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PETROGRAPHICAL, THERMOCHRONOLOGICAL, AND GEOCHEMICAL ANALYSIS OF PAN-AFRICAN AGE METAMORPHIC AND SHEAR ZONE ROCKS IN WESTERN ETHIOPIA AND SOUTHERN SRI LANKA

A thesis submitted to the Kent State University Graduate College in partial fulfillment of the requirements for the degree of Master of Science

by

Chelsea A. Lyle

May, 2014

Thesis written by

Chelsea A. Lyle

B.A. SUNY Geneseo, 2010

M.S. Kent State University, 2014

Approved by

Daniel Holm, Advisor

Daniel Holm, Chair, Department of Geology

Janis Crowther, Dean, College of Arts and Sciences

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TABLE OF CONTENTS

LIST OF FIGURES ...... v

LIST OF TABLES ...... vi

ACKNOWLEDGEMENTS ...... vii

Summary ...... 1

Chapter I. Introduction ...... 3 Pan-African ...... 6 Arabian-Nubian Shield ...... 7 Geology of Western Ethiopia ...... 10 Geology of Central and South-Eastern Sri Lanka...... 16 Study Area ...... 18

II. Petrographic Characteristics ...... 22 Sample MR8-Ethiopia ...... 24 Sample MR1-Ethiopia ...... 26 Sample MR2-Ethiopia ...... 28 Sample MR3-Ethiopia ...... 30 Sample MR4-Ethiopia ...... 32 Sample MR6-Ethiopia ...... 34 Sample SLR2-Sri Lanka ...... 36 Sample SLR3-Sri Lanka ...... 38 Sample SLR6-Sri Lanka ...... 40 Sample SLR7-Sri Lanka ...... 42 Summary ...... 44

III. 40Ar/39Ar Thermochronology ...... 45 Methodology ...... 45 Results ...... 47 Summary ...... 55

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IV. Lithogeochemsitry and Magnetic Mineralogy ...... 56 Methodology ...... 57 Lithogeochemistry ...... 57 Magnetic Mineralogy ...... 58 Results ...... 60 REE Geochemistry...... 60 Harker Diagrams ...... 63 Isocon Plots ...... 65 Curie Point Temperatures ...... 68 Low-Temperature Magnetic Transitions ...... 71 Summary ...... 74

V. Conclusion ...... 75 Ethiopia ...... 75 Sri Lanka ...... 78

REFERENCES ...... 79

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LIST OF FIGURES Figure

1. Map of East and West during the Pan-African Orogeny ...... 4 2a. Map of Ethiopia ...... 5 2b. Map of Sri Lanka ...... 6 3. Simplified tectonic evolution of the Arabian Nubian Shield ...... 9 4. Map showing location areas for previous studies and the Mengi River megashear zone ...... 12 5. Geologic Map of Mengi River megashear zone ...... 20 6. Outcrop of Mengi River megashear zone L-S tectonics showing a strong stretching lineation ...... 23 7. MR8- photomicrograph and hand sample...... 25 8. MR1- photomicrograph and hand sample...... 27 9. MR2- photomicrograph and hand sample...... 29 10. MR3- photomicrograph and hand sample...... 31 11. MR4- photomicrograph and hand sample...... 33 12. MR6- photomicrograph and hand sample...... 35 13. SLR2- photomicrograph and outcrop ...... 37 14. SLR3- photomicrograph and outcrop ...... 39 15. SLR6- photomicrograph ...... 41 16. SLR7- photomicrograph ...... 43 17. Ethiopian sample (MR3) age and K/Ca spectra ...... 48 18. Sri Lankan sample (SLR2, SLR3, SLR6, SLR7) age and K/Ca spectra ...... 49 19. REE concentration plot for MR1, MR2, MR4, and MR8 ...... 63 20. Harker diagrams for MR1, MR2, and MR4 ...... 64-65 21. Isocon plots ...... 68 22. Curie point temperature plots...... 69-70 23. Low-temperature magnetic transitions ...... 72-73

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LIST OF TABLES

Tables

1. Summary of stages of tectonism during the Pan-African Orogeny ...... 16 2. Age results and isotopic data from Ar/Ar dating analysis ...... 50-52 3. Summary of plateau and total gas ages from 40Ar/39Ar data ...... 52 4. Known low-temperature magnetic transitions ...... 59 5. Known Curie and Néel temperatures ...... 59 6. Oxide weight percent values for sample MR1, MR2, MR4, and MR8 ...60 7. Rare earth and Trace element concentrations in ppm for sample MR1, MR2, MR4, and MR8 ...... 61-62 8. Scale vectors and scaled values used to produce isocon plots ...... 67

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ACKNOWLEDGEMENTS

I wish to express my sincerest appreciation to all those who helped me throughout my thesis project. Firstly, I would like to thank my advisor Daniel Holm for his guidance, patience, and encouragement throughout all of my studies and research at Kent

State University. I would also like to thank my committee members Donald Palmer and

David Hacker for their assistance in my research.

I owe many thanks to Yonathan Admassu for his knowledge, enthusiasm, and resources on Ethiopian geology. Also, a special thanks to Hasanthi Widanagamage for allowing me to use her samples and for her help with the geology of Sri Lanka.

I would like to thank my friends and colleagues at both Kent State University and

SUNY Geneseo, many of whom supported me and listened to me as I worked through my research. Not only have I made great friendships while at school, I have met incredibly intelligent people who are always willing to help in any way possible. It is this support that keeps me driven to keep going every day.

I must thank the many professors at SUNY Geneseo for making me learn to love research and inspiring me to continue on to graduate school. Their continued support has helped me to succeed more than they know.

Lastly, I need to thank my family for their love and support. Especially to my parents, who continue have faith in me and remind me I can do anything.

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SUMMARY

Petrographical, thermochronological, and geochemical analyses were performed on shear zone related Pan-African samples in west Ethiopia and southeast Sri Lanka.

Five sheared samples of either metagranites or and two samples of undeformed granitic rock were collected from the Mengi River megashear zone (MMSZ) in western Ethiopia. Four gneisses were collected from within and around the Highland

Complex- Vijayan Complex boundary zone in central to eastern Sri Lanka.

The MMSZ samples contain abundant fine-grained recrystallized quartz and feldspar grains with secondary late seams of euhedral to anhedral magnetic minerals.

These magnetic minerals are identified as dominantly magnetite with some possible titanomagnetite based on Curie point temperature and low-temperature crystallographic phases. The textures described indicate significant late post-shearing fluid alterations.

40Ar/39Ar dating of hornblende minerals were used to establish a likely age near the end of shearing for the MMSZ. Single grains of hornblende minerals produced older ages of 602 Ma and 630 Ma, whereas bulk separates of hornblende grains produced younger total gas ages of ~480 Ma and ~540 Ma. Younger bulk hornblende ages are likely related to impurities associated with inclusion of lower temperature phases within the hornblende minerals. Biotite grains from the MMSZ produced highly discordant ages between 250 and 350 Ma. Localized late fluid alteration at lower temperatures would explain why these micas are reset compared to micas from previous studies in Ethiopia. It

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is probable that reset ages in the MMSZ are due to fluid transport and localized resetting caused by the tectonomagmatic activity in the Great Valley.

A full rock chemical analysis was obtained for sheared Ethiopian samples.

Varying REE concentrations exist between the samples, indicating varying degrees of alteration. When analyzing oxide weight percents, from least altered to most altered, there is an overall decrease in weight percent of Al2O3 MgO, CaO, Na2O, and P2O5, and an overall increase in Fe2O3, K2O, and MnO. Overall there is an enrichment of trace elements. Variation of concentrations and element mobility are likely due to the late alteration from fluid flow.

Sri Lankan Proterozoic samples exhibit unaltered granoblastic textures of coarse grained pristine minerals, containing primary euhedral magnetic minerals. The study by Widanagamage (2011) on these same rocks revealed higher temperature results

(>600°C) indicating peak of the Highland Complex at 572 Ma, followed shortly by retrograde metamorphism during exhumation at 558 Ma. 40Ar/39Ar ages of bulk hornblende and biotite grains ranged between 490 Ma and 470 Ma, indicating the age of lower temperature cooling (~500-300°C) at the end of the Pan-African Orogeny.

The relatively concordant Ar/Ar ages indicate rapid lower temperature cooling of both the Highland Complex and the Vijayan Complex 60-80 million years after lower crustal exhumation of the Highland Complex. Collectively, the data indicates that following tectonic juxtaposition via accretion, this area has behaved as a single coherent tectonic block by 500 Ma, and has remained unaltered since then.

INTRODUCTION

Growth of by accretion and/or collision results in juxtaposition of deformed and metamorphosed terranes bounded by large-scale shear zones or sutures.

Shear zones are localized, discrete structures that consist of highly deformed rocks, which can provide insight into the kinematics, timing, and conditions during tectonism. During collision, fabrics form reflecting the amount of strain experienced during deformation

(Davis and Reynolds, 1996). Investigation of shear zones and surrounding metamorphic rocks can provide information about pressure-temperature conditions, sense of shear, the amount of displacement, and amount of strain that occurred throughout the shearing event. Additionally different age dating techniques, including 40Ar/39Ar or U/Pb, can be applied to document the time and possibly duration of the shearing event. In this study, I investigate the Mengi River megashear zone (MMSZ) located within the Arabian-Nubian

Shield (ANS), in western Ethiopia and rocks bordering the Highland Complex- Vijayan

Complex boundary zone in Sri Lanka (Figure 1 and 2a,b).

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Figure 1. Map of West (blue) and East (yellow) Gondwana during the Pan-African Orogeny, focusing on the East African Orogen (EAO) collision. The Arabian- Nubian shield (ANS) is located in the northern section of the EAO (modified from Avigad and Gvirtzman, 2009). SF: São Francisco, RP: Rio de la Plata, Mad: Madagascar, SL: Sri Lanka, W,S,N: West, South, and North Australian shields. Approximate study locations are circled in red.

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Figure 2a. Approximate extent of general lithologies in Ethiopia. Western Ethiopia is dominated by (PЄ) rock units. Cenozoic (Cz) and Mesozoic (Mz) rocks are dominant throughout the rest of Ethiopia. The Mengi River Megashear Zone (MMSZ) approximate location is represented by the green. The Great Rift Valley (GRV) trends NE-SW through Ethiopia (Image modified from Google Earth).

