Science, Irving K. Barber Faculty of (Okanagan)
Earth, Environmental and Geographic Sciences, Department of (Okanagan)
DSpace
UBCO
Nweke, Nchekwube Dieudonne
Master of Science - MSc
University of British Columbia
Lithium-cesium-tantalum (LCT) pegmatites are a key source of economic lithium. The timing of LCT pegmatite crystallization within the context of an orogenic system (i.e., pre-, syn- and post-orogenic) appears to vary globally and exerts a primary influence on the distribution of lithium mineralization in orogenic belts. The recent advances in in-situ Rb–Sr mica geochronology have enabled the direct dating of lepidolite, a major phase in LCT pegmatites. When combined with in-situ Rb–Sr, Lu-Hf, and U–Pb geochronology on other related major and accessory phases, the timing of LCT pegmatite crystallization can be accurately determined and placed in the context of a region’s magmatic and thermometamorphic history. Here, we present new in-situ Rb–Sr, Lu–Hf, and U–Pb geochronology data and results of petrological modeling from an east to west transect of the Benin-Nigeria shield to constrain the timing of magmatism, metamorphism, and LCT pegmatite crystallization related to the Pan-African orogeny. Crystallization ages of Pan-African granites across our transect range from ca. 625–600 Ma. Magmatism is followed by a thermal metamorphic peak at ca. 550–530 Ma, as recorded by Lu-Hf systematics of post-kinematic garnet and monazite U–Pb. The southwesteern Benin-Nigeria shield cooled through biotite Rb–Sr closure temperatures at ca. 500 Ma. Lepidolite in an LCT pegmatite swarm found in the western part of our transect crystallized at ca. 489 Ma, consistent with post-orogenic pegmatite genesis. In the absence of the wide-scale detachment structures associated with orogenic collapse, we propose that the LCT pegmatites were generated by remelting of highly fractionated Pan-African granite due to post-orogenic thermal maturation of the Benin-Nigeria shield. Garnet-staurolite schists record a static increase in geothermal gradients from ~1040 °C/GPa to ~1160 °C/GPa. The cause of this late thermal pulse is not yet determined but may be associated with lithosphere delamination, the break-off of the subducting slab, or post-magmatic radiogenic heating.
Crustal Evolution of The Southern Benin-Nigerian Shield: Insights from Late Post-Orogenic Lithium-Cesium-Tantalum (LCT) Pegmatite Mineralization by Nchekwube Dieudonne Nweke B.Sc., Chukwuemeka Odumegwu Ojukwu University, 2019 M.Sc., Chukwuemeka Odumegwu Ojukwu University, 2023 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE COLLEGE OF GRADUATE STUDIES (Earth and Environmental Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Okanagan) June 2025 © Nchekwube Dieudonne Nweke, 2025 ii The following individuals certify that they have read, and recommend to the College of Graduate Studies for acceptance, a thesis entitled: Crustal Evolution of the Southern Benin-Nigerian Shield: Insights from Late post-orogenic Lithium-Cesium-Tantalum (LCT) Pegmatite Mineralization submitted by Nchekwube Dieudonne Nweke in partial fulfillment of the requirements of the degree of Master of Science in Earth and Environmental Sciences Dr. Brendan Dyck, Irving K. Barber Faculty of Science – Department of EEGS Supervisor Dr. Kyle Larson, Irving K. Barber Faculty of Science – Department of EEGS Supervisory Committee Member Dr. Alessandro Ielpi, Irving K. Barber Faculty of Science – Department of EEGS Supervisory Committee Member Dr. Richard M. Palin, University of Oxford External Examiner iii Abstract Lithium-cesium-tantalum (LCT) pegmatites are a key source of economic lithium. The timing of LCT pegmatite crystallization within the context of an orogenic system (i.e., pre-, syn- and post-orogenic) appears to vary globally and exerts a primary influence on the distribution of lithium mineralization in orogenic belts. The recent advances in in-situ Rb–Sr mica geochronology have enabled the direct dating of lepidolite, a major phase in LCT pegmatites. When combined with in-situ Rb–Sr, Lu-Hf, and U–Pb geochronology on other related major and accessory phases, the timing of LCT pegmatite crystallization can be accurately determined and placed in the context of a region’s magmatic and thermometamorphic history. Here, we present new in-situ Rb–Sr, Lu–Hf, and U–Pb geochronology data and results of petrological modeling from an east to west transect of the Benin-Nigeria shield to constrain the timing of magmatism, metamorphism, and LCT pegmatite crystallization related to the Pan-African orogeny. Crystallization ages of Pan-African granites across our transect range from ca. 625–600 Ma. Magmatism is followed by a thermal metamorphic peak at ca. 550–530 Ma, as recorded by Lu-Hf systematics of post-kinematic garnet and monazite U–Pb. The southwesteern Benin-Nigeria shield cooled through biotite Rb–Sr closure temperatures at ca. 500 Ma. Lepidolite in an LCT pegmatite swarm found in the western part of our transect crystallized at ca. 489 Ma, consistent with post-orogenic pegmatite genesis. In the absence of the wide-scale detachment structures associated with orogenic collapse, we propose that the LCT pegmatites were generated by remelting of highly fractionated Pan-African granite due to post-orogenic thermal maturation of the Benin-Nigeria shield. Garnet-staurolite schists record a static increase in geothermal gradients from ~1040 °C/GPa to ~1160 °C/GPa. The cause of this late thermal pulse is not yet determined but may be associated with lithosphere delamination, the break-off of the subducting slab, or post-magmatic radiogenic heating. iv Lay Summary The Benin-Nigeria shield is a Precambrian shield that constitutes one of the oldest rocks in West Africa. The shield has been affected by multiple deformational episodes that are associated with the Liberian orogeny (ca. 2700 Ma), Eburnean orogeny (ca. 2200 Ma), Kibaran orogeny (ca. 1100 Ma), and the Pan-African orogeny (ca. 550 Ma). These orogenic activities were accompanied by crustal reworking, realignment of the basement lithologic units, and the emplacement of widespread magmatism. Here, this study investigates the timing associated with granite intrusion, peak metamorphism, and the timing of lithium-cesium-tantalum pegmatite mineralization. Findings from this study show that granitic intrusion occurred between ca. 623–600 with metamorphic peak at ca. 550–530 Ma and the crystallization of LCT pegmatites at ca. 489 Ma. v Preface This master’s thesis is the result of my own work. The analytical methods employed were developed by Dr. Brendan Dyck, who also served as supervisor for this research. Fieldwork was conducted by me in February and March 2023, with the valuable assistance of Dr. Ademuyiwa Adetunji, Enechukwu Ebenezer Obuzor, and Mr. Emeka Chukwunenye from the Nigerian Geological Survey Agency, Cross River State zone. Sample preparation and analysis, including in-situ mica Rb–Sr, Lu–Hf, and U–Pb geochronology, electron microprobe analysis, and petrographic studies were carried out by me at the Fipke Laboratory for Trace Element Research (FiLTER), University of British Columbia, Okanagan. Analytical support and assistance with data processing were provided by Dr. Kyle Larson, Dr. Mark Button, and Dr. Sudip Shrestha, as well as by Dr. Carl Beno (currently with the United States Geological Survey, Tucson, Arizona). No generative artificial intelligence (GenAI) tools were used in any part of this thesis. The work presented herein constitutes a novel analytical approach not previously applied within the study regions and reflects the efforts of both the student and the supervisor. vi Contents Abstract ........................................................................................................................................................ iii Lay Summary ............................................................................................................................................... iv Preface .......................................................................................................................................................... v Contents ....................................................................................................................................................... vi List of Tables ............................................................................................................................................. viii List of Figures .............................................................................................................................................. ix Acknowledgements ..................................................................................................................................... xii Chapter One .................................................................................................................................................. 1 1.1. Background of study .......................................................................................................................... 1 1.2. Statement of problem ......................................................................................................................... 2 1.3. Research aims and objective .............................................................................................................. 3 1.4. Scope of study .................................................................................................................................... 3 1.5. Significance of study .......................................................................................................................... 4 Chapter Two.................................................................................................................................................. 5 2.1 Regional geology and tectonic history of Northwest Africa ............................................................... 5 2.2 Geology of the Benin-Nigerian shield ................................................................................................ 9 2.3 First-order lithostratigraphy of the Nigerian basement complex ...................................................... 10 2.3.1 Older metasedimentary rocks (>2500 Ma) ................................................................................ 10 2.3.2 Migmatite-gneiss complex (ca. 2200–550 Ma) ......................................................................... 10 2.3.3 Nigerian schist belts (ca. 1100–800 Ma).................................................................................... 11 2.4 Pan-African granites (“Older granites”) (ca. 950–600 Ma) .............................................................. 17 Chapter Three.............................................................................................................................................. 19 3.1 Fieldwork .......................................................................................................................................... 19 3.2 Sample collection .............................................................................................................................. 19 3.3 Petrography ....................................................................................................................................... 19 3.3.1 Petrographic analysis ................................................................................................................. 19 3.4 X-ray fluorescence (XRF) spectroscopy ........................................................................................... 19 3.4.1 Phase maps ................................................................................................................................. 20 3.5 Geochronology .................................................................................................................................. 20 3.5.1 87Rb/87Sr analysis ....................................................................................................................... 20 3.5.2 176Lu/176Hf analysis .................................................................................................................... 27 3.5.3 U-Pb ............................................................................................................................................... 30 3.5.3.1 Analytical procedure and data processing............................................................................... 30 vii 3.6 Mineral chemistry ............................................................................................................................. 32 3.7 Petrological modelling ...................................................................................................................... 32 Chapter Four ............................................................................................................................................... 33 4.1. Oban Massif ..................................................................................................................................... 33 4.1.1 Lithologic and petrographic description of rock units ............................................................... 33 4.1.2. In-situ mica 87Rb/87Sr geochronology ....................................................................................... 37 4.2. Igarra schist belt ............................................................................................................................... 38 4.2.1. Lithologic and petrographic description of rock units .............................................................. 38 4.2.2. In-situ mica 87Rb/87Sr geochronology ....................................................................................... 46 4.2.3. In-situ garnet 176Lu/177Hf geochronology ...................................................................................... 49 4.2.4 In-situ U-Pb geochronology ........................................................................................................... 49 4.3. Ife-Ilesha schist belt ......................................................................................................................... 52 4.3.1. Lithologic and petrographic description of rock units .............................................................. 52 4.3.2. In-situ mica 87Rb/87Sr geochronology ....................................................................................... 62 4.3.3. In-situ garnet 176Lu/177Hf geochronology .................................................................................. 64 4.3.4 Petrological modelling ............................................................................................................... 65 Chapter Five ................................................................................................................................................ 68 5.1 Interpretation of in-situ geochronology ............................................................................................ 68 5.1.1 Oban Massif ............................................................................................................................... 68 5.1.2 Igarra schist belt ......................................................................................................................... 69 5.1.3. Ife-Ilesha schist belt .................................................................................................................. 70 5.2 Pressure–temperature (P-T) evolution .............................................................................................. 72 5.3 Thermotectonic history of the southern Benin-Nigeria shield .......................................................... 73 5.4 Model for the formation of late Pan-African LCT pegmatites.......................................................... 74 Chapter Six.................................................................................................................................................. 79 Chapter Seven ............................................................................................................................................. 80 7.1 Limitations ........................................................................................................................................ 80 7.2 Recommendation for future studies .................................................................................................. 80 References ................................................................................................................................................... 82 Appendices .................................................................................................................................................. 97 Appendix A ............................................................................................................................................. 97 Appendix B ........................................................................................................................................... 112 Appendix C ........................................................................................................................................... 115 Appendix D ........................................................................................................................................... 116 viii List of Tables Table 3. 1. Laser and ICP-QQQ-MS parameters ........................................................................................ 21 Table 3. 2. Reproduced dates of reference materials via ChrontourR (Larson, 2022) after 2 analytical runs. .................................................................................................................................................................... 26 Table 3. 3. Laser and ICP-QQQ-MS parameters as adopted after Larson et al (2024) .............................. 27 Table 3. 4. Table Reproduced dates of Lu-Hf reference materials via IsoplotR (Vermeesch, 2018) ........ 28 Table 3. 5. Table Reproduced dates of U-Pb reference materials via IsoplotR (Vermeesch, 2018) .......... 30 Table 4. 1. Representative mineral compositions (wt.% oxides and cations per formula unit) for the single sample used for this study. .......................................................................................................................... 58 ix List of Figures Figure 2. 1: Gondwana configuration showing Pan African belts (after Johansson 2014) ........................... 5 Figure 2. 2: Geologic map of Northwest Africa showing the major continental blocks and the Trans-Saharan Mobile Belt (modified after Liégeois 2019) ................................................................................... 6 Figure 2. 3: a) Structural map of Northwest Africa showing the Pan African nappe system and the Benin-Nigerian shield; b) Map of the Nigerian basement complex (modified after Ugwuonah et al 2019). .......... 8 Figure 4. 1. Geological map of the Oban Massif showing the sample locations (modified after Ekwueme 1987; the Nigerian Geological Survey Sheet map of Cross River State).................................................... 33 Figure 4. 2. Quarry exposure of blasted rocks from the Mark-Sino quarry site. a) freshly blasted unit; b) amphibolite unit with gold-like spots of pyrite; c) deformed mica Schist; Sample ND021B..................... 34 Figure 4. 3. a) mineral phase map of sample ND021A showing different mineral phases; b) photomicrograph taken under crossed-polars showing microcrystalline quartz as the background texture of the rock; c) photomicrograph taken under crossed-polars showing folded micaceous foliation; d) photomicrograph taken under crossed-polars showing mica fish. .............................................................. 