction. The mean direction from all samples from the dyke itself falls at D=103°, I=-47° (k=29, α95=6°)

(Figure 3-14). After removal of the overprint, the isolation of this magnetic vector sometimes produced some instability at higher temperatures and/or alternating fields (>545°C; 30 mT).

Curie temperature runs showed that magnetite was the primary signal carrier in the dyke and dykelets although samples showed alteration on heating – consistent with the presence of overprint directions and sometimes unstable characteristic direction isolation.

Another baked contact test (I1715) on the same dyke (the ‘large’ Bhima Kunda dyke;

I1435, I1642, I1644, I1714) examined its relationship with a smaller, more northwesterly- trending dyke (‘bridge’ dyke; I1436, I1643) well-exposed in the same river section. The two dykes coalesce just before an abrupt hillslope, and the field relationships do not allow a clear answer as to which dyke cuts which. Given the substantially different paleomagnetic directions recovered from both dykes away from the contact, a paleomagnetic test of their relative age was undertaken. Multiple transects of cores were drilled at the intrusive intersection of the two dykes

(Figure 3-14). A remarkably consistent dataset emerged, showing that the WNW-trending dyke cut and baked the NW-trending dyke. All samples were overprinted (2 completely) by a relatively high-temperature overprint with D=334°, I=+35 ° (k=10, α95=12°), which is consistent with the general ‘far-field’ overprint results from the NW-trending dyke and WNW-trending dyke (Figure 3-14). The underlying magnetization, however, was an easterly, intermediate up- polarity direction consistent with ‘far-field’ results from the WNW-trending dyke.

The easterly paleomagnetic direction preservation at Bhima Kunda shows that this is a surviving primary remanent magnetization, however, the absolute age is uncertain. The virtual geomagnetic pole calculated from this mean direction falls at 10° S, 200°E (A95=16°). This represents a small sampling of dykes from around Singhbhum craton, and requires more data to ensure it has averaged secular variation and can be considered a paleomagnetic pole.

3.3.6 Northerly, Intermediate Normal Direction

Five sites (4 dykes) throughout the Singhbhum craton have an intermediate normal inclination, northerly declination paleomagnetic direction (Table 3-5b). The site (I148) in the northern Singhbhum craton carries a consistent NW-shallow overprint D=314°, I=-12° (α95=10°) up to moderate temperatures (Figure 3-12a), while the other sites in the southern Singhbhum craton exhibit only minor low-temperature overprints (Figure 3-13b). The remanence is carried

by magnetite, but shows alteration on heating (Figure 3-13f). The mean direction from these six dykes falls at D=352°, I=50° (k=30; α95=17°) (Figure 3-13c).

4/5 dykes trend NNE-NE, only the dyke sampled at I148 trends northwest. The virtual geomagnetic pole calculated from these directions is at 77° N, 50° E (k = 30, A95=20°). No favorable field test contacts were exposed for baked contact tests. This direction is steeper than the present day field (PDF) in the region.

3.4. Discussion

3.4.1 Magnetic Relationships within Singhbhum Craton

As seen from the magnetic data discussed above, there are two main paleomagnetic directions recovered from Singhbhum Craton in previous paleomagnetic studies of dykes of

~2762 Ma and ~1765 Ma (Kumar et al., 2017; Shankar et al., 2017), which yielded, respectively, steep dual-polarity and shallow up-inclination data. These reports argued for a simple magnetic story for Singhbhum craton. Our work, however, shows that this simple story is incomplete.

First, we discuss the paleomagnetic data from the Neoarchean dykes (3.2.1). The work on these dykes by Kumar et al., (2017) considered the steep direction to be primary based on several arguments: dual-polarity magnetization, passage of a reversals test, and no amphibolite- grade metamorphism in the craton post-dyke-emplacement (Nelson et al., 2014). In relation to the first evidence, we point out that remagnetizations can be of dual-polarity. Given the geologically “instantaneous” nature of a reversal, it would indeed be remarkable if many protracted remagnetization events did not record a reversal. We also want to point out that a reversals test, in any iteration (Merrill and McElhinney, 1990; Heslop and Roberts, 2018), merely tests for a common mean. Also, although we agree that tectonothermal events in the

Singhbhum may not have brought the cratonic interior above the blocking temperature of magnetite (Nelson et al., 2014; Kumar et al., 2017), remagnetization can take place at lower

temperatures over extended periods (Pullaiah et al., 1975) or as the result of fluid flow

(Geissman and Harlan, 2002). In their petrographic examination of the dykes, (Kumar et al.,

2017) noted alteration of both pyroxene and plagioclase. The alteration of the dykes, in fact, has been previously attributed to hydrothermal activity (Sengupta et al., 2014). Various lines of rock magnetic evidence – demagnetization spectra and Curie temperature analysis – also suggest that many of the steep magnetic data are from altered rocks. The multiple separate intrusion events in Singhbhum craton post-dating the Neoarchean each would provide another opportunity to thermally alter the Neoarchean dykes, as well as refocusing and concentrating fluid flow.

Multiple baked contact tests (discussed extensively in Section 3.2) sometimes provide weak evidence that the steep direction in northerly-trending dykes is primary, but they are seldom unquestionably convincing. Another interesting point is that steep paleomagnetic directions are recovered from dykes of various trends and from dyke margins with contradictory interior directions across Singhbhum craton. In other words, they are not solely restricted to northerly- trending Neoarchean dykes as simply envisioned in previous work (Section 3.2). Indeed, given the multiple episodes of Paleoproterozoic dyke emplacement delineated by Srivastava et al.

(2018), there are likely various pulses of activity represented in the total intermediate-steep paleomagnetic directional dataset. These magnetizations may have little to do with the emplacement ages of the dykes!

However, some of the steeply-inclined, dual-polarity paleomagnetic data exhibit

‘excellent’ rock magnetic behavior, with hard-shouldered Curie temperature curves and high temperature/coercivity components of magnetization. If we break the intermediate-steep data into the two groupings proposed in Section 3.2.4, an argument can be made for two separate primary Neoarchean remanences at 2800 Ma and at ~2762 Ma. However, given the

hydrothermal alteration pervasive in Singhbhum craton, as well as the complicated relationship with the NW-shallow magnetic remanence, attributing all steep magnetic remanence in

Singhbhum craton to Neoarchean times is premature. Indeed, given the multiple negative baked contact tests, it is incorrect.

The easterly, intermediate up-polarity direction – with one antipodal direction – also has an interesting relationship with the current paradigm of dyke ages and trends. At Bhima Kunda, it is seen in a WNW-trending late-stage dyke, however, it is highly altered and overprinted.

However, it is definitely younger than a NW-trending dyke at the same outcrop as shown by a detailed baked contact test (I1715). At a different site, on a different, northerly-trending dyke

(I1635), in a different area, this direction displayed only minor overprinting, stable high- temperature/high coercivity magnetic behavior (Figure 10), and exhibited a reversible Curie temperature heating/cooling curve. The dyke at site I1635 is cross-cut by the large 1765 Ma dyke, which suggests that this remanence is older than 1765 Ma. Given that the current paradigm based on geochronological and satellite imagery is that the WNW-trending dykes in

Singhbhum craton are part of the ~1.77 Ga Pipilia swarm (Srivastava et al., 2018), and that we have relatively reliable paleomagnetic results for dykes of this age, this deserves further scrutiny.

This paleomagnetic grouping of dykes also underscores that dyke trends, especially the trend of smaller dykes within the craton, may not correlate well with broad trend-as-age determinations.

The northwest shallow, reverse polarity magnetic direction occurs in multiple places throughout Singhbhum craton. We have shown that this magnetic component is the primary signature for the 1765 Ma dyke swarm, agreeing with Shankar et al. (2017), but also likely results from the growth of a high-magnetic-coercivity phase (pyrrhotite in this case) during the

emplacement of that dyke swarm. This has the advantage of also explaining the regional preponderance of NW-shallow magnetic overprints, typically associated with pyrrhotite.

A detailed look at the densely emplaced dykes around the Kanjhari Reservoir (Figure 3-

15) provides examples of almost every paleomagnetic remanence present in Singhbhum Craton.

In this area, a large dyke dated to 1765 Ma (Shankar et al., 2017) and a number of generally older north-northeasterly trending dykes outcrop along an outflow channel south of a reservoir.

Notably, this area provides some possibilities as to the relative age relationships of magnetic remanences tied to magnetic mineralogy. As discussed before, paleomagnetic analysis of the

1765 Ma dyke, which cuts all other dykes in the area, reveals a northeasterly, shallow negative with thermal treatment, but its remanence trends along a great-circle towards northwesterly- shallow directions with alternating-field treatment. Another northerly-trending dyke (I174) exhibits the same split-behavior, with sharp drops at 350 °C moving its magnetic remanence from a northwesterly shallow to a steep direction. Alternating field however, does not dispel the northwesterly direction. A small northerly-trending dyke (I172) sampled within the ‘baked’ zone of the 1765 Ma dyke is the only site in the area to show a northwest-shallow direction after both thermal and alternating field treatment.

The best conclusion from the available evidence is that the northwest-directed shallow inclination magnetization dates to 1765 Ma. This view is supported by the baked contact test, and the combination of alternating field and thermal demagnetization techniques.

Accompanying the acquisition of this magnetization, the growth of new magnetic minerals took place – principally high-coercivity pyrrhotite – carrying a northwesterly-shallow remanence.

This explains the pyrrhotite components seen in I174 (and other places throughout Singhbhum craton), both of which are dispelled by thermal but not by alternating field demagnetization.

This magnetic mineral growth event may have been due to the 1765 Ma swarm being particularly sulfur-rich, and been focused along older planes of weakness during fluid movement.

In the cases where the northwesterly direction coexists with steep directions (e.g. sites I176,

I1720, and I1734), this paradigm views the steep direction as secondary, and the northwesterly direction surviving as a primary remanence. This is consistent with the susceptibility profiles through the dykes, as well as the alteration along fractured, relatively permeable dyke chilled margins. A similar situation was noted by Halls et al. (2001) in an Uruguayan dyke swarm.

We acknowledge that this is a rather complex magnetic story. However, alternating field magnetization does support this case – we see the removal of a large magnetite component of variable direction followed by survival of a northwesterly-shallow remanence in many AF- treated samples. The removal of large components of remanence before isolation of a preferred direction is well-illustrated in Figure 3 of Shankar et al. (2017). Our site I1734 samples, for instance, show very similar behavior to their shown site K14 samples during AF- demagnetization. However, without details as to the directions that were removed, it is hard to elucidate more information.

3.4.2 Comparison with Other South Indian Block Cratons

With a better understanding of the paleomagnetic data from Singhbhum craton, it is now possible to assess its relationship to the other South Indian block cratons. There are no primary

Neoarchean paleomagnetic data from either Dharwar or Bastar cratons, although metamorphosed dykes of this age do exist in the other cratons (Srivastava et al., 2018). Therefore, accepting the steep direction poles for 2800 Ma and 2762 Ma dykes as primary, there are no options for comparison at this time. We note that the ostensibly ‘Neoarchean’ Singhbhum craton poles are spatially coeval with early Paleoproterozoic poles from Dharwar craton. However, simply based on geochronologic information, the spatiotemporal match is off by several hundreds of

millions of years. Furthermore, it is very likely that both the intermediate-steep magnetizations from Singhbhum craton are not primary.