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Figure 2b. Approximate extent of Proterozoic domains in Sri Lanka. Northern portion of Sri Lanka is covered by Mesozoic (Mz) and Cenozoic (Cz) rock units. Sri Lanka is dominated by Precambrian (PЄ) rock units, which are divided into three crustal units: Wanni Complex (WC), Highland Complex (HC), and Vijayan Complex (VC). The study area by Sajeev et al. (2010) is indicated by the red box. The study area by Widanagamage (2011) is indicated by the blue box (Image modified from Google Earth; Mathavan et al., 1999).

PAN-AFRICAN OROGENY

The Pan-African Orogeny occurred in the Neoproterozoic to early Paleozoic

(~900-450 Ma) and consisted of multiple involving periods of collisions and extensions of continental and oceanic plates (Kroner and Stern, 2005). Collectively, this poly-orogenesis involved tectonic, magmatic, and metamorphic events that ultimately led to the formation of the supercontinent Gondwana (Figure 1). Gondwana formed after the

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breakup of , a large Neoproterozoic supercontinent. East Gondwana and West

Gondwana were continents which formed shortly after breakup of Rodinia and prior to the supercontinent Gondwana formation. West Gondwana was composed of the West

African, Saharan, Amazonian, São Francisco, Congo, Rio de la Plata, and Kalahari ; roughly modern day Africa and . East Gondwana was composed of the Australian, Indian, Madagascar, and East Antarctic shields; roughly modern day

Australia, India, and (Figure 1, Avigad and Gvirtzman, 2009). Many of these smaller cratons and shields were separated by a network of Neoproterozoic orogens.

The Pan-African Orogeny culminated with the East African Orogen, which resulted from collision between West and East Gondwana (Figure 1). The East African

Orogen is a suture composed of the Arabian Nubian Shield (ANS) to the north and the

Mozambique Belt (MB) to the south. Much of the East African Orogen is comprised of an aggregate of juvenile Neoproterozoic . This juvenile crust formed mostly due to island arc accretion that followed a period of sea floor spreading. Collision of East and

West Gondwana led to the final amalgamation of Gondwana in the Cambrian (Avigad and Gvirtzman, 2009).

ARABIAN-NUBIAN SHIELD

The ANS, which occupies the northern part of the East African Orogen, is a relatively young shield composed of low grade metamorphic and island-arc rocks compared to the in the southern part of the East African Orogen

(Kroner and Stern, 2005). The ANS covers a 3000 km long by 500 km wide region

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extending from the Middle East in the North to Ethiopia in the south (Kroner and Stern,

2005).

Tectonic processes leading to the formation of the ANS in north-east Africa began with rifting of the supercontinent Rodinia at ca. 850 Ma (Figure 3; Abdelsalam and Stern,

1996). Continental rifting led to seafloor spreading and ultimately formation of the

Mozambique Ocean. Around 750 Ma, the Mozambique Ocean began to close resulting in formation of arc and back-arc basins, simultaneous with aggregation of the western and eastern Gondwana continents. occurred about 650 Ma during the

Eastern African Orogeny uniting the western and eastern terranes during formation of

Greater Gondwana. Shortening continued after collision resulting in formation of new shear zones, reactivation of older shear zones, and rotation (Abdelsalam and Stern, 1996).

Around 600-560 Ma, gravitational tectonic collapse occurred due to weakening of thickened orogenic crust. Extension continued until about 530 Ma, resulting in structures throughout the ANS such as extensional basins, NE-SW trending dikes, and intrusion of

A-type mantle-derived granites (Blasband et al., 2000).

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Figure 3. Simplified tectonic evolution of the Arabian Nubian Shield. Not to scale. (Modified from Johnson et al., 2011)

Formation of the Arabian-Nubian Shield is characterized by two periods of magmatism during the Neoproterozoic (Morag et al., 2011). Early magmatism lasted from about 870-740 Ma, primarily resulting from island-arc cycling. Positive zircon eps hf values ranging from +1 to +13 indicate a depleted mantle source consistent with a dominantly juvenile arc source.

Arc magmatism was followed by a period of reduced activity from 740-680 Ma.

In this minimally active period, metabasic dikes intruded into structures formed during the older island-arc cycle. The origin of these dikes, based on an average zircon eps hf

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value of +10, suggests depleted mantle source. Dike intrusions likely occurred during extension associated with crustal thinning and mantle upwelling (Morag et al., 2011).

The final magmatic cycle, which lasted from about 680-580 Ma, was a late/post- collisional cycle characterized by pulses of calc-alkaline granitic plutons (Morag et al.,

2011). Post-collisional zircons also yielded positive eps hf values ranging from +6 to +9, indicating a mantle source with some possible reworking of young crust (Morag et al.,

2011). Avigad and Gvirtzman (2009) suggest post-collisional delamination of the over- thickened mantle lithosphere to partial melting of the ANS lower crust and upper mantle during final formation of ANS (Avigad and Gvirtzman, 2009; Morag et al., 2011).

GEOLOGY OF WESTERN ETHIOPIA

A few geologic studies in western Ethiopia provide critical information on the tectonic and metamorphic history of the region (Figure 4). A study by Ayalew et al.

(1990) was one of the first to date samples in western Ethiopia. They used both Rb-Sr and U-Pb techniques on samples from an area south of the MMSZ. Their samples came from three domains found in the Western Ethiopian Shield (WES); the Baro domain to the west, the Birbir domain located between Baro and Geba domains, and the Geba domain to the east (Ayalew et al., 1990). Each domain is a region of lithology that is composed of a similar geologic unit. The Birbir domain consists of lower grade volcanic, plutonics, wackes, and pelites, surrounded by tectonites. In contrast, the Baro and Geba domains are dominantly gneissic and high-grade rocks. Ayalew et al. (1990) found evidence of three separate plutonic events, as well as periods of metamorphism. Zircons

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collected from the Birbir domain gave concordant U-Pb dates of 830-810 Ma, documenting formation of the earliest intrusions, most likely subduction-related (Ayalew et al., 1990). Zircons from the Baro Domain yielded ages of 780 Ma, interpreted as a second period of plutonism, possibly associated with regional metamorphism due to continental collision. Finally, relatively young zircon U-Pb dates from quartz monzonites and mylonites from Baro and Birbir ranged from 570-550 Ma (Ayalew et al., 1990).

These youngest ages are interpreted as evidence of the final period of plutonism and the end of tectonic activity. Whole-rock and mineral Rb-Sr isotope data from the Birbir quartz diorite yielded ages of 630 Ma and 760 Ma, interpreted as periods of regional metamorphism (Ayalew et al., 1990).

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Figure 4. General sketch of study locations in north-east Africa. Approximate location of MMSZ is shown relative to previous studies by Ayalew et al. (1990, Alemu and Abebe (2000), Grenne et al. (2003), Johnson et al. (2004), Tsige and Abdelsalam (2005), and Abdelsalam et al. (2008). (Modified from Abdelsalam et al., 2008).

A structural and fabric analysis was completed on an area south of the MMSZ in the Aba Sina domain, Chochi domain, and Katta Domain by Alemu and Abebe (2000).

The Aba Sina domain, which is lithologically similar to the more southern Geba domain,

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consists of high-grade gneisses and migmatites. The Chochi domain, which is lithologically similar to the Baro domain, consists of high-grade gneissic rocks. Lastly, the Katta domain, which is lithologically similar to the Birbir domain, consists of low grade metavolcanic and metasedimentary rocks (Alemu and Abebe, 2000).

Three periods of deformation are evident in these domains, as well as in the highly strained Tulu Dimtu Belt located between the Aba Sina and Katta domain. The first period of deformation involves west-verging thrust faults, foliation, and tight to isoclinal folding, likely related to closure of an oceanic basin (Alemu and Abebe, 2000).

The second period of deformation involves steepening of previously deformed features by upright folding and dominantly N-S to NE-SW sinistral strike-slip shearing events.

Kinematic indicators, such as rotated porphyroblasts and porphyroclasts, were found throughout these domains. Crenulation cleavage was also found in the Katta domain

(Alemu and Abebe, 2000). The final third period of deformation involves NW-SE shear zones and faults, which overprint the first and second deformational features present in the domains. The most dominant structure from this deformation is the ductile and sinistral Didesa shear zone, which is located between the Chochi and Katta domain. The

Didesa shear zone is the youngest deformational feature in the area (Alemu and Abebe,

2000).

More recently, Grenne et al. (2003) used REE geochemical, isotope, and age data to test the proposed accretionary history for the ANS east of the MMSZ (Figure 4). U-Pb dating was performed on zircon and titanite mineral grains that were separated from samples from the Kilaj intrusive complex, the Duksi synkinematic intrusion, and the Dogi

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synkinematic intrusion. The Kilaj intrusive complex yielded an age of 866 ± 20 Ma, relating to an early period of plutonism (Grenne et al., 2003). The Duksi synkinematic intrusion yielded an age of 699 ± 2 Ma, and the Dogi synkinematic intrusion yielded an age of 651 ± 5 Ma. These intrusive bodies were also enriched in incompatible elements with positive eps Nd values and therefore interpreted as partial melts at the base of the crust from a collision period during the Pan-African Orogeny. The age gap between the

Duksi and Dogi intrusions suggests that the continental collision was long-term beginning earlier than previously thought and supports the idea of the Mozambique

Ocean closure having occurred between 750-650 Ma (Grenne et al., 2003).

Finally, in a study by Johnson et al. (2004), analysis of the WES was performed though investigation of Sm-Nd, U-Pb, and Rb-Sr data to form a three stage tectonic model (Figure 4). Sm-Nd analysis of samples collected from the Baro and Birbir domains yielded an average age of 1.04 Ga. This period is their pre-accretionary first stage of the tectonic model involving formation in an oceanic environment. This was followed by plutonism 830-815 Ma to form the Birbir Domain. Metamorphism from back-arc spreading soon after formed the Baro and Geba domains. The second stage of the proposed model was the accretionary period, which involved collision between the continental portion of the Geba Domain with the continental margin of the Birbir and

Baro domains, leading to isotopic resetting (Johnson et al., 2004). The final period involved cratonization after plutonism and regional metamorphism. According to this model and based on Rb/Sr ages, tectonic processes causing thermal resetting possibly continued to as late as 420 Ma (Johnson et al., 2004).