35 Figure 4. 4. a) phase map of specimen ND021A showing major mineral phases; b) photomicrograph taken under crossed-polars showing sphene porphyroblast surrounded with hornblende; c) photomicrograph taken under crossed-polars of biotite porphyroblast surrounded with plagioclase and hornblende. .......... 36 Figure 4. 5. Photomicrographs of sample ND025. (a–b) shows sericitization in feldspars and chloritisation of hornblende. (c–d) Remnant hornblende cleavage is preserved in the core or chlorite-altered domain. . 37 Figure 4. 6. Mica Rb/Sr isochron diagrams for sample ND021A, a mica Schist from the Oban Massif, Southeastern Nigeria. a) Biotite Rb/Sr result for biotite spot analysis; b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in biotite; c) Muscovite Rb/Sr result for muscovite spot analysis showing multiple regression across different muscovite textures; d) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in muscovite. .......................... 38 Figure 4. 7. Geological map of the Igarra Schist Belt showing the sample locations (modified after Nweke et al. 2024) .................................................................................................................................................. 39 Figure 4. 8. (a–b) Road cut exposure of garnet-biotite Schist. ................................................................... 40 Figure 4. 9. a) Phase map of Igarra L1A b) photomicrograph taken under crossed-polars of biotite, quartz, and garnet porphyroblast; c) photomicrograph taken under crossed-polars showing polygonal quartz in a quartz-rich microlithon; d) photomicrograph taken under crossed-polars showing schistosity of mica in a microcrystalline quartz background. ........................................................................................................... 41 Figure 4. 10. a) Phase map of specimen Igarra L2; b) photomicrograph taken under crossed-polars showing post-kinematic garnet porphyroblastic garnet; c) photomicrograph in plane-polarized light showing the microcrystalline texture and main foliation of the rock as defined by parallel biotite matrix; d) photomicrograph taken under crossed-polars showing quartz, plagioclase, and biotite matrix. ............ 42 Figure 4. 11. Phase map of specimen Igarra 1C; a) photomicrograph taken under crossed-polars showing lepidolite megacrysts, b) lepidolite surrounded by quartz and plagioclase, c) acicular plagioclase cluster; and d) spongy lepidolite microstructure. ..................................................................................................... 43 Figure 4. 12. Granitic samples from the Akoko-Edo metamorphic complex. (a) biotite granite containing metapelites xenolith; (b) granodiorite; (c) weathered granite ..................................................................... 44 Figure 4. 13. a) Phase map of Igarra L9; photomicrograph taken under crossed-polars showing b) photomicrograph taken under plane-polarized light showing titanite megacryst; c) allanite surrounded with quartz and chlorite; and d) microcline-twinned K-feldspar. ............................................................... 45 x Figure 4. 14. Backscatter electron images showing two microstructural populations of titanite in sample Igarra L9. (a–c) represent the larger-sized idiomorphic population. (d–f) represent the non-idiomorphic and smaller-sized population. ..................................................................................................................... 46 Figure 4. 15. Mica Rb/Sr isochron diagrams for samples from the Igarra Schist, Southwest Nigeria. a) biotite Rb/Sr result for biotite spot analysis on biotite schist (Igarra L1A); b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in biotite; c) Biotite Rb/Sr result for biotite spot analysis on biotite granite (Igarra L9); d) post-ablation photomicrograph taken under plane-polarized light showing ablation pits biotite e) Lepidolite Rb/Sr result for lepidolite spot analysis on LCT pegmatite (Igarra 1C); f) post-ablation photomicrograph taken under plane-polarized light showing ablation pits lepidolite. ................................................................................................................................ 48 Figure 4. 16. a) Garnet Lu/Hf isochron diagrams for sample Igarra L1A; b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in garnet. ................................ 49 Figure 4. 17. U/Pb Tera wasserburg diagrams for monazite and titanite samples from the Igarra Schist, Southwest, Nigeria. a) Tera Wasserburg diagram showing lower intercept date of 63 spot analysis on monazite in the biotite schist (Igarra L1A); b) U-Pb weighted mean date showing over-dispersed monazite regression; c) Tera Wasserburg diagram showing single dispersed lower intercept date on 71 spot analysis on titanite in the biotite granite (Igarra L9); d) U-Pb weighted mean date showing multiple inseparable titanite populations; e) U-Pb weighted mean date of 40 spot analysis on idiomorphic titanite matrix; f) Tera Wasserburg diagram showing lower intercept date of 31 spot analysis on non-idiomorphic titanite matrix. ......................................................................................................................................................... 51 Figure 4. 18. Geological map of the Ife-Ilesha Schist Belt showing the sample locations (Modified after Ige et al. 1998). ........................................................................................................................................... 52 Figure 4. 19. Leucogranite intruded by pegmatite ...................................................................................... 53 Figure 4. 20. a) Phase map of specimen ND007A; a-d) photomicrographs collected under crossed-polars showing undeformed granitic microstructures. ........................................................................................... 54 Figure 4. 21. a) shows mineral phases in specimen ND007B; b) photomicrograph taken under crossed-polars showing igneous microstructures. .................................................................................................... 55 Figure 4. 22. Garnet quartzo-feldspathic schist. (a) shows crenulation cleavages and garnet porphyroblast on the schistose unit; (b) boudins formed by quartzo-feldspathic veins; (c-d) shows multiple quartzo-feldspathic lineations in the outcrop; (e) multiple staurolite porphyroblast; (f) multiple micro isoclinal folds ............................................................................................................................................................ 56 Figure 4. 23. a) Phase map of specimen ND003-1; b) Photomicrographs collected under plane-polars showing staurolite and garnet association; c) Shows metapelite and quartzo-feldspathic domains with garnet, staurolite, and biotite association; d) Photomicrograph taken under plane-polarized light expressing the main foliation of the rock and shows garnet – biotite – chlorite- association. .................... 58 Figure 4. 24. a) Phase map of specimen ND003-2; b) Cross-polar photomicrograph showing twin staurolite porphyroblast; c) Cross-polar photomicrograph of garnet, biotite, muscovite, and quartz association; d) Plane-polar image showing garnet overgrowing fabric on mica-rich domains in specimen ND003-2. .................................................................................................................................................... 61 Figure 4. 25. Garnet compositional line profiles. a) Rim-to-rim profiles of garnet end-member mole fractions for ND003. b) Plane polar micrograph of a garnet porphyroblast surrounded by phyllosilicates and staurolite matrix. Location of probe points marked by red circles. ...................................................... 61 Figure 4. 26. Rb/Sr isochron diagrams for sample ND003-1 from the garnet-staurolite schist from Ife-Ilesha Schist Belt, Southwest, Nigeria. a) Rb/Sr isochron result for biotite spot analysis; b) post-ablation photomicrograph taken under plane-polarized light showing biotite analytical spots; c) Rb/Sr isochron xi result for muscovite spot analysis; d) post-ablation photomicrograph taken under plane-polarized light showing muscovite analytical spots. ........................................................................................................... 63 Figure 4. 27. Mica Rb/Sr isochron diagrams for sample ND007A from the Ife-Ilesha Schist, Southwest, Nigeria. a) Muscovite Rb/Sr isochron result for muscovite spot analysis on Pan-African granite; b) post-ablation photomicrograph taken under plane-polarized light showing muscovite analytical spots; c) Biotite Rb/Sr isochron result for biotite spot analysis on Pan-African granite; b) post-ablation photomicrograph taken under plane-polarized light showing biotite analytical spots; ............................... 64 Figure 4. 28. a) Garnet Lu/Hf isochron plot for sample ND003-1 from the Ife-Ilesha Schist, Southwest, Nigeria; b) post-ablation photomicrograph taken under plane-polarized light showing garnet analytical spots. ........................................................................................................................................................... 65 Figure 4. 29. Calculated P–T pseudo section for sample ND003 using MAGEMin (Riel et al. 2022) using ds_62 (White et al 2014). Relevant fields are contoured for pyrope and grossular isopleths mode (legend, bottom right). The green polygon represents the garnet core, while the red polygon represents the rim composition. ................................................................................................................................................ 66 Figure 4. 30. Calculated P–T pseudo-section for sample ND003 using ds_55 (Powell and Holland 1988). Relevant fields are contoured for pyrope and grossular isopleths mode (legend, bottom right). The purple polygon represents the garnet core, while the orange polygon represents the rim composition. ................ 67 Figure 5. 1. Summary of crustal events in the southern Benin-Nigerian Shield ......................................... 72 Figure 5. 2. Tectonic model of the southern Benin–Nigeria Shield illustrating a three-stage evolution associated with the Pan-African orogeny. Stage I corresponds to the magmatic emplacement of peraluminous granites (~621–600 Ma). Stage II is characterized by slab break-off, fractional crystallization of peraluminous granites, a static increase in geothermal gradient, and peak regional metamorphism (~550–530 Ma). Stage III represents post-orogenic thermal cooling and the subsequent emplacement and mineralization of LCT pegmatites (~530–480 Ma). ...................................................... 78 xii Acknowledgements This thesis reflects the collective efforts, guidance, and encouragement of many individuals to whom I am deeply indebted. The journey that brought me here was not without its challenges—I vividly recall the over one hundred rejections I faced while seeking educational opportunities abroad. It feels like only yesterday. First and foremost, I would like to extend my sincere gratitude to my supervisor, Dr. Brendan Dyck, whose unwavering support and mentorship were instrumental in bringing this research to fruition. His belief in my potential and the research infrastructure he provided transformed a long-held dream into a tangible reality. I am especially thankful for the opportunity he gave me, one that changed the course of my academic journey and life. I am also grateful to Dr. Kyle Larson for his invaluable input and consistent support throughout the course of this work. His analytical insights and access to lab facilities significantly enriched the quality of my research. A special acknowledgment goes to Dr. Richard M. Palin, who played a formative role in the early stages of this project. Although I was unable to join his research group at the University of Oxford, his continued support and encouragement were critical in shaping the direction of this study and ultimately in facilitating my move to Canada. I deeply appreciate the assistance of research lab managers, Dr. Mark Button and Dr. Sudip Shrestha, as well as the camaraderie and collaboration shared with my research colleagues: Alix Osinchuk, Danny Richard, Shreya Roy, Finnegan Vanderkam, and many others. I am also thankful to Deborah Barnett and Christa Nyarko for their administrative support, which greatly eased my transition and academic responsibilities. Lastly, I offer my heartfelt thanks to my family for their unwavering love and support throughout this journey. Their prayers, guidance, and financial sacrifices provided the foundation that enabled me to pursue and realize this goal. 1 Chapter One Introduction 1.1. Background of study The Pan-African orogeny (a.k.a, the Pan-African orogenic system) is a long-lived composite orogen that was involved in a series of distinct thermo-tectonic events associated with the amalgamation of the Gondwana supercontinent (ca. 950–550 Ma). The Pan-African orogeny was caused by the convergence and collision of 12 major continental blocks (cratons). It involved the closure of the Goias Pharusian Ocean and the formation of numerous (up to ten) distinct mobile belts along the margins of the various continental blocks and oceanic terranes (Rino et al 2008; Kroner and Stern 2004). At the time of its formation, the Pan-African orogenic system spanned over 3,000 km from northwestern Africa into western South America (Ferre et al., 2002; Kroner and Stern, 2004). During the convergent period of the orogen, the continental crustal blocks and their margins experienced voluminous felsic magmatism and substantial tectonometamorphic reworking (Kennedy 1964; Kroner 1980; Rino et al. 2008). The Trans-Saharan Orogenic Belt is one of the Pan-African aged mobile belts that is found in present-day northwestern Africa. The Trans-Saharan Orogenic Belt was formed during east-west convergence between the West African craton, Congo craton, the Sahara metacraton, along with the Benin-Nigerian shield (Abdesalam et al. 2003). This convergent event resulted in east-dipping subduction along the western margin of the West African craton and the emplacement of voluminous Pan-African granitoids throughout West and Central Africa, with significant granitic plutonism concentrated in the Benin-Nigeria shield (Jahn et al. 2001). The Benin-Nigerian shield has a long geological history, with a polymetamorphic Paleoarchean to Paleoproterozoic migmatite-gneiss basement complex at its core and a series of in-folded supracrustal belts of disputed origin and age (McCurry 1976; Turner 1983; Fitches et al. 1985). Grant (1978) suggested that the in-folding of supracrustal sediments within the basement complex may have occurred before the Pan-African orogeny. Whereas Caen-Vachette and Ekwueme (1988) argued that the in-folding event was Pan-African in age. What is known from the limited studies is that the Benin-Nigerian shield has experienced multiple phases of tectonic reworking, associated with the Liberian orogeny (ca. 2800–2500 Ma), Eburnean orogeny (ca. 2200–2000 Ma), Kibaran orogeny (ca. 1100–900 Ma) Ma and the Pan-African event (ca. 950–550 Ma) (Fitches et al. 1985; Ekwueme 1987; Ajibade 1988; Ekwueme and Caen-Vachette 1992; Kroener et al. 2001). The record of the pre-Pan-African orogenic events is patchy, whereas the deformation fabrics, metamorphism, and magmatism associated with the Trans-Saharan Orogenic Belt of the Pan-African event are found to have pervasively affected the Benin-Nigerian shield (Ball 1980; Ekwueme 1987; Ajibade 1988; Ephraim et al. 2009; Njanko et al. 2010). 2 Within the Benin-Nigerian shield, the Pan-African orogeny led to crustal thickening, emplacement of a voluminous (mostly) calc-alkaline suite of felsic intrusives and associated lithium-cesium-tantalum (LCT) pegmatites, the realignment of the major basement fabrics and lithological contacts to northeast–southwest direction, and the formation of multiple major dextral strike-slip faults across the shield (McCurry and Wright 1977; Egbuniwe et al. 1985; Attoh et al. 1997; Ferre et al. 2002; Girei et al. 2019). This complex history of magmatism, tectonism, and metamorphism has all contributed to the current mineral/metal endowment of the Benin-Nigerian shield, which is a topic of acute societal relevance to the quarter of a billion individuals living in the region. In general, the tectonic evolution of the Benin-Nigerian shield has been poorly studied relative to other Pan-African terranes, and many fundamental geological problems remain to be solved. The impact of the various orogenic events on the basement complex has not been fully understood, resulting in the proposal of numerous conflicting models for the evolution of the Benin-Nigerian shield. There have been particularly few studies on the crustal evolution of the southernmost exposure of the Benin-Nigerian shield, and as a result, there is very limited information regarding the timing of deformation, emplacement of felsic magmas, peak metamorphic age, and the timing of mineralization of LCT pegmatites. 1.2. Statement of problem There is a general dearth of bedrock maps and modern geochronological data collected from the Benin-Nigerian shield, and as such, there are numerous avenues upon which to focus this research. Given the societal importance of LCT pegmatites and the connection that these units have to magmatism, tectonism, and metamorphism (Kinnaird 1984, Matheis 1987; Goodenough et al. 2014; Alebiosu and Omotoso 2024; Olade 2025), I have chosen to center my research on understanding the link between the thermomagmatic evolution of the Benin-Nigerian shield and LCT pegmatite crystallization. Globally, the petrological processes that lead to LCT pegmatite formation are not well known. Various models have been proposed for the formation of LCT pegmatite melts, all of which either require extreme crystal fractionation (Deveaud et al. 2015; Li et al 2023; Gardiner et al. 2024) or remelting of fractionated felsic intrusions (Simmons and Webber 2008; Simmons et al. 2016; Koopman et al. 2023). One of the best ways to differentiate between these models is to determine the timing of LCT pegmatite crystallization relative to the timing of felsic magmatism and orogenesis. In the case of pegmatites formed during extreme crystal fractionation, the pegmatite crystallization ages will closely match those of the regional magmatism. In the case of remelting of fractionated intrusion, the crystallization ages of the LCT pegmatites will post-date magmatism, and the lag in between the magmatic and pegmatite ages may be indicative of the tectonic processes that cause the remelting event. 3 The southern Benin-Nigerian shield is particularly enriched with (LCT) pegmatites. A single preliminary geochronological study of magmatism in the northern Benin-Nigerian shield by Goodenough et al (2014) concluded that the LCT pegmatites formed at ca.560–450 Ma, after the last main pulse of Pan-African magmatism within the Trans-Saharan Orogenic Belt. Constraining the timing of LCT pegmatite intrusion relative to deformation/plutonism/metamorphism in the southern Benin-Nigerian shield and identifying the characteristics of viable source rocks, as well as the paleodepths where LCT pegmatites were intruded, would all be very valuable in understanding how these mineral systems behave. Generating new geochronological data and elucidating the metamorphic evolution of the Benin-Nigerian shield would also further our understanding of the role that this crustal block played in the formation of Gondwana. 