Athough we do not have a direct age constraint on it, the age of the easterly-intermediate up-inclination paleomagnetic direction and associated VGP at 10° S, 200°E (A95=16°) is relatively constrained as younger than the Neoarchean dykes and older than the 1765 Ma dyke event. Known ages of dyke intrusion (Srivastava et al., 2018) would place this VGP age as sometime in the early Paleoproterozoic. What is its actual age, though? Given the 2250 Ma age constraints on the NE-trending Kaptipada dyke (Srivastava et al., 2018), the best tentative comparison is with the Dharwar craton 2250 Ma pole at 16° N, 109° E (A95=14°) of Nagaraju et al., (2018). We see that these two poles are quite longitudinally mismatched, implying the South

Indian Block was not together at this time. However, this may be a premature comparison due to the uncertain age of this Singhbhum VGP.

Recent work on dykes in Bastar craton (Liao et al., 2019) showed a geochronological and petrochronological match of Dharwar and Bastar at 2370 Ma. Was Singhbhum with them at this time? Current evidence is insufficient to test that particular scenario, but further focus on the geochronological framework of Paleoproterozoic Singhbhum dykes will doubtless answer that question.

Another Paleoproterozoic point of comparison for Singhbhum with Dharwar craton comes at ~1770 Ma. Either paleomagnetic directional option for Singhbhum craton dykes at

1765 Ma – either northeasterly or northwesterly does not temporally match with extant paleomagnetic data from other SIB cratons. The large 1885 Ma swarm (French et al., 2008) has paleomagnetic data from both Dharwar (Belica et al., 2014; Nagaraju et al., 2018) and Bastar

(Meert et al., 2011; Radhakrishna et al., 2013). Preliminary paleomagnetic work on 1794 Ma

dykes (Soderlund et al., 2018) in the Dharwar is needed to allow us to further test Dharwar –

Singhbhum convergence during the 1888 – 1765 Ma interval.

3.5. Conclusions

The Singhbhum craton has a complex magnetic history. With the pervasive occurrence of thermal and hydrothermal alteration within Singhbhum craton all reported Precambrian paleomagnetic directions require rigorous field tests to ensure their stability and primary nature.

Additionally, particularly for small dykes, dyke trends may not be reliably correlative with ages.

Our sampling sites corresponded with recently published paleomagnetic results from the

Singhbhum craton in a number of areas. We feel that further sampling, especially for geochronology, in Singhbhum craton is warranted to interrogate prior results, and conclusively characterize the intricate rock magnetic and demagnetization behavior of the mafic dykes in this area.

The reported “key” 1765 Ma paleopole for the Singhbhum craton (Shankar et al., 2017) is indeed primary. This paleomagnetic direction also appears as an overprint at low temperatures associated with pyrrhotite, but can survive to high alternating fields. It has a complicated relationship, therefore, with other paleomagnetic data from the craton, but it well-established as primary.

The paleomagnetic results of Kumar et al. (2017) on NNE-trending Neoarchean mafic dykes of the Singhbhum craton found a steep, dual-polarity direction. They viewed the magnetization as primary based on what we consider to be unsatisfactory postulates. Our multiple baked contact tests and rock magnetic results show that the survival of primary

Neoarchean magnetizations at both 2800 Ma and 2762 Ma is highly uncertain, and likely these data represent younger remagnetizations.

Figure 3-1. Tectonic sketch map of India, modified from Meert et al. (2010), showing major features from the Precambrian geologic history of India. PCSZ = Palghat-Cauvery Shear Zone, WDD = Western Dharwar Domain, EDD = Eastern Dharwar Domain, PG-R = Prahnita-Godavari Rift, EGMB = Eastern Ghats Mobile Belt, MR = Mahanadi Rift, SIB = South Indian Block. The aggregate South Indian Block is outlined, made up of the Dharwar, Bastar, and Singhbhum cratonic nuclei.

Figure 3-2. Simplified geological map of Singhbhum craton and Google Earth traced dykes (modified after Kumar et al. (2017). Inset map to the top left indicates location of Singhbhum craton within peninsular India: SC: Singhbhum craton, BC: Bastar craton, DC: Dharwar craton, SGT: Southern Granulite Terrane, DT: Deccan Traps, AC: Aravalli craton, BuC: Bundelkhand Craton. Boxes indicate location detailed paleomagnetic site location shown in Figure 3.

Figure 3-3. Paleomagnetically sampled dykes in the (a) northern Singhbhum craton and (b) southern Singhbhum craton, trends as determined via field observations and satellite imagery. Detailed insets of heavily sampled locales are shown: (c) Khanjari dam area and (d) Bhima Kunda area. Radiometrically dated dykes and locations of baked contact tests are shown with stars and squares respectively.

Table 3-1. Overprint directions from Singhbhum craton, most pronounced in north near Jamshedpur Site Lat Lon Trend n Dec Inc a95 k I145 22.60768 86.07898 130 10 105.7 8.6 10.3 22.9 I146 22.63347 86.07445 30 9 189.1 -21.7 5.9 76.4 I148 22.63165 86.05995 130 8 314 -11.9 9.5 35.1 I1410 22.40768 86.14523 150 11 25.3 38.6 12 15.4 I1420 21.56652 85.69683 20 3 71.6 7.5 16.9 54.1 I1427 21.55063 85.66183 140 8 10.1 50.2 11.6 23.6 I1429 21.40388 85.73985 24 10 159.3 5.6 11.7 18 I1433 21.64277 85.65068 5 4 11.2 22.9 22.4 17.8 I1442 21.83168 85.86118 20 5 359.1 23.7 3.1 599.9 I1446 22.53617 85.90393 1 7 299.6 14.4 6.4 89 I1636 21.5921 85.7251 10 10 317.2 22.8 13.3 14.1 I1637 21.5932 85.7263 120 11 61.7 2.9 19.7 6.3 I1639 21.5656 85.7053 30 5 295.8 35.6 12.2 40.4 I1641 21.5526 86.0172 120 8 32.7 19.2 16.4 12.3 I1643 21.5514 86.0173 140 15 304.2 -2.8 24.7 3.4 I1642 21.5519 86.0174 120 21 348.5 8.7 27.2 2.3 I1645 21.5516 86.0184 20 6 358.8 11.2 11.9 32.9 I1647 21.5534 86.0178 140 12 340.5 -19.1 18.1 6.7 I1647 21.5534 86.0178 40 13 295.6 -56.6 26.2 3.5 I1650 21.5205 86.0171 125 8 13.8 27.5 17.8 10.6 I176 21.59828 21.59828 21 12 234.6 63.2 8.4 27.4 I177 21.68422 21.68422 21 12 353.8 -13.4 26.9 3.6 I178 21.68292 21.68292 21 8 25.1 75.1 28.1 4.8 I1718 21.57325 21.57325 21 12 5.5 42.4 16 8.3 I1727 21.35139 21.35139 21 8 357.9 34.3 5.3 110.8 I1730 21.35164 21.35164 21 15 273 -4.8 2.8 189.9 I1732 21.38781 21.38781 21 9 359.6 39.6 5.2 100.5 Notes: Site = name of cooling unit Lat = site latitude, Lon = site longitude, Trend = trend of dyke, n = number of samples, Dec = declination, Inc = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953)

Figure 3-4. General overview of overprint data from Singhbhum craton. Demagnetization curves (a,b) showing drops at ~330°C (Curie temperature of pyrrhotite; Dunlop and Ozdemir, 1997). Curie temperature experiments showing susceptibility versus temperature curves (c) similarly displaying drops at pyrrhotite and alteration on cooling. Well-grouped overprint directions (Table 1) illustrated on a stereoplot. Present-day magnetic field shown as a star. Orange directions illustrate the most prevalent overprint in the Singhbhum craton, and correspond with the data of Shankar et al., (2018). In the northern Singhbhum craton, some overprints were complete (e), while others were only partial (f).

Table 3-2. NW-shallow magnetic component in Singhbhum craton

Site Lat Lon Trend n Dec Inc a95 k VGP VGP Lon Lat (N) (E)

I172 21.5935 85.7269 20 2 333 -19 13 389 49 308

I176 21.5976 85.7467 307 2 323 -16 33 58 43 320

I1720 21.5171 85.9113 297 2 333 -19 13 389 49 308

I174 21.5943 85.7292 10 10 339 -28 10 24 48 297

I1729 21.3513 86.0065 20 16 329 5 GC GC 55 328

K11 21.5932 85.7437 302 10 333 -35 7 56 41 301

K1 21.5516 86.0177 291 10 308 -22 2 482 30 328

K14 21.3725 86.1479 305 10 346 -32 7 45 49 287

K15 21.4302 86.1813 298 8 327 -22 5 129 43 314

SKJ10 21.5492 85.8223 300 10 339 -23 4 216 51 299

K30 21.5969 85.8852 303 10 343 -21 6 65 54 294

Notes: Site = name of cooling unit Lat = site latitude, Lon = site longitude, Trend = trend of dyke, n = number of samples, Dec = declination, Inc = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), VGP Lat = virtual geomagnetic pole latitude, VGP Lon = virtual geomagnetic pole longitude (these last two calculated from directional data and site location)

Figure 3-5. Summary of the NW shallow inclination paleomagnetic direction recovered from the Singhbhum craton. Stereoplot (a) showing the combination of our data (in blue) and that of Shankar et al. (2017; in orange). Representative Zjiderveld plots (b), and Curie temperature experiments showing this remanence is sometimes carried by magnetite, shown by semi-reversible heating/cooling curves (d), with slight altering on heating. This component is also carried by pyrrhotite, as shown in Figure 3.4.

Figure 3-6. Baked contact test on a northerly-trending dyke cut by a 1765 Ma dyke with conflicting paleomagnetic data. The geometry of the sampling shown in (a), with sites I1637 and most of I171 taken from 10s of meters to the west. Summary of resultant directions from dyke I172, showing a beautiful positive baked contact test. Furthermore, samples from the large dyke baked by the northerly-trending dyke get pulled toward the NW-shallow component, but with a interesting disparity between blocking temperature and coercivity.