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Tectonic gravitational collapse of the Pan-African Orogeny is evident in

Neoproterozoic shear zones, in southern Ethiopia (Figure 4, Tsige and Abdelsalam, 2005, and Abdelsalam et al., 2008). For instance, the Chulul Shear Zone is a low-angle oblique normal shear zone that formed during tectonic collapse following crustal thickening and folding from the collision of the Melka Guba and Chulul domains. This tectonic gravitational collapse occurred about 550-500 Ma, based on K/Ar, 40Ar/39Ar, U/Pb,

Pb/Pb, and Th-U-Pb dating techniques (Tsige and Abdelsalam, 2005). Gravitational collapse marks the final tectonic event of the Pan-African Orogeny.

A summary of the previous studies of western Ethiopia from Ayalew et al. (1990),

Alemu and Abebe (2000), Grenne et al. (2003), and Johnson et al. (2004) is outlined in

Table 1.

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Stages of Western Ayalew et al. Alemu and Grenne et al. Johnson et al. Ethiopian (1990) Abebe (2000) (2003) (2004) Tectonism west-verging thrust faults, Preaccretionary- foliation, Earliest Early period of 1.04 Ga; tight/isoclinal 1 intrusions- plutonism- plutonism folding 830-810 Ma 866 ± 20 Ma occurred 830- (possibly related 815 Ma to oceanic basin closure) Accretionary- N-S to NE-SW collision sinistral strike- between the Regional slip shearing, Collisional continental metamorphism rotated phase- Geba Domain 2 possibly due to porphyroblasts/ 699 ± 2 Ma and the collision porphyroclasts, 650 ± 5 Ma amalgamation 780 Ma crenulation of continental cleavage Birbir and Baro Domains NW-SE shear Cratonization- zones and faults thermal End of tectonic overprinting resetting 3 activity- previous possibly 570-550 Ma features; Didesa continued until shear zone 420 Ma Table 1. This is a summary of the results from Ayalew et al. (1990), Alemu and Abebe (2000), Grenne et al. (2003), and Johnson et al. (2004). Each study proposed stages of tectonism during the Pan-African Orogeny.

GEOLOGY OF CENTRAL AND SOUTH-EASTERN SRI LANKA

The island nation of Sri Lanka was located just east of the Mozambique Belt during the Pan-African Orogeny (Figure 1). Sri Lanka is composed of mostly

Proterozoic rocks aggregated during the Pan-African Orogeny. The island is divided geologically into the three lithotectonic units of the 2-3 Ga central Highland Complex

(HC), the 1-2 Ga eastern Vijayan Complex (VC), and the 1-2 Ga western Wanni

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Complex (WC) (Figure 2b; Cooray, 1994, Kröner et al., 2003). In Sri Lanka, a major thrust boundary zone separates the Highland Complex from the Vijayan Complex

(Kröner, 1991). This thrust zone represents an area where collision occurred between

East and West Gondwana during the Pan-African Orogeny.

In a study by Sajeev et al. (2010), zircon and monazite grains from two different collected from the Highland Complex were dated using SHRIMP U/Pb geochronology techniques. One sample (3107B) contained both prograde and retrograde assemblages, whereas the second sample (0505E) was mostly dominated by retrograde assemblages. Sample 3107B consisted of both zircon and monazite grains, while the second sample, 0505E, had only zircon grains. Sajeev et al. (2010) found zircon core ages of about 1700 Ma and 800-1000 Ma. These zircon cores were surrounded by overgrowths yielding two different ages. The first overgrowth surrounding the core was found to be about 570 Ma and represented peak metamorphism. During this time, there was also textural evidence of garnet growth. The second overgrowth age found was about

551 Ma and represented retrograde metamorphism, most likely during exhumation.

Textural evidence suggests garnet consumption during retrogression.

Widanagamage (2011) performed EMPA monazite dating on high grade metamorphic samples collected in and around the Highland-Vijayan boundary Zone, Sri

Lanka. Her study found that peak metamorphism occurred about 571 Ma, and is a record of continued collision. Retrograde metamorphism, representing exhumation, was found to be about 558 Ma in the eastern Highland Complex and 551 Ma in the western

Highland Complex. These ages from Widanagamage (2011) are very similar to the

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previous study done by Sajeev et al. (2010). Interestingly, the study by Widanagamage

(2011) also found a range of ages from 595-635 Ma preserved in the monazite grains collected from the boundary zone.

The studies by Sajeev et al. (2010) and Widanagamage (2011) produced very similar ages for peak metamorphism and retrograde metamorphism in Sri Lanka. The

635-595 Ma ages from the boundary zone may represent juxtaposition of the HC and the

VC due to terrane collision and crustal thickening at the end of the Pan-African Orogeny.

This collision then led to the ca. 570 Ma peak metamorphism in the HC rock units, followed by the ca. 558-551 Ma retrograde metamorphism from rapid exhumation. Both of these studies provide ages for initiation of collision and continued shearing in Sri

Lanka through high temperature mineral dating. Dating of lower temperature minerals are needed to establish a minimum age for the end of shearing associated with thrusting.

STUDY AREAS

Six samples collected from the central portion of north-northwest trending Mengi

River megashear zone in western Ethiopia provide an excellent opportunity to study large-scale deformation related to the construction of the Arabian-Nubian shield. This particular shear zone is over 20 km long and 1 km wide, located east of Mt. Undo, west of Mt. Fadungo and is crosscut by the Sirkole and Menge Rivers (Figure 5, Mewesha and

Bireda, 1997). The MMSZ cuts through mildly deformed plutonic units of metagranites, schists, quartzite, and metaconglomerates. The northern and southern portions of the

MMSZ juxtapose low grade greenstone units against dominantly metaigneous units. All

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these units surrounding the MMSZ are Late Proterozoic. The MMSZ was possibly formed during the late gravitational collapse stage of the Pan-African Orogeny, as it follows similar trends of shear zones already determined as collapse related.

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LEGEND

Alkali trachyte and phonolite Cenozoic Basalt

Metagranite

Late Metagranodiorite Proterozoic Metaquartz diorite

Biotite/graphite/chlorite schists, quartzite, metasandstone, metaconglomerate, and quartzofeldspathic schist Middle Biotite-hornblende gneisses, muscovite-biotite schist and quartzofeldspathic gneiss Proterozoic

Figure 5. Approximate location of study area of the Mengi River megashear zone (MMSZ) outlined in black. Samples collected where Sirkole River crosses shear zone (red circle). (Image modified from Mewesha and Bireda, 1997)

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Four samples were selected from the study by Widanagamage (2011) for further analysis based on their abundance of fresh amphibole and biotite crystals. These samples were collected from within and around the Highland Complex- Vijayan Complex boundary zone in central to eastern Sri Lanka. One sample was selected from the

Highland Complex, two from the Vijayan Complex, and one from within the Highland

Complex- Vijayan Complex boundary zone. These crustal units are predominantly

Proterozoic (2.0-1.0 Ga), consisting of various gneisses, migmatites, and pegmatites

(Widanagamage, 2011). These samples provide an opportunity to study the deformational history of Sri Lanka during the final stages of the Pan-African Orogeny, specifically during the collision of East and West Gondwana.

PETROGRAPHIC CHARACTERIZATIONS

Five samples were collected from the Mengi River megashear zone by Daniel

Holm and Yonathan Admassu in 2006. The samples are L-S tectonics with a strong stretching lineation oriented steeply to the strike of the foliation (Figure 6). Petrographic analysis can yield important information on kinematics and conditions of shearing that occurred during the late Proterozoic/Cambrian Pan-African Orogeny. A few undeformed to slightly deformed country rock samples from outside the shear zone were also collected. These relatively undeformed samples were also selected for thin section analysis for comparison with the strongly deformed samples (Figure 7-12).

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Figure 6. Outcrop of MMSZ L-S tectonics showing a strong stretching lineation oriented steeply and obliquely to the strike of the foliation. Lineation indicated here in red. Sample collectors shown are Dr. Daniel Holm (left) and Dr. Yonathan Admassu (right) (Photo Credit: Dr. Yonathan Admassu).

Twenty one samples were collected for the study by Widanagamage (2011) from within and adjacent to the Highland Complex- Vijayan Complex boundary zone in Sri

Lanka. Four of those samples were selected for further analysis (Figure 13-16).

Deformed samples were cut, when possible, parallel to lineation and perpendicular to foliation for microstructural analysis. The MMSZ sample thin sections were prepared by Quality Thin Sections in Arizona. Analysis of the shapes and patterns

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formed in the fabrics of samples can yield a detailed history of the area. Three different categories are used to describe fabrics based on what strain information they retain from the shearing event. They are strain-sensitive, strain-insensitive, and composite shape fabrics. Strain-sensitive fabrics ideally contain all record of strain, while strain- insensitive fabrics contain incomplete records. Composite fabrics are more complicated and are not guaranteed to either have or not have a complete record of finite strain (Davis and Reynolds, 1996). Specific structures may also give details about shearing, such as porphyroblasts, porphyroclasts, mica fish, and pressure fringes or shadows to name a few.

For example, a porphyroclast can have wings form during deformation that can determine if sense of shearing from an oriented sample formed during dextral or sinistral movement

(Hammer and Passchier, 1991).

SAMPLE MR8-ETHIOPIA

Sample MR8 is a granitic rock, which is a mildly deformed sample collected from outside the MMSZ. MR8 is mostly coarse grained 45-55% quartz and 15-20% plagioclase, and 20-25% potassium feldspar. A fresh surface of MR8 has shades of white, light brown, grey, and salmon. This sample weathers to a light brown and yellow color. The hand sample reveals a weakly developed fabric (Figure 7a and b).

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A. B.

C. D.

Quartz

Plagioclase

Plagioclase Quartz

Figure 7. A. Hand sample of MR8 showing the weathered surface. B. Hand sample of MR8 showing a fresh cut surface. C, D. Photomicrograph of sample MR8 in XPL in 10X, containing quartz and feldspar. Feldspars are showing alteration by sericitization.

In thin section, anhedral 0.5-6.0 mm quartz and feldspar crystals exhibit moderate undulatory extinction (Figure 7). Many of the plagioclase crystals also have polysynthetic twinning. A few feldspars exhibit perthitic/antiperthitic texture. Most of the plagioclase and potassium feldspars have been mildly sericitized. Along some of these coarser grains of quartz and feldspar are some slender 0.1-1.0 mm amphiboles and

26

interstitial biotites. Minor anhedral biotite and very small opaque crystals are found between coarser feldspar grains.

SAMPLE MR1-ETHIOPIA

Sample MR1 is a strongly lineated and foliated metagranite. The fresh surface of this sample is yellowish white to light orange and green with black speckles (Figure 8).