1.3. Research aims and objective The overarching aim of my research is to generate new quantitative metamorphic and geochronological data that can be used to further elucidate the magmatic and metamorphic evolution of the Benin-Nigerian shield. This will be done by addressing the following specific objectives: 1. Determine the timing of Pan-African granite magmatism within my specific study areas. 2. Determine the metamorphic conditions of the southern Benin-Nigerian shield as recorded by the metasedimentary lithologies of the various Nigerian schist belts. 3. Determine the timing of the LCT pegmatites of the southwestern Benin-Nigerian shield. Taken together, the results associated with each objective will enable me to reinterpret the thermal history of the southern Benin-Nigerian shield as it relates to the late-to-post-Pan-African magmatic and metamorphic episodes. 1.4. Scope of study This study is focused on three major regions that were accessible by an east-west transect of the southern Benin-Nigerian shield. The regions include the Ife-Ilesha Schist belt of the central southwestern Nigerian basement block, the Igarra Schist belt at the eastern edge of the southwestern Nigerian basement block, and the southern Oban Massif (Figure 2.3a). The main focus of my thesis research is on the units of metasedimentary rocks, granites, and pegmatites of the basement complex that were affected during the Pan-African orogen. The bedrock is not well exposed in some regions of the study transect due to thick vegetation and overburden, so I have focused on regions with the best exposure. I also restricted the scope of my study to southern Nigeria, where it was easier for me to gain information about outcrop exposure and access from local groups, and because travel in the central and northern parts of Nigeria was unsafe due to high rates of kidnapping. These restrictions also prevented the traversing of the far western region as well as the more northern locations. 4 1.5. Significance of study This research will reveal the tectono-thermal and crustal evolution of the Pan-African orogeny in southern Nigeria, which is currently unknown. It will constrain the peak period of granitic emplacement, the metamorphic events associated with the later stages of the Pan-African orogeny, and the timing of LCT pegmatite formation. This new information will inform a new model for the crustal evolution and geological history of the Benin-Nigerian shield and will also serve to guide future exploration of granite/pegmatite-associated resources. 5 Chapter Two Geological Background 2.1 Regional geology and tectonic history of Northwest Africa The Benin-Nigerian shield was pervasively affected by the Pan-African orogeny, which spanned from 950–550 Ma (Ekwueme 1987; Ajibade 1988; Ekwueme and Caen-Vachette 1992). The term Pan-African was introduced by Kennedy (1964) to describe the thermo-tectonic event in Africa which led to the formation of series of Neoproterozoic mobile belts between cratons during the formation of Gondwana supercontinent (Fig. 2.1). These Neoproterozoic belts were formed due to multiple collisions between stable continental blocks (cratons) and the closure of intervening ocean basins. The Pan-African orogeny is primarily recognized to have affected the western Gondwanaland supercontinent (Fig. 2.1). Within West Gondwana, the Pan-African belt spans over 3000 km from northwest Africa through Brazil down to Rio de la Plata in present-day Argentina (Fig. 2.1) (Caby 1989; Ferre et al. 1995; Kröner and Stern 2004; Ganade de Araujo et al. 2014). Figure 2. 1: Gondwana configuration showing Pan African belts (after Johansson 2014) 6 In northwest Africa, the main Pan-African mobile belt is the Trans-Saharan Mobile Belt (TSMB). The Trans-Saharan Mobile Belt is bordered by the West African craton to the west, the Saharan meta-craton to the northeast, and the Congo craton to the south of the Central Africa Fold Belt (Fig. 2.2). In addition to those three large cratons, the Trans-Saharan Mobile Belt also encompasses the Tuareg Shield in Algeria, the Air Shield in Niger, and the Benin-Nigerian Shield (Fig. 2.2). The present position of this mobile belt is generally regarded to be part of the terrane of the Precambrian Goias Pharussian ocean which was closed during the onset of the Pan African orogeny (ca. 950–500 Ma) (Ferre et al. 2002). The collisional episodes between the now passive cratons (West African craton, Congo craton, and Sahara meta-craton) in the Trans-Saharan Mobile Belt resulted in the formation of a pervasive north-south trend to the map-scale geologic structures. This collisional event also led to the formation of numerous dextral shear zones and strike-slip fault systems trending from northwest Africa to South America. (Fig. 2.3a). Figure 2. 2: Geologic map of Northwest Africa showing the major continental blocks and the Trans-Saharan Mobile Belt (modified after Liégeois 2019) 7 Detailed studies have been carried out to investigate the northern segment (Tuareg shield) of the Trans-Saharan Mobile Belt (Caby 1970; Fabre et al. 1982; Black et al 1994; Caby and Monie 2003). Black et al. (1994) identified 23 terranes within the Tuareg segment of the TSOB as well as ophiolitic complexes within some of the identified terranes. At present, very little is known about the southern segment (Benin-Nigerian shield) of the Trans-Saharan Mobile Belt in comparison to the amount of research that has been conducted in the north. 8 Figure 2. 3: a) Structural map of Northwest Africa showing the Pan African nappe system and the Benin-Nigerian shield; b) Map of the Nigerian basement complex (modified after Ugwuonah et al 2019). 9 2.2 Geology of the Benin-Nigerian shield The Benin-Nigerian shield is characterized as a Precambrian basement complex with exposure of Paleoarchean through Neoproterozoic magmatic and metamorphic units (Oshin and Rahaman 1986). The shield is well-exposed in Nigeria, where it makes up approximately two-thirds of the country’s surface geology (Oshin and Rahaman 1986). The Benin-Nigerian shield lies at the lower end of TSOB and is bordered to the west by the >2.0 Ga West African and Congo cratons to the southeast, respectively (Turner 1983; Woake et al. 1987; Caby 1989; Kröner et al. 2001; Ukwang et al. 2003) (Fig. 2.3a). The Nigerian basement has been grouped into three main exposed domains: the northcentral basement, the southwestern basement, and the southeastern basement (Rahaman 1973; Odeyemi 1976, 1977; Rahaman et al. 1988; Olarewaju and Rahaman 1982; Ekwueme and Ekwere 1989, Ekwueme 1990) (Fig. 2.3b). In its entirety, the basement complex has experienced polyphase deformation associated with the Liberian Orogeny (ca. 2700 Ma), the Eburnean Event (ca. 2200 Ma), the Kibaran orogeny (ca. 1400 Ma), and the Pan-Africa orogenic system (ca. 950–550 Ma) (Odeyemi 1981). Relics of these past tectonic events have been noted by various previous workers in rocks across Nigeria (Fitches et al. 1985; Ekwueme 1987; Ajibade 1988; Ekwueme and Caen-Vachette 1992; Kroener et al. 2001; Ekwueme and Kröner 2006). Odeyemi (1981) and Ephraim et al. (2009) have emphasized that the imprints of older orogenic events within the Benin-Nigeria shield have been overprinted by the pervasive magmatism and deformation associated with the Pan-African event. The Pan-African event folded and realigned the majority of lithological contacts and internal rock fabrics towards an approximate north-south direction with a predominance for dextral strike-slip deformation (Ferre et al. 2002). However, the Pan-African tectonism did not completely transpose the fabric and regional-scale structures of the Nigerian basement, with some of the earlier deformational structures of past orogenic events still preserved in metamorphic complexes across the basement (Oyawoye 1972; Rahaman 1976; Onyeagocha and Ekwueme 1982; Ekwueme 1987, 1994). The pervasive effect of the Pan-African event on the Nigerian basement has obscured much of the pre-Pan-African rock record, and as such, numerous diverging models have been proposed to explain the origin of much of the crust that now constitutes the Nigerian basement complex (e.g., Ajibade and Wright 1989; Ferre et al. 1996, 2002; Caby and Boesse 2001). Ajibade and Wright (1989) proposed that the Nigerian basement is composed of island arcs and older continental fragments that were fused during a sandwich-type collision between the West African Craton, the Sahara Metacraton, and the Congo Craton. This model explains why the oldest rocks in West Africa, the Kabala gneiss (3571 ± 3 Ma), are found within the northcentral Nigerian basement (Kröner et al. 2001). The northcentral basement complex has been further subdivided into the Eastern Terrane and the Western Terrane (Ferre et al., 1996, 2002). The grouping was based on the presence of a 500 km long north-south structural lineament that cuts across the basement 10 at approximately longitude 8 °E (Fig. 2.4b). This structural lineament was first described by Ananaba and Ajakaye (1987), based on the occurrences of spatial lineament density maps that were observed on Landsat imagery. The Western Terrane is dominated by synclinal schist belts that are infolded into the upper-amphibolite faces migmatite basement (Oyawoye 1972), while the Eastern Terrane is rich in Uranium deposits and dominated by Paleoproterozoic plutons, Pan-African granitoids, younger granites, all intruded into a granulite-facies basement (Dickson et al. 1991; Ferre et al. 1996; Ukwang et al. 2003; Ugwuonah et al. 2019). These younger granites are alkaline in their chemistry with crystallization ages ranging between ca. 310 Ma in the north of the northcentral basement to ca. 140 Ma in the south of the northcentral basement (Rahaman et al. 1984; Umeji and Caen-Vachette 1991). They are thought to be caused by a sub-continental hotspot and extend from the area of the Air region in Niger into Nigeria with the youngest unit emplaced at the southern tip of the northcentral basement at Nassarawa-Egon (Umeji and Caen-Vachette 1991). 2.3 First-order lithostratigraphy of the Nigerian basement complex 2.3.1 Older metasedimentary rocks (>2500 Ma) The term older metasedimentary series was first introduced by Oyawoye (1964) and reused by Rahaman (1976) to describe rocks of sedimentary origin that were deposited in the Archean and were not entirely reworked during subsequent tectonic episodes (Ekwueme and Kalsbeek 2012). Generally, it is believed that the basement complex evolved from the rejuvenation of older continental materials by polycyclic deformational episodes associated with earlier orogenic events (Caen-Vachette and Umeji 1987). They represent the oldest supracrustal material within the Nigerian basement complex, although they largely exist as rafts and xenoliths within younger intrusions. 2.3.2 Migmatite-gneiss complex (ca. 2200–550 Ma) The migmatite-gneiss complex constitutes the volumetric majority of the Nigerian basement. As a map unit, the migmatite-gneiss complex can be subdivided into constituent granitic migmatites, banded gneisses, augen gneisses, and granitic gneisses. The age, pressure, and temperature of metamorphism in the migmatite-gneiss complex vary across Nigeria due to multiple reworking events. Deformation in the older basement as described by Cooray (1974) and McCurry (1976) trends mainly east-west. This east-west trend is common with rocks associated with the 2200–2000 Ma Eburnean orogeny in West Africa (Rollinson 1978). Caen-Vachette and Umeji (1987) reported Rb-Sr whole rock isochron Eburnean ages (ca. 2197 + 31 Ma) and (2256 ± 172 Ma) for banded gneisses from the south-central basement (east of 11 on Fig. 2.3b). In southwestern Nigeria, Grant et al. (1969) worked on the granite gneiss from the Ibadan area using the whole-rock Rb-Sr method; they obtained an isochron age of 2190 ± 30 Ma. Rahaman et al. (1983) reported Rb-Sr whole rock isochron age, which 11 suggests a late-Eburnean age of ca. 1904 ± 18 Ma for augen gneisses from the Igbeti area of southwestern Nigeria. 2.3.3 Nigerian schist belts (ca. 1100–800 Ma) The Nigerian schist belts are packages of supracrustal units that overlie and are infolded into the migmatite-gneiss basement (Fig. 2.3b). The schist belts are often associated with major fault/shear zones as well as slivers of mafic (i.e., pillow basalt) and ultramafic (i.e., serpentinite) rocks (McCurry 1973, 1978; Wright 1976, 1988; Ajayi 1982; Baer 1982). Although the initially published works that describe the schist belts report that the schist belts are restricted to west of longitude 8°E, this has since been shown to be incorrect, with schist belts now identified in other parts of Nigeria, e.g., Oban Massif and Keffi area of the Eastern Terrane (Ekwueme and Onyeagocha 1985; Ekwueme 1987; Ugwuonah and Obiora 2011; Ugwuonah et al. 2017). The schist belts are dominated by low to medium-grade metamorphic rocks of semi-pelite to pelitic composition interbedded with psammitic rocks, calcareous rocks (marble and dolomite), and calc-gneisses (Elueze 1982; Turner 1983). However, relatively little quantitative metamorphic research has been done on these units, so the absolute conditions and timing of metamorphism in the schist belts are poorly constrained (Oyawoye 1965; McCurry 1970; Caby and Boesse 2001). Many of the schist belts as described as synclinal structures (e.g., Oyawoye, 1972), and Olade and Elueze (1979) proposed that they originated as geosynclines that were steeply folded into the migmatite basement in a sandwich-type collision between continental blocks during the pan-African event and are now preserved as synclinorial keels. Alternatively, the schist belts may have originated as thrust nappes or infolded klippe units that were moved into their present position during the pan-African event (Caby and Boesse, 2001). The schist belts form distinct mineralogic provinces and are the host to much of Nigeria’s mineral resources. The primary resource types range from metals like gold, tin, and iron ore, to minerals like lepidolite and columbite (Woakes et al, 1987). Within the Nigerian basement, there are eleven well-defined shist belts that all trend mainly North–South to North of Northeast and South of Southwest (Turner 1982; Wright et al. 1985). These eleven shist belts can be divided into two groups based on regional occurrences into the northcentral and the southwestern schist belts (Fig. 2.3b). The northcentral schist belts, according to Turner (1983); Wright (1985), and Wright et al. (1985), include Zungeru-Birnin Gwari schist, Kusheriki-Kushaka schist, Karaukarau schist, Kazaure schist, Wonaka schist, Maru schist, Anka schist, and Zuru schist. Based on earlier studies (Turner 1983; Wright 1985), there are three schist belts within the southwestern basement, which include the Iseyin Oyan River schist belt, the Ife-Ilesha schist belt, and the Igarra schist belt. No detailed work has proven the exact depositional nor provenance ages of the schist belts in western Nigeria. Relative ages have been assigned to them based largely on the timing of the subsistence stage of the Pan-12 African orogeny (Grant 1969; McCurry 1973; McCurry and Wright 1971; Fitches et al. 1985). Most of the published ages from the schist belts are based on cross-cutting and/or host relationships with granitic or migmatitic gneisses (e.g., Grant 1978). In lower central Nigeria below the confluence of Rivers Niger and Benue, Caen-Vachette and Ekwueme (1988) investigated the age of schist occurring southeast of the Lokoja area using whole rock Rb-Sr and report dates of 1315 ± 72 Ma, 1005 ± 35 Ma and 678 ± 13 Ma. The authors interpreted the older dates as relics of Kibaran orogeny and the younger as thermal resetting during the Pan-African orogeny (Ekwueme, 1988). Limited work has been done on the supracrustal units in the eastern half of Nigeria (i.e., Eastern Terrane; Fig. 2.3b), east of longitude 8°E. In part, this is due to geopolitical reasons, but also reflects the fact that the Eastern Terrane is covered with dense vegetation and overburden. The first report on the occurrence of schists in the terrane east of longitude 8°E was the River Andu schist described in Onyeagocha (1984). Subsequent research (Ekwueme and Onyeagocha 1985, 1986; Ekwueme 1987; Mucke 2005; Ugwuonah and Obiora 2011; Ekwueme and Kalsbeek 2013; Elatikpo et al. 2013; Andongma et al. 2021) has also reported the occurrences of schist within the Eastern terrane. Schistose units within the Uwet area of the Oban massif in SE Nigeria were dated using whole rock Rb-Sr by Ekwueme (1987). Ekwueme (1987) reported a date of 572 ± 16 Ma for a kyanite-sillimanite schist and 672 ± 24 Ma for a mica schist within the Kwa Fall region. These dates were interpreted by Ekwueme (1987) to be related to two distinct metamorphic and deformation events in the Uwet area during the Pan-African orogeny. Studies by Kalsbeek et al. (2013) and Ekwueme and Kalsbeek (2014) carried out a zircon U-Pb geochronological study on schist from Kokona within the Akwanga area of the northcentral basement. From their zircon study, the 207Pb/206Pb probability diagram yielded two populations at ca. 2200–1700 Ma and ca. 1100–600 Ma. The analyzed zircons show variation from core to rim. The older population, i.e., core (Archean, Meso–Neoproterozoic), was interpreted by Ekwueme and Kalsbeek (2013) as being detrital, with the younger population, i.e., rim (ca. 640–630 Ma), being a mixture of detrital and metamorphic. The interpretation of their results did not specify the potential maximum depositional age of the metasedimentary rocks. 