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Table 3-3. Intermediate-steep dual polarity paleomagnetic results from both NNE-trending and WNW-trending dykes of Singhbhum craton Site Lat Lon Trend n Dec Inc a95 k Group VGP VGP Lat (N) Lon (E) I177+ 21.6842 85.8540 17 2 11 -66 16 263 1 20 258 I1733+ 22.5584 85.8920 20 16 95 66 5 57 1 13 129 I1413 22.4053 86.1475 40 10 110 58 4 123 2 0 134 I1429 21.4039 85.7399 24 9 68 66 4 178 2 30 132 I1441 22.2062 86.1811 25 7 235 -54 10 36 2 -40 330 I1450 22.4230 86.0780 40 10 228 -67 6 67 2 -44 308 I1636 21.5921 85.7251 10 7 327 -81 13 24 2 -7 275 I1639 21.5656 85.7053 30 5 172 -67 6 156 2 -62 255 I1645 21.5516 86.0184 20 6 350 83 7 95 2 36 83 I1647+ 21.5534 86.0178 40 11 342 76 8 34 2 47 74 I1649 21.5539 86.0175 20 7 141 84 6 98 2 12 94 I1651 22.3823 86.0863 40 6 230 78 7 95 2 7 69 I173 21.5944 85.7282 10 11 142 -71 6 64 2 -46 235 I174 21.5944 85.7292 10 10 61 69 10 27 2 34 125 I178 21.6840 85.8540 13 11 192 67 9 26 2 -18 78 I1718 21.5733 85.9943 175 6 151 -78 6 139 2 -41 252 I1721 21.5983 85.8927 10 14 325 -78 4 96 2 -2 279 I1724 21.3678 85.9800 20 15 204 71 3 244 2 -11 72 I1725 21.3643 85.9860 50 15 92 86 2 299 2 21 94 I1726 21.3532 85.9929 30 13 10 73 3 222 2 52 95 I1727 21.3514 85.9952 10 11 258 -42 6 58 2 -19 336 I1728 21.3513 85.9956 0 7 132 81 5 170 2 9 99 I1732 21.3878 85.8436 30 14 157 70 3 243 2 -12 100 I149 22.6214 86.0049 140 7 22 73 6 115 3 51 104 I1426 21.5676 85.6510 140 9 229 68 8 46 3 -5 58 I1427 21.5506 85.6618 140 13 66 78 8 27 3 29 110 I1451 22.4112 86.0751 130 10 84 60 5 99 3 19 139 I1641+ 21.5526 86.0172 120 16 74 66 5 64 3 26 132 I1643+ 21.5514 86.0173 140 4 261 -45 14 46 3 -17 333 I1646+ 21.5534 86.0178 140 7 63 68 6 90 3 33 128 I1650 21.5205 86.0171 125 8 74 74 2 589 3 27 118 I1414 22.5481 86.1586 35 6 237 68 11 40 4 -1 54 I1433 21.6428 85.6507 5 6 91 50 12 35 4 10 146 I176 21.5976 85.7467 307 6 173 79 16 20 4 0 88 I1720 21.5171 85.9113 297 3 95 72 10 148 4 15 120 I1734 21.3725 86.1479 305 5 135 -75 13 38 4 -39 241 Notes: Site = name of cooling unit Lat = site latitude, Lon = site longitude, Trend = trend of dyke, n = number of samples, Dec = declination, Inc = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), Group – trend, age or geologic data groups, VGP Lat = virtual geomagnetic pole

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latitude, VGP Lon = virtual geomagnetic pole longitude (these last two calculated from directional data and site location)

Figure 3-7. Steep paleomagnetic data from Singhbhum craton. Different demagnetization behavior shown in Zjiderveld plots (a,b,d,e). All steep-intermediate data from both NNE-NE-trending and WNW-trending dykes is shown in (c). A rough grouping can be delineated. Purple dots indicated known Neoarchean dykes. Curie temperature analysis prevailingly showed alteration on heating (f).

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Figure 3-8. Paleomagnetic stability tests on NNE-trending dykes in Singhbhum craton. These tests were on relatively large dykes. Test show similar directional data between dykes and host granite. The test in (a) has two samples (1 next to dyke, 1 well away) showing exactly the same direction, rendering this contact likely negative. The (b) test is also conclusively negative, whereas (c) shows somewhat different behavior from granites away from the dyke.

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Figure 3-9. Paleomagnetic stability tests for WNW-trending dykes in Singhbhum craton. Both tests took place in the south of the craton. Both took place on relatively small dykes, with (a) on a 5-meter width dyke and (b) on a 1-meter width dyke. Neither test is conclusive, although (a) is most conclusive. Notably, (b) does not preserve any evidence of baking by the latest dyke. Both WNW-dykes have an intermediate, somewhat easterly mean direction.

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Figure 3-10. Two apparent groupings in Neoarchean paleomagnetic data for Singhbhum craton. Intermediate-inclination data with E-W declinations contains a 2800 Ma dyke, while all dykes of the ~2760 Ma swarm have a steeper-inclination, N-S declination consistent with the results of previous workers. Each of these groups pass a reversals test with ‘C’ classification, although paleomagnetic stability tests are largely inconclusive.

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Table 3-4. NE-SW shallow inclination magnetic components in Singhbhum craton VGP VGP Site Lat Lon Trend n Dec Inc a95 k Lat (N) Lon (E) 1765 - 21.5932 85.7263 120 16 57 8 24 28 195 dyke 11 I1410 22.4077 86.1452 150 8 252 21 8 52 -13 13 - I1415 22.6400 86.0094 1 3 73 7 300 13 190 15 I1419 21.5665 85.6968 130 6 227 -5 11 40 -40 12

I1420 21.5665 85.6968 20 9 60 12 10 28 30 181 I1430 21.3333 85.2867 170 16 206 -5 6 42 -59 28 I1431 21.2293 85.7592 45 5 37 5 9 76 50 198 I1446 22.5362 85.9039 1 8 215 27 2 734 -40 40 I1449 22.6179 85.9994 10 14 66 -7 2 379 20 189 I498 22.5561 86.1521 40 7 218 -2 7 7 -47 21 Notes: Site = name of cooling unit Lat = site latitude, Lon = site longitude, Trend = trend of dyke, n = number of samples, Dec = declination, Inc = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), VGP Lat = virtual geomagnetic pole latitude, VGP Lon = virtual geomagnetic pole longitude (these last two calculated from directional data and site location)

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Figure 3-11. Summary of the NE-SW, shallow inclination paleomagnetic direction recovered from the Singhbhum craton. Representative Zjiderveld plots (a,b), and stereoplot showing dual-polarity site mean directions (c), this remanence is typically carried by magnetite, shown with semi-reversible heating/cooling curves (d).

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Figure 3-12. Baked contact test at site I1637, showing respresentative Zijderveld diagrams and stereoplots of vectors from the central dyke (a), through the chilled margin (b,c), and out into the baked and unbaked (less baked) granite (d,e). Also included is a plot of bulk magnetic susceptibility, with specimens shown in the figure circled, along with a schematic of sampling location for (a – e).

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Figure 3-13. Mean directional data for both intermediate, single polarity groupings, both northerly and easterly (c). Representative Zijderveld diagrams and stereoplots of vectors (a,b,d,f), and Curie temperature data (f) are color coded based on which grouping the mean directions belong to.

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Figure 3-14. Baked contact tests on the easterly-intermediate direction, particularly at Bhima Kunda. Data from the large dyke (and associated dykelets) were quite altered (as seen from susceptibility analysis). Nonetheless, the combination of sampling at separate locations on and around the dyke (a,b) suggests that the direction is primary, albeit altered. This additionally suggests that there are at least 2 phases of northwesterly-trending dyke emplacement, since the ‘large’ dyke has baked the ‘bridge’ dyke, but both trend roughly similarly.

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Table 3-5. Northerly and easterly intermediate single polarity magnetic components from Singhbhum craton Site Lat Lon Trend n Dec Inc a95 k VGP Lat VGP (N) Lon (E) Easterly Intermediate BKBig 21.5519 86.0174 120 21 103 -47 6 30 -21 199 I1635 21.5891 85.7286 10 5 75 -39 7 138 5 202 I1442 21.8317 85.8612 20 5 113 -45 5 253 -29 195 I1437 21.5526 86.0172 55 8 56 -51 3 455 15 219 I1452 22.6135 86.1838 40 8 100 -43 12 23 -18 197 I1730 21.3516 86.0132 0 5 274 58 7 118 16 31 Northerly Intermediate Site Lat Lon Trend n Dec Inc a95 k VGP Lat VGP (N) Lon (E) I148 22.6317 86.0600 130 8 327 59 20 9 57 36 I1443 21.6783 85.8952 10 3 346 36 12 107 77 351 I1447+ 22.5592 85.8760 40 14 2 50 5 78 82 101 I1648 21.5538 86.0176 60 7 8 53 10 36 76 115 Notes: Site = name of cooling unit Lat = site latitude, Lon = site longitude, Trend = trend of dyke, n = number of samples, Dec = declination, Inc = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953), VGP Lat = virtual geomagnetic pole latitude, VGP Lon = virtual geomagnetic pole longitude (these last two calculated from directional data and site location)

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Figure 3-15. Dense dyke emplacement at Kanjhari Reservoir, along with stereoplots of magnetic directions from the dykes here. We recover several stable directions from dykes here.

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CHAPTER 4 ASSESSING THE INTERSECTION/REMAGNETIZATION PUZZLE WITH SYNTHETIC APPARENT POLAR WANDER PATHS*

4.1 Introduction

The Van der Voo quality criteria (Van der Voo, 1990) have guided a general increase in robustness of paleomagnetic results since their inception. The seven guidelines (hereafter VQ1—

VQ7; Table 4-1) provide a useful yardstick to make qualitative assessments of paleomagnetic data. The original formulation (Van der Voo, 1990) grew primarily from observations centered on a relatively well-populated Phanerozoic database. As the Precambrian paleomagnetic database becomes more populated and the Van der Voo criteria continue to be utilized to assess data, a reappraisal of the seven tenets is warranted. Certain of the Van der Voo criteria are more straightforward than others; precise ages satisfy VQ1 and stepwise demagnetization is the standard in modern studies (VQ3). An assessment and update of VQ2 - the statistical demands for paleomagnetic datasets - by Deenen et al. (2011) focused on the number of samples (N) and reconciling statistical reliability with modern understanding of geomagnetic field behavior. Other criteria are more complex, e.g. field tests designed to reconcile rock age with age of magnetization (VQ4) and assessing tectonic coherence of the study lithology with the larger host region (VQ5). Criterion VQ7 requires that there is “…no resemblance to paleopoles of younger age (by more than a period)”. The rationale for establishing VQ7 was that such a resemblance should prompt the suspicion of remagnetization (Van der Voo, 1990). This is not an unreasonable suspicion. There are well-characterized North American examples of older paleopole resemblance to younger poles due to remagnetization e.g. early Paleozoic Appalachian

*This chapter is reprinted from a published article in Geophysical Journal International with permission from A.F.P., and can be cited as: Pivarunas, A.F., Meert, J.G., Miller, S.R., 2018. Assessing the intersection/remagnetization puzzle with synthetic apparent polar wander paths, Geophysical Journal International 214, 1164-1172, https://doi.org/10.1093/gji/ggy216.

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basin rocks (McCabe and Elmore, 1989) or Rocky Mountain rocks (Geissman and Harlan, 2002) that both show a Late Paleozoic remagnetization.

Given that the continents have moved throughout geological time, is resemblance to younger paleopoles likely to occur without remagnetization? Veikkolainen et al. (2014) proposed that VQ7 was contraindicated for Precambrian results based on the existence of various self- closing APWP loops where there is otherwise no evidence for pervasive remagnetization. The likelihood that an APWP will contain overlapping poles/segments over the long-term is intuitively simple: given an increasingly long timescale of movement; the dipolar nature of the geomagnetic field, and the error envelope around a mean pole, a continent is likely to occupy the same latitude in a similar orientation. Remagnetization, although possible, may not be the only reason that an older pole falls along a younger segment of the APWP. Bazhenov et al. (2016) took a more pessimistic approach to the issue based on an analysis of Precambrian data from

Baltica. In their analysis, a remagnetization ‘alarm band’ was plotted around the Phanerozoic

APWP for Baltica. Not only did a majority (>60%) of Precambrian poles fall within the ‘alarm band’, but they also formed distinct age clusters along the Phanerozoic APWP. Bazhenov et al.