Weathered surfaces appear more dark orange to brownish in color. The rock consists predominantly of coarse grains of 55-65% quartz and 20-25% potassium feldspar, 5% plagioclase, with finer grains of 5% biotite and <5% of epidote and disseminated fine- grained opaques. Sample MR1 has experienced relatively high strain which is why it is a finer grained foliated and lineated rock compared to other samples collected from the

MMSZ.

27

A.

B. D.

C. Epidote Epidote

E. Epidote F. Epidote

Quartz Quartz

Figure 8. A. Hand sample of MR1 showing the weathered surface. B. Hand sample of MR1 showing a fresh surface cut perpendicular to foliation. C. Photomicrograph of sample MR1 in XPL at 10X, containing quartz ribbons, epidote-rich layers, and late opaque crystals. D. Photomicrograph of sample MR1 in PPL at 10X (same view as C). E. Photomicrograph of sample MR1 in XPL at 5X, containing quartz ribbons, epidote-rich layers, and abundance of scattered opaque crystals. F. Photomicrograph of sample MR1 in PPL at 5X (same view as E).

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In thin section, there are parallel layers of 0.1-0.2 mm wide bands of slightly elongated recrystallized quartz ribbons (Figure 8). The quartz grains are 0.01-1 mm anhedral with undulose extinction. Between the quartz ribbons are very fine grains of recrystallized 0.01-0.1 mm anhedral quartz, feldspar, and epidote. Sample MR1 contains epidote rich layers consisting of mostly epidote and some quartz crystals. MR1 contains some 0.1-1 mm anhedral plagioclase feldspars with deformation twins and fine quartz inclusions. Fine to coarse 0.1-2 mm potassium feldspars exhibit undulose extinction. Fine

0.1-0.2 mm diameter euhedral ferromagnetic crystals overgrow the foliation planes.

These opaque minerals tend to be concentrated in the epidote rich areas and are also more euhedral in these epidote rich areas. Opaque crystals in the quartz and feldspar rich layers are more subhedral to anhedral. Through thin section analysis, there are a few visible

<0.1 mm needle-like slender biotite grains aligned in bands along the foliation.

SAMPLE MR2-ETHIOPIA

MR2 is a strongly lineated and foliated metagranite. The fresh surface of this sample is yellow and light brown with speckles of black (Figure 9). MR2 weathers to a yellowish brown color. This metagranite is predominantly composed of coarse grains of about 40-50% quartz, 5-10% plagioclase and 30-40% potassium feldspars, with fine grains of <10% amphibole. Ferromagnetic minerals and epidote are also present as fine grained minor accessory minerals. Compared to other samples collected from the MMSZ, sample MR2 has experienced relatively moderate strain.

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A. B.

C. D. Quartz

Epidote Quartz

Feldspar

Figure 9. A. Hand sample of MR2 showing the weathered surface. B. Hand sample of MR2 showing a fresh cut surface. C. Photomicrograph of sample MR2 in XPL at 10X, containing perthitic feldspar and quartz. D. Photomicrograph of MR2 in PPL at 10X, containing anhedral magnetic crystals in association with high relief epidote.

In microphotograph, there are parallel layers of 0.1-0.5 mm wide bands of elongated recrystallized quartz ribbons. Between these quartz ribbons are zones of quartz and feldspar. The recrystallized quartz and feldspar crystals are 0.01-1.0 mm anhedral with undulose extinction. A few plagioclase feldpsars that are <0.5 mm exhibit polysynthetic twinning and a few microclines <0.5 mm exhibit tartan twinning. Some

30

feldspars also show flame perthite. There are also a few anhedral perthitic feldspars, ranging from 0.1-1.0 mm. Elongated <0.1 mm anhedral slender amphiboles are found in bands along the foliation. Medium to fine (<0.1-0.5 mm) anhedral opaque crystals can be found overgrown on the foliation planes (Figure 9). These magnetic crystals tend to concentrate in bands that contain sparse amounts of anhedral epidote.

SAMPLE MR3-ETHIOPIA

MR3 is a sheared sample from the MMSZ of an (Figure 10). This hand sample is foliated and lineated with a fresh surface of dark grey to black color.

Weathered surface of MR3 is a golden brown color. MR3 is predominantly 60-75% coarse grained amphiboles. It also consists of fine grains of 20-25% biotite, 5% feldspar, and 5% quartz. MR3 also has a short 0.2-0.8 mm wide quartz vein.

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A.

B. C.

Hornblende

Figure 10. A. Hand sample of MR3 showing the weathered surface. B. Photomicrograph of sample MR3 in XPL at 10X, containing hornblende, biotite, quartz, and opaques. C. Photomicrograph of sample MR3 in PPL at 10X (same view as B).

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In thin section, there are 0.1-2.0 mm anhedral amphiboles and 0.1-1.0 mm anhedral slender elongated biotites aligned along the foliation planes, some of which are interstitial biotite grains. Some poikilitic amphiboles are present containing very fine quartz grains. Recrystallized anhedral 0.1-0.5 mm quartz and feldspars exhibit undulose extinction throughout this sample. There are anhedral to euhedral 0.1-0.5 mm opaques aligned within the foliation. A few euhedral opaques have a bladed morphology (Figure

10).

SAMPLE MR4-ETHIOPIA

Sample MR4 is a strongly foliated and lineated metagranite (Figure 11). This sample is coarse grained, consisting predominantly of about 40-50% quartz, 5% plagioclase, and 40-45% potassium feldspar crystals. Biotite and clay minerals make up about <10% of the sample found along foliation. A fresh surface consists of cloudy to clear, salmon, and dark brown colors. MR4 weathers to a brown to rusty orange color.

Compared to other samples collected from the MMSZ, sample MR4 has is coarser grained and has experienced relatively low strain.

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A.

B.

C.

Feldspar

Quartz

D. E.

0.1 mm Quartz

Figure 11. A. Hand sample of MR4 showing the weathered surfaces. B. Hand sample showing fresh surface. C, D, and E. Photomicrograph of sample MR4 in both XPL and PPL in 10X, containing quartz, feldspars, and a seam of opaque crystals (D and E are the same view).

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In thin section, there is some banding of recrystallized elongated quartz and feldspars, ranging from 0.1-2 mm. Recrystallized <0.1 mm grains of feldspar and quartz are located between the foliation planes (Figure 11). The majority of the crystals have undulatory extinction. Some crystals are coarse anhedral poikilitic feldspars containing quartz. MR4 also contains many coarse anhedral feldspars with perthitic texture. MR4 has anhedral <0.2 mm magnetic crystals that are present in thin wavy seams and as tiny isolated crystals disseminated thoughout the rock. Another fine anhedral <0.1 mm mineral exhibiting high relief may be epidote grains.

SAMPLE MR6-ETHIOPIA

Sample MR6 is a strongly foliated and lineated metagranite (Figure 12). This sample is highly weathered and fractured with abundant clay material between grains.

MR6 contains mostly coarse grains of 40-50% quartz, 25-35% potassium feldspar, 10-

15% amphibole, and about 5-10% fine grained biotites. The fresh surfaces of MR6 are light salmon to white colored. Weathered surfaces are brown colored.

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A. B.

C.

Quartz Feldspar

D. E.

Figure 12. A. Hand sample of MR6 showing the weathered surface. B. Hand sample of MR6 showing a fresh cut surface. C. Photomicrograph of sample MR6 in XPL at 10X, containing quartz and feldspar. D. Photomicrograph of sample MR6 in XPL at 10X, containing quartz, feldspar, biotite, and epidote. E. Photomicrograph of sample MR6 in PPL at 10X (same view as D).

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In thin section, elongated bands of recrystallized 0.1-1 mm quartz ribbons form along foliation planes (Figure 12). Along these foliation planes are aligned thin wavy seams of highly fractured anhedral elongated biotites, ranging from <0.1-1 mm. These biotite crystals are highly weathered and altered. Some biotites have relics of amphiboles in them. Recrystallized fractured anhedral <0.1-1 mm potassium feldspar and quartz crystals are found with undulatory extinction throughout the thin section. Perthitic texture is found in some coarser grained feldspar crystals. There are some aligned, anhedral, high relief, <0.1 mm minerals present that may possibly be epidote crystals.

SAMPLE SLR2-SRI LANKA

Sample SLR2 is strongly foliated charnockite gneiss from a locality of high grade metamorphic rocks from the Highland Complex. A fresh surface consists of black and greyish green colors. SLR2 is medium grained and is 10-15% plagioclase, 35-40% potassium feldspar, 20% hypersthene, 10% amphibole, 10% biotite, and 10% quartz

(Figure 13).

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A.

B. C. Biotite Biotite

Plagioclase Plagioclase K-feldspar

D.

E.

Hypersthene Hypersthene Plagioclase

Figure 13. A. Outcrop view showing weathered surface of sample SLR2. B. Photomicrograph of sample SLR2 in PPL at 10X, containing feldspars, opaques, and biotite crystals. C. Photomicrograph of sample SLR2 in XPL at 10X (same view as B). D. Photomicrograph of sample SLR2 in PPL at 4X, containing hypersthene and plagioclase. E. Thin section of sample SLR2 in XPL at 4X (same view as D).

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In thin section, there are anhedral to subhedral, 0.5-1.0 mm orthoclase (30%) and plagioclase (20%) grains. The plagioclase grains show polysynthetic twinning. A few of the orthoclase exhibit simple twinning and/or undulatory extinction. Anhedral, 0.5-0.8 mm quartz also exhibits undulatory extinction. Some finer recrystallized quartz grains form the matrix. There is also some myrmekite intergrowth between some quartz and feldspar grains. SLR2 contains high relief, anhedral to subhedral, fractured, 0.5-1.0 mm hypersthene grains. Euhedral to subhedral prismatic 0.3-1.0 mm biotites are aligned with the foliation plane (Figure 13). SLR2 has subhedral 0.5-1.0 mm amphibole grains. In thin section, there are also visible accessory minerals, such as opaques, apatite, calcite, and zircon. The opaque crystals are subhedral and bimodal, with some crystals <0.1mm and some crystals about 0.5 mm in length.

SAMPLE SLR3-SRI LANKA

Sample SLR3 is a garnet bearing charnockite gneiss from a locality of highly sheared rock from the boundary zone between the Highland Complex and the Vijayan

Complex. A fresh surface of this sample is white with black spots. SLR3 is strongly foliated consisting of coarse grains of 10-15% garnet, 15-25% amphibole, 15-20% quartz,

5-10% plagioclase, and 30-35% potassium feldspar (Figure 14).

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A.