2.3.3.1 Northwest Nigeria schist belts In the following section, I briefly describe the eleven documented Nigerian schist belts. Zuru schist belt The Zuru schist belt extends over 200 km in length and is up to 30 km in width (McCurry and Wright, 1977). Zuru schist belt is represented as 1 in (Fig. 2.3b) and is the largest schist belt in Nigeria. The geology, geochemistry, petrology, and geochronology of rocks within and adjacent to this belt have been 13 reported in detail in McCurry (1976), McCurry and Wright (1977), and Ogezi (1977). McCurry and Wright (1977) described the granite suites within the Zuru area as high-K calc-alkaline granites. Whole rock K-Ar data obtained from dacite within the belt, as presented in McCurry (1976), suggest the granitoids are late-Pan-African in age (ca. 500 Ma); however, this date may well represent a cooling age rather than primary crystallization. McCurry and Wright (1976) and Ajibade and Wright (1989) suggest that the fault system of the Zuru Schist belt could be one of the Pan-African sutures. Salawu et al. (2019), through aeromagnetic studies, identified two deformational fabrics on the east and west of the area, separated by the major fault. The Northwest and Southwest of the fault have a dominant E-W ductile deformation, confirming the Eburnean event in the area, while in the SE and NE, there is a dominant NE–SE ductile deformation, confirming the area was affected by the Pan-African event. Over the past few years, a lot of mineral prospecting, especially gold mining, has been carried out within the Zuru schist belt. Garba (2002, 2003) described gold mineralized trends occurring in NW Nigeria as alluvial and eluvial placers and mostly in veins. The major faults within this area have also been linked with the occurrences of gold mineralization (Garba 2002, 2003; Salawu et al. 2023). Recent studies have applied diverse techniques in mineral exploration (Salawu et al. 2019; Olasunkanmi et al. 2020; Fagbohun et al. 2020, 2021) involving geochemical anomaly and low-latitude aeromagnetic surveys. Other structural studies (Ramadan and Abdel Fattah 2011; Salawu et al. 2019; Olasunkanmi et al. 2020; Fagbohun et al. 2020) suggest that mineralization is structurally controlled by the regional faults within the area. Anka schist belt The Anka schist belt, located in northwest Nigeria, is marked as number 2 in Figure 2.3b. It is within the northwestern end of the northcentral basement. The Anka schist belt is separated from the Zuru and Maru schist belts by migmatite and Pan-African granitoids (Turner 1983). Holt et al. (1978), Turner (1983), and Fitches et al. (1985) listed and described the member units of the Anka schist belt as primarily dominated by slate, phyllite, metagraywacke, and metavolcanic, along with a distinct meta-conglomerate unit. The depth of the meta-conglomerate unit was estimated by Turner (1983) to be within the range of 150–250 m with intercalations of metagraywacke. Holt (1982) also described the presence of a distinct turbidite unit. McCurry and Wright (1977) studied the geochemistry of volcanic rocks from the areas around this schist belt and northwestern Nigeria and concluded that the magmas were potassic, intermediate to felsic, and plotted mostly within the calc-alkaline field on an AFM diagram. Ogezi (1977) reported ultra-mafic complexes (serpentinite and amphibolite) around the major transcurrent faults of this area, 14 interpreting them as remnants of ophiolites. A single Rb-Sr isochron date of 485 ± 50 Ma has been reported from the region, and it comes from a metagraywacke sample (Ogezi 1977). Maru schist belt The Maru schist belt is marked as number 3 in Figure 2.3b and is one of the most explored schist belts in the Nigerian basement complex. The belt is 200 km long and ranges in width from 12–19 km. The Maru schist belt appears to be separated from the Anka and Zuru schist belts by granitic intrusions of suspected Pan-African age (Holt et al. 1978; Turner 1983). Holt et al. (1978) and Adekoya (1998) report that the main lithology of the Maru schist belt comprises amphibolite, orthoquartzite, semi-pelite, pelite, psammite, and banded ironstone. Deformation within the Maru schist belt is marked by north-south trending cleavage in phyllite (Holt et al. 1978). The dominant deformation fabric is marked with axial planar to the tightly folded structure that produced crenulation cleavages (Holt et al. 1978; Turner 1983; Adekoya 1998). Adekoya (1998) reported a greenschist-facies assemblage of muscovite, chlorite, quartz, and accessory biotite, chlorite, and epidote as well as tourmaline, pyrite, graphite, and magnetite in pelitic schists. Ogezi (1977) reported a whole-rock Rb-Sr isochron date of 1068 ± 65 Ma for a micaceous pelite from the Maru schist belt. An earlier study by Hurley (1966) reported a whole-rock Rb-Sr isochron date of 619 ± 36 Ma for the Kanoma pluton, which lies within the schist belt. Snelling (1964) reported a 780 ± 40 Ma hornblende K-Ar date from an amphibolite unit within the schist belt. Wonaka schist belt The Wonaka schist belt is located northeast of the Maru schist belt and northwest of the Karaukarau schist belt; it is represented as number 4 in Figure 2.3b. The Wonaka schist belt comprises hornfels schist, quartz-biotite rocks, quartzite, biotite-muscovite schist, ferruginous quartzite, and calc-silicate rocks (McCurry 1973; Turner 1983; Usman and Ibrahim 2017). The common minerals associated with rocks from the area include sillimanite, cordierite, and or garnet (Turner 1983). Turner (1983) described metamorphism as high-temperature and low-pressure metamorphism, potentially due to contact metamorphism from widespread granitic intrusion forming contact aureoles. Kazaure schist belt Kazaure schist belt, marked as number 5 in Figure 2.3b, is in northern Nigeria and follows the major structural lineament of the basement. The major lithological units within this schist belt are quartzite 15 meta-conglomerate, metapelite, metavolcanic, banded iron formation, and Pan-African intrusive (Danbatta 2010). Karaukarau schist belt The Karaukarau schist belt is located within the Kusheriki area of the northcentral basement (number 6 in Figure 2.3b). The Karaukarau schist belt largely consists of quartzite intercalated with layers of calc-silicate amphibolite and muscovite–cordierite schists (McCurry 1973; Turner 1983). Deformation within the Karaukarau schist belt as described in early studies (McCurry 1973; Grant 1978; Turner 1983) has been divided into two phases: an earlier recumbent folding event and a later folding event that are evident in the fold axis and lineation as they recline at every angle in a subvertical north-trending plane (Grant 1978). Kushaka schist belt The ~ 50 km long Kushaka schist belt, marked as number 7 in Figure 2.3b, lies west of the Karaukarau schist belt in the northcentral basement (Truswell and Cope 1963; Turner 1983; Garba 2002). The main lithological units of this schist belt include garnet-staurolite schist, graphitic schist, banded garnet-grunerite iron formation, and amphibolite (Turner 1983). The Kushaka schist belt is cut across by a major dextral fault, the Kalangai fault. According to Garba (2002), this fault system has produced a series of NNE-trending isoclinal fold structures within the Kushaka schist best. A strong axial plane foliation was earlier reported within the schist belt, which could have formed due to the fault system (Russ 1957; Truswell and Cope 1963). Zungeru-Benin Gwari schist belt The ~150 km long Zungeru-Birnin Gwari schist belt is located within the southwest of the northcentral basement, belt number 8 in Figure 2.3b. The schist belt is made up of quartzo-feldspathic rocks, amphibolite, quartzite, and a muscovite-biotite schist (Turner, 1983). Little work has been conducted on the Zungeru-Benin Gwari schist belt, although Turner (1983) noted that it is indeed separated from the Kushaka schist belt by migmatitic gneisses and Pan-African plutons. 2.3.3.2 Southwest Nigeria schist belts Ife-Ilesha schist belt The Ife-Ilesha Schist belt (one of my study areas) is in southwest Nigeria and is represented by 10 in Figure 2.3b. The lithologic units of the Schist belt consist of a gneissic basement, mafic-ultramafic amphibolite units, metapelites (staurolite-garnet schist, muscovite schist, quartz-mica schist, talc schist), 16 and quartzite (Ige et al. 1998). The gneisses occur as grey gneiss and orthogneiss. The grey gneisses are Archean in age (ca. 2.6 Ga; U-Pb upper intercept zircon age) while the orthogneisses are Paleoproterozoic in age (ca. 1850 Ma; U-Pb upper intercept zircon age) (Rahaman, 1988). The mafic-ultramafic amphibolite shows color and textural variation. They occur as mesocratic or leucocratic in color and may show foliation. The ages of the other lithologic units are presently unknown. The Schist belt is cut across north-south by the Ifewara fault, resulting in the mylonitization of the gneisses that were cut through. Petrography of the metapelites suggests they were metamorphosed to Amphibolite facies grade (garnet-staurolite-biotite-muscovite; andalusite-staurolite-garnet-biotite-muscovite; sillimanite-andalusite-garnet-biotite-muscovite-chlorite; cordierite-staurolite-garnet-biotite; cordierite-garnet-biotite-staurolite) (Zeh and Holness, 2023). High-level deformation is commonly observed in quartzites within the metapelite as they show asymmetric recumbent and isoclinal folds with the fold axes trending north-northeast (Caby and Boesse, 2001). The folds bear the imprints of the Pan-African orogeny (north-south, north-northeast), suggesting that the Ife-Ilesha Schist belt was affected by the Pan-African event. The schist belt is suggested to have experienced a rifting stage around ca. 1.85 Ga, which is associated with the Trans-Saharan belt. This stage led to the emplacement of mafic-ultramafic amphibolite, which shows geochemical signatures of mid-ocean ridge basalt and island arc tholeiitic (Olade and Elueze 1979; Elueze 1982; Ige et al. 1998; Caby and Boesse 2001). Rahaman et al. (1988) and Ige et al. (1998) suggest that amphibolite was formed from a back-arc setting. Recent studies (e.g., Olajide-Kayode et al. 2022, 2023) have confirmed that the amphibolite is ortho-amphibolite of tholeiitic and calc-alkaline affinity and could have been derived from different sources of magma, coupled with their tectonic field plotting in Within-Plate-Basalt. Caby and Boesse (2001) proposed a nappe system for the Ife-Ilesha Schist belt based on geologic structures preserved as tectonic contacts and thrust zones. The schist belt was part of the areas affected by the Pan-African orogeny in southern Nigeria. Zeh and Holness (2003) reported on post-kinematic garnets and reaction overstep on garnet in metapelites from the schist belt. The P-T condition of metapelite from the Schist belt has been estimated at 550–610°C and 3–5 ± 0.5 kbar (Ige et al. 1998; Zeh and Holness 2003). Olajide-Kayode et al. (2022, 2023) estimated the pressure and temperature of amphibolite using a plagioclase-hornblende thermobarometer and obtained P-T values of 0.5-4.3 kbar and 578 - 642°C. Iseyin-Oyan River schist belt The Iseyin-Oyan River schist belt is the south westernmost schist belt in Nigeria, almost at the Western border with the Benin Republic, it is represented with 9 (Fig. 2.3b). It is the closest schist belt to the Dahomey belt. It was folded into Archean to Lower Proterozoic rocks, which extend to the center of 17 Ibadan (Turner 1983). The schist belt is not as exposed as many other Nigerian schist belts. It is covered by a very thick tropical forest and overburden. It shows similarities with the Ife-Ilesha schist belt, which is located east of the Iseyin Oyen River schist belt. The similarities with the Ife-Ilesha schist lie in the presence of mica schist, quartzite, thin discontinuous layers to massive amphibolite, and granitic intrusions that intruded both the Archean basement and schist belt. The area is also dominated by amphibolite which has been suggested to be of higher metamorphic grade than most Nigerian schist belts (Rahaman 1976; Turner 1983). Recent studies (Salawu et al. 2020) suggest the metamorphic grade within metapelite in the Iseyin Oyan schist belt is within lower to upper Amphibolite facies. Igarra schist belt Igarra schist belt (one of my study areas) forms the smallest schist belt in southwestern Nigeria. It trends about 50 km NNW (Turner 1983) and is located at the eastern end of the southwestern Nigerian basement; it is represented with 11 (Fig. 2.3b). It is the most unique schist belt in Nigeria as described in (Turner 1983). It is dominated by mica schist, garnet mica schist, quartzite schist, migmatite-gneiss, calc-gneiss, and granite. It shows the major difference from other Nigerian schist in terms of the presence of calcareous rocks and the occurrence of meta-conglomerate units. The Igarra schist belt is highly mineralized, and most research within the area has been on mineral potential assessment, and structure of the schist belt. Kolawole et al. (2021) identified the spread of columbite/tantalite mineralization in some pegmatite from the Bunu area of the Igarra schist belt and suggested a minimal degree of mineralization. A possible epigenetic mineralization trend has been identified within the Dagbala-Atte area of the Igarra schist belt using Landsat-8 Lineament analysis (Adepoju et al 2021). There are different studies on petrography and geochemistry of charnockite within the Igarra schist belt (Ogunyele et al. 2022), deformation and structure (Ajigo et al. 2020; Adetunji et al 2020; Obiadi et al. 2013), and aeromagnetic studies (Faruwa et al. 2021; Salawu et al. 2021) on the schist belt. Earlier studies (Omitogun et al. 1991) used mineral assemblages as geothermobarometers to estimate the pressure and temperature of schist and migmatite within the Igarra schist belt. There are no major studies on petrogenesis, whole rock geochemistry, mineral chemistry, and geochronology on the lithological units of this schist belt. 2.4 Pan-African granites (“Older granites”) (ca. 950–600 Ma) The tectonic evolution of the pan-African granites in Nigeria has sparked controversies that have lasted over four decades (Van Breemen et al. 1977; Lenoir et al. 1994; Ferre et al. 1998; Kwekam et al. 2020). In the literature, the pan-African intrusions are also referred to as the “Older granites”, however, for clarity I will use the term pan-African granites. The Nigerian basement constitutes most of the Benin-Nigeria shield, an N-S trending shield sitting on the southern edge of the Pan-African belt in Northwest 18 Africa. To the west of the Benin-Nigerian shield lies the Pan Africa suture which was formed by the eastern subduction of the West African craton during the Pan African orogeny (Fig. 2.3a). The pan-African granitoids represent the youngest unit in the Nigerian basement complex and are found across the whole of Nigeria (Fig. 2.3b). As their name implies, they are products of the Pan-African orogeny. These granites are found to intrude both the migmatite-gneiss complex and the supracrustal schist belts. The pan-African intrusive bodies are variably deformed and are typically characterized as either syn-kinematic or post-kinematic (Ferre et al. 2002; Girei et al 2019). Many of the pan-African granites are mapped as deep-seated batholiths. They are highly potassic and occur mainly as monzogranite and granodiorite. They have a wide age range across the basement complex ca. 665–579 Ma (Ekwueme and Kroner 1998; Goodenough et al. 2014; Adetunji et al. 2016; Bute et al. 2020) Pan-African intrusions occur in the area surrounding the Pan-African suture in Ghana to as far as Cameroon spanning an area over 1300 km (Turner 1983). Early research on the older granite confirmed they are high K, calc-alkaline in origin and plots within an orogenic field during geochemical characterization (McCurry and Wright 1976; Egbuniwe et al. 1985; Fitches et al. 1985; Girei et al. 2019). In northwest Nigeria, McCurry (1971), observed that the older granite was emplaced mostly along the multiple NNE dextral fault system which was formed during the Pan-Africa orogeny. Bearing in mind the position of these dextral fault systems relative to the positions where the granitoids were emplaced, McCurry and Wright (1976) suggested the fault systems could be other Pan African sutures. A recent study by Girei et al. (2019) also reported calc-alkaline granites in NE Nigeria within the Hawal massif. The area surrounding the Hawal massif is cut across by the Massenya Ouinanga gravitational anomaly which was discussed in (Freeth 1984) and interpreted as a possible suture zone (Fig. 2.3a and b). 19 Chapter Three Methodology 3.1 Fieldwork Fieldwork was carried out across southern Nigeria in March 2023. The fieldwork involved an eleven-day truck-supported east-west traverse of southern Nigeria with a particular focus on two major schist belts (Ife-Ilesha schist belt and Igarra schist belt) in the southern basement complex and the Oban massif in the southeast basement complex. Structural data of deformation fabric and major fracture orientations were collected at each outcrop and are presented in Appendix A., Supplementary Table 1. 3.2 Sample collection A total of 34 rock samples were collected from the transect across the Nigerian basement complex. The rock units sampled include migmatite, gneiss, granite, charnockite, pegmatite, talc schist, biotite schist, biotite-muscovite schist, amphibolite, and quartzite. Care was taken to collect representative samples from all of the major units within the three main study areas. 3.3 Petrography A total of 17 thin sections were made from a subset of the samples with the most suitable mineral assemblages and microstructures for pressure-temperature determination and geochronology. Mineral abbreviations are after Whitney and Evans (2010). 3.3.1 Petrographic analysis Thin sections were studied in plane and cross polars using a NIKON 550S LV-UEPI microscope housed in the Fipke Laboratory of Trace Element Research (FiLTER) at the University of British Columbia, Okanagan, Canada. The purpose of the petrographic analysis was to confirm the presence of suitable mineral phases for in-situ geochronology and mineral chemical determination. 3.4 X-ray fluorescence (XRF) spectroscopy To ascertain the modal mineralogy of the 17 rock samples analyzed, I analyzed thin sections using an IXRF Atlas X micro-XRF housed in the Filter Research Center at the University of British Columbia, Okanagan, Canada. Major and minor elements were mapped in 2D across all 17 thin sections. Analyses were performed at a working (focus) distance of approximately 10 mm. A point dwell time of 10 ms and a time constant of 0.5 were used for all analyses. The X-ray voltage and current were set at 50 kV and 800 µA, respectively. Post-acquisition, the data file generated by this method is output as a hyperspectral SMA file, which can be processed into various file formats depending on the intended usage. Since my intended 20 focus for the use of the XRF data was to construct phase maps, the files were converted from SMA files to element-intensity text files. 3.4.1 Phase maps Xmaptools version 4.3 was used to carry out modal mineral analysis of the samples analyzed. Phase maps were produced using a supervised learning algorithm that involved the manual selection of regions of interest within element intensity maps for each mineral phase. The regions of interest were selected on all identifiable minerals in the sample and informed/confirmed by petrographic observations. The generated phase maps were used to target areas during subsequent geochronology and mineral chemistry analyses. They are presented in Appendix A. 3.5 Geochronology To determine the timing of magmatism and metamorphism in my study areas, we applied in-situ geochronology involving mica Rb/Sr, garnet Lu-Hf, and U-Pb on monazite/titanite. All the in-situ analyses were carried out at the Fipke Laboratory of Trace Element Research (FiLTER) at the University of British Columbia, Okanagan campus. Six thin sections from the three study locations, spanning east to west across Nigeria, were selected for geochronological analyses. The thin sections include three metasedimentary samples (ND003-2, Igarra L1A, and ND021A) from Ife-Ilesha and Igarra Schist Belts and the Oban Massif, two granitic samples (ND07A and L9) from Ife-Ilesha and Igarra Schist Belts, and an LCT pegmatite from Igarra Schist Belt. 3.5.1 87Rb/87Sr analysis 87Rb/86Sr geochronology on mica (biotite, muscovite, and lepidolite) was carried out in-situ in thin sections of metasediments, granite, and pegmatite using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A total of six samples were analyzed in two different runs (five samples on biotite and muscovite, and one sample on lepidolite). Each run consisted of three thin sections for analysis. Four major mineral phases were targeted in each of the six samples, including biotite, muscovite, plagioclase, and lepidolite. Analytical spot locations on each sample were chosen to represent an extensive spatial and textural variability of the microstructural complexity within each thin section. Mica (biotite, muscovite, and lepidolite) and plagioclase were targeted during each run using a circular laser spot. During each run, ~80 spot analyses of mica and ~15 spot plagioclase of unknown were analyzed. A 65µm diameter circular laser spot was used during the analysis with a repetition rate of 5 Hz and a ~ 3J/cm2 fluence. The parameters of the laser and ICP-MS are described in Table 3.1. The 87Rb/86Sr data of the reference materials are presented in Table 3.2, while the full data of the analysis is presented in Appendix D, Supplementary file. 21 Table 3. 