(2016) argued that VQ7 should remain an important consideration in Precambrian paleomagnetic studies even when a particular pole determination was deemed to be primary via field tests.

Furthermore, it was correctly noted in Bazhenov et al. (2016) that there are no statistical measures for how often an APWP might loop back upon itself besides intuition. Intuition is a non-scientific path to wander along. There is a necessity for a more testable basis on which to base discussion of the utility (or lack thereof) in continued usage of “no resemblance to younger paleopoles” as a primary quality factor.

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The use of random-walk simulations in evaluation of the inclination frequency test of the

GAD hypothesis (Meert et al., 2003; McFadden, 2004; Evans and Hoye, 2007) and more sophisticated geodynamic modelling (Rolf and Pesonen, 2018) have provided interesting results on that problem. In this study, we use a similar random-walk approach to create synthetic apparent polar wander paths (APWPs). By testing these APWPs for self-intersection, we provide a more rigorous test on the likelihood of pole resemblance arising from long-term movement.

4.2 Random Walk Construction

We start with single paleomagnetic poles at a fixed (90 °N, 0 °E) or random point on the

Earth surface. The pole was subsequently moved in a random azimuthal direction (0 – 360°) with a random speed (1 – 10 cm/year). These random picks were drawn from the integer distribution between the stated limits. This segmented model (SM) is broadly similar to the random-walk model of Evans and Hoye (2007). Our “speed limit” is conservative on its upper end (Meert et al., 1993; Domeier and Torsvik, 2014), since higher drift speeds (up to 24 cm/year) have been proposed for Laurentia in the late Mesoproterozoic (Swanson-Hysell et al., 2014). These parameters in fact represent the movement of the block that generates this paleomagnetic pole, which we viewed as “fixed” to the block. Although this 1:1 correlation between block and pole is a rather naive view of apparent polar wander paths (see discussion), it is useful as a first order approximation. The pole moves along the chosen azimuth and constant speed for 10 million years (Myr). This yields an arc distance and direction on the Earth surface. The Vincenty formula direct solution (Vincenty, 1975) was used to calculate the ending point of this arc. This process was repeated (i.e. with the original azimuth and speed) in 10 Myr segments until an assigned plate reorganization time was reached. An equivalent result could be achieved by calculating an arc distance in a given azimuth for movement during the full span of time between plate reorganization. The utility of this segmented approach was in the granular path provided.

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The period of time between major plate reorganization posited in previous “random walk” simulations designed to test the GAD hypothesis range from 75 Myr (Meert et al., 2003) to 100

Myr (Evans and Hoye, 2007). The Meert et al. (2003) 75 Myr estimation was based on analysis of the Laurentia APWP. Since our model produced synthetic APWPs, their approach is directly applicable to our method. To test both reorganization time periods, the model was run with plate reorganization at either 70 Myr or 100 Myr. Our representation of plate reorganization was simple: a change in azimuth (0 – 360°) and a change in speed (1-10 cm/year). We note that our methodology might allow the azimuth and speed to repeat; however, an exactly repeated outcome for two steps in a row is highly improbable.

The duration for any model run is chosen by the operator. In this study, APWPs of 500

Myr and 1000 Myr in length were generated. We believe these periods are ideal for the illustration of apparent polar wander path behavior, as discussed in the results. An illustration of the data generated in singular APWP runs is shown in Figure 4-1(a,b). Each APWP was analyzed for self-intersection. The model was then repeated to produce a set number of APWPs.

The number of paths analyzed in a given model run was varied to test model dependency on this parameter. Any given model run was repeated 10 times in order to assess repeatability of results.

The flexibility of the quasi-geodynamic tuning of our random walk allowed various movement scenarios to be tested. An illustrated flowchart of our hierarchical methodology is shown in

Figure 4-2.

We also ran models using a ‘random Euler pole method’ (Meert et al., 2003). Every 10 million years, a pole was positioned at 90° N, 0° E and rotated about a random Euler pole to generate an evolutionary APWP that always terminates at the geographic North Pole (see Figure

4-3 and 4-4). The length of each run was either 500 million or 1000 million years (with “re-

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organization” intervals of either 70 or 100 million years) and the percentage of intersecting paths was noted. Each iteration generated 30 APWPs that were checked for self-intersection. The means for each particular run were based on 5 separate iterations using the same model parameters (Table 4-2).

4.3 Results

The 500 Myr runs were initially simulated with changes in both speed and direction

(“reorganization”) every 70 Myr, following the observations of Meert et al. (2003). The results are summarized in Table 4-2. The starting point was held fixed for the majority of runs, however, model behavior was identical when starting points were generated randomly (Table 4-2).

Reported uncertainties on our final intersection probabilities represent two sample standard deviations of the repeated models runs. We found that, on average, 69 ± 9% of synthetic APW paths intersect in 500 Myr (Table 4-2). The number of APW paths generated was typically 100, but the model behaved consistently across an order of magnitude (50-500) (Table 4-2). In model runs with plate reorganization set at 100 Myr as in Evans and Hoye (2007); the probability of intersection drops to 54 ± 6%. The probability of intersection was highly dependent on plate velocity. Constraining the path (which is synonymous with the plate in our model) to a relatively low average velocity (2 cm/year) resulted in 20 – 35% chance of intersection, with the lower end based on 100 Myr reorganization interval and the higher end based on 70 Myr reorganization interval. Although the lower velocity leads to the most unlikely self-intersection scenario in our analysis, such a low speed over the entirety of a 500 Myr period may be equally unlikely.

Conversely, a relatively high average speed (8 cm/year) resulted in a 75 – 80% chance of intersection given the same reorganization intervals. Crucially, these values bracket the model runs wherein the speed was allowed to vary between 1 – 10 cm/year.

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1000 Myr of apparent polar wander was also generated with our model. This length of time is greater than the extant (semi)continuous APW paths of all major continental blocks except for Baltica and Laurentia (Veikkolainen et al., 2014). As above, the model was first run with 70 Myr reorganization, which yielded consistent results of 97 ± 2% of paths intersecting

(Table 4-2). A change in reorganization time to 100 Myr did not radically change these results as those model runs yielded 93 ± 5% of paths intersecting.

The random Euler path method (REM) was used as an independent check of the more robustly determined segmented model (SM) cited above (see Table 4-2; Figure 4-3). In the REM simulations, 30 APWPs were generated for several situations. In the 500 Ma runs with 70 Ma

(500/70) plate reorganization, the paths intersected 65.3 ± 3.9% of the time (versus 69.2% for the

SM). For the 500 Ma runs with 100 Ma (500/100) plate reorganization the REM intersected 52.4

± 2.8% of the time (versus 54.8% for the SM). The 1000/100 REM runs yielded a 90% intersection (versus 93.7 for the SM) and the 1000/70 yielded intersections 96.4 ± 1.5% of the time (versus 97.1% for the SM). We also checked the REM using 2 cm/year velocities using length/reorganization patterns of 500/70 (33.6 ± 2.2% versus 34.5% for the SM) and 500/100

(22.1 ± 1.6% versus 20.2% for the SM). The final REM model used a length/reorganization pattern of 500/50 and yielded intersection 83.1 ± 2.4% of the time (versus 81.4% for the SM).

The strong agreement between the two approaches strengthens the confidence in our conclusions.

Given these results, our simulation predicts that path intersection is highly probable for

APWPs spanning 1000 Myr. We consider this to be a strong statistical argument that older paleomagnetic poles are likely to overlap with younger segments of the APWP, given that our model assumes an idealized, primary, apparent polar wander sequence. APWP sequences for

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durations longer than 1000 Myr were not run, but longer sequences would result in a higher likelihood of self-intersection. Since the likelihood of self-intersection is already ~95 percent, any extension of our APWP simulation would simply move it (unnecessarily) closer to absolute certainty. We found that in the case of paths with self-intersections, the frequency of multiple intersections varied with duration of the APWP. For paths with 500 Myr length, 1 intersection was most likely. At 1000 Myr, ~4 path intersections were most frequently observed. This result follows intuitively from the premises; greater path length means more opportunity for intersection. Although this is an interesting result, we note that this is only the most likely number of intersections – “real” APWPs may show more or less intersections for a given path length and still accord with our model.

4.4 Discussion

Our models are relatively straightforward and monitor duration, interval of reorganization, and the rate of continental motion. A “real” apparent polar wander path reflects the latitudinal and rotational motion of a rigid block with respect to the spin axis (absolute plate motion) in addition to motion of the entire lithosphere and mantle with respect to the spin axis

(True Polar Wander). Large-scale true polar wander (e.g. Evans, 2003) was not input into the models, but the incorporation of large-scale true polar wander would result in large, geologically rapid, shifts in our synthetic apparent polar wander paths. Qualitatively, TPW would likely result in a higher probability of self-intersection since it results in increased apparent block velocity.

Perhaps more problematic in an ideological sense is the assumption of a 1:1 correlation between

APW paths and continental block movement. This, of course, does not reflect paleomagnetic reality. Certain plate movements, such as stalling on a pole or purely longitudinal motion are irresolvable with apparent polar wander paths. This is well-appreciated among paleomagnetists; longitudinal uncertainty remains an appreciable hurdle to Precambrian reconstructions (Meert,

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2014a). The opposite problem results from a block rotating on the equator. This purely rotational motion could result in significant apparent polar wander without appreciable continental motion.

These situations confound each other, each either underestimating or overestimating continental movement purely based on an APWP. The model(s) employed in this study yield geologically reasonable APWPs while remaining faithful to some basic geodynamic considerations such as speed and changes in direction. We view these results as a necessary, parsimonious first step in providing a more rigorous evaluation of VQ 1-7 (Box, 1979).

Our models are also conservative because we place no error estimates on the paleomagnetic poles that make up the APWP. As even a cursory glance at the raw data of a compilation such as Torsvik et al. (2012) reveals, real paleomagnetic data more closely resemble a smeared, messy, path-like jumble of poles and errors. Various averaging and smoothing methods (moving window averages, spline fits, etc.) can reduce these data to a single

‘GAPWaP”, but we note that GAPWaP contains multiple intersection points over the past 320

Ma (see Figure 4-4a). An analysis of the confidence limits for APWPs was provided in

Bazhenov et al. (2016). Their empirical case study on Precambrian paleomagnetic poles from

Baltica was compared with the Phanerozoic APWP and provided a framework for future spatiotemporal comparisons. They constructed a Fisher statistic “alarm band” around

Phanerozoic and Precambrian APWPs from Baltica, and methodically assessed older data falling into these bands. Their approach was self-described as “reserved”, and ended by stressing the importance of VQ7 (no resemblance to younger poles) to the evaluation of Precambrian paleomagnetic data.