B. C. Amphibole Amphibole

Feldspar Feldspar

Figure 14. A. Outcrop view of sample SLR3 showing the weathered surface. B. Photomicrograph of sample SLR3 in XPL at 10X, containing feldspar, amphibole, and opaque crystals. C. Photomicrograph of sample SLR3 in PPL at 10X (same view as B).

In thin section, there are subhedral, 1.0-2.0 mm, fractured garnets. SLR3 has subhedral, 0.1-1.0 mm plagioclase with polysynthetic twinning. Subhedral, <0.1 mm microclines exhibit tartan twinning. SLR3 contains anhedral 0.1-1.0 mm potassium feldspars with undulatory extinction. Many feldspars exhibit well preserved perthitic/antiperthitic texture. SLR3 contains subhedral, 0.1- 1.5 mm hornblende grains

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(Figure 14). Quartz grains are anhedral to subhedral, 0.1-0.5 mm with moderate undulatory extinction. There are euhedral to subhedral, elongate <0.5 mm magnetic crystals. Some opaque fine grained crystals are bladed. Zircon and/or monazite, and apatite are found as accessory minerals in SLR3.

SAMPLE SLR6-SRI LANKA

Sample SLR6 is a foliated garnet bearing charnockite gneiss from a locality of medium to high grade rocks from the Vijayan Complex. SLR6 is coarse grained and is

30-35% plagioclase, 20-25% hypersthene, 10% biotite, 5-10% garnet, 5-10% quartz, and

5-10% orthoclase (Figure 15). A fresh surface is dark and light greens with spots of red and black.

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A. B.

Hypersthene Hypersthene

Biotite Biotite

C. D.

Hypersthene Hypersthene

Plagioclase

Biotite Biotite

Figure 15. A. Photomicrograph of sample SLR6 in XPL at 10X, containing hypersthene, biotite, feldspar, and opaque crystals. B. Photomicrograph of sample SLR6 in PPL at 10X (same view as A). C. Photomicrograph of sample SLR6 in XPL at 10X, containing hypersthene, biotite, feldspar, and elongated opaque crystals. D. Photomicrograph of sample SLR6 in PPL at 10X (same view as C).

In thin section, subhedral to anhedral, 0.1-2.0 mm plagioclase crystals exhibit polysynthetic twinning. Anhedral 0.1-1.0 mm potassium feldspars exhibit undulatory extinction. A few feldspars also exhibit simple twinning. Anhedral, 0.1-2.0 mm quartz

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also exhibit undulatory extinction. There is high relief, subhedral, 0.5-2.0 mm, fractured hypersthene. SLR6 also has euhedral to subhedral, 0.05-0.5 mm, slender, prismatic biotite grains (Figure 15). Subhedral to euhedral, 0.1-0.5 mm opaque crystals are spatially associated with the mafic minerals. There are subhedral, 0.1-0.5 mm garnets throughout SLR6.

SAMPLE SLR7-SRI LANKA

Sample SLR7 is foliated biotite gneiss from a locality of medium to high grade rocks from the Vijayan Complex. SLR7 is coarse grained and predominantly 20-30% plagioclase, 10-15% quartz, 20-25% biotite, and 20-25% orthoclase (Figure 16). SLR7 has titanomagnetite, garnet, phlogopite, and muscovite as accessory minerals. A fresh surface consists of white and black colors. SLR7 weathers to a dark grey color.

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A. Plagioclase B.

Biotite Biotite

C. D.

Biotite Biotite

Orthoclase Orthoclase Quartz Quartz

Figure 16. A. Photomicrograph of sample SLR7 in XPL at 10X, containing quartz,

plagioclase, biotite, and opaque crystals. B. Photomicrograph of sample SLR7 in PPL at 10X (same view as A). C. Photomicrograph of sample SLR7 in XPL at

10X, containing quartz, orthoclase, biotite, and magnetic crystals. D. Photomicrograph of sample SLR7 in PPL at 10X (same view as C). Feldspars exhibit minor alteration by sericitization.

In thin section, subhedral, 0.1-1.0 mm plagioclase grains exhibit polysynthetic twinning. SLR7 has subhedral, 0.01-1.0 mm orthoclase grains, some with moderate undulatory extinction. Anhedral 0.01-1.0 mm quartz crystals exhibit undulatory

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extinction. There are subhedral, 0.01-0.5 mm, slender biotites (Figure 16). Much of this sample is highly weathered and altered. Some alteration of biotite to chlorite is present.

Many of the coarser feldspar crystals are sericitized. There are also very few sparse anhedral to subhedral, <0.3 mm ferromagnetic grains, some of which are bladed.

SUMMARY

The MR suite of sheared Pan-Africa rocks from Ethiopia exhibit textural indications of late fluid alteration. MR samples contain an abundance of fine grained recrystallized quartz and feldspars. Magnetic phase minerals appear as euhedral to anhedral secondary late seams.

The SLR suite of high grade metamorphosed Pan-African rocks from Sri Lanka exhibit unaltered textures. SLR samples contain coarse grained pristine minerals.

Euhedral opaque minerals present appear to be primary.

40Ar/39Ar THERMOCHRONOLOGY

Mineral age data were collected via 40Ar/39Ar dating in an effort to establish a time near the end of shearing for the Mengi River megashear zone in Ethiopia and end of thrusting in the thrust boundary zone in Sri Lanka. 40Ar/39Ar dating provides ages of cooling and/or recrystallization based on the minerals closure temperature. Closure occurs at lower temperature ranges where daughter products of radioactive decay start to be retained in the minerals crystal lattice, which for hornblende and biotite minerals are

500ºC and 300ºC respectively. The process of 40Ar/39Ar dating first involved separating biotite and hornblende minerals from an amphibolite (sample MR3) from the MMSZ.

Biotite grains were also collected from three Sri Lankan gneiss samples (SLR2, SLR6, and SLR7) and hornblende grains were collected from one Sri Lankan gneiss sample

(SLR3). These samples were selected for 40Ar/39Ar analysis because of their appropriate grain size and minimum alteration of biotite and/or hornblende grains.

METHODOLOGY

Samples were crushed using first the Braun Chipmunk VD67 and then the BICO

Model UA Disk Pulverizer. Machines were thoroughly cleaned between each use to avoid contamination. Samples were then sieved using sieve sizes of 30, 45, 60 and 80.

These mineral grains were then washed and oven-dried overnight. Once dried, a hand magnet was used to remove magnetic minerals from the sieved mineral grains to avoid

45

46

picking the magnetic grains for 40Ar/39Ar analysis. Once the magnetic minerals were removed, the remaining mineral grains were used to collect the biotite and hornblende separates. Mineral grains that were retained by the 45 and 60 sieve were used. Grains retained by the 30 sieve did not have any singular clean grains. A minimum of 50 grains, either biotite and/or hornblende, from each sample was hand-picked using a microscope.

The largest and least altered grains of biotite and hornblende were selected.

Samples were sent to and analyzed in the New Mexico Geochronological

Research Laboratory. Mineral separates of biotite and honblende were loaded into aluminum discs and irradiated for 40 hours at the U.S Geological Survey’s TRIGA

Reactor in Denver, Colorado. A neutron flux monitor of Fish Canyon Tuff sanidine (FC-

2), which has a known age of 28.02 Ma, was also irradiated with the biotite and honblende separates (Renne et al., 1998). At the New Mexico Geochronological Research

Laboratory, samples were analyzed using the Thermo-Fisher Scientific ARGUS VI mass spectrometer on line with an automated all-metal extraction system. The mass spectrometer used had a sensitivity of 1E-16 mol/fA. Samples were analyzed using an incremental step-heating method with a 75W Photon-Machine 810 nm diode laser.

Released Ar gas was cleaned of reactive gases using a 1 SAES GP-50 getter operated at

450ºC for five minutes. Mass spectrometer blanks and backgrounds were measured throughout analysis, with total system blank and background of 25 ± 1%, 0.04 ± 20%,

0.05 ± 100%, 0.30 ± 85%, 0.10 ± 1.5%, x10-17 moles for masses of 40, 39, 38, 37, and 36, respectively. Through extrapolation of the sanidine flux monitor, neutron flux parameters, known as J-factors, were determined. These J-factors have a precision of

47

~±0.01% determined through use of a CO2 laser-fusion analysis performed on 6 individual crystals from each of 4 radial positions around the irradiation tray. K-glass and CaF2 were used to determine correction factors for interfering nuclear reactions.

40 39 36 37 There corrections were ( Ar/ Ar)K = 0.008068 ± 0.000068, ( Ar/ Ar)Ca = 0.000273 ±

39 37 0.0000002, and ( Ar/ Ar)Ca = 0.000698 ± 0.0000078.

The specific closure temperatures for the minerals analyzed are 500ºC for the hornblende grains and 300ºC for the biotite grains (McDougall and Harrison, 1999).

Plateau ages were determined based on the relatively flattest portion of each age spectrum when large portions of 39Ar gas were released.

RESULTS

The age spectra of the dated biotite and hornblende grains for both the Ethiopia and Sri Lanka samples are shown in Figures 17 and 18, respectively, and analytical results presented in Tables 2 and 3. When possible, plateau ages should be defined by at least 50% of the argon gas released during the incremental step-heating. However, the bulk hornblende MR3, biotite MR3, SLR3 and SLR6 samples did not release over 50% of the argon gas in what was determined as the flattest part of the age spectra.

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Figure 17. Ethiopian sample MR3 age and K/Ca spectra. (a) bulk hornblende and biotite, (b) bulk hornblende, (c,d) single crystal of hornblende (Modified from Heizler, 2013).

49

Figure 18. Sri Lankan sample age and K/Ca spectra. (a) sample SLR3 hornblende, (b) sample SLR2 biotite, (c) sample SLR6 biotite, (d) sample SLR7 biotite (Modified from Heizler, 2013).

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Table 2. Age results and isotopic data from Ar/Ar dating (Modified from Heizler, 2013).

51

Table 2 (continued)

52

Table 2 (continued).

Table 3. Summary of plateau ages and total gas ages from the samples dated (Modified from Heizler, 2013)

53

Results from the Ethiopia samples (Figure 16) show a very complex spectra with varying ages. The MR3 (61457-01) biotite mineral grains produced incremental ages between about 250-350 Ma and had an age spectrum with no concordant consecutive heating steps and therefore no meaningful plateau age. The 40Ar/39Ar total gas age (TGA) for the MR3 biotite is 288.90 ± 0.14 Ma.