1. Laser and ICP-QQQ-MS parameters Laser (ESL 193 nm Excimer with TwoVol3) Value Unit Spot size 65 μm Rep rate 5 Hz Fluence 3 J/cm2 He flow rate 350 mL/min Ablation/ washout time 25/20 s ICP-MS/MS (Agilent 8900) RF Power 1550 W RF matching 1.3 V Sample depth 5 mm Ar gas - mixed with laser He 0.9 L/min Extract lens 1 −15 V Extract lens 2 −250 V Omega bias −120 V Omega lens 6.8 V Q1 Entrance −50 V Q1 Exit −1 V Cell focus 0 V Cell entrance −50 V Cell Exit −60 V Deflect 4 V Plate bias −60 V Q1 Bias −2 V Q1 prefilter Bias −5.5 V Q1 Postfilter Bias −10 V N2O flow (4th cell gas) 0.37 (25%) mL/min OctP Bias −5 V Axial Acceleration 2 V OctP RF 170 V Energy Discrimination −10 V Wait time offset 2 ms 22 3.5.1.1 Instrumentation settings and optimization The instrumentation settings used for the 87Rb/86Sr analyses were followed as reported by Larson et al. (2021). The laser ablation system consisted of an ESL193 Excimer laser ablation system fitted with two Vol 3 sample chambers attached to an Agilent 8900 Triple Quadrupole ICP-MS with tandem mass spectrometry capacity. The ICP-MS is connected to an Argon nebulizer gas supply, which mixes with Helium gas from the Laser through a synthetic polymer “Y” connector. This goes through an in-house signal smoothing device before being passed to the plasma. Before the start of the analysis, the ICP-MS was tuned to single quadrupole mode for some hours under robust plasma conditions with NIST 610 as a reference to generate maximum sensitivity on the other hand adhering to the oxide (monitored as 232Th16O) ratios below 0.2% and isotope ratio equivalence (monitored as 238U/232Th). As soon as optimal sensitivity was attained, the ICP-MS was switched into Mass-Shift mode (MS/MS mode) using nitrous oxide (N2O) as the reaction gas following the procedure outlined in Hogmalm et al. (2017), which has proven that N2O produced is ~10 times more sensitive when reacting to Sr+ than other gases (e.g., SF6 and O2) (Hogmalm et al. 2017). The flow of carrier gases (Argon and Helium) and the reaction gas (N2O) were further optimized to generate a stable signal and sensitivity. The Agilent 8900 has a special feature (axial acceleration) when in mass-shift mode that enables the system to prevent a decrease in sensitivity in the hexapole collision/reaction cell due to a drop in kinetic energy of the reaction products towards the second quadrupole (Q2). Adopting (Redaa et al. 2021; Larson et al. 2023), we added a 2V value of axial acceleration to maintain sensitivity in the collision cell. 3.5.1.2 Analysis method and data processing All the analytical runs contained thin sections (specimens) to be analyzed (an unknown) with reference materials (RMs) of known age and/or composition. The RMs are used to monitor and correct temporal instrument drift in signal during analysis, matrix differences between various reference materials and the unknowns, and possible laser-induced downhole element fractionation (Zack and Hogmalm, 2016; Larson et al, 2023; Stijn et al, 2024). The thin sections containing the minerals to be analyzed were introduced into the ICP-MS, and spot locations were selected randomly across the thin sections. The chosen spots were ablated using the laser beam. All the ablated particles were transported by helium gas to the Argon plasma, where the particles are ionized. The plasma strips off the outer electron shells of the particles, leaving them with only positively charged particles. After ionization in the plasma torch, ions are extracted into the mass spectrometer through a sample skimmer cone under high vacuum. They are directed into an ion optics system where focusing, acceleration, and mass separation occur. The selected isotope moves towards the ion lenses, which have different mass-charges ratios. The ions are then fired down from the lenses to the first quadrupole, Q1. The 23 first quadrupole functions as a mass filter to filter only a certain ion (ions of interest) based on their mass-to-charge ratio (m/z) and allows them (Rb and Sr) to pass through to the collision cell. For the Rb/Sr analysis, N2O was used as the reaction gas inside the collision cell following the recommendation of Hogmalm et al. (2017), where it has been proven through online separation that Rb does not react with N2O. Rather, Sr reacts with N2O to produce 87Sr16O+, removing the isobaric interference that normally occurs between parent and daughter isotopes. The newly formed isotopes are transported from the collision cell to the second quadrupole, Q2, which also functions as another mass filter. Q2 filters the transported ions based on their mass-to-charge ratio (m/z), leaving only the desired ions to hit the detector. All the ions hit by the detector are measured for their abundance. There are two modes within the detector: a pulse mode detector and an analog mode detector. The detector in the ICP-MS Agilent 8900 displays a dynamic range of 11 orders of magnitude, meaning it can count from millions to trillions. The pulse detector counts to about 1.5 million counts per second, while the analog mode detector counts to a much higher range. When the pulse detector gets hit by so many element isotopes exceeding its counting range, it switches to a robust detector (analog detector) with the ability to count at a very high range. These two detector modes cover the dynamic ranges present in a given sample. For all analyses, we applied the standard bracketing technique reported in recent studies (Dyck and Larson 2023; Larson et al. 2024), where a group of standards (RMs) was placed after every ~15 unknowns to monitor and correct instrument drift during the analytical sessions. The NIST610 glass was used as the Primary Reference Material (PRM) for all the analytical sessions following the recommendation of Stijn et al. (2024) due to its homogeneity as compared with the inhomogeneous compressed phlogopite nano-powdered pellet, Mica Mg, used by previous research (Zack and Hogmalm, 2016; Hogmalm et al., 2017; Rosel and Zack, 2022; Redaa et al, 2021; Larson et al., 2023; Dyck and Larson, 2023). The phlogopite, Mica-1B, reported by Camacho et al. (2012), was used as the matrix correction reference material (MCRM) for all the analytical sessions; however, there was an exceptional unknown (sample Igarra 1C, a lepidolite pegmatite) that required the use of a different MCRM (Tanco, a lepidolite pegmatite) that has the same crystalline structure. Hence, we used Tanco, an Archean lepidolite pegmatite that was reported in Camacho et al. (2012) as the MCRM for the analysis involving the lepidolite pegmatite because they have the same crystalline structure. These MCRMs were used alongside other RMs and in-house reference materials (Mica Mg, Mica Fe, Mica 1B, Mica 1O, MA1, and MA2). Secondary standards were used to verify the certainty of the ages obtained since they have well-established ages. All generated data were processed in the Iolite software program v.4.5 after Panton et al. (2011) with a recent modification of 87Rb/87Sr_Isotope data reduction scheme, which was modeled after Redaa et al. (2021) by Joe Petrus and Bence Paul. The recent Data Reduction Scheme on Iolite by Larson (2024) 24 enables calculations involving error correlation of raw isotopic ratios for each sweep. The ICP-MS files (data and laser log files) were uploaded into Iolite for the Data Reduction Scheme (DRS), which involves the calculation of isotopic ratios 87Sr/86Sr (Larson 2024). Before the calculations, the log signals are re-evaluated and reprocessed by repacking the signals on the signal plateaus for each spot analysis. This is done for reference materials and the unknowns, after which DRS is crunched on NIST610. The calculation of isotopic ratios involves four analytical protocols as detailed in Stijn et al. (2024) which are related to; (1) background subtraction; (2) correction of downhole fractionation (DHF) against NIST610; (3) averaging the corrected downhole ratios for each sweep in the analysis; (4) normalize to NIST610 to correct temporal signal drift during analysis and matrix correction. The 87Sr/86Sr ratios for the two runs were corrected from the differences between the measured and expected value of NIST610 (0.709699) based on Woodhead and Hergt (2001). Excess dispersion during the measuring of isotopic ratios (87Rb/87Sr and 87Sr/86Sr) as measured in NIST610 for the individual analysis was propagated in quadrature between 0–1%. No dispersion of (87Rb/87Sr and 87Sr/86Sr) was propagated for the first run; however, 1% dispersion on (87Rb/87Sr) was propagated for the second run. The second analysis involved three unknowns (two granites and a lepidolite pegmatite). The lepidolite pegmatite was processed separately because a different RM (Tanco, Camacho et al. 2012) was used for matrix correction. 1% dispersion of 87Sr/86Sr was propagated for the lepidolite pegmatite with 0% dispersion of 87Rb/87Sr. We applied the Powell et al. 2020 robust isochron regression model, which is referred to as “spine,” where the authors used the robust statistical approach of Huber (1981). Unlike the York (1969) model, which is guided by the mean standard weighted deviation, MSWD, which requires that all uncertainties associated with any analytical data be Gaussian distributed to be valid, following this, datasets with meaningful results are often disregarded because they are not Gaussian distributed (Powell et al. 2020). The Powell et al 2020 robust regression calculations model does not require that the uncertainty in the dataset be truly Gaussian, provided that the central part of the uncertainty’s overall pattern of the dataset defines a spine (Spine = s), irrespective of whether it has fat tails at either end (Powel et al. 2022). The spine method evaluates the scatter of data points around the isochron line using a statistic denoted as "s". A value of s ≤ 1 indicates that the data scatter is within expected analytical uncertainties, suggesting a reliable isochron age, whereas values >1 indicate overdispersion (errorchron). Error correlation (rho) is not utilized for the Rb-Sr isochron dates calculated. The high Rb/Sr ratios due to low Sr concentrations in some specimens lead to high uncertainties in the measured 86Sr. Because 86Sr occurs in the denominator of both ratios in a typical isochron plot, the uncertainty in that measurement dominates the error correlation and, when high, can result in error correlations approaching 1. Such correlations have essentially no geological meaning and 25 only serve to exacerbate the apparent dispersion of the data as they result in needle-like ellipses elongate parallel to the regression line. The analysis of all the reference materials in run 1 (e.g., Mica 1B, Mica 1O, Mica Fe, Mica Mg, GA1150, MA1, and MA2) returned dates that overlap the expected ages. However, in run 2, only the analysis of Mica 1B, Mica 1O, Mica Fe, Mica Mg, and Tanco produced dates that overlap with the expected dates. MA1 and MA2 produced dates that are older than expected. This is because the laser ablated on glass, and the analysis is bad, so we do not trust the dates that they return. The reproduced dates on the reference materials are reported in Table 3.2; the numbers in brackets specify the analytical runs. The errors and uncertainties associated with the data are 2SE. 26 Table 3. 2. Reproduced dates of reference materials via ChrontourR (Larson, 2022) after 2 analytical runs. S=spine for robust isochron calculation regression model after (Powell et al. 2020) Analytical Run Reference Material Author Ndi/Ns Spot size (μm) Age (Ma) This study 1 Mica Mg Hogmalm et al. (2017) 0.726 65 519.4 ± 6.5 Ma 500 ± 3 Ma (S=1.09) 1 Mica Fe Rosel and Zack (2022) 0.71 65 305 ± 2 Ma 304 ± 3 Ma (S=0.48) 1 Mica 1B Camacho et al. (2012) 0.7035 65 990 ±5 Ma 988 ± 10 Ma (S= 1.1) 1 Mica 1O Camacho et al. (2020) 0.735 65 986 ± 5 Ma 966 ± 15 Ma (S=0.52) 1 MA1 A Camacho, Pres. Comm. 0.75 65 350 – 360 Ma 366 ± 3 Ma (S=1.27) 1 MA2 A Camacho, Pres. Comm. 0.75 65 350 – 360 Ma 361 ± 4 Ma (S=1.16) 1 GA1150 0.7049 65 98 105 ± 3(S=0.5) 2 Mica Mg Hogmalm et al. (2017) 0.726 65 519.4 ± 6.5 Ma 505 ± 4 Ma (S=0.57) 2 Mica Fe Rosel and Zack (2022) 0.71 65 305 ± 2 Ma 302 ± 3 Ma (S=0.55) 2 Mica 1B Camacho et al. (2012) 0.7035 65 990 ±5 Ma 998 ± 11 Ma (S= 0.57) 2 Mica 1O Camacho et al. (2020) 0.735 65 986 ± 5 Ma 991 ± 19 Ma (S=0.2) 2 MA1 A Camacho, Pres. Comm. 0.75 65 350 – 360 Ma 370 ± 3 Ma (S=0.33) 2 MA2 A Camacho, Pres. Comm. 0.75 65 350 – 360 Ma 380 ± 4 Ma (S=0.98) 2 Tanco Camacho et al. (2012) 0.7 65 2640 Ma 2560 ± 110 Ma (S=0.13) 27 3.5.2 176Lu/176Hf analysis In-situ 176Lu/176Hf on garnet was carried out in thin sections of metasediments from Ife/Ilesha and the Igarra Schist belts of the Benin-Nigeria shield using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A total of two thin sections were analyzed in a single run. Analytical spot locations on each sample were chosen randomly across the thin sections to avoid inclusions such as zircons, which absorb and retain Hf. The parameters of the Laser and ICP-MS are described in Table 3.3. The 176Lu/176Hf data of the reference materials are present in Table 3.4, while the full data of the analysis is presented in Appendix D, Supplementary file. Table 3. 3. Laser and ICP-QQQ-MS parameters as adopted after Larson et al (2024) Laser (ESL 193nm Excimer with TwoVol3) Value Unit Spot sizes 35, 65, 150, 220 µm Rep rate 10 Hz Fluence 4 J/cm2 He flow rate 350 mL/min Ablation/ washout time 40/25 s ICP-MS/MS (Agilent 8900) RF Power 1550 W RF matching 1.3 V Sample depth 3.5 mm Ar gas - mixed with laser He 0.9 L/min Extract lens 1 -15 V Extract lens 2 -250 V Omega bias -120 V Omega lens 6.8 V Q1 Entrance -50 V Q1 Exit -1 V Cell focus 0 V Cell entrance -50 V Cell Exit -60 V Deflect 4 V Plate bias -60 V Q1 Bias -2 V Q1 prefilter Bias -5.5 V Q1 Postfilter Bias -10 V 28 Laser (ESL 193nm Excimer with TwoVol3) Value Unit NH3 flow (3rd cell gas) 0.37 (25%) mL/min OctP Bias -5 V Axial Acceleration 2 V OctP RF 170 V Energy Discrimination -10 V Wait time offset 2 ms Isotopes monitored Mass/mass shift Dwell time (ms) Al 27 -> 27 2 Ca 43 -> 43 2 Ti 47 -> 47 2 Y 89 -> 89 2 Zr 90 -> 90 2 Ce 140 -> 140 2 Yb 172 -> 172 10 Yb 172 -> 254 5 Lu 175 -> 175 10 Lu 175 -> 191 20 Lu 175 -> 257 100 Hf 176 -> 258 150 Hf 178 -> 260 150 Table 3. 4. Table Reproduced dates of Lu-Hf reference materials via IsoplotR (Vermeesch, 2018) Reference Material Author Spot size Age (Ma) This study Hogsbo Garnet Simpson et al (2021) 75 µm 1030 – 1037 Ma 1022 ± 24 Ma; MSWD = 1.2 Gwa Rebeiro et al (2024) 150 µm 936 ± 1.5 Ma 929 ± 24; MSWD = 1.5 Skaha Larson et al (2024) 150 µm 70 ± 1 Ma 79 ± 11 Ma; MSWD = 1.2 3.5.2.1 Instrumentation settings and optimization The instrumentation settings used for 176Lu/176Hf analyses were after (Simpson et al. 2021, 2022; Larson et al. 2024). The laser ablation system consisted of an ESL193 Excimer laser ablation system fitted with two Vol 3 sample chambers attached to an Agilent 8900 Triple Quadrupole ICP-MS with tandem mass spectrometry capacity. The ICP-MS is connected to an Argon nebulizer gas supply, which mixes with Helium gas from the Laser through a polypropylene “Y” connector, and it goes through an in-house signal smoothing device before being passed to the plasma. Before the start of the analysis, the ICP-MS was tuned 29 to single quadrupole mode for some hours under robust plasma conditions with NIST 610 as a reference to generate maximum sensitivity while adhering to the oxide (monitored as 232Th16O) ratios below 0.2% and isotope ratio equivalence (monitored as 238U/232Th). As soon as optimal sensitivity was attained, the ICP-MS was switched into Mass-Shift mode (MS/MS mode) using NH3 as reaction gas in the hexapole collision/reaction cell (Simpson et al. 2021, 2023; Larson et al. 2024). 3.5.2.2 Analytical procedure and data processing The determination of Lu-Hf isotopic ratios without isobaric interference of parent and daughter isotopes (176Lu, 176Yb, and 176Hf) has been successful using ammonia as a reaction gas (NH3) in analyzing aqueous samples (Woods, 2017). However, more recently, it has proven to be more useful in in-situ analysis involving diverse mineral systems (e.g., garnet, apatite, xenotime, calcite, etc.) using laser ablation inductively coupled plasma mass spectrometry (Simpson et al. 2021, 2022, 2023; Tamblyn et al 2021; Brown et al 2022, Stijn et al. 2023; Kharkongor et al 2023; Larson et al. 2024). The analytical procedure adopted herein followed the analytical protocols outlined in (Simpson et al. 2021, 2022; Larson et al 2024). Garnet porphyroblasts in two thin sections were analyzed in a single analytical run. The standard bracketing technique was applied where a group of unknowns (<16 unknown) are bracketed within a range of primary and secondary reference materials of well-established ages to correct temporal instrument drift, elemental fraction, and matrix correction (Dyck and Larson 2023; Simpson et al. 2023; Larson et al 2023, 2024; Kellett et al 2024). The NIST610 (Nebel et al 2009; Jochum et al 2011) was used as the primary reference with the 1030–1037 Ma Hogsbo garnet (Simpson et al. 2021) serving as the matrix correction reference material along with two garnets in-house reference materials derived from granites, which were used to check for analytical accuracy. Hogsbo garnet was derived from the Hogsbo pegmatite in Gothenburg, Sweden, yielding a Columbite U-Pd date of 1029 ± 1.7 Ma (Romer and Smeds, 1996). The reference materials and the unknowns were all analyzed using a 150 µm laser spot size, except for the Hogsbo garnet, which required a smaller spot because it contains a higher concentration of Lutetium. The output data from the analysis was analyzed using Iolite Software version 4 (Paton et al. 2012) for drift correction and a data reduction scheme. The in-house Lu-Hf data reduction scheme of Larson et al. (2024) with reference to Simpson et al. (2021) was also used. Detector correction between pulse and analog mode was done for the reference materials and unknowns. Excess dispersion (1.5%) measured on the isotopic ratios in the primary reference material (NIST610) was propagated in quadrature for all the unknowns. Laser spot analysis that was on glass was excluded. The analysis of all the reference materials returned dates that overlap the expected ages and are reported in Table 3.2. The analytical process involved the analysis of two unknowns (Igarra 1A and ND003) in one analytical session. During the garnet analysis, spot locations were well spread across multiple garnet grains, targeting areas without zircon, monazite, and 30 other inclusions while also avoiding cracks within the grains. For Igarra 1A, 69 spot 176Lu/177Hf analyses were measured, while 81 spot 176Lu/177Hf analyses were measured for ND003, all in a single analysis. The errors and uncertainties associated with the data are 2SE. 3.5.3 U-Pb In-situ U-Pb isotopic analysis on titanite and monazite was carried out using LA-ICP-MS. The analytical sessions involved the measurement of 238/206U-207/206Pb isotopic ratios and trace elements in both titanite and monazite in two separate analytical runs, respectively. The two analytical sessions required two separate instrument settings, which are presented in Appendix D, Supplementary file. The U-Pb data of the reference materials are presented in Table 3.5. Table 3. 