Van der Voo (1990) argued that any older pole that falls along a younger segment of the

APWP should raise concerns about its primary nature. That concern is justified by several well-

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established examples. Large-scale remagnetization has occurred throughout geologic time: southern Finland in the Proterozoic (Mertanen et al., 2008) and in the eastern and western United

States during the late Paleozoic (McCabe and Elmore, 1989; Geissman and Harlan, 2002; Van der Voo and Torsvik, 2012). A global review of remagnetization is beyond the scope of this paper, but we mention examples from South China (Huang and Opdyke, 1996; Zhang et al.,

2015), Africa and South America (Tohver et al., 2006), and New Zealand (Rowan and Roberts,

2008) to emphasize the scope of the problem. Remagnetization can usually be tied to orogenic activity (Bazhenov et al. 2016), sometimes quite distal to the rocks in question such as in the

Alleghenian orogeny (McCabe and Elmore, 1989). Orogenic activity is a potent cause of remagnetization since it can create high temperature conditions during the large-scale metamorphic activity (Pullaiah et al., 1975) or cause fluid migration with associated resetting

(Geissman and Harlan, 2002), although other burial diagenetic processes can often be invoked as regards the latter (Elmore et al., 2001).

Loops and self-intersection of APWPs occur in extant data quite frequently. Laurentia, with the most populous paleomagnetic database, contains a number of temporally separated but spatially similar poles that require the APWP to loop back upon itself multiple times.

Paleomagnetic data from the Ectasian and Stenian ages (Mesoproterozoic), Cryogenian and

Ediacaran ages (Neoproterozoic), and Cambrian all occupy low-latitude positions over a relatively narrow latitudinal band (Figure 4-5a). The path geometries and paleogeographic considerations implied by these data have been discussed at length (Ernst and Buchan, 1993;

Weil et al., 1998; Fairchild et al., 2017; Figure 4-5b), although certain loop behavior remains troubling (McCausland et al., 2011; Figure 5c). Other continents are not immune to APWP intersections, such as the early Paleozoic Gondwana APWP (Torsvik et al., 2012; Figure 4-4b) or

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the complex back and forth behavior of Baltica (Meert, 2014b). Our synthetic tracks replicate these APWP path behaviors (Figure 4-1 and 4-3). It may be that as apparent polar wander paths for more blocks and cratons become well-constrained in space and time for longer uninterrupted periods, self-intersection may be seen to be a normal occurrence. Such a finding would be excellent support of our model; it is indeed predicted by our results.

Self-intersections in these loops are not synonymous with remagnetization provided field tests are able to tightly constrain the age of magnetization. Compelling field tests include baked contact tests, syn-sedimentary slump tests, fold tests on strata where the age of folding is close to the age of the rocks, and intra-formational conglomerate tests (McElhinny and Merrill, 1999).

We concur that thorough assessment of potential remagnetization should be an ongoing concern of all paleomagnetic investigations. We argue that VQ7 should not be applied indiscriminately to reject a pole simply because of its resemblance to a younger pole. A resemblance to a younger paleopole spatiotemporally adjacent to a large-scale orogenic event is strong evidence for remagnetization but does not provide a priori proof of remagnetization. Van der Voo (1990) attempted to soften the apparently binary nature of VQ7, stating that resemblance is strong indication of remagnetization “…Unless field tests are available to constrain the age of magnetization”. Given our results, we believe that resemblance in the absence of more suspicious circumstances need not be a “strong” indication for remagnetization, but we otherwise concur with the notion that field tests are the premier constraint on the age of magnetization.

4.5 Conclusions

Apparent polar wander simulations provide insight into the probability of a paleomagnetic pole falling along a younger segment of the apparent polar wander path and

‘violating’ the VQ7 criterion. The self-intersection of our synthetic apparent polar wander paths

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is strongly dependent on rate of movement and somewhat dependent on reorganization time. For realistic scenarios of movement (randomly-varying) and reorganization times (70-100 Myr) we find it is more likely than not (54–69%) for poles to coincide given 500 Myr of APW and virtually certain (93–97 %) for poles to coincide given 1000 Myr of APW. These modelled pole resemblances are completely independent of remagnetization, and simply governed by the movements of blocks on the Earth surface. Our simulation lays the groundwork for evaluating the evolution of APWPs through time as well as its implications for VQ7. Future model refinements might incorporate various geodynamic and paleomagnetic concerns touched on in our discussion, but we feel that our conservative approach provides important statistical constraints on self-intersection. Based on these results, we believe that VQ7 should focus on suspicion of remagnetization and paleomagnetic tests related to those suspicions rather than just a resemblance to younger poles. We fall somewhere in between the conclusions of Bazhenov et al. (2016) and Veikkolainen et al. (2014). Our simulations show that over long intervals of geological time, it is probable for an APWP to loop back upon itself. Given that probability, we suggest that any older pole that overlaps with a younger segment of the APWP should be carefully examined for evidence supporting its primary nature. While the previous statement seems simple (and perhaps self-obvious), it has created issues amongst paleomagnetists tasked to evaluate Precambrian data. On the one hand, it has been argued that VQ7 is straightforward and therefore any pole that falls along a younger segment should be viewed as suspicious. On the other hand, there is a strong intuitive argument that VQ7 is too restrictive for Precambrian data because of the increased likelihood of an older pole falling along a younger path segment. Our models indicate that the latter argument is correct and that evaluating VQ7 must include a more in-depth evaluation of those data.

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Table 4-1. The Van der Voo (1990) quality criteria as outlined in the original paper. Our work focuses on criterion VQ7. Number Brief description 1 Well-determined rock age and a presumption that magnetization is the same age 2 Sufficient number of samples (N > 24), k (or K) ≥ 10 and α95 ≤ 16.0° 3 Adequate demagnetization that demonstrably includes vector subtraction 4 Field tests that constrain the age of magnetization 5 Structural control and tectonic coherence with craton or block involved 6 The presence of reversals 7 No resemblance to paleopoles of a younger age (by more than a period)

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Figure 4-1. An illustration of typical results from the segmented model. (a) shows 1000 Myr of APWP with several prominent path self-intersections shown with red. (b) dots shows 500 Myr of APWP, we see no intersection in this APWP. Latitude/longitude lines are at 30° intervals.

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Figure 4-2. A schematic view of the flow of our synthetic APWP model. The model is implemented in Matlab, with data visualization taking place using Python. The four parameters at the first step are operator controlled, giving the model its flexibility. The two parameters within the program are randomly generated within the given range (although they can also be controlled if desired).

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Table 4-2. Numerical model results for both approaches, Euler pole approach results boxed. Model parameters as discussed in the section on construction. Starting APWP Length "reorganization" N (paths APWP intersection 2σ Point (Myr) interval (Myr) generated) (%) (%) 500 Myr interval, 70 Myr reorganization (90,0) 500 70 500 68.3 3.6 (90,0) 500 70 100 69.9 6.7 (90,0) 500 70 50 68.2 16.4 random 500 70 100 70.0 9.7

mean 69.1 9.1 (90,0)- 500 70 30 65.3 3.9 REM

1000 Myr interval, 70 Myr reorganization (90,0) 1000 70 100 97.6 2.3 (90,0) 1000 70 500 97.0 1.7 random 1000 70 100 96.8 2.1 mean 97.1 2.0 (90,0)- 96.4 1.5 REM 1000 70 30

100 Myr reorganization, different intervals (90,0) 500 100 100 54.8 6.7 (90,0) 500 100 500 53.4 6.1 (90,0) 1000 100 100 93.7 6.7 (90,0) 1000 100 500 93.2 3.5 (90,0)- 500 100 30 52.4 2.8 REM

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Table 4-2. Continued Testing model dependence on speed

Starting APWP Length "reorganization" N (paths APWP intersection 2σ

Point (Myr) interval (Myr) generated) (%) (%) random 500 70 100 34.5 8.4 2 cm/yr random 500 70 100 81.2 9.9 8 cm/yr random 500 100 100 20.2 6.2 2 cm/yr random 500 100 100 74.5 9.1 8 cm/yr random- 500 70 30 33.6 2.2 2 cm/yr REM random- 500 100 30 22.1 1.6 2 cm/yr REM

Testing model dependence on reorganization time random 500 50 100 81.4 8.4 random 500 30 100 94.0 5.8 (90,0)- 500 50 30 83.1 2.4 REM

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Figure 4-3. Illustrations of the random Euler pole model (Meert et al., 2003). (a) shows a non- intersecting case of the model while (b) shows an intersecting case (intersections shown by red dots). (c) An assortment of 100 paths of the model, all colored differently, illustrates the random in this quasi-random walk model. Latitude/longitude lines are at 30° intervals.

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Figure 4-4. Large Phanerozoic paleomagnetic datasets provide useful real-world examples of apparent polar wander path architecture. (a) the Global Apparent Polar Wander Path for the past 320 Myr of Torsvik et al. (2012), global poles rotated to African coordinates with inclination-correction, showing several self-intersections, along with a shared segment at the 140 Ma cusp. (b) Another example of a closed APWP loop in Torsvik et al. (2012) from the early-mid Phanerozoic Gondwana path.

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Figure 4-5. Case studies of intersections in a real paleomagnetic data set: Laurentia. The crowded intersection area of Laurentian paleomagnetic data (a) from the Mesoproterozoic to the Cambrian. Specific examples of self-intersection in the Laurentian path: (b) Laurentian paleomagnetic data from Grenvillian time, the Logan Loop moving into the southerly Keweenawan Track before swinging east then west, self-intersecting in the Grenville Loop. Data are from the 2017 Nordic Paleomagnetism Workshop Laurentia key-pole compilation. (c) shows the intractable Ediacaran-Cambrian APWP from Laurentia (data after McCausland et al., 2011) exhibiting an odd affinity for sudden equatorial shifts and crossing itself (general implied path shown in alphabetical sequence). All latitude/longitude lines are at 30 degree intervals.

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CHAPTER 5 PROTRACTED MAGMATISM AND MAGNETIZATION AROUND THE MCCLURE MOUNTAIN ALKALINE IGNEOUS COMPLEX*

5.1 Introduction

Rapid cooling and the potential for both relative and absolute dating render dykes as highly favorable targets for paleomagnetic research. Conducting integrated studies to simultaneously recover both magnetizations and emplacement ages for these intrusive bodies allows accurate spatiotemporal relationships to be determined throughout the entire range of geologic time. A number of external factors complicate the retention of a primary magnetization. These factors include heat (Pullaiah et al., 1975), fluid flow (McCabe and

Elmore, 1989; Geissman and Harlan, 2002), viscous remanent magnetizations (Kent, 1985), instability of magnetic phases, and lightning (Graham, 1961).

We present here an integrated paleomagnetic and geochronologic reinvestigation of dykes surrounding the McClure Mountain igneous complex (MMIC) in order to better constrain the temporal evolution of the complex. Previous paleomagnetic studies (Larson and Mutschler,

1971; French et al., 1977; Lynnes and Van der Voo, 1984) yielded multiple paleomagnetic directions from dykes in and around the complex but lacked reliable ages. Although the main syenite body of the MMC has been well-dated (Schoene and Bowring, 2006), and its later thermochronological history explored at medium temperatures (Samson and Alexander, 1987;

Renne et al., 1998; Spell and McDougall, 2003; Schoene and Bowring, 2006) and low temperatures (Anderson et al., 2017; Weisberg et al., 2018), the temporal progression of the end- stage MMIC dyke intrusions are poorly constrained. Defining a temporal framework for the

*This chapter is under review at Lithosphere.