Two bulk hornblende samples were also dated from sample MR3. The first MR3

(61456-01) bulk hornblende initially produced an age around 550 Ma and followed by a drastic decline to about 350 Ma before gradually increasing back up to about 550 Ma.

This sample of MR3 (61456-01) hornblende has no clear flat portion and therefore does not yield a meaningful plateau age. MR3 (61456-01) bulk hornblende has a 40Ar/39Ar total gas age of 478.36 ± 0.09 Ma. The second aliquot, MR3 (61456-02) bulk hornblende, yielded more consistent age increments between 500 and 550 Ma, with a near-plateau age of 556.6 ± 1.2 Ma which was calculated from 45.3% of 39Ar gas released from seven consecutive plateau steps. MR3 (61456-02) bulk hornblende has a 40Ar/39Ar total gas age of 538.48 ± 0.07 Ma.

Two single crystals of hornblende were also dated from sample MR3. The first

MR3 (61456-03) hornblende single crystal produced a plateau age of 602.1 ± 0.4 Ma calculated from 88.4% of 39Ar gas released from five consecutive plateau steps. MR3

(61456-03) hornblende single crystal has a total gas age of 632.31 ± 0.64 Ma. The second aliquot, MR3 (61456-04) hornblende single grain, has a plateau age of 630.1 ± 0.6

Ma calculated from 89.1% of 39Ar gas released from four consecutive plateau steps.

MR3 (61456-04) hornblende single crystal has a total gas age of 662.39 ± 0.53 Ma.

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For the Sri Lankan samples dated, 40Ar/39Ar ages were well-behaved with both hornblende and biotite grains giving consistent age increments between 470-490 Ma.

Only one aliquot of hornblende grains were dated from the Sri Lankan samples. SLR3

(61459-01) hornblende grains produced a pseudo-plateau age of 490.3 ± 0.1 Ma which was calculated from 38.4% of 39Ar gas released from two consecutive plateau steps.

SLR3 (61459-01) has a total gas age of 488.70 ± 0.06 Ma.

Biotite grains were dated from three different samples from Sri Lanka. Sample

SLR2 (61458-01) produced a biotite plateau age of 478.12 ± 0.05 Ma calculated from

68.4% of 39Ar gas released from two consecutive plateau steps. Sample SLR2 (61458-01) has a total gas age of 479.14 ± 0.05 Ma. Sample SLR6 (61460-01) produced a biotite near-plateau age of 488.10 ± 0.55 Ma which was calculated from 44.4% of 39Ar gas released from two consecutive steps. Sample SLR6 (61460-01) has a total gas age of

483.75 ± 0.07 Ma. Lastly, Sample SLR7 (61461-01) produced a biotite plateau age of

477.76 ± 0.53 Ma calculated from 63.9% of 39Ar gas released from seven consecutive steps. Sample SLR7 (61461-01) has a total gas age of 472.68 ± 0.05 Ma.

K/Ca ratios were also determined for each age increment. For the MR suite, K/Ca ratios all fell within a small range of ratio values indicating purity of phases present.

Sample MR3 biotite shows a decline in K/Ca at the highest temperature fractions, possibly indicating the presence of minor amounts of hornblende. The rest of the MR3 samples ranged between 0 and 4.0. For the SLR suite, a K/Ca ratio for SLR3 hornblende was a very small range from 0.10 to 0.46. Biotite samples SLR2 yielded K/Ca ratios

55

typical for this high K phase. No K/Ca ratios were calculated for samples SLR6 and

SLR7.

SUMMARY

Overall, the MR suite from Ethiopia has a complex history with samples producing a wide range of ages (Table 3). Single grains of hornblende minerals produced the oldest ages of 602 Ma and 630 Ma. Bulk separates of hornblende produced significantly younger total gas ages of ~480 and 540 Ma. MR3 biotite grains produced ages that ranged from 250 Ma to 350 Ma. The complex spectra produced are consistent with petrographic evidence for late alteration. The age difference between the hornblende grains and biotite grains is consistent with their different closure temperatures. Late alterations likely occurred at lower temperatures, which would more heavily affect the biotites.

The SLR series from Sri Lanka produced more precise ages that ranged between

490 Ma and 470 Ma. These SLR sample ages, along with the petrographic analysis, appear unaltered and may indicate the age of low temperature cooling at the end of the

Pan-African Orogeny.

LITHOGEOCHEMISTRY AND MAGNETIC MINERALOGY

Lithogeochemical methods were used to obtain a full rock chemical analysis on four samples selected from the MR suite of rocks from the MMSZ in Ethiopia. The four samples chosen consisted of similar mineralogical compositions. Three of the samples

(MR1, MR2, and MR4) were sheared samples collected from within the MMSZ. One sample (MR8) was an undeformed to mildly deformed country rock collected from outside the MMSZ. Comparative analysis is based on the assumption that sample MR8 represents the original country rock that was variably sheared within the MMSZ. Grain size reduction in hand sample analysis (Figure 8, 9, and 11) suggests that sample MR1 experienced the most relative strain, sample MR2 experienced moderate relative strain, and sample MR4 experienced the least relative strain. The goal is to test whether or not element mobility relates to intensity of strain or fluid flow following deformation

(Rolland et al., 2003).

Identification of ferromagnetic minerals can aid in the interpretation of petrographical and geochemical analyses of rock suites. Because of their low concentrations, fine grain size, and opacity, optical identification of magnetic minerals can be both difficult and limited. In this chapter I use Curie temperature and the Quantum

Design Magnetic Property Measurement System (MPMS-7) to quantitatively analyze and identify magnetic minerals present in samples from the MR suite of rocks from Ethiopia.

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57

The MR suite rocks exhibit texturally late alteration and coarse to fine-grained ferromagnetic minerals.

METHODOLOGY

Lithogeochemistry

Samples were processed at Activation Laboratories Ltd. (Actlabs) in Ontario,

Canada. At least 5 g of each of the four samples (MR1, MR2, MR4, and MR8) were sent for analysis. Selected samples were fresh and unweathered. Rock samples were pulverized using mild steel mills to avoid Cr and Ni contamination (code RX4).

Pulverized samples must have at least 95% pulp passing the 150 mesh (105 microns)

(Actlabs.com, 2013).

Pulverized samples were then analyzed under code 4litho, which is a combination of lithium metaborate/tetraborate fusion inductively coupled plasma (ICP) whole rock and trace element inductively coupled plasma mass spectrometry (ICP/MS). For the lithium metaborate/tetraborate fusion ICP, samples were analyzed in batches containing a reagent blank, certified reference material and 17% replicates. Pulverized material was combined with a flux of lithium metaborate and lithium tetraborate. Material is fused in a furnace and then mixed and completely dissolved into a 5% nitric acid solution. ICP yields oxide weight percent for SiO2, Al2O3, Fe2O3, MgO, MnO, CaO, TiO2, Na2O, K2O,

P2O5, and loss on ignition (LOI). Oxide weight percent should total between 98.5% and

101%. ICP also yields trace element amounts in ppm for Ba, Sr, Y, Zr, Sc, Be, and V. For

ICP/MS, the fused samples used for ICP were diluted and each group of sample was

58

analyzed along with three blanks and five controls. Analysis was done with the Perkin

Elmer Sciex ELAN 6000, 6100, or 9000 ICP/MS and yielded trace and rare earth element data with detection limits in ppm (Actlabs.com, 2013).

Magnetic Mineralogy

Samples were also processed at the New Mexico Highlands University paleomagnetic-rock magnetic laboratory using the MPMS-7 that measures the magnetic moment of particles. Selected rock chips were fresh and unweathered containing ferromagnetic minerals. Rock samples were analyzed using zero-field cooling/warming experiments in order to quantitatively determine the composition of the magnetic phases present in the samples. Zero-field cooling experiments began by inducing magnetization in a 1.5 Tesla field at 300 Kelvin. Samples were then cooled and remanence measurements were taken at 10 Kelvin increments until reaching the final temperature of

10 Kelvin. Zero-field warming experiments also involved inducing magnetization in a 1.5

Tesla field at 20 Kelvin with remanence recorded every 10 Kelvin as the sample was warmed to 300 Kelvin. The MPMS is used primarily for magnetic mineral identification based on low-temperature crystallographic transitions (Rochette et al., 2011; Table 4).

These transitions are related to a change in the crystallographic structure which induces a change in the net magnetic moment of the sample. For example, pyrrhotite will transition from the standard room temperature monoclinic structure to a triclinic structure at an extremely low temperature of 32 K (Rochette et al., 2011; Wolfers et al., 2011).

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Mineral Temperature (K) Transition Name

Pyrrhotite 32 K Besnus

Magnetite 118 K Verwey

Hematite 265 K Morin

Table 4. Known low-temperature magnetic transitions (Rochette et al., 2011; Besnus and Meyer, 1964; Nagata et al., 1964; Kosterov, 2007).

Evaluation of magnetic mineralogy was also assessed on the basis of estimating their Curie point (or Néel) temperatures (Tauxe, 1998; Table 5). Continuous low-field susceptibility versus temperature experiments were conducted on samples placed in an

Argon atmosphere using a CS4 furnace attachment for a multi-function kappabridge

(MFK1-A). For this, stepwise heating and cooling occurred from 25ºC to 700ºC to 40ºC.

These experiments can also assist in determining mixtures of magnetic phases within each sample.

Mineral Curie (or Néel) Temperature (ºC)

Pyrrhotite 325ºC

Magnetite 580ºC

Maghemite 590 - 675ºC

Hematite 675ºC

Table 5. Known Curie and Néel temperatures for ferromagnetic minerals used for thermomagnetic analysis (Tauxe, 1998)

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RESULTS

All oxide weight percent values, rare earth element (REE), and trace element data collected for the four Ethiopia samples are presented in Tables 6 and 7. These data were analyzed though normalized REE concentration plots, Harker diagrams, and isocon graphs, shown in Figures 19, 20, and 21, respectively.

REE Geochemistry

Analyte Unit Detection Analysis Symbol Symbol Limit Method MR1 MR2 MR4 MR8

SiO2 % 0.01 FUS-ICP 74.47 76.87 75.66 71.52

Al2O3 % 0.01 FUS-ICP 13.35 11.49 11.27 15.41

Fe2O3(T) % 0.01 FUS-ICP 1.83 3.29 2.6 3.2

MnO % 0.001 FUS-ICP 0.021 0.053 0.033 0.025

MgO % 0.01 FUS-ICP 0.1 0.05 0.04 0.42

CaO % 0.01 FUS-ICP 2.6 0.29 0.18 0.35

Na2O % 0.01 FUS-ICP 5.21 3.53 3.25 4.77

K2O % 0.01 FUS-ICP 0.49 4.71 4.68 2.23

TiO2 % 0.001 FUS-ICP 0.187 0.199 0.155 0.329

P2O5 % 0.01 FUS-ICP 0.02 0.01 0.01 0.1

LOI % FUS-ICP 0.1 0.1 0.2 1.16

Total % 0.01 FUS-ICP 98.37 100.6 98.08 99.51

Table 6. Oxide weight percent values for sample MR1, MR2, MR4, and MR8.