5. Table Reproduced dates of U-Pb reference materials via IsoplotR (Vermeesch, 2018) Reference Material Author Spot size Age (Ma) This study M_KED1 Spandler et al 2016 50 µm 1517.3 ± 0.3 Ma 1507 ± 16 Ma; MSWD = 2.1 McClure Schoene and Bowning (2006) 50 µm 523.3 ± 1.3 Ma 527.2 ± 4.6 Ma; MSWD = 2.6 Mud Tank Fisher et al (2020) 50 µm 319.2 ± 0.4 Ma 320 ± 11 Ma; MSWD = 2.1 Mz_44069 Aleinikoff et al (2006) 25 µm 424.9 ± 0.4 Ma 424 ± 2.5 Ma; MSWD = 0.029 Mz_Stern Wiedenbeck et al (1995) 25 µm 507 ± 4 Ma 515 ± 2.5 Ma; MSWD = 6.2 M_Trebilcock Tomascak et al (1996) 25 µm 273 Ma 275.6 ± 1.4 Ma; MSWD = 5.9 3.5.3.1 Analytical procedure and data processing All titanite grains were ablated using a 50 µm laser spot size with 3.5 J/cm2 fluence and 5.00 Hz ablation frequency and dwell time of 25. Monazite grains were targeted using a 15 µm laser spot size with an ablation frequency of 8.00 Hz and 3.5 J/cm2 fluence. Pre-ablation shots were carried out for 2 seconds to remove carbon coatings on the surfaces of the thin sections before the analysis. The standard bracketing technique was used, and groups of reference materials were placed at intervals between the unknowns to correct instrument drift, elemental fractionation, and matrix correction. The homogeneous NIST612 glass was used as the primary reference material for the titanite analysis. The accuracy of the analysis was assessed by using the inhomogeneous T_MKED1 of Spandler et al. (2016) as the matrix correction reference material. Two standard secondary reference materials whose ages are well established were used to test the accuracy of the analysis; the reference materials include Mud Tank, which was reported by Fisher et al. (2020), and McClure, which was analyzed by Schoene and 31 Bowning (2006). The analysis of T_MKED1 yielded a U-Pb concordant date (1507 ± 16 Ma; MSWD = 2.1) which overlaps with the reported age (1517.3 ± 0.3 Ma; Spandler et al. 2016). The analysis of Mud Tank returned U-Pb concordant age (320 ± 11 Ma; MSWD = 2.1) which overlaps the reported age (319.2 ± 0.4 Ma; Fisher et al. 2020) while the analysis of McClure yielded concordant age (527.2 ± 4.6 Ma; MSWD = 2.6) and it overlaps the reported date (523.3 ± 1. Ma; Schoene and Bowning, 2006). All the reference materials returned dates within 3% uncertainty. Mz_44069 was used as the primary reference material for the monazite analysis. The Mz_Stern of Wiedenbeck et al. (1995), an inhomogeneous material, was used as the correction reference material to assess the accuracy of the analysis. Two standard reference materials (Mz_44069; Aleinikoff et al. 2006 and Trebilcock; Tomascak et al. 1996) with well-established ages were used to test the accuracy of the analytical procedure. The analysis of Mz_Stern yielded a U-Pb concordant age (515 ± 2.5 Ma; MSWD = 6.2) which overlaps with the actual age (507 ± 4 Ma; Wiedenbeck et al. 1995). The analysis of Primary reference material, Mz_44069, yielded U-Pb concordant age (424 ± 2.5 Ma; MSWD = 0.029) and this date overlaps with the reported U/Pb ID TIMS age (424.9 ± 0.4; Aleinikoff et al. 2006) while Trebilcock returned a U-Pb concordant age (275.6 ± 1.4 Ma; MSWD = 5.9), which overlaps with the date (273 Ma) reported in Tomascak et al. 1996. The dates returned by the standards are within 3% uncertainty. The data obtained via LA-ICP-MS analytical runs were processed on Iolite Software Version 4 (Panton et al. 2012). The processing accounts for downhole fractionation, drift correction, and data reduction schemes. The first step in data processing involves the repacking of stable signal portions of 206Pb/207Pb ratios in the analysis. This requires a comparison of 206Pb/207Pb ratios on each spot analysis. During spot analysis, the grains to be analyzed might explode during ablation, and the signals obtained may include some record of the grain, while the rest might be a glass analysis. Typically, this results in a very noisy signal showing instability throughout the analysis. A stable section is followed by an unstable section, or the reserve may be the case. In such cases, the best stable portion of the 206Pb/207Pb ratio signal for each analysis is selected. To account for natural variability and to provide accuracy and meaningful interpretation of the dataset obtained, dispersion in 206Pb/207Pb and 238U/206Pb was calculated on Mz_44069, the primary reference material for the titanite analysis, whereas for the monazite analysis, it was calculated on Trebilcock, a secondary standard. Dispersion values ranging between 0.2–1.7% were added to the titanite and monazite analysis using ChrontourR software after Larson (2022). Tera-Wasserburg diagrams for the titanite and monazite analysis were made from IsoplotR (Vermeesch, 2018) and ChrontourR (Larson, 2022). The common 206Pb/207Pb and lower intercept dates for the monazite and titanite analysis were obtained on the Tera-Wasserburg diagram. The errors and uncertainties associated with the data are 2SE. 32 3.6 Mineral chemistry The chemical composition of major mineral phases in a thin section (ND003) from the Ife-Ilesha Schist belt, with a suitable mineral assemblage for pressure and temperature determination, was obtained using a CAMECA SX-Five electron microprobe analyzer (EMPA). The microprobe analyzer is housed at the Fipke laboratory for trace element research at the University of British Columbia, Okanagan campus. The probe operational conditions include an accelerating voltage of 15 kV, 20 nA beam current, 20 s counting time on the peaks, 10 s background position, and a 5µm beam spot size. Complete information on mineral calibration is provided in Appendix D, Supplementary Table 6. The EMPA dataset reduction was done following the steps outlined in Pouchou and Pichoir (1985). Spot locations for analysis on all mineral phases were chosen to be representative of the variety of petrographic positions (i.e., matrix, porphyroblast core, and rim). Multiple spots were collected across the thin section to identify both inter- and intra-crystalline compositional variety. The full EMPA dataset is presented in Appendix C, Supplementary file, and the associated site backscattered electron images are presented in Appendix C. 3.7 Petrological modelling To determine the pressure and temperature (P-T) conditions at which the rocks of the southwestern Nigerian basement were metamorphosed, I performed phase diagram-based petrological modeling. To construct phase diagrams, I used MAGEMin, a mineral assemblage Gibbs free energy minimization program developed by Riel et al (2022), which employs the thermodynamic dataset of Holland et al (2018). Phase diagrams were modeled in the Na2O–CaO–K2O–FeO–MgO–MnO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system using the activity–composition relations of White et al (2014), which were developed for pelitic bulk compositions. MAGEMin employs a gridded minimization algorithm, where the number of calculations begins with an 8x8 grid and then, in each subsequent grid, the calculations increase by a factor of two. I used an initial grid subdivision of three and six refinement levels to calculate the final phase diagram. To test the accuracy of the calculations, I ran the same bulk rock composition through the well-established Theriak-Domino Gibbs minimization software (de Capitani and Petrakakis 2010), using both tc_ds55 (Powell and Holland 1998) as well as SPaC databases (Pattisson and Debuhr 2015). 33 Chapter Four Results In this chapter, I present the petrographic, geochronological, and petrologic modeling results for each of the three study areas, ordered from east to west. Given the variety of lithologies encountered on this transect and the inherent differences in how amenable these samples are to the geochronological and petrological modeling techniques I applied, it was not possible to apply the same methods uniformly to all three study areas. Instead, I focused on generating the highest quality geochronological and pressure–temperature data for each study area that I could, with the aim of interpreting these data collectively in chapter 5. 4.1. Oban Massif 4.1.1 Lithologic and petrographic description of rock units The Oban Massif consists of gneiss and migmatite to the east and supracrustal rocks to the west, with both units intruded by post-kinematic Pan-African age granitoids and charnockite. My transect across the Oban Massif (Fig. 4.1) cut across three main supracrustal units, including mica schist, amphibolite, and charnockite. Figure 4. 1. Geological map of the Oban Massif showing the sample locations (modified after Ekwueme 1987; the Nigerian Geological Survey Sheet map of Cross River State). 34 Sample ND021A is a mica schist collected from the Mark-Sino quarry site along the Calabar-Ikom highway in Awi Town, Cross River State, Nigeria. The sample is of a freshly blasted rock face from the quarry site that has previously mapped by Ekwueme (1987) as being a kyanite schist (Fig. 4.2). However, the thin section made from this sample does not contain kyanite and instead contains quartz (~42%), muscovite (~38%), biotite (~17%), and orthoclase (~5%) (Fig. 4.3). Biotite and muscovite define the main penetrative foliation. Biotite and muscovite both occur as matrix grains and as porphyroclasts. Biotite porphyroblasts range between ~200 and ~600 μm across, while muscovite porphyroclasts are typically larger, between ~580 and 1800 μm across (Fig. 4.3 b–d and 4.6d). Textural evidence of disequilibrium at the edges of muscovite porphyroblasts suggests ongoing or incipient reaction with the fine-grained biotite matrix, which defines the regional foliation (Fig. 4.3 b and d). Orthoclase occurs mostly as fine grains along quartz veins and in quartz-rich areas. Figure 4. 2. Quarry exposure of blasted rocks from the Mark-Sino quarry site. a) freshly blasted unit; b) amphibolite unit with gold-like spots of pyrite; c) deformed mica Schist; Sample ND021B 35 Figure 4. 3. a) mineral phase map of sample ND021A showing different mineral phases; b) photomicrograph taken under crossed-polars showing microcrystalline quartz as the background texture of the rock; c) photomicrograph taken under crossed-polars showing folded micaceous foliation; d) photomicrograph taken under crossed-polars showing mica fish. Sample ND021B is an amphibolite that was collected from the same quarry as sample ND021A. The rock is medium-grained and dominated by dark green hornblende. The thin section made from this sample contains hornblende (~76%), plagioclase (~12%), biotite (~10%), and accessory minerals sphene (~2%) and apatite (~1%) (Fig. 4.4). ND021B exhibits a foliation defined by a preferred shape orientation of hornblende and biotite (Fig. 4.4 a–c). Biotite occurs as fine-grained (200–400 μm) foliation-parallel matrix grains and as inclusions in plagioclase. Plagioclase occurs as fine-grained clusters with biotite and forms a fine-grained matrix (Fig. 4.4c). Sphene occurs mostly as porphyroblasts within hornblende crystals. 36 Figure 4. 4. a) phase map of specimen ND021A showing major mineral phases; b) photomicrograph taken under crossed-polars showing sphene porphyroblast surrounded with hornblende; c) photomicrograph taken under crossed-polars of biotite porphyroblast surrounded with plagioclase and hornblende. Sample ND025 is an anorthosite that forms a large mesocratic to leucocratic subcrop throughout the Oban Massif. Rocks from this unit commonly show spheroidal weathering (i.e., onion back peeling). The thin section made from this sample is made up of anorthite (~ 84%), pyroxene (~ 9%), and hematite (~ 7%) with minor amphibole and chlorite (<1%). The thin section exhibits a relict igneous microstructure with coarse interlocked grains. The feldspars are partially replaced by sericite (Fig.4.5 a–b). Whereas amphibole is locally replaced by chlorite (Fig.4.5 c–d). 37 Figure 4. 5. Photomicrographs of sample ND025. (a–b) shows sericitization in feldspars and chloritisation of hornblende. (c–d) Remnant hornblende cleavage is preserved in the core or chlorite-altered domain. 4.1.2. In-situ mica 87Rb/87Sr geochronology All laser analytical spots that ablated on the glass slide and grain analysis that exploded were excluded while making the isochron plots, as they do not yield the results of valid/correct analysis. Sample ND021A was targeted for Rb-Sr isotopic age determination using biotite and muscovite. A total of one hundred and seventy-eight (178) spot analyses were performed on biotite, muscovite, and plagioclase. 15 spot analyses on plagioclase were excluded because the grains exploded. Eighty-one (81) spot analyses on biotite define an isochron date of 496 ± 5 Ma (S=0.82). The 87Rb/86Sr ratio in biotite ranges between 10–777, forming a single isochron with a little spread and an initial 87Sr/86Sri ratio of 0.7326 ± 0.0049 (Fig. 4.6 a). Eighty-two (82) spot analyses on multiple muscovite textures (porphyroblast and main foliation) define an isochron date of 465 ± 20 Ma (S=0.79) with an initial 87Sr/86Sri ratio of 0.7575 ± 0.0043 (Fig. 4.6 b). While the datapoints for a single statistical population using the robust regression of Powell et al. (2020), applying a York (1969) regression to the data indicates significant overdispersion (mean squared weighted deviates = 2.95). The datapoints spread between reference isochrons of ca. 430 and ca. 530 Ma. 38 Figure 4. 6. Mica Rb/Sr isochron diagrams for sample ND021A, a mica Schist from the Oban Massif, Southeastern Nigeria. a) Biotite Rb/Sr result for biotite spot analysis; b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in biotite; c) Muscovite Rb/Sr result for muscovite spot analysis showing multiple regression across different muscovite textures; d) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in muscovite. 4.2. Igarra schist belt 4.2.1. Lithologic and petrographic description of rock units The Igarra Schist Belt, like most other parts of the Nigerian basement, is predominantly made up of migmatite-gneiss with minor exposure of infolded supracrustal rocks, all of which are intruded by Pan-African intrusives. My field traverses within the Igarra Schist belt, cut across both the main supracrustal 39 exposures and various granitic intrusions (Fig. 4.7). The intrusive lithologies include biotite granite, leucogranite, and LCT pegmatite. Figure 4. 7. Geological map of the Igarra Schist Belt showing the sample locations (modified after Nweke et al. 2024) Samples Igarra L1A and L2 are biotite schists that outcrop along a village road off Ojirami-Oke Road, NE of Igarra, Edo state, Nigeria (Fig. 4.8 a–b). The outcrop dips at 88° southwest and strikes 15° northeast. The schist is fine-grained and shows a pervasive and uniform foliation that strikes 015 ° and dips 88 °. Garnet porphyroblast occurs in clusters that are heterogeneously distributed across the outcrop (Fig. 4.9 b). The thin section made from sample Igarra L1A constitutes plagioclase (~43%), biotite (~32%), quartz (~23%), garnet (~1%), and accessory zircon (1%). The thin section cut from sample Igarra L2 contains plagioclase (~56%), quartz (~25%), biotite (~18%), and garnet (~1%). In both sections, biotite defines the main foliation (Fig. 4.9–4.10). The fine-grained matrix quartz and plagioclase are polygonal 40 and show no grain shape elongation; however, microliths of quartz are elongated parallel with the foliation. Post-kinematic garnet porphyroblasts are distributed across both thin sections (4.9 a–d; 4.10 a–d). Figure 4. 8. (a–b) Road cut exposure of garnet-biotite Schist. 41 Figure 4. 9. a) Phase map of Igarra L1A b) photomicrograph taken under crossed-polars of biotite, quartz, and garnet porphyroblast; c) photomicrograph taken under crossed-polars showing polygonal quartz in a quartz-rich microlithon; d) photomicrograph taken under crossed-polars showing schistosity of mica in a microcrystalline quartz background. 42 Figure 4. 10. a) Phase map of specimen Igarra L2; b) photomicrograph taken under crossed-polars showing post-kinematic garnet porphyroblastic garnet; c) photomicrograph in plane-polarized light showing the microcrystalline texture and main foliation of the rock as defined by parallel biotite matrix; d) photomicrograph taken under crossed-polars showing quartz, plagioclase, and biotite matrix. Sample Igarra L1C is of a Lithium-cesium-tantalum (LCT) pegmatite unit that outcrops approximately 500 meters east of the Igarra-Ekpeshi Road in Ekpeshi, Edo state, Nigeria (Fig. 4.7). The thin section made from this sample contains lepidolite mica (~60%), quartz (~24%), plagioclase (~15%), and microlite (~1%) with accessory zircon and monazite (<0.5%). The thin section exhibits an igneous microstructure with patches of acicular feldspar as well as graphic intergrowths of quartz and feldspar (4.13c). Coarse-grained flakes of lepidolite exhibit the diagnostic low relief and second order interference colors as well as both euhedral and spongy microstructures (Fig. 4.11b–d). 43 Figure 4. 11. Phase map of specimen Igarra 1C; a) photomicrograph taken under crossed-polars showing lepidolite megacrysts, b) lepidolite surrounded by quartz and plagioclase, c) acicular plagioclase cluster; and d) spongy lepidolite microstructure. Samples Igarra L7, Igarra L8, and Igarra L9 are all granitic samples from the Igarra town at the center of the Igarra Schist Belt (Fig. 4.12). The granite bodies are well-exposed NE of Igarra and southeast of Semilika Hill around the Akuku and Ojirami area close to the Ojirami Dam. The rocks I sampled came from the foot of the 450-meter-high Semolika hill at the Akuku and Ojirami area. After a preliminary evaluation of their mineralogy, I chose to focus on sample Igarra L9 because it contains abundant titanite, which can be dated using the U-Pb system. Sample L9 is a coarse-grained biotite granite that, in outcrop, contains xenoliths of the neighboring schistose units (4.12 a). The unit is locally deformed, leading to a foliation that is defined by quartz ribbons and the weak alignment of biotite. The thin section made from sample L9 contains plagioclase (~34%), orthoclase (~26%), quartz (~29%), biotite (~4%), chlorite (~3%), hornblende (~3%) and accessory minerals allanite and titanite (~1%) (Fig. 13 a–d). Two microstructural populations of titanite are found throughout the sample. One population is made up of 44 idiomorphic (diamond-shaped) grains that are ~800–1600 μm in diameter (Fig. 4.14a–c). The second population of titanite is smaller, with an average diameter of 90–4500 μm and is distinctly non-idiomorphic (Fig. 4.14d–e). Chlorite locally replaces hornblende, and plagioclase is locally replaced by sericite (Fig. 4.13d). K-feldspar exhibits microcline twinning (Fig. 4.13d). Figure 4. 12. Granitic samples from the Akoko-Edo metamorphic complex. (a) biotite granite containing metapelites xenolith; (b) granodiorite; (c) weathered granite 45 Figure 4. 13. a) Phase map of Igarra L9; photomicrograph taken under crossed-polars showing b) photomicrograph taken under plane-polarized light showing titanite megacryst; c) allanite surrounded with quartz and chlorite; and d) microcline-twinned K-feldspar. 46 Figure 4. 14. Backscatter electron images showing two microstructural populations of titanite in sample Igarra L9. (a–c) represent the larger-sized idiomorphic population. (d–f) represent the non-idiomorphic and smaller-sized population. 4.2.2. In-situ mica 87Rb/87Sr geochronology All laser analytical spots that ablated on the glass slide and grain analysis that exploded were excluded while making the isochron plots, as they do not yield the results of valid/correct analysis. Samples Igarra L1A, L9, and L1C were all targeted for Rb-Sr isotopic age determination. The three rock samples were analyzed in two different runs. A total of eighty-two (82) spot analyses involving biotite and plagioclase were carried out on sample Igarra L1A. The analyses define an isochron date of 497 ± 4 Ma (S=0.51) (Fig. 4 .15a). The analysis yielded an 87Rb/87Sr ratio that extends to high amounts (10–4080) and an initial 87Sr/86Sri ratio of 0.7188 ± 0.0075. For sample Igarra L9, ninety-nine (99) spot analyses were targeted on biotite and plagioclase. The analysis returned an isochron date of 550 ± 5 Ma (S=0.44) and an initial 87Sr/86Sri ratio of 0.7108 ± 0.0014 47 (Fig. 4.15c). As with sample L1A, the analyses for L9 also extend to high amounts of 87Rb/87Sr ratios, 0.01–6370. For the sample Igarra L1C, ninety-one (91) spot analyses were targeted at lepidolite mica and plagioclase. In combination, all analyses yield an isochron date of 489 ± 6 Ma (S=0.6) (Fig. 4 .15e). The sample is characterized by very high amounts of 87Rb/87Sr ratios, between 0.0001–396984. During the Rb/Sr ratio processing, the uncertainties (i.e., rho) in both the x and y axes were not quantified. It was observed that there were very few counts of 86Sr, which affected the calculation of rho. Consequently, rho was not used to plot the final isochron as doing so would have led to an artificially high mean standard weighted deviation, MSWD. As a result, the data are plotted without any error correlation (Fig. 4.15e). 