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McClure Mountain igneous complex will allow us to better ascertain the regional-scale tectonic relationships of the southern Colorado intrusive episodes.

5.2 Setting

5.2.1 Regional Geology

Proterozoic basement rocks (migmatitic gneisses and granites) of the Wet Mountains of southern Colorado (Taylor et al., 1975a, 1975b; Armbrustmacher and Hedge, 1982; Bickford et al., 1989) host a number of alkaline igneous complexes of early Cambrian age: McClure

Mountain, Gem Park, Democrat Creek, and Iron Mountain (Olson et al., 1977; Armbustracher and Hedges, 1984; Bickford et al., 1989; Figure 5-1). The McClure Mountain igneous complex

(MMIC) is the largest of these (Taylor, 1970). The complex is roughly bimodal, with a mafic- ultramafic complex emplaced in the northeast followed by a larger alkaline complex (Parker and

Hildebrand 1963; Shawe and Parker 1967; Figure 1). The youngest igneous activity in the

MMIC is represented by dykes of varied mineralogy (i.e. lamprophyric, carbonatitic, and trachytic) intruding the main complex and the host rocks to the west (Heinrich and Dahlem,

1969; Alexander, 1980). The area is cut by a number of faults including the Texas Creek Fault to the west of the complex and the Ilse Fault to the east as the largest features (Taylor, 1975).

Weisberg et al. (2018) suggest that Phanerozoic normal displacement on the Ilse fault was minimal (see also Kelley and Chapin, 2004), although Noblett et al. (1987) argue for at least 16 km of strike-slip displacement. This amount of strike-slip deformation, if coupled to offsets along the numerous smaller faults (Taylor et al. 1975a,b), might result in significant structural rotations through the area. The Texas Creek fault also shows little evidence for major displacements (French et al., 1977), although detailed examination of the Phanerozoic history of the fault is lacking.

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The numerous minor faults throughout the region exhibit minimal offsets although they may serve as conduits for fluid flow (French et al., 1977; Heinrich and Dahlem, 1968). Our field observations on dykes along the Texas Creek fault (Highway 65; Fig 5-1) confirm that the dykes are highly-altered and sheared along their margins. Dykes distal to faults are less altered.

5.2.2 Previous Paleomagnetic Studies

Two paleomagnetic studies were conducted on intrusive rocks throughout Southern

Colorado (Larson and Mutschler, 1971; Larson et al., 1985) and two subsequent studies specifically targeted igneous complexes in the Wet Mountains (French et al., 1977; Lynnes and

Van der Voo, 1984).

The first paleomagnetic study on intrusive rocks of southern Colorado was conducted by

Larson and Mutschler (1971). That study sampled three localities: dykes in the Black Canyon of the Gunnison, a gabbroic dyke in the Powderhorn alkalic complex, and major units of the

MMIC. The exact sampling locations were not documented in their study. Samples in that study were subjected to limited alternating field (AF) and thermal demagnetization and yielded a south-southeast, shallowly inclined paleomagnetic direction. Larson and Mutschler (1971) calculated separate poles for the means of the AF-treated and thermally treated samples.

Unfortunately, the authors did not provide site or sample directional data in their publication.

The two virtual geomagnetic poles (VGPs) are not significantly different (AF: 37° N, 122° E

A95=8°; thermal: 44° N, 100° E, A95=8°) and we therefore combine the data to compute a new paleomagnetic pole at 41° N, 112° E. The same workers examined the paleomagnetism of Black

Canyon dykes in an attempt to link the southern Colorado intrusives to the Southern Oklahoma

Aulacogen (SOA) (Larson et al., 1985). They reported a mean D=157°, I =+19° from nine dykes. The paleomagnetic pole calculated from the mean direction falls at 37° N, 102° E; and coincides with their results from the MMIC.

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Two papers focused exclusively on the paleomagnetism of the McClure Mountain igneous complex (French et al., 1985; Lynnes and Van der Voo, 1984). French et al. (1977) sampled dykes around the complex (29 sites) along with the ultramafic-mafic portion of the

MMIC. Their detailed demagnetization experiments revealed two groupings of directions. The first, poorly clustered grouping combines 21 sites (I and II directions) with a mean D=104°, I=-

2° (a95=11°, k = 9). French et al. (1977) suggested the poor grouping resulted from temporal differences between the dykes. This magnetization appears to be carried by magnetite, and yields a pole at 12°N, 156°E (A95=12°).

The second grouping (III directions) in that study is coincident with the results of Larson and Mutschler (1971) and Larson et al. (1985). The mean direction from seven sites is D=159°,

I=-2° (a95=16°, k = 15). In their discussion of this result, the workers note that the majority of sites with this characteristic magnetization were taken meters from the Texas Creek fault (French et al., 1977). Lynnes and Van der Voo (1984) targeted trachytic dykes along with the main syenite bodies of the complex. Although Lynnes and Van der Voo (1984) were unable to isolate stable vector endpoints, their great-circle analysis, yielded a mean direction with D=152°, I=+2°; that is nearly identical to the group III directions of French et al., (1977) as well as the studies of

Larson and Mutschler (1971) and Larson et al. (1985). A brick-red color in the trachytes, high blocking temperature, and thin-section analysis suggested hematite as the carrier of this remanence. Lynnes and Van der Voo (1984) interpreted the direction to reflect a fluid-triggered

Pennsylvanian-age remagnetization.

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5.3 Methods

5.3.1 Geochronological Methods

Geochronological hand samples were taken from two extracomplex dykes: site 1

(trachyte dyke), and site 8 (lamprophyric dyke). Standard mineral separation techniques

(crushing, sieving, heavy liquids, and magnetic methods) were used to isolate zircons from the dyke samples. Samples were hand-picked for zircon grains under a binocular microscope then transferred to double-sided tape in preparation for mounting. The grains were set in epoxy then polished to expose interiors. Zircons were examined on an SEM with backscatter election (BSE) and cathode luminescence (CL) imaging. Epoxy plugs were then washed and sonicated in an

HNO3 solution prior to LA-ICP-MS analysis.

U-Pb isotopic analyses were carried out using the Nu-Plasma multi-collector plasma source mass spectrometer at the Department of Geological Sciences at the University of Florida.

The LA–ICP–MS is equipped with a custom-designed collector block for simultaneous acquisition of 204Pb (204Hg), 206Pb and207Pb signals on the ion detectors and235U and238U on the

Faraday detectors (Mueller et al., 2008). Prepared zircon grains were laser ablated using a New-

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

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5.3.2 Paleomagnetic Methods

Over 100 oriented core and hand samples were collected from 12 dykes and intrusions around the McClure Mountains igneous complex in the Wet Mountains area in Southern

Colorado. Paleomagnetic core samples were collected in the field with a water-cooled, gasoline- powered drill and oriented with magnetic and sun compass. Deep erosion of dykes as compared with host rocks (Heinrich and Dalhem, 1968) rendered drilling unfeasible at certain sites; oriented hand samples were taken at these sites. All samples were returned to the University of

Florida where they were trimmed to a standard size. Natural remanent magnetization (NRM) directions were collected using either a Molspin spinner magnetometer or 2G-77R cryogenic magnetometer. Pilot samples from all sites were demagnetized by thermal and alternating methods using either an ASC TD-48 thermal demagnetizer or DTech 2000 AF demagnetizer.

Subsequent demagnetization treatment was optimized based on the results from the pilot samples. Paleomagnetic vector directions were determined via principal component analysis

(Kirschvink et al., 1980) using IADP software (Torsvik et al., 2016). Additional analysis was carried out using PmagPy (Tauxe et al., 2016). Powdered/crushed material from several samples at each site was analyzed with a KLY-3S Kappabridge with a CS-3 furnace attachment and a vibrating sample magnetometer (VSM) in order to ascertain magnetic carriers and magnetic domain characteristics.

5.4 Results

5.4.1 Geochronologic Results

5.4.1.1 Ordovician trachyte

The latest stage of intrusion at the McClure Mountain igneous complex (MMIC) are a series of prominent red trachytic dykes. These dykes were dated (four-point Rb-Sr isochron) by

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Olson et al. (1977) as Cambro-Ordovician at 495±10 Ma. K-Ar whole-rock dates from Lynnes and Van der Voo (1984) were scattered, with the youngest Pennsylvanian age (~300 Ma) interpreted by the authors as evidence of fluid alteration at this time.

A hand sample from a trachyte dyke sampled paleomagnetically yielded over 50 zircons.

The paleomagnetic data from this dyke is discussed below. Over 80 analyses were conducted on the zircons from the trachyte dyke. To confidently calculate the crystallization age of this dyke, we rejected any analysis that was more than 3% discordant. This reduced the number of analyses to 18, on 13 zircons. The weighted mean age (206Pb/238U; 2σ uncertainty) of the 18 analyses under 3% discordance was 483.6 ± 1.8 Ma (Figure 5-2a,b). We regard this result as a more reliable update to the older Rb-Sr and K-Ar ages, fixing the age of intrusion of this dyke as

Tremadocian. Notably, this dyke post-dates the main body of the complex by ~40 Myr (524 Ma;

Schoene and Bowring, 2006). However, one zircon from the trachyte did produce an age comparable to the main complex body (Figure 5-2a), which may represent inheritance. Our result is consistent with the interpretation of Olson et al. (1977), who viewed the red trachyte dykes as a late intrusive stage of the MMIC.

5.4.1.2 Cambrian lamprophyre

Lamprophyric dykes are another constituent of the late-stage MMIC magmatism. These dykes cut all major units of the complex (Heinrich and Dalhem, 1966; Alexander, 1980). Their relationship with other late-stage dykes (i.e. the red trachytes) are unclear based solely on field relationships.

A hand sample from a paleomagnetically sampled extracomplex lamprophyre dyke yielded three zircons suitable for analysis. One of these zircons was inherited, with a

Paleoproterozoic age (~2430 Ma). The other two zircons yielded concordant, Cambrian ages.

The weighted mean age (206Pb/238U; 2σ uncertainty) of the 2 analyses was 525.6 ± 7.9 Ma (Figure

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5-2a). Although this date is based on only two zircons, it overlaps with the reliable MMIC syenite age of Schoene and Bowring (2006). We consider the age of intrusion for this dyke as on the “young-uncertainty” side of our analyses (i.e. probably younger than the MMIC syenite), but allow for the possibility that lamprophyric intrusion was coeval as well as older than the main

McClure Mountain syenite intrusion.

5.4.2 Paleomagnetic Results

Ten of thirteen sites sampled for paleomagnetic analysis all yielded suitably consistent

(i.e. stable) paleomagnetic directions. Our paleomagnetic results confirm earlier results in the area (Larson and Mutschler, 1971; French et al., 1977; Lynnes and Van der Voo, 1984; Figure 5-

3a). The main magnetic carrier was magnetite (Figure 5-3c), with some exceptions discussed below. Ubiquitous steep magnetic directions (also noted by French et al., 1977) suggest

Cenozoic to recent remagnetizations around the complex (Figure 5-3b).

5.4.2.1 Southeasterly paleomagnetic data

Six sites in our study, including both sites with geochronological control (see above), had southeasterly, shallow-medium paleomagnetic high-temperature/coercivity directions (Table 5-2 and 5-3). The dykes show a steep-downwardly directed low-temperature overprint with a mean

D=52°, I=+78° (a95=5, k=14). In order to best compare our data with that of previous studies, we break them into groups based on declinations (after French et al., 1977; Figure 5-4a).

Three dykes, including both sites with geochronological control (see above), yielded east- southeast, shallow paleomagnetic high-temperature directions (Table 5-2). The dykes show a steep-downwardly directed low temperature overprint with a mean D=65°, I=+67° (a95=6°, k=18) (Figure 5-3b, 5-4b). The mean high-temperature direction from these three dykes yields a mean D=118°, I=-7° (k=6, a95=55°). The poor grouping in our study (when combined with previous studies) may reflect a variety of complications. The most obvious explanation for this

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disparity is the ~40 Myr time span separating the different dykes. Although we can compute a mean pole by combining our data with the group I and II directions isolated by French et al.

(1977), the result is relatively meaningless because our geochronological data confirm that the dykes may have a 40+ Myr age range.

We can further demonstrate that the combined directions reflect a ‘blend’ of Cambro-

Ordovician directions because the mean pole (from all dykes in Table 5-2) falls at 13° S, 336° E

(A95=8°, k=15). The calculated paleopole falls along the smoothed Laurentian apparent polar wander path (APWP) (Torsvik et al., 2012) at 500 Ma (Figure 5-6). The mean pole position almost exactly splits the difference in end-member ages (C161 trachyte and C168 lamprophyre) from dykes with these paleomagnetic signatures.

A baked contact test at site C168 (dated at 525 Ma; this study) is problematic (Figure 5-

5a). Unbaked samples are identical to those at the dyke-granite contact and exhibit a mean

D=69°, I=+65° (k=37, a95=7°). Samples from the dyke were also heavily overprinted (some completely) by this steep direction; however, the majority showed a distinct, high-temperature direction at D=129°, I=+21° (a95=9°, k = 45). Low-temperature/coercivity magnetic vectors overlap with the present-day field (PDF) direction in southern Colorado (reference direction is

D=8°, I=+65°). The granite and medium-temperature dyke magnetic components lie along a great-circle between the PDF overprint and the high temperature/coercivity direction of the dyke

(Figure 5-5a). The baked contact test here is therefore negative. We see no overprinting of the granitic contact rocks by the dyke.

Three sites (2 dykes) yielded south-southeasterly, shallow-medium negative inclination paleomagnetic directions. These characteristic directions are overprinted by a steep, positive inclination direction with a mean D=14°, I=+84° (a95=6°, k=16) (Figure 5-3b,c). The mean

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direction from our sites is D=153°, I=-23° (a95=33°, k=15). This mean is similar to the Group

III directions of French et al. (1977) and the trachyte grouping of Lynnes and Van der Voo

(1984). When our results are combined with those of French et al. (1977), they yield a grand mean direction from 18 sites in the complex of D=150°, I=-5° (a95=8°, k=19) with a corresponding paleomagnetic pole at 51 S, 292 E. Because only mean directions were reported, we are unable to incorporate site mean data from Lynnes and Van der Voo (1984) or

Mutschler and Larson (1971). We do note that the VGP (43 S, 294 E) reported for red trachyte dykes in Lynnes and Van der Voo (1984) falls close to our pole. Similarly, the VGP (41 S, 312

E) of Larson and Mutschler (1971) from various area alkaline complex rocks is consistent with our pole. Also the pole (37 S, 282 E) from Black Canyon dykes discovered by Larson et al.

(1985) falls near ours.

A single dyke from this grouping (Group III; French et al., 1977) shows a reversed polarity direction (D=338, I=+8; Figure 5-4a). The dykes that exhibit this direction were all sampled along the Texas Creek fault zone. The paleomagnetic pole calculated from our mean direction falls at 51° S, 292° E (A95=10°, k=24). The pole coincides with the Laurentian apparent polar wander path (APWP) (Torsvik et al., 2012) at ~300 Ma (Figure 5) and is consistent with a K-Ar whole rock age on a trachyte from Lynnes and Van der Voo (1984) at 300

± 11 Ma and a less precise fission track date from Olson et al. (1977) at 293 ± 62 Ma. Lynnes and Van der Voo (1984) argued that this direction was acquired during a Late Paleozoic hydrothermal event. Low-temperature diffusion studies on the MMIC syenite (Anderson et al.,

2017; Weisberg et al., 2018a,b) support a low-temperature for the MMIC syenite during the Late

Paleozoic. The Late Paleozoic alteration is also confirmed by regional-scale paleomagnetic studies (Geissman and Harlan, 2002). Geissman and Harlan (2002) discovered that the late

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Paleozoic remagnetization is carried by hematite and was acquired primarily via fluid remagnetization at low (<200 °C) temperatures which is consistent with our observations and those of Lynnes and Van der Voo (1984).

A baked contact test at site C1611 was conducted on a small (0.75 m wide) dyke adjacent to the Texas Creek fault zone, several hundred meters north of sites C164 and C1611. All of these dykes exhibited variable alteration, quartz/calcite veining, and shear along their margins.

The dyke (C1611) yielded a high temperature/coercivity direction with a mean D=55°, I=+76°

(a95=7°, k=43). Low temperature/coercivity overprints yield a mean direction of D=357°,

I=+75° (a95=4°, k=128). Granite samples taken at the contact with the dyke carry a hematite- carried, south-southeast, shallow inclination component with a mean D=158°, I=0° (a95=6°, k=59; Figure 5-5c). These data illustrate a negative baked contact test at site C1611.

5.4.2.2 Steep paleomagnetic data: recent, but how recent?

5 sites (5 dykes) carried a dual-polarity, steeply-inclined paleomagnetic direction as their characteristic magnetic component (Figure 5-3b,c) This component is consistent with the overprints on dykes that carry shallow, southeasterly paleomagnetic directions (Figure 5-3c).

These sites carry less prominent secondary magnetizations (Figure 5-3b) which fall more in line with a present day field direction.

A baked contact test was attempted at site C1610 (Figure 5-5b). This site was part of a network of dyke and sill gabbroic intrusions sampled near Canon City, Colorado (sites C162,

C163, and C1610). Sites C162 and C163 displayed univectorial, steep positive inclination magnetizations. C1610 was more complex. A well-exposed contact with the host granite made a baked contact test possible. The mafic intrusion samples carried a southeasterly, steep negative inclination magnetization (D=152°, I=-55°). However, approaching the contact (i.e. samples 5-

7), the demagnetizations progressed through this negative inclination along a great circle towards

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a steep, positive inclination at high (>580 °C) temperatures. An intrusive sample from directly at the contact was unstable. Samples from the granite had uncomplicated, steep magnetizations with D=288°, I=+88°. All rocks (both intrusion and host) had low-temperature/low coercivity components with a mean D=8°, I=+70°. The area around this site is tectonized by Cenozoic-age folding and faulting (Taylor et al., 1975b).

The mean of the northerly, steep inclination paleomagnetic direction (with one site inverted) has a D=8°, I=+73° (a95=15°, k=26). Present day field directions have slightly shallower inclinations (I=+58 GAD assumption, I=+65 PEF). Based on the concordance of site mean steep directions with the generally steep, positive inclination overprints (Figure 3d), and the agreement of both with expected Cenozoic to the known present day magnetic field, we consider the northerly, steep inclination direction to represent Cenozoic–present remagnetizations.

5.5 Discussion

5.5.1 Deformation and Magnetization

A striking feature of the Paleozoic paleomagnetic data from around the MMIC (French et al., 1977; Lynnes and Van der Voo, 1984; this study) is the directional scatter. As mentioned earlier, this is likely a feature of a combination of factors; faulting, fluid flow, and age scatter are the most prominent. Although large-scale movement on the small faults mapped in the immediate vicinity of the MMIC is unlikely, localized deformation cannot be ruled out. The Ilse fault (6 kilometers to the east) accommodated 16 kilometers of right-lateral strike-slip during

Laramide deformation (Noblett et al., 1987), which suggests that the small-scale faulting around the MMIC may have triggered coeval vertical axis rotations. Fault trends, especially in the smaller faults, are highly variable as are the offsets. This variability suggests that any rotations are less likely to be systematic in the region resulting in differential rotations throughout the

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study area. Based on the scattered paleomagnetic directions along with more recent assessments of fault movement (e.g. Noblett et al., 1987), we update the conclusion of French et al. (1977) of no structural disturbance to “likely structural disturbance”. An analysis of declination and dyke trend revealed no consistent pattern of deformation.

Group III dykes of French et al. (1977) along with the trachyte data of Lynnes and Van der Voo (1984) were interpreted by the original authors and others (Geissman and Harlan, 2002) to be indicative of a late Paleozoic remagnetization associated with fluid migration during the formation of the Ancestral Rockies. This remagnetization is prominent along the Texas Creek fault zone – a likely conduit for fluid movement at that time.

The question may be fairly asked given the structural disturbance, fluid remagnetization, and scattered paleomagnetic directions: is it even worthwhile to separate these southern Colorado paleomagnetic data into different groups? That is, might the entire southeasterly, shallow- inclination dataset be a product of differential rotations and various levels of remagnetization?

Assessing the data overall, a natural break in declination is notable between the directional groupings (Figure 5-4a). Additionally, the more south-southeasterly declination paleomagnetic data are concentrated in sites taken from along the Texas Creek fault zone (French et al., 1977; this study).

A comparison to the North American apparent polar wander path (NAM APWP; Torsvik et al., 2012) further reinforces the data break, with the mean of the east-southeasterly directions falling along the late-Cambrian/Ordovician segment of the path, while the interpreted remagnetization falls, as expected, around the late Paleozoic. The data polarity are also suggestive of separate ages. The first grouping of data (which has geochronologic control ranging in age from the Cambrian to Ordovician) contain multiple sites of contrasting polarity,

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whereas the late Paleozoic data are dominantly southwardly-directed and shallow (with one exception) consistent with remanence acquisition during the Kiaman Superchron (262-318 Ma;

Opdyke and Channell, 1996). Overall, the paleomagnetic data are consistent with a protracted history of magnetization, remagnetization, and deformation. Our overall assessment of these data is that although they are broadly consistent with the NAM APWP, the likely impact of deformation and large range of ages render them unsuitable for direct incorporation into the

North American APWP.

5.5.2 Far-field Tectonic Implications

The McClure Mountain igneous complex and other Cambrian complexes have been temporally linked with the Southern Oklahoma Aulacogen (Loring and Armstrong, 1980; Larson et al., 1985; Bickford et al., 1989; Hanson et al., 2013; Bruseke et al., 2016; Weisberg et al.,

2018), although such associations are controversial (Van der Voo, 1986; Loring et al., 1986;

McLemore, 1987; Purucker, 1988; McMillan and McLemore, 2004).

A number of recent papers on the Southern Oklahoma Aulacogen (SOA) (Hanson et al.,

2013; Brueseke et al., 2016) have mentioned the temporal correlation of central/southwestern diabase and alkali intrusives with the SOA. Precise age constraints on the bimodal igenous units of the Southern Oklahoma Aulacogen are also limited, but gabbros, granites, and rhyolites of the

Wichita and Arbuckle Mountains return early Cambrian (539 – 530 Ma) ages (Wright et al.,

1996; Hames et al., 1998; Hanson et al., 2009; Wall et al., 2018). Geophysical data provide support for the connection (Keller and Stephenson, 2007), with discontinuous gravity anomalies traceable from the SOA into Utah. The southern Colorado/New Mexico intrusive rocks are geochemically indicative of an extensional environment/mantle plume (McMillan and

McLemore, 2004), similar to the Wichita igneous province suite (Hanson et al., 2013).

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However, the most reliable ages for Southern Colorado/New Mexico intrusive rocks are appreciably younger than those of the Southern Oklahoma Aulacogen (539-530 Ma): the main syenite at McClure Mountain dated to 523.98 ± 0.12 Ma by Schoene and Bowring (2006), the

Florida Mountains at 503 ± 10 Ma (Clemons, 1987), and trachyte dykes of the McClure

Mountain igneous complex at 484 ± 2 Ma (this study). Ages in the K-Ar system are more scattered, but generally fall within the same age range (McMillan and McLemore, 2004).

These data suggest that either rift-related magmatism along the lengthy aulacogen proposed by Larson et al. (1985) was severely protracted (over 40 Myr), well into passive margin development in the western United States (Dalziel, 1991; Yonkee et al., 2014), or the tectonic setting is more complex. McMillan and McLemore (2004) proposed a “New Mexico aulacogen”, analogous to the SOA but differently oriented. Mantle plume impingement along the SOA has been proposed (Hanson et al., 2013; Brueseke et al., 2016), which may help explain the age disparity between the two regions.

Testing the upper and lower age limits of the early Paleozoic intrusions in southern

Colorado and New Mexico, as well as further investigation of the SOA Wichita igneous province are necessary to fully explore the tectonic setting of central-southwestern North America in the late Paleozoic. Given the current relative age disparity (exacerbated by the early Ordovician dyke age of our present study), a direct relationship between the Cambrian complexes of southern Colorado and the SOA demands further scrutiny. A large number of putatively

Paleozoic intrusives in the southwest lack reliable age constraints (McMillan and McLemore,

2004). Providing reliable ages for these bodies provides a continuing challenge for the future, with the promise of untangling the apparently busy tectonic regime in the area during the transition to a passive margin.

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5.6 Conclusions

Dykes around the McClure Mountain igneous complex record at least three different paleomagnetic directions. We tentatively consider these as earliest Paleozoic (Cambrian-

Ordovician), Late Paleozoic (Permo-Carboniferous), and Cenozoic in age. Due to the strong likelihood of local rotations along faults in the region, none of these poles are suitable as reference data for the North American APWP. An early Ordovician U-Pb age of 483.6 ± 1.8 Ma on trachytic dykes around the MMIC suggest a protracted interval of magmatism in the area post-emplacement of the main syenite intrusion at 524 Ma. At present, given the protracted magmatism illustrated in the McClure Mountain igneous complex, it seems possible that magmatism in southern Colorado was generated in a different tectonic setting than southern

Oklahoma in the early Paleozoic.

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Figure 5-1. Map of sampling areas in the Wet Mountains. Simplified geologic map based on mapping of Taylor et al., (1975a,b) and Wobus et al., (1979). Note that the area has a significant number of small-scale faults that are not shown on this simplified map, details shown in original maps. Black squares indicated sampling locations.

Figure 5-2. (a) Plot of 206Pb/238U zircon ages for trachyte (blue) and lamprophyre (orange). Vertical bars show 2σ error, horizontal line is at the weighted mean age calculated. Only analyses with <3% discordance were considered for the trachyte zircons, although this did not significantly change the age calculation (as compared to other discordance filters or naïve mean calculation). (b) Back-scatter electron images of zircons from the trachyte dyke with representative analytical spots shown.

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Figure 5-3. (a) equal area stereonet of our data as compared with other studies (French et al., 1977; Lynnes and Van der Voo, 1984). Mean directions of previous studies with associated confidence intervals shown. (b) A number of dykes only carried a steep directions (c), these correspond with overprints on other dykes. Dykes with steep directions were mentioned but not reported by French et al. (1977). Magnetite (c) was the primary magnetic carrier in all dykes.

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Table 5-1. Combined paleomagnetic data from this study and French et al. (1977) VGP Lat VGP Lon Site Lat Lon Dec Inc a95 k n/N (N) (E) *C161* 38.3832 -105.461 108.7 -0.9 5.4 72.6 11 -15 333 *C168* 38.3377 -105.573 128.3 21.4 9.1 44.9 7 -21 310 *C169* 38.3613 -105.588 115.8 -40.8 6.8 79.9 7 -35 348 FCA1A 38.3 -105.5 124 -1 11.8 33 6 -26 322 FCA2A 38.3 -105.5 121 -7 21.7 9 7 -26 327 FCA2B 38.3 -105.5 121 21 1 -16 316 FCA3A 38.3 -105.5 126 14 53 7 3 -22 315 FCA3B 38.3 -105.5 105 14 26.7 22 3 -7 330 FCA5 38.3 -105.5 132 19 27.7 21 3 -24 308 FCA6 38.3 -105.5 124 13 15.3 26 5 -21 317 FCA7 38.3 -105.5 99 7 24.4 11 5 -5 336 ACC75 38.3 -105.5 308 -8 1 26 136 ACC102 38.3 -105.5 108 4 1 -13 332 ACC287 38.3 -105.5 294 -32 1 7 136 ACC294 38.3 -105.5 266 -30 1 -13 154 FCA4 38.3 -105.5 100 -18 46 8 3 -14 346 ACC13 38.3 -105.5 86 -17 1 -2 354 ACC72 38.3 -105.5 269 20 1 6 173 ACC119 38.3 -105.5 89 -10 1 -2 349 ACC158 38.3 -105.5 254 30 1 -2 187 ACC184 38.3 -105.5 279 32 1 18 173 ACC291 38.3 -105.5 250 20 1 -9 185 ACC292 38.3 -105.5 264 13 1 -1 173 ACC293 38.3 -105.5 283 29 1 20 169 Mean 105 -2 9 10 24 -13 336 Site = name of site in original papers (entries with asterisks our data, other entries from French et al. (1977), Lat = site latitude, Lon = site longitude, D = declination, I = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953, n/N = number of samples/number of sites, VGP Lat = latitude of VGP calculated from direction and site data, VGP Lon = longitude of VGP calculated from direction and site location data

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Table 5-2. Combined paleomagnetic data from this study and French et al. (1977) VGP Lat VGP Lon Site Lat Lon Dec Inc a95 k n/N (N) (E) - *C164* 38.3552 147.5 -33 8 56.4 7 -55 318 105.591 - *C165* 38.3552 151.7 -37.6 5.7 40.5 17 -60 317 105.591 - *C1611* 38.3529 159 -0.2 6 59.3 11 -47 286 105.594 FCA1B 38.3 -105.5 168 -26 17.3 29 4 -63 281 ACC25 38.3 -105.5 143 6 1 -36 303 ACC78 38.3 -105.5 152 16 1 -37 290 ACC285 38.3 -105.5 338 8 1 50 110 ACC286 38.3 -105.5 142 -9 1 -42 310 ACC288 38.3 -105.5 188 4 1 -49 242 ACC289 38.3 -105.5 165 3 1 -48 277 Mean 157 -8 14 13 10 -51 292 Site = name of site in original papers (entries with asterisks our data, other entries from French et al. (1977), Lat = site latitude, Lon = site longitude, D = declination, I = inclination, a95 = cone of 95% confidence about the mean direction, k = kappa precision parameter (Fisher, 1953, n/N = number of samples/number of sites, VGP Lat = latitude of VGP calculated from direction and site data, VGP Lon = longitude of VGP calculated from direction and site location data

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Figure 5-4. (a) equal area stereonet of notable Paleozoic magnetic directions from around the McClure Mountain igneous complex. Note the declination break between the two groupings. (b) Representative Zjiderveld diagrams from these directions – showing the steep overprint directions.

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Figure 5-5. A summary of baked contact test results from around the igneous complex and environs. The geometries are discussed in more detail in the corresponding sections. (a) Baked contact test at site C168, dated at 526 Ma. Overprint and granite directions are steep, while the dyke characteristic directions are shallow east-southeasterly. (b) Baked contact test at site C1610, from gabbroic intrusions near Canon City, putatively Cambrian in age. The gabbroic directions are primarily upward inclination, although marginal samples trend along a great circle towards the steep direction in the granites at high temperatures. (c) Baked contact test at site C1611. Here the late Paleozoic remagnetization direction survives in the granite, while the dyke itself has been more recently remagnetized.

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Figure 5-6. Comparison of Paleozoic data from the McClure Mountain igneous complex with the reference North American apparent polar wander path, along with a schematic summary of age constraints on intrusions comprising the MMIC. Light gray stars (and associated confidence intervals are from prior studies: LM (Larson and Mutschler, 1971), F1+2 and F3 (French et al., 1977), and LT (Lynnes and Van der Voo, 1984).

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CHAPTER 6 CLOSING REMARKS

Geology in the Precambrian has a fundamental data issue. While geologists always work from incomplete datasets, certain time periods and geographic areas either lack data at all or are only constrained by data of questionable quality. Paleomagnetism is a singularly useful technique for constraining ancient paleogeography; it is the best quantitative method for doing so. However, paleomagnetic data taken alone is subject to a host of confounding issues over the long geologic term: remagnetization, structural context, age, stability of magnetic behavior, imperfection of the recording media, measurement error…the list goes on. In an effort to push for a more reliable paleomagnetic database, Van der Voo (1990) proposed seven basic criteria for paleomagnetic data that have guided the ongoing quest for high-quality results in the discipline. A major outcome has been the increased regularity of combining paleomagnetic data with geochronologic data at the site level – thereby directly tying paleomagnetic results with precise ages.

This dissertation has been investigated multiple periods of Earth history, and shown how new spatiotemporal data can confirm or deny large-scale tectonic questions. Yes, the Salem

Block was together with Dharwar Craton. No, magnetization does not survive from the

Neoarchean in Singhbhum craton. Yes, polar wander paths will intersect without remagnetization. No, the Southern Oklahoma Aulacogen may not extend into southern

Colorado.

The questions that we can answer – or models we can generate – are only as good as the data we have. While Precambrian tectonics may lack constraints, this dissertation provides steps forward on the path to answering some of its large questions.

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BIOGRAPHICAL SKETCH

Anthony Pivarunas was introduced to geology when he was told that he could “do physics, but outside”. Taking this literally, he majored in geophysics at SUNY-Geneseo in central New York. Further developing an unhealthy attachment to numbers, he also minored in mathematics. In May of 2015, he received a Bachelor of Arts degree in geophysics, with a minor in mathematics. After being introduced to paleomagnetic research during his undergraduate thesis with Scott Giorgis, Anthony decided to pursue an advanced degree in this discipline under the mentorship of Joe Meert at the University of Florida. A long period of drilling rocks and analyzing magnetic data from three continents ensued, and Anthony realized that the ratio of

“outside” field work to “inside” laboratory work was fatally skewed in favor of the latter. By then, however, it was too late. In the spring of 2019, Anthony received his PhD in geology from the University of Florida.

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