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Analyte Unit Detection Analysis Symbol Symbol Limit Method MR1 MR2 MR4 MR8

Ag ppm 0.5 FUS-MS 1.5 5.7 1.4

In ppm 0.2 FUS-MS < 0.2 < 0.2 < 0.2 < 0.2

Sn ppm 1 FUS-MS 3 7 3 < 1

Sb ppm 0.5 FUS-MS 0.7 0.6 0.8 0.6

Cs ppm 0.5 FUS-MS < 0.5 0.6 < 0.5 0.5

Ba ppm 3 FUS-ICP 190 256 191 1339

La ppm 0.1 FUS-MS 42.9 86.4 48.1 23.2

Ce ppm 0.1 FUS-MS 83 194 111 41.1

Pr ppm 0.05 FUS-MS 8.42 23.1 13.6 3.8

Nd ppm 0.1 FUS-MS 30.8 90.1 53 13.7

Sm ppm 0.1 FUS-MS 5.7 20.6 10.5 2.1

Eu ppm 0.05 FUS-MS 0.68 2.6 1.12 0.81

Gd ppm 0.1 FUS-MS 5.6 21.6 9.4 1.9

Tb ppm 0.1 FUS-MS 1 3.7 1.4 0.3

Dy ppm 0.1 FUS-MS 6 22.6 8 1.6

Ho ppm 0.1 FUS-MS 1.2 4.6 1.5 0.3

Er ppm 0.1 FUS-MS 3.7 13.5 4.4 0.9

Tm ppm 0.05 FUS-MS 0.56 2 0.68 0.15

Yb ppm 0.1 FUS-MS 3.7 13.1 4.6 1

Lu ppm 0.04 FUS-MS 0.57 2.09 0.78 0.18

Hf ppm 0.2 FUS-MS 5.8 24.3 13 3.9

Ta ppm 0.1 FUS-MS 1.4 3.1 1 0.5

W ppm 1 FUS-MS < 1 < 1 < 1 2

Tl ppm 0.1 FUS-MS 0.1 0.3 0.3 0.1

Pb ppm 5 FUS-MS 8 25 8 13

Bi ppm 0.4 FUS-MS < 0.4 < 0.4 < 0.4 < 0.4

Th ppm 0.1 FUS-MS 8.3 12.2 4.9 6.7

U ppm 0.1 FUS-MS 3.6 4.9 1.6 1.5

Table 7. Rare earth and trace element concentrations in ppm for sample MR1, MR2, MR4, and MR8.

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Analyte Unit Detection Analysis Symbol Symbol Limit Method MR1 MR2 MR4 MR8

Sc ppm 1 FUS-ICP 2 < 1 < 1 4

Be ppm 1 FUS-ICP 2 4 3 2

V ppm 5 FUS-ICP 34 < 5 5 23

Cr ppm 20 FUS-MS < 20 < 20 < 20 < 20

Co ppm 1 FUS-MS 1 1 1 4

Ni ppm 20 FUS-MS < 20 < 20 < 20 < 20

Cu ppm 10 FUS-MS 10 < 10 < 10 10

Zn ppm 30 FUS-MS < 30 120 30 40

Ga ppm 1 FUS-MS 18 28 26 16

Ge ppm 1 FUS-MS 2 3 2 2

As ppm 5 FUS-MS < 5 < 5 < 5 18

Rb ppm 2 FUS-MS 7 97 89 52

Sr ppm 2 FUS-ICP 282 19 22 307

Y ppm 2 FUS-ICP 34 120 30 8

Zr ppm 4 FUS-ICP 171 1010 656 155

Nb ppm 1 FUS-MS 13 41 17 9

Mo ppm 2 FUS-MS < 2 6 < 2 < 2

Table 7 (continued).

Normalized REE concentrations for the four bulk samples analyzed (Figure 19) show slight enrichment of the LREE relative to the HREE, which is a typical distribution for felsic igneous rocks. The deformed samples (MR1, MR2, and MR4) all yield negative Eu anomalies, whereas sample MR8 yields a slight positive Eu anomaly.

Additionally, all of the deformed samples show high elevated concentrations of REE compared to sample MR8 (again with the exception of Eu). The difference in Eu pattern suggests that sample MR8 may not be the protolith as was originally assumed.

Differences in REE concentration between the three sheared samples all exhibit a similar

63

fractionation trend common for felsic rocks. This suggests that the REE were largely immobile during shearing and/or fluid alteration.

Figure 19. Normalized REE concentrations for samples MR1, MR2, MR4, and MR8.

Harker Diagrams

Petrographic analysis of sheared MR series rocks (Figure 8, 9, 11, and 12) indicates late fluid alteration post-dating shear zone formation. Pristine unaltered layers of quartz, feldspar, and epidote with euhedral fine disseminated opaques (Figure 8) are hydrothermally altered to varying degrees as shown in figures 9, 11, and 12.

Comparison of weight percent of oxides for the three MR sheared rock samples

(MR1, MR2, and MR4) were performed using Harker diagrams (Figure 20). This allows

64

for analysis of chemical variations between the unaltered sample MR1 (Figure 8) and the more altered samples MR2 (Figure 9) and MR4 (Figure 11). For the SiO2 weight percents, MR1 has the lowest percent of 74.47%. Both MR2 and MR4 have higher SiO2 weight percents of 76.87% and 75.66%, respectively. Compared to unaltered sample

MR1, both altered samples MR2 and MR4 show a decrease in weight percent of Al2O3,

MgO, CaO, Na2O, and P2O5, and an overall increase in K2O, Fe2O3, and MnO. TiO2 is relatively unchanged.

14 4

13 3

wt%

wt%

3

3 2 MR1

O

O 2

2 12 1 Al Fe MR2 11 0 MR4 74 75 76 77 74 75 76 77

SiO2 wt% SiO2 wt%

0.06 0.15

0.04 0.1 MR1

0.02 0.05 MgO wt% MgO MnO wt% MnO MR2 0 0 MR4 74 75 76 77 74 75 76 77

SiO2 wt% SiO2 wt%

3 6

2 4

MR1 O wt% O

1 2 2 CaO wt% CaO

Na MR2 0 0 MR4 74 75 76 77 74 75 76 77

SiO2 wt% SiO2 wt%

Figure 20. Harker diagrams of oxide weight percents for samples MR1, MR2, and MR4.

65

6 0.3

4 0.2

wt%

2 MR1 O wt% O

2 2 0.1 K TiO MR2 0 0 MR4 74 75 76 77 74 75 76 77

SiO2 wt% SiO2 wt%

0.03

0.02

wt%

5 MR1 O

2 0.01

P MR2 0 MR4 74 75 76 77

SiO2 wt%

Figure 20 (continued).

Isocon Plots

First proposed by Grant (1986), isocon diagrams are used to graphically analyze changes in volume and/or concentration during hydrothermal alteration. Isocons represent equal concentration of elements between two samples, therefore isocon diagrams allow for an overall observation of varying concentration trends. Any elements that plot above the isocon represent enrichment, while any elements that plot below the isocon represent depletion. Isocon diagrams have been used in previous studies, such as

Rolland et al. (2003), to show element mobility related primarily due to fluid-rock interactions in shear zones.

For this study, isocon diagrams were used to analyze the concentration changes between the minimally altered sample, MR1, versus the altered samples, MR2 and MR4

(Figure 21). To construct isocon diagrams, an arbitrary scaling vector was selected for

66

each element concentration so that scaled concentrations lie between the range of 2 to 40.

Selected scaling vectors are shown in Table 8. The 1:1 isocon line depicted represents the values for no mass or volume change. Deviation of a data point from this 1:1 isocon line represents a concentration change from the unaltered sample MR1 (x-axis) and the corresponding altered sample (y-axis). Data points that line up with the origin are connected as an isocon, where the slope of the line defines the mass change due to the alteration. For MR2 versus MR1, Co, TiO2, and SiO2 all plot along a straight line through

2 the origin, with an R value of 0.9994. For MR4 versus MR1, SiO2, Pb, Co, and Ba all plot along a straight line through the origin, with an R2 value of 1. For both MR2 and

MR4, there is a negligible to very slight gain in mass, with slopes of 1.0381 and 1.0149, respectively. Overall, both plots show much of the data plotting above the isocon lines, representing overall more enrichment in predominantly trace elements.

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Analyte Scaling MR1 MR2 MR4 Symbol Vector Scaled Scaled Scaled SiO2 0.5 37.235 38.435 37.83 Al2O3 1 13.35 11.49 11.27 Fe2O3(T) 10 18.3 32.9 26 MnO 500 10.5 26.5 16.5 MgO 95 9.5 4.75 3.8 CaO 15 39 4.35 2.7 Na2O 5.2 27.092 18.356 16.9 K2O 5 2.45 23.55 23.4 TiO2 100 18.7 19.9 15.5 P2O5 300 6 3 3 Be 9.9 19.8 39.6 29.7 V 1 34 5

Co 5 5 5 5 Ga 1 18 28 26 Rb 0.4 2.8 38.8 35.6 Sr 0.12 33.84 2.28 2.64 Y 0.25 8.5 30 7.5 Zr 0.03 5.13 30.3 19.68 Nb 0.9 11.7 36.9 15.3 Ag 6 9 34.2

Sb 30 21 18 24 Ba 0.02 3.8 5.12 3.82 Hf 1.5 8.7 36.45 19.5 Ta 12 16.8 37.2 12 Tl 100 10 30 30 Pb 1 8 25 8 Th 1 8.3 12.2 4.9 U 8 28.8 39.2 12.8

Table 8. Scale vectors and scaled values used to produce isocon plots.

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40 Si y = 1.0381x 35 R² = 0.9994 Fe 30 Mn 25 K Trace elements and Oxides 20 Ti MR2 Na Co, TiO2, SiO2 15 Linear (Co, TiO2, SiO2) Al 10

5 Mg Ca P 0 0 10 20 30 40 MR1 40 y = 1.0149x Si 35 R² = 1 30

25 Fe

K Trace Elements and Oxides 20 MR4 Mn Na SiO2, Pb, Co, Ba 15 Ti Linear (SiO2, Pb, Co, Ba) 10 Al

5 P Mg Ca 0 0 10 20 30 40 MR1 Figure 21. Isocon plots to analyze concentration changes between samples.

Curie Point Temperatures

Curie point temperature estimates in Celsius were determined by the Hopkinson

Peak method (Moskowitz, 1981). Preferred estimates were used to interpret which

69

common magnetic mineral phases were present in each sample. Curie point plots are shown in Figure 22.

MR1

Susceptibility

MR2

Susceptibility

Figure 22. Curie point temperature plots. Red=Heating; Blue=Cooling.

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MR3

Susceptibility

MR4

Susceptibility

Figure 22 (continued).

Analysis of sample MR1 yielded a single preferred Curie temperature of about

585ºC. This is interpreted as magnetite, which has a known Curie point temperature of about 580ºC. Between heating and cooling, sample MR1 exhibits irreversible behavior

71

indicating a chemical change has occurred during heating. Irreversible behavior is exhibited by the difference in susceptibility between the heating and cooling curves. For sample MR1, the cooling curve has a higher susceptibility than the heating curve. During heating, sample MR1 has an increase in susceptibility.

Sample MR2 yields a preferred Curie temperature of about 575ºC, also interpreted as magnetite. The thermomagnetic curve for sample MR2 has the same Curie point temperature on both the heating and cooling curves, exhibiting reversible behavior.

Reversible behavior indicates there is no alteration to the magnetic mineral during the heating and cooling cycle.

Thermomagnetic curves for samples MR3 are very irregular but appear to have a

Curie point temperature around 580ºC, again indicating magnetite. Sample MR3 was also weakly magnetized, possibly indicative of titanomagnetite. This sample has reversible behavior during the heating and cooling curves.

Sample MR4 was found to have a preferred Curie of about 580-585ºC, again interpreted as magnetite. The Curie point temperature exhibits irreversible behavior following heating, suggestive of a permanent chemical reaction. For sample MR4, the cooling curve has a higher susceptibility than the heating curve.

Low-Temperature Magnetic Transitions

Low-temperature magnetic transitions are another analysis performed on samples with remanence, where the transitions at specific Kelvin temperatures are indicative of a specific magnetic mineral. This is used here to provide additional information on the

72

identification of magnetic minerals present in the samples through the use of the Verwey transition for magnetite, Morin transition for hematite, and the recently proposed Besnus transition for pyrrhotite (Rochette et al., 2011; Table 7). Low-temperature crystallographic transition plots are shown in Figure 23.

1 MR1 0.8

0.6

0.4

0.2 Normalized Long Moment Long Normalized 0 0 50 100 150 200 250 300 Temperature (K)

1 MR2 0.8

0.6

0.4

0.2 Normalized Long Moment Long Normalized 0 0 50 100 150 200 250 300 Temperature (K)

Figure 23. Low-temperature magnetic transitions in Kelvin.

73

1 MR3 0.8

0.6

0.4

0.2 Normalized Long Moment Long Normalized 0 0 50 100 150 200 250 300 Temperature (K)

1 MR4 0.8

0.6

0.4

0.2 Normalized Long Moment Long Normalized 0 0 50 100 150 200 250 300 Temperature (K)

Figure 23 (continued).

Sample MR1exhibits a sharp transition at 120 K, which is indicative of magnetite crystals undergoing the Verwey transition. In contrast, sample MR2 lacks an abrupt transition. It does begin a transition at a lower temperature around 10 K and decreases gradually until about 115 K. This smear of data may indicate a mixture of fine superparamagnetic minerals but with no clear identification. For sample MR3, there is a low-temperature transition which occurs around 120 K, indicative of magnetite. Lastly,

74

sample MR4 exhibits a sharp transition around 100-115 K, also interpreted as magnetite

(Figure 23).

SUMMARY

Based on REE concentration plots and the difference in Eu anomalies, sample

MR8 is not likely the protolith of the sheared MR suite of rocks. However, there is a clear pattern within the deformed samples that is consistent with chemical variations related to hydrothermal fluid alteration evident in petrographic analysis. The hydrothermally altered samples both show a net loss of Al2O3 MgO, CaO, Na2O, and

P2O5, and a gain in Fe2O3, K2O, and MnO. Trace elements show overall enrichment.

These concentration changes and element mobility are likely due to late alteration by fluid flow.

Curie point temperature experiments reveal magnetite as the dominant ferromagnetic mineral in samples MR1, MR2, MR3, MR4 and possibly titanomagnetite in sample MR3. From the low-temperature crystallographic transitions, Verwey transitions around 118 K, indicating magnetite, were present clearly in samples MR1,

MR3, and MR4.

CONCLUSION

Shear zones are ductile deep-seated crustal deformation zones commonly associated with end of orogeny and/or periods of gravitational orogenic collapse. They can also be reactivated tectonically or act as conduits for fluid flow long after orogenesis.

Deformation and metamorphism studies were performed through petrographical, thermochronological, and geochemical analysis on Pan-African samples. Samples were collected from the Mengi River megashear zone in western Ethiopia and from within and around the Highland Complex- Vijayan Complex boundary zone in central to eastern Sri

Lanka. Ethiopian samples consisted of five sheared samples of either metagranites or amphibolites and two samples of undeformed granitic rock. Sri Lankan samples consisted of four gneisses, all chosen due to the samples mineralogical composition comprising of amphibole or biotite grains for lower temperature thermochronology purposes.

ETHIOPIA

Samples collected from the MMSZ in Ethiopia contain an abundant amount of fine grained recrystallized quartz and feldspar grains. This texture is indicative of late fluid alterations. Fluid alteration in the MMSZ is due to a localized event. Petrographical analysis of Ethiopian samples from nearby Pan-African shear zones from previous studies show no evidence of fluid alteration. Samples from the MMSZ also contain secondary late seams of euhedral to anhedral magnetic minerals. Based on Curie point

75

76

temperature and low-temperature crystallographic phases, these magnetic minerals consist dominantly of magnetite, with some possible traces of titanomagnetite grains in sample MR3.

40Ar/39Ar dating was used to establish an age near the end of shearing for the

MMSZ using hornblende and biotite minerals, using closure temperatures of 500ºC and

300ºC, respectively. 40Ar/39Ar dating has been used in only a few other studies to produce age results in western Ethiopia and Eritrea. Tsige and Abdelsalam (2005), in southern

Ethiopia, reported one hornblende age of 511 Ma, five biotite and muscovite ages between 506 and 534 Ma, and one biotite age of 421 Ma. Ghebreab et al. (2005) in eastern Eritrea, north of Ethiopia, reported seven hornblende ages between 565 and 594

Ma, and four muscovite ages between 563 and 576 Ma. For my study, single grains of hornblende minerals produced older ages of 602 Ma and 630 Ma, whereas bulk separates of hornblende grains produced younger total gas ages of ~480 Ma and ~540 Ma. Younger bulk hornblende ages are likely related to impurities associated with inclusion of lower temperature phases within the hornblende minerals. The Neoproterozoic hornblende dates from my study are consistent with nonreset cooling ages interpreted in the previous studies by Tsige and Abdelsalam (2005) and Ghebreab et al. (2005).

Biotite grains from my study produced highly discordant ages that ranged between 250 Ma and 350 Ma. These micas are clearly reset compared to the micas from previous studies. The complex range of ages from the biotite grains is consistent with the late altered textures described in Chapter 2 MR samples. Late fluid alteration at lower temperatures would explain why the biotites are more heavily altered. This portion of

77

Africa has experienced no Phanerozoic tectonic activity until plume related break-up and formation of the Great Rift Valley. Interestingly, locations closer to the rift have not experienced lower temperature resetting. Due to the distance from the Great Rift Valley and the lack of reset grains in the previous studies, it is probable that reset ages in the

MMSZ are due to fluid transport and localized resetting.

A full rock chemical analysis was obtained for sheared samples MR1, MR2, and

MR4, and undeformed sample MR8. Samples MR1, MR2, and MR4 yielded REE concentration plots of similar patterns with a negative Eu anomaly. The REE concentration plot for sample MR8 followed a similar pattern but with a positive Eu anomaly, making MR8 an unlikely protolith of the sheared MR samples. Varying REE concentrations exist between sheared samples, indicating MR1 has the least alteration and

MR2 has the most alteration of the three samples. When analyzing oxide weight percents, with MR1 considered least altered, there is an overall decrease in weight percent of Al2O3

MgO, CaO, Na2O, and P2O5, and an overall increase in Fe2O3, K2O, and MnO. Overall there is an enrichment of trace elements. Variation of concentrations and element mobility are likely due to the late alteration of fluid flow which was visible in the texture of the MR sheared samples.

Both Curie point temperature and low-temperature crystallographic transition experiments were used to determine the magnetic mineralogy of the samples from the

MMSZ. Based on Curie point temperature, the dominant ferromagnetic mineral in samples MR1, MR2, MR3, and MR4 is magnetite. There is also a possibility that

78

titanomagnetite is present in sample MR3. Based on low-temperature crystallographic transitions, samples MR1, MR3, and MR4 contain magnetite.

SRI LANKA

Sri Lankan Proterozoic basement samples exhibit unaltered metamorphic textures of coarse grained pristine minerals. These Sri Lankan Pan-African samples contain primary euhedral magnetic minerals.

Not surprisingly, 40Ar/39Ar dates for the Sri Lankan samples are more precise since the samples are unaltered. Ages of the bulk hornblende and biotite grains ranged between 490 Ma and 470 Ma, indicating the age of lower temperature cooling (~500-

300°C) at the end of the Pan-African Orogeny. The study by Widanagamage (2011) on these same rocks revealed higher temperature results (>600°C) indicating peak metamorphism of the Highland Complex at 572 Ma, followed shortly by retrograde metamorphism during exhumation at 558 Ma. Very similar ages were also reported for ultrahigh temperature metamorphic rocks of the Highland Complex (Sajeev et al., 2010).

The relatively concordant Ar/Ar ages obtained here indicate rapid lower temperature cooling of both the Highland Complex and the Vijayan Complex 60-80 million years after lower crustal exhumation of the Highland Complex. Collectively, the data indicates that the Highland Complex and the Vijayan Complex behaved as a single coherent tectonic block by 500 Ma, and have remained unaltered since then.

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