48 Figure 4. 15. Mica Rb/Sr isochron diagrams for samples from the Igarra Schist, Southwest Nigeria. a) biotite Rb/Sr result for biotite spot analysis on biotite schist (Igarra L1A); b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in biotite; c) Biotite Rb/Sr result for biotite spot analysis on biotite granite (Igarra L9); d) post-ablation photomicrograph taken under plane-polarized light showing ablation pits biotite e) Lepidolite Rb/Sr result for lepidolite spot analysis on LCT pegmatite (Igarra 1C); f) post-ablation photomicrograph taken under plane-polarized light showing ablation pits lepidolite. 49 4.2.3. In-situ garnet 176Lu/177Hf geochronology Sample Igarra L1A was also targeted for garnet Lu-Hf isotopic age determination. Sixty-nine (69) spot analyses across multiple garnet porphyroblasts (including core and rim domains) in Igarra 1A yield a single isochron date of 532 ± 62 Ma (MSWD = 1.4; Fig. 4.16). The garnet grains contained up to 430 ppm Lu and yielded 176Lu/177Hf ratios between 5–34. Figure 4. 16. a) Garnet Lu/Hf isochron diagrams for sample Igarra L1A; b) post-ablation photomicrograph taken under plane-polarized light showing ablation pits in garnet. 4.2.4 In-situ U-Pb geochronology All laser analytical spots that ablated on the glass slide and grain analysis that exploded were excluded while making the isochron plots, as they do not yield the results of valid/correct analysis. Monazite in Igarra L1A and titanite in Igarra L9 were targeted for U-Pb isotopic age determination. A total of sixty-three (63) spot analyses were carried out on several monazite grains in sample Igarra L1A. The analysis yielded a lower intercept date of 554 ± 2 Ma, MSWD = 9 (Fig. 4.17 a), and the regression is over-dispersed. To further observe how over-dispersed the data is, we plotted a U-Pb weighted mean, and it yielded a mean age of 558 ± 1, MSWD =10 (Fig. 4.17b). The monazite grains have U concentrations ranging between 1027–5369 ppm with a mean value of 3054 ppm, Pb concentration of 6–1626 ppm with a mean value of 686 ppm, Th concentration of 137–41193 ppm with a mean value of 17341 ppm, and U/Th ratio of 0.07–3.94 ppm with a mean value of 0.64 ppm. A total of seventy-one (71) spot analyses were targeted on multiple titanite grains in sample Igarra L9; seventy-one (71) spot analyses define multiple inseparable populations, yielding a U-Pb lower intercept date of 611 ± 2 Ma with an MSWD of 22 (Fig. 4.17c). The regression is over-dispersed; hence, we plotted 50 the U-Pb weighted mean to confirm whether there are multiple statistical populations. The U-Pb weighted mean yielded a mean date of 597 ± 2 Ma with an MSWD = 49, showing two clusters of titanite populations (Fig. 4.17d). If treated separately, the analysis on idiomorphic grains yields a U-Pb weighted mean date of 623 ± 2 Ma (Fig.4 .17e), whereas the analysis on non-idiomorphic grains yields a U-Pb lower intercept date of 563 ± 4 Ma, MSWD = 15 (Fig. 4 .17f). Uranium concentrations ranged between 10–257 ppm with a mean value of 112 ppm, Pb concentration of 2–31 ppm with a mean value of 14 ppm, Th concentration of 3–505 ppm with a mean value of 200 ppm, and U/Th ratio of 0.2–27 ppm with a mean value of 6 ppm. 51 Figure 4. 17. U/Pb Tera wasserburg diagrams for monazite and titanite samples from the Igarra Schist, Southwest, Nigeria. a) Tera Wasserburg diagram showing lower intercept date of 63 spot analysis on monazite in the biotite schist (Igarra L1A); b) U-Pb weighted mean date showing over-dispersed monazite regression; c) Tera Wasserburg diagram showing single dispersed lower intercept date on 71 spot analysis on titanite in the biotite granite (Igarra L9); d) U-Pb weighted mean date showing multiple inseparable titanite populations; e) U-Pb weighted mean date of 40 spot 52 analysis on idiomorphic titanite matrix; f) Tera Wasserburg diagram showing lower intercept date of 31 spot analysis on non-idiomorphic titanite matrix. 4.3. Ife-Ilesha schist belt 4.3.1. Lithologic and petrographic description of rock units The Ife-Ilesha Schist Belt, southwestern Nigeria, is unique amongst the Nigerian Schist Belts in that it was offset by a major strike-slip shear zone called the Ifewara shear zone (Fig. 4.18). The bedrock east of the shear zone is dominated by infolded supracrustal rocks, Pan-African intrusive, and granite gneisses, while the bedrock west of the shear zone consists of amphibolite, ultramafic rocks, infolded supracrustal rocks, and Pan-African intrusive. My field traverse across the Ife-Ilesha Schist Belt cuts across schist, amphibolite, and Pan African granite and pegmatite (Fig. 4.18) Figure 4. 18. Geological map of the Ife-Ilesha Schist Belt showing the sample locations (Modified after Ige et al. 1998). 53 Sample ND007A is a muscovite-biotite leucogranite that outcrops at the Itanrin quarry site about 500 m east of the Ile-Ife Road near Ife Town, Osun State, Nigeria. The quarry site exposes over 500 m of the leucogranite, which is intruded by a pegmatitic biotite granite, sample ND007B (Fig. 4.19). The leucogranite is medium grained with equant (i.e., platy) grains of muscovite and biotite giving the outcrop a vitreous luster. The thin section made from sample ND007A contains quartz (~33%), orthoclase (~30%), plagioclase (~24%), biotite (~4%), muscovite (~8%), and accessory apatite (<1%). The rock is undeformed and exhibits an igneous microstructure (Fig. 4.20). The pegmatitic biotite granite exhibits similar mineral modes yet with a coarser microstructure (Fig. 4.21). Figure 4. 19. Leucogranite intruded by pegmatite 54 Figure 4. 20. a) Phase map of specimen ND007A; a-d) photomicrographs collected under crossed-polars showing undeformed granitic microstructures. 55 Figure 4. 21. a) shows mineral phases in specimen ND007B; b) photomicrograph taken under crossed-polars showing igneous microstructures. Sample ND003 is a garnet-staurolite schist (Fig. 4.22). The schist outcrops to the east of the Ilesha-Akure expressway within the Osu area, Osun State, Nigeria. The rock is fine-grained, porphyroblastic, and has a consistent foliation striking 178 ° north–south with a dip between 44–54 ° E. The schist is locally intruded by foliation-aligned quartz veins (Fig. 4.22 a-f). The quartz veins are highly deformed and exhibit boudinage and ptygmatic structures (Fig. 4 .22a, b, and d). Two thin sections were cut from this unit (ND003-1 and ND003-2). The first thin section ND003-1 contains quartz (~30%), plagioclase (~28%), biotite (~17%), muscovite (~19%), garnet (~2%), staurolite (~2%), chlorite (1%) and accessory mineral appetite (~1%) (Fig. 4.23a). The foliation is defined by parallel to subparallel biotite and muscovite as well 56 as the alignment of flattened quartz and feldspar domains (Fig. 4.23a). Garnet forms post-kinematic porphyroblasts (up to 2 mm in diameter) throughout the section, whereas staurolite, which also overgrows the foliation, is only found along the margins of quartz veins. The second thin section ND003-2 contains biotite (~30%) quartz (~30%), garnet (~1%), muscovite (~17%), staurolite (~4%), plagioclase (~17%), and chlorite (~1%) (Fig. 4.24). It has the same mineral assemblage as section ND003-1, however, staurolite occurred mostly as a twinned poikiloblast (5 mm) with quartz inclusions (Fig.4.24 a–b). Figure 4. 22. Garnet quartzo-feldspathic schist. (a) shows crenulation cleavages and garnet porphyroblast on the schistose unit; (b) boudins formed by quartzo-feldspathic veins; (c-d) shows multiple quartzo-feldspathic lineations in the outcrop; (e) multiple staurolite porphyroblast; (f) multiple micro isoclinal folds The results of the microprobe analysis on ND003-1 are presented in Table. 4.1. The garnet porphyroblasts exhibit a bell-shaped manganese zonation from rim to rim (Fig. 4.25 a-b). The garnet cores have end-member molar fraction compositions of ~0.65 almandine (molar Fe / Fe + Mg + Ca + Mn), ~0.085 57 pyrope (molar Mg / Mg + Fe + Ca + Mn), ~0.07 grossular (molar Ca / Ca + Mg + Fe + Mn) and ~0.19 spessartine (molar Mn / Mn + Mg + Ca + Fe). Within the garnet rims there is an increase in almandine content to ~0.74 and pyrope to ~0.10 with a corresponding decrease in grossular to ~0.05 and spessartine to ~0.10. Biotite within the matrix have XMg = molar Mg / (Mg + Fe) = 0.46–0.47, Ti = 0.09–0.10 cation per formula unit (cpfu), and Aliv = 5.27–5.30 whereas biotite adjacent to garnet grains have XMg = 0.45–0.48, Ti = 0.09–0.09 cpfu and Aliv = 5.31–5.39. Biotite that are adjacent to staurolite exhibit slightly higher XMg values of 0.49–0.50, with Ti = 0.08–0.09 cpfu and Aliv = 5.31–5.33. Muscovite in the matrix have XMg = 0.30–0.31, Aliv = 4.88–4.95 and XNa = molar Na/(Na + K) = 0.11–0.18, wheras muscovite adjacent to garnet exhibit greater variation in XMg = 0.23–0.31, with Aliv = 4.94–4.95 and XNa = 0.11–0.13. Throughout the section chlorite have XMg values of 0.50–0.54, Fe = 2.2–2.3, Mn = 0.011–0.015, Ti = 0.006–0.01, and Mg = 2.3–2.4 (cpfu) Staurolite XMg range from 0.15–0.18, Fe = 1.66–1.8, Mn = 0.026–0.037, Ti = 0.050–0.059, and Mg = 0.29–0.39 (cpfu) The matrix plagioclase have XAb = values of 0.66–0.69 with Xorth values of 0.003–0.006. 58 Figure 4. 23. a) Phase map of specimen ND003-1; b) Photomicrographs collected under plane-polars showing staurolite and garnet association; c) Shows metapelite and quartzo-feldspathic domains with garnet, staurolite, and biotite association; d) Photomicrograph taken under plane-polarized light expressing the main foliation of the rock and shows garnet – biotite – chlorite- association. Table 4. 1. Representative mineral compositions (wt.% oxides and cations per formula unit) for the single sample used for this study. Sample ND003 Mineral Grt Grt Bt Ms Chl St Pl Location Core Rim Matrix Matrix Matrix Matrix Matrix Analysis 1 1 25 9 35 24 26 SiO2 36.53 37.38 35.13 45.54 24.14 26.87 60.66 TiO2 0.04 0.02 1.56 0.44 0.09 0.51 0.00 Al2O3 21.17 21.28 18.82 35.08 23.10 53.69 25.15 FeO 29.03 33.23 19.92 2.32 25.12 14.35 0.02 MnO 8.35 4.65 0.08 0.01 0.15 0.26 0.01 CaO 2.40 1.75 -0.02 -0.01 0.01 0.00 6.68 Na2O 0.05 0.03 0.30 0.93 0.01 0.02 8.04 MgO 2.11 2.61 10.02 0.55 15.00 1.63 0.00 K2O -0.01 0.01 8.85 8.15 0.05 0.01 0.08 P2O5 -0.02 0.00 0.00 0.01 0.00 0.00 0.01 59 Mineral Grt Grt Bt Ms Chl St Pl Location Core Rim Matrix Matrix Matrix Matrix Matrix F 0.00 0.00 0.39 0.24 0.27 0.44 0.00 Cl 0.00 0.00 0.00 0.01 0.01 0.00 0.00 O=F, CL 0.00 0.00 -0.16 -0.10 -0.11 -0.19 0.00 Total 99.66 100.96 95.08 93.2775 87.99 97.80 100.66 Cation Proportion Si 0.61 0.62 0.58 0.76 0.40 0.45 1.01 Ti 0.00 0.00 0.02 0.01 0.00 0.01 0.00 Al 0.42 0.42 0.37 0.69 0.45 1.05 0.49 Fe 0.40 0.46 0.28 0.03 0.35 0.20 0.00 Mn 0.12 0.07 0.00 0.00 0.00 0.00 0.00 Ca 0.04 0.03 0.00 0.00 0.00 0.00 0.12 Na 0.00 0.00 0.01 0.03 0.00 0.00 0.26 Mg 0.05 0.06 0.25 0.01 0.37 0.04 0.00 K 0.00 0.00 0.19 0.17 0.00 0.00 0.00 P 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Total 1.64 1.66 1.70 1.70 1.58 1.75 1.88 Oxygen 12 12 11 11 14 24 8 Proportion SiO2 1.22 1.24 1.17 1.52 0.80 0.89 2.02 TiO2 0.00 0.00 0.04 0.01 0.00 0.01 0.00 Al2O3 0.62 0.63 0.55 1.03 0.68 1.58 0.74 FeO 0.40 0.46 0.28 0.03 0.35 0.20 0.00 MnO 0.12 0.07 0 0.00 0.00 0.00 0.00 CaO 0.04 0.03 0 0.00 0.00 0.00 0.12 Na2O 0.00 0.00 0 0.01 0.00 0.00 0.13 MgO 0.05 0.06 0.25 0.01 0.37 0.04 0.00 K2O 0.00 0.00 0.09 0.09 0.00 0.00 0.00 P2O5 0.00 0.00 0 0.00 0.00 0.00 0.00 Total 2.46 2.50 2.39 2.71 2.21 2.73 3.01 C.P.F.U Si 2.97 2.99 2.69 3.08 2.54 3.93 2.68 Ti 0.00 0.00 0.09 0.02 0.01 0.06 0.00 Al 2.03 2.01 1.7 2.80 2.87 9.26 1.31 Fe 1.97 2.22 1.28 0.13 2.21 1.76 0.00 Mn 0.58 0.32 0.01 0.00 0.01 0.03 0.00 Ca 0.21 0.15 0 0.00 0.00 0.00 0.32 60 Mineral Grt Grt Bt Ms Chl St Pl Location Core Rim Matrix Matrix Matrix Matrix Matrix Na 0.01 0.00 0.04 0.12 0.00 0.01 0.69 Mg 0.26 0.31 1.14 0.05 2.36 0.36 0.00 K 0.00 0.00 0.87 0.70 0.01 0.00 0.00 P 0.00 0.00 0 0.00 0.00 0.00 0.00 Total 8.02 8.01 7.82 6.91 10.02 15.39 5.01 XMg 0.11 0.12 0.47 0.30 0.52 0.17 Alm. (Fe) 0.65 0.74 Py. (Mg) 0.08 0.10 Grs. (Ca) 0.07 0.05 Spe. (Mn) 0.19 0.10 An 0.31 Alb 0.68 Orth 0.00 Plagioclase XCal 0.31 PlAl/Si 0.31 61 Figure 4. 24. a) Phase map of specimen ND003-2; b) Cross-polar photomicrograph showing twin staurolite porphyroblast; c) Cross-polar photomicrograph of garnet, biotite, muscovite, and quartz association; d) Plane-polar image showing garnet overgrowing fabric on mica-rich domains in specimen ND003-2. Figure 4. 25. Garnet compositional line profiles. a) Rim-to-rim profiles of garnet end-member mole fractions for ND003. b) Plane polar micrograph of a garnet porphyroblast surrounded by phyllosilicates and staurolite matrix. Location of probe points marked by red circles. 62 4.3.2. In-situ mica 87Rb/87Sr geochronology All laser analytical spots that ablated on the glass slide and grain analysis that exploded were excluded while making the isochron plots, as they do not yield the results of valid/correct analysis. Samples ND003-1 and ND007A were targeted for Rb-Sr isotopic age determination. Eighty (80) spot analyses on biotite and plagioclase grains in ND003-1 yield an isochron date of 496 ± 3 Ma (S=0.99) (Fig. 4.26a), whereas ninety-four (94) spot analyses on muscovite and plagioclase yielded an isochron date of 521 ± 17 Ma (S=0.7) (Fig. 4.26b). The mica analyzed has a low Rb and Sr between 0.2–254 ppm and 0.4–110 ppm, respectively. For sample ND007A, ninety-five (95) muscovite and plagioclase spot analyses yielded an isochron date of 583 ± 3 Ma (S=0.73) (Fig. 4.27 a), while eighty-one (81) biotite and plagioclase spot analyses yielded 557 ± 6 Ma (S=0.3). The sample contains high Rb 8150 ppm, which resulted in high Rb 87Rb/87Sr ratios of up to 15000 (Fig. 4.27 b). 63 Figure 4. 26. Rb/Sr isochron diagrams for sample ND003-1 from the garnet-staurolite schist from Ife-Ilesha Schist Belt, Southwest, Nigeria. a) Rb/Sr isochron result for biotite spot analysis; b) post-ablation photomicrograph taken under plane-polarized light showing biotite analytical spots; c) Rb/Sr isochron result for muscovite spot analysis; d) post-ablation photomicrograph taken under plane-polarized light showing muscovite analytical spots. 64 Figure 4. 27. Mica Rb/Sr isochron diagrams for sample ND007A from the Ife-Ilesha Schist, Southwest, Nigeria. a) Muscovite Rb/Sr isochron result for muscovite spot analysis on Pan-African granite; b) post-ablation photomicrograph taken under plane-polarized light showing muscovite analytical spots; c) Biotite Rb/Sr isochron result for biotite spot analysis on Pan-African granite; b) post-ablation photomicrograph taken under plane-polarized light showing biotite analytical spots; 4.3.3. In-situ garnet 176Lu/177Hf geochronology Sample ND003-1 was also targeted for Lu-Hf isotopic age determination. Eighty-one (81) spot analyses across multiple garnet porphyroblasts in ND003-1 yield an isochron date of 545 ± 8 Ma (MSWD = 1.9; Fig. 4.28). The garnet data presented includes analyses across the rim and core domains of garnet grains. High Lu concentration was observed in the garnets of up to 2200 ppm, which produced high 176Lu/177Hf ratios of up to 5–920. 65 Figure 4. 28. a) Garnet Lu/Hf isochron plot for sample ND003-1 from the Ife-Ilesha Schist, Southwest, Nigeria; b) post-ablation photomicrograph taken under plane-polarized light showing garnet analytical spots. 4.3.4 Petrological modelling ND003 – Lower amphibolite facies schist Sample ND003 was selected for pressure (P) and temperature (T) determination using isochemical phase equilibria. The reasons for selecting this single sample for P–T analyses are that 1) it was the only meta-aluminous supracrustal sample found in all three study regions, and 2) the post-kinematic garnet and staurolite porphyroblasts offered an opportunity to determine the crustal depth and thermal gradients at the time of static garnet growth. Two phase diagrams were calculated with the same constructed bulk rock composition using two thermodynamic datasets (ds_55 and ds_62), each with different activity-composition models for solution phases. This was done to be able to assess how sensitive the low-temperature stability of garnet is to the choice of dataset. Both of the phase diagrams calculated for ND003 are presented in (Fig. 4.29 & 4.30). Both phase diagrams are contoured for garnet end-member isopleths (pyrope and grossular). Using dataset ds_55 (Powell and Holland 1998), garnet first appears at ~3 kbar and 525°C, while with ds_62 (White et al 2014), the garnet-in isomode is below 3 kbar and 400°C. Comparing the measured garnet chemistry to the calculated isopleths of pyrope and grossular, the garnet core equilibrated at 5.3 kbar and 560°C when using ds_55, while when using ds_62, it equilibrated at 5.5 kbar and 560°C. The final equilibration of garnet occurred at 5.5 kbar, 590°C, and 5.2 kbar, 610°C for the ds_55 and ds_62 datasets, 66 respectively. Phyllosilicate minerals remained stable throughout the metamorphic growth of the rock with the formation of staurolite between 525–550°C. Figure 4. 29. Calculated P–T pseudo section for sample ND003 using MAGEMin (Riel et al. 2022) using ds_62 (White et al 2014). Relevant fields are contoured for pyrope and grossular isopleths mode (legend, bottom right). The green polygon represents the garnet core, while the red polygon represents the rim composition. 67 Figure 4. 30. Calculated P–T pseudo-section for sample ND003 using ds_55 (Powell and Holland 1988). Relevant fields are contoured for pyrope and grossular isopleths mode (legend, bottom right). The purple polygon represents the garnet core, while the orange polygon represents the rim composition. 68 Chapter Five Interpretation and Discussion 5.1 Interpretation of in-situ geochronology In this section, the geochronological data obtained from in-situ analyses of mica (Rb–Sr), garnet (Lu–Hf), monazite, and titanite (U–Pb) across lithologic units of the southern Benin-Nigerian Shield are interpreted spatially from east to west. All dates are individually attributed as representing either magmatic crystallization, peak metamorphism, or the closure of the isotopic system during cooling. The geochronological significance of these results is discussed in the subsequent sections. 5.1.1 Oban Massif Previous studies on the Oban Massif (e.g., Ekwueme 1989; Ekwueme et al. 1990; Ekwueme and Matheis 1995) have dated samples from the region using whole-rock Rb–Sr systematics and returned dates of 676 ± 20 Ma for kyanite schist and 572 ± 16 Ma for a retrogressed kyanite schist containing chlorite. The in-situ Rb-Sr ages presented here provide additional temporal resolution to the metamorphic and tectonic evolution of the Oban Massif. In sample ND021A from the Oban Massif, in-situ Rb–Sr isochron dates of 496 ± 5 Ma and 465 ± 20 Ma were obtained for biotite and muscovite analyses, respectively. The dates obtained in this study postdate earlier metamorphic ages from the same region (676 ± 20 Ma and 572 ± 16 Ma) and are therefore interpreted to record the final cooling phase of the Oban Massif. The observed discrepancy between the biotite and muscovite dates is counter intuitive. Given the larger size of the muscovite (600–1800 μm versus 200–600 μm), and its expected higher closure temperature (~500–600 °C versus ~350–450 °C) compared to biotite (Verschure et al., 1980; Del Moro et al., 1982; von Blanckenburg et al., 1989; Freeman et al., 1997; Jenkin et al., 2001), it would be expected to yield an older date than biotite when recording a simple cooling history. It follows, then, that the muscovite does not record a single cooling event. The complexity in the recorded muscovite history is reflected in the spread in the Rb-Sr data, which fall between two isochrons separated by 100 Myr (430 and 530 Ma). It is possible that the spread in data reflects partial resetting/equilibration of Rb-Sr systematics during a late tectonothermal event (ca. 430 Ma). The presence of hydrothermal alteration features in surrounding intrusions, including pervasive sericitization of plagioclase and chloritization of hornblende, is consistent with possible fluid-assisted resetting of the Rb–Sr system. 69 5.1.2 Igarra schist belt The in-situ U–Pb analysis of titanite from sample Igarra L9, collected from the Igarra Schist Belt, yielded multiple overlapping age populations within a single dataset (611 ± 2 Ma; MSWD = 22; Fig. 4.17c). The high mean square weighted deviation (MSWD) indicates significant dispersion, rendering the bulk data geologically meaningless if interpreted as a single event. However, a U-Pb weighted mean plot of the analysis reveals two distinct age populations: one grouping above ca. 600 Ma and another below ca. 550 Ma (Fig. 4.17d). These two age clusters correspond to contrasting microstructural titanite populations in the sample, idiomorphic porphyroblasts (~800–1600 μm) and smaller, non-idiomorphic grains (90–450 μm) (Fig. 4.14). When assessed separately, the idiomorphic titanite grains yield a U-Pb weighted mean date of 623 ± 2 Ma (MSWD = 2.6) (Fig. 4.17e). We interpret this date as the timing of magmatic titanite crystallization and the emplacement age of the granite. In contrast, the non-idiomorphic titanite grains provide a younger date of 563 ± 4 Ma, MSWD = 15 (Fig. 4.17f), which likely represents a later hydrothermal or metamorphic (re)crystallization event. Links to hydrothermal activity are consistent with extensive sericitization observed in plagioclases across multiple samples from the study area, including Igarra L9, L8, and L7. Petrographic observations confirm that sericitization is widespread in granitoids from this region. Such alteration is typically attributed to low-grade metamorphic processes, likely within the zeolite to greenschist facies, driven by fluid interaction (Verati and Jordan, 2014). Additionally, sample Igarra L9 is notably deformed and contains pelitic xenoliths enclosed within the granite. The presence of these xenoliths indicates assimilation of partially melted metapelitic host rocks during granite emplacement, providing further evidence for crustal interaction during pluton evolution (Fig. 4.12a). We interpret the in-situ U–Pb date of 554 ± 2 Ma (MSWD = 9) obtained from monazite in sample Igarra L1A as representing the timing of metamorphic (re)crystallization (Fig. 4.17a). The U-Pb weighted mean date shows over-dispersed regression, suggesting a protracted metamorphic recrystallization (Fig. 4. 17b). However, in the absence of additional data, such as compositional zoning or detailed chemical analysis of the monazite, it remains uncertain whether this age corresponds to a prograde, peak, or retrograde metamorphism. We interpret the in-situ garnet Lu–Hf date of 532 ± 62 Ma (MSWD = 1.4) obtained from sample Igarra L1A as reflecting the timing of peak metamorphism (Fig. 4.16a). Microstructural observations indicate that the garnets are post-kinematic, having grown over the foliation planes of biotite. As such, this date post-dates the main deformation event for the region. We interpret the in-situ mica Rb–Sr date of 550 ± 6 Ma (S = 0.44), obtained from biotite in sample Igarra L9, as recording the timing of metamorphic (re)crystallization associated with hydrothermal fluids 70 (Fig. 4.15a). Some of the biotite matrix occurs perpendicular to the planes of chlorite, while the majority occurs as clusters around the non-idiomorphic titanite grains. In support of this interpretation, the mica Rb-Sr age overlaps with the garnet date (532 ± 62), monazite date (554 ± 2 Ma), and the date of non-idiomorphic titanite grains (563 ± 4 Ma). This suggested a protracted metamorphic (re)crystallization within the Igarra Schist belt. The Igarra Schist Belt hosts several economic mineral deposits, and these events may be linked to the genesis of such mineralization. We interpret the in-situ mica Rb–Sr date of 497 ± 4 Ma (S = 0.51), obtained from biotite in sample Igarra L1A, to record the timing of cooling in the host metasedimentary rocks (Fig. 4.15a). This cooling phase postdates the metamorphic recrystallization events recorded by the biotite mica Rb–Sr, titanite U-Pb (non-idiomorphic titanite), monazite U–Pb, and garnet Lu–Hf systems. Following that interpretation, the data indicate that, following peak metamorphism, the Igarra Schist Belt entered a period of thermal and tectonic quiescence between ~550 and 497 Ma, with no evidence of subsequent magmatic or metamorphic activity nor fabric-forming deformation. We interpret the in-situ mica Rb–Sr date of 489 ± 6 Ma (S = 0.6), obtained from lepidolite in sample Igarra L1C, to represent the timing of LCT pegmatite crystallization (Fig. 4.15e). Although this date overlaps with the cooling of the adjacent metapelites, which occurred between ca.550 and ca. 497 Ma. As such, by the time the pegmatite was emplaced, the surrounding crustal levels had already cooled below the closure temperature for Rb–Sr in mica (Igarra L1A). Although the lepidolite Rb–Sr age could potentially reflect a cooling event, it might have also been intruded at a high level into the cold crust, leading to rapid cooling. However, the relatively higher closure temperature of strontium in white mica as opposed to biotite supports the interpretation that the lepidolite date reflects a crystallization age rather than a post-crystallization cooling age. 5.1.3. Ife-Ilesha schist belt We interpret the in-situ mica Rb–Sr dates of 583 ± 3 Ma (S = 0.73) and 557 ± 6 Ma (MSWD = 0.3), obtained from muscovite and biotite in sample ND007A from the Ife-Ilesha Schist Belt, as recording two distinct thermal events. The muscovite dates are interpreted to represent magmatic crystallization associated with the emplacement of the leucogranite (Fig. 4.27a), whereas the biotite date represents subsequent thermal cooling (Fig. 4.27c). This elucidates the retentivity between the two micas (Larson et al, 2023). The ca. 26 Myr gap between the muscovite and biotite ages outlines a protracted cooling history, with the granitic body gradually cooling after its emplacement. We interpret the in-situ garnet Lu–Hf age of 545 ± 8 Ma (MSWD = 1.9), obtained from sample Igarra ND003-2, to represent the timing of peak metamorphism within the Ife-Ilesha Schist Belt (Fig. 71 4.28a). Microstructural analysis indicates that both garnet and staurolite are post-kinematic, indicating peak metamorphism postdates the development of the main tectonic fabric in this region. Garnet crystals are observed to have nucleated along the more aluminous foliation planes of biotite and muscovite, while some garnet and staurolite grains are partially wrapped by these micas, further supporting their syn to post-tectonic growth. The observed mineral assemblage, Grt + St + Ms + Bt + Chl + Ilm + Pl + Qtz, reflects metamorphic conditions typical of the garnet-staurolite zone, and lower amphibolite facies grade metamorphism. We interpret the in-situ mica Rb–Sr dates of 496 ± 3 Ma (S = 0.99) and 521 ± 17 Ma (S = 0.7), obtained from biotite and muscovite in sample ND003-1 from the Ife-Ilesha Schist Belt, to represent the cooling history of the host metasedimentary rocks. These dates postdate the garnet Lu–Hf metamorphic age. The data are interpreted to show that following peak metamorphism, the Ife-Ilesha Schist Belt underwent a phase of tectonic quiescence between approximately ca. 540 and 490 Ma, with no evidence of further magmatic or metamorphic activity. The entire region is interpreted to have cooled below the closure temperature of Rb-Sr in mica by ca. 490 Ma. The crustal evolution of the southern Benin-Nigerian Shield is interpreted as a sequence of temporally and spatially constrained tectonothermal events (Fig. 5.1). Magmatic crystallization occurred between ca. 625 and 580 Ma within the Igarra and Ife-Ilesha Schist Belts, respectively. The earliest evidence of post-magmatic cooling is recorded in the Ife-Ilesha Belt and is interpreted to reflect the thermal relaxation of Pan-African intrusive bodies. Contemporaneously, the Igarra Belt underwent regional metamorphism, as indicated by U–Pb ages from non-idiomorphic titanite and monazite in metapelitic rocks, as well as Rb–Sr ages from biotite in syn- to post-tectonic granitoids. Peak metamorphic conditions are recorded by garnet in the metapelites of both belts. This was followed by a thermally quiescent period characterized by the absence of significant magmatic or metamorphic activity, during which the crust cooled progressively. This phase culminated in the emplacement of Li–Cs–Ta (LCT) pegmatites, marking the final stages of crustal reworking. 72 Figure 5. 1. Summary of crustal events in the southern Benin-Nigerian Shield 5.2 Pressure–temperature (P-T) evolution The pressure–temperature model interpreted here is specifically for the westernmost study site (Ife-Ilesha Schist belt). No pressure–temperature studies were carried out in the other two study sites (i.e., Oban Massif and Igarra Schist belt) due to the absence of suitable mineral phases in our rock samples from these regions. We interpret the pressure–temperature results presented in Figures 4.29–4.30 for sample ND003-1 to represent a single prograde metamorphic growth. The microprobe transects conducted on garnet porphyroblasts from the Ife-Ilesha Schist belt recorded similar chemical zonation from rim to rim (Fig. 4.25). The garnet line profile (Fig. 4.25) shows a bell-shaped profile in the manganese end member, which is consistent with a single episode of prograde metamorphic growth (Kohn and Spear 2000; Dyck et al, 2021). The pressure–temperature (P–T) evolution of the Ife-Ilesha Schist Belt records a static isobaric thermal overprint at approximately ~5.0 ± 0.5 kbar, with temperatures increasing from ~520 ± 50 °C to ~580 ± 50 °C. This evolution also reflects prograde garnet growth and aligns with previous findings for peak metamorphic conditions in the region (Zeh and Holness, 2003). The isobaric overprint precedes the stabilization of both garnet and staurolite and is interpreted as a post-deformational thermal event, occurring after the development of the regional foliation and following the emplacement of late Pan-African granitic bodies. 73 In keeping with numerous recent studies (Bowie et al, 2025; McKenzie et al, 2024) that compare the output of ds_55 and ds_62, we find that ds_55 better reproduces the negative Clausius–Clapeyron slope of garnet at temperatures below ~500 °C, within the chlorite-bearing field. As such, we favor the results generated with ds_55. However, both model results are consistent with our interpretation of static isobaric prograde garnet growth. This thermal overprint corresponds to a modest increase in the regional geothermal gradient, from approximately 1040 °C/GPa to 1160 °C/GPa, indicative of enhanced crustal heating in the absence of significant burial or exhumation. Such a P–T path is consistent with static thermal re-equilibration of the crust and may be attributed to lithospheric mantle delamination, increased radiogenic heat production or direct heating of the lower crust by upwelling asthenosphere. Overall, the P–T data document a post-tectonic thermal pulse, likely driven by residual heat from Pan-African magmatism or localized heat advection, rather than continued tectonic thickening or regional metamorphic reworking. 5.3 Thermotectonic history of the southern Benin-Nigeria shield The Benin-Nigeria Shield exhibits a polychronous tectonothermal history, having been subjected to multiple phases of deformation associated with at least four distinct orogenic events (McCurry, 1976; Turner, 1983; Fitches et al., 1985). The cumulative effects of these tectonic episodes were ultimately overprinted by the Pan-African orogeny, which left the most pervasive thermal and structural imprint across the region. Magmatic activity within the shield is largely attributed to this late Neoproterozoic event, during which widespread syn- and post-orogenic granitoid emplacement occurred (Van Breemen et al., 1977; Tubosun et al., 1984; Egbuniwe et al., 1985; Ferré et al., 1998; Goodenough et al., 2014; Adetunji et al., 2016, 2018; Bute et al., 2020, 2022). While the northern segments of the shield (north-central, northeast, and northwest) have been relatively well studied, the southern portion remains comparatively under-investigated, particularly concerning its magmatic, metamorphic, and cooling history (e.g., Tubosun et al., 1984; Rahaman et al., 1991; Ekwueme et al., 1990; Adetunji et al., 2011). This study contributes new geochronological and petrological insights into the thermo-tectonic evolution of the southern Benin-Nigeria Shield, based on in-situ geochronology using Rb–Sr (biotite, muscovite, lepidolite), Lu–Hf (garnet), and U–Pb (monazite and titanite) systems, alongside pressure–temperature (P–T) modeling of metapelitic assemblages. The compiled data reveal a tripartite evolution of the crust in this region: Stage I (ca. 623–600 Ma). This phase is characterized by voluminous granite magmatism. The crystallization ages of the granites dated in this thesis align with existing regional data and suggest syn-74 tectonic emplacement of granites, consistent with the foliated nature of sample Igarra L9 (e.g., Tubosun et al., 1984; Ferre et al., 2002; Adetunji et al., 2018). The timing also corresponds with ca. 620 Ma ultrahigh-pressure coesite-bearing assemblages reported along the Pan-African suture zone (Jahn et al., 2001), consistent with subduction-related magmatism and a regional-scale crustal reworking event during the late Neoproterozoic. Stage II (ca. 550–530 Ma). This interval marks peak metamorphic conditions, as recorded by in-situ Lu–Hf dating of garnet and U–Pb dating of titanite and monazite in granite and schist, respectively. Garnet and staurolite display post-kinematic textures, having overgrown the foliation planes of biotite and muscovite. This metamorphic pulse postdates the magmatic crystallization by approximately 70–90 million years and follows the closure of the Rb–Sr system in metapelites (ca. 521–497 Ma). Thermodynamic modeling indicates isobaric garnet growth, suggesting localized crustal heating without evidence of significant exhumation. Such heating may reflect lithospheric delamination and asthenospheric upwelling beneath the thinned lithosphere. Stage III (530–480 Ma). This stage corresponds to regional cooling and the emplacement of lithium–cesium–tantalum (LCT) pegmatites. The Rb–Sr cooling ages obtained from mica across the study transect suggest spatial variations in the cooling history. In the eastern Oban Massif, biotite and muscovite record cooling ages of 496 ± 5 Ma and 459 ± 18 Ma, respectively. The central Igarra Schist Belt records a biotite cooling age of 497 ± 4 Ma, while the westernmost Ife-Ilesha Schist Belt yields biotite and muscovite cooling ages of 496 ± 3 Ma and 521 ± 17 Ma, respectively. This pattern indicates progressively later cooling from east to west, implying deeper exhumation along the eastern margin of the shield. Collectively, these data provide the first comprehensive geochronological and metamorphic constraints for the southern Benin-Nigeria Shield. The integration of field relationships, in-situ geochronology, and P–T modeling highlights a dynamic crustal evolution influenced by syn-tectonic magmatism, post-tectonic thermal overprints, and regional-scale crustal cooling. These findings significantly improve our understanding of the Pan-African tectonothermal regime in West Africa and underscore the need for continued investigation in the underexplored southern segment of the shield 5.4 Model for the formation of late Pan-African LCT pegmatites. Two principal types of rare-metal mineralization associated with tin (Sn), niobium–tantalum (Nb–Ta), lithium (Li), beryllium (Be), and zinc (Zn) have been identified within the Benin-Nigeria Shield (Kinnaird, 1984). These mineralization types differ significantly in terms of age, tectonic setting, mineralogy, and geochemistry. They are hosted in two major lithological suites: Proterozoic pegmatites and Jurassic-aged granites, commonly referred to as the “Younger Granites” or ring complexes (Kinnaird, 1984; 75 Matheis & Emeforieta, 1990). While the Proterozoic pegmatites are an integral part of the Benin-Nigeria Shield, the Younger Granites comprising alkaline anorogenic suites emplaced during the Jurassic are considered distinct, having been intruded along hotspot-related lineaments of the Trans-Saharan mobile belt, from the Aïr region in Niger to the Afu complex in Nigeria (Wright, 1985). Kinnaird (1984) further classified the Pan-African-related Paleozoic pegmatites into two major groups: barren and mineralized types. These pegmatites occur predominantly as dykes and sheets intruding metasedimentary rocks (e.g., schists), amphibolites, paragneisses, migmatites, and granitic terrains across the shield (Kinnaird, 1984; Adetunji & Ocan, 2010). Pegmatite emplacement postdates the regional syn to post-tectonic granites (emplaced between ca. 640–580 Ma) and occurred between ca. 560–450 Ma (Ferré et al., 2002; Goodenough et al., 2014; Bute et al., 2020; Cao et al., 2024). Early classifications based on mineral assemblages include the microcline–quartz, microcline–quartz–mica, and quartz–mica pegmatites (Jacobson & Webb, 1947; Kinnaird, 1984; Adetunji & Ocan, 2010). A dominant northeast–southwest trend of pegmatite mineralization has been documented, extending from Ife–Ilesha through Jos. This trend is interpreted to reflect structural control by Pan-African dextral shear zones within the Benin-Nigeria Shield (Kinnaird, 1984; Woakes et al., 1987; Adetunji & Ocan, 2010; Goodenough et al., 2014; Cao et al., 2024). Importantly, the mineralization grade appears to increase from the southern to the north-central portion of the shield (Wright, 1970; Adetunji & Ocan, 2010). However, mineralization is not restricted to this structural trend, as pegmatites have also been reported in other localities such as the Oban Massif and the Obudu Plateau in southeastern Nigeria. These off-trend occurrences suggest the presence of multiple pegmatite districts within the shield, potentially derived from different rare-metal-enriched sources, as reflected by variations in emplacement age (560–450 Ma). A recent study, Cao et al (2024) reported U-Pb ages ranging from 570–548 Ma for lithium-bearing pegmatites in the northcentral segment of the Benin-Nigerian shield. These dates coincide with the emplacement age of some granites in the area (Bute et al. 2022), and they postdate the date reported by this study (ca. 489 Ma), suggesting that a different source could have influenced their formation. Economically significant pegmatites are, however, typically associated with metasedimentary rocks, particularly within the schist belts aligned along the Pan-African north–south structural framework (Kinnaird, 1984). This study documents, for the first time, an LCT (lithium–cesium–tantalum) pegmatite within the established structural corridor that is not dominated by the typical microcline–quartz–mica assemblage. Instead, the pegmatite from the Igarra Schist Belt is primarily composed of plagioclase feldspar, quartz, and lepidolite mica, suggesting crystallization within the wall or intermediate zones of a zoned pegmatite system (Cameron et al., 1949; London, 2014; Simmons et al., 2016). The pegmatite intrudes metasedimentary rocks and yields an in-situ lepidolite Rb–Sr isochron crystallization age of 489 ± 6 Ma. 76 This age falls within the recognized post-orogenic pegmatite window across the Benin-Nigeria Shield (560–450 Ma). The significant temporal gap (>100 Ma) between the emplacement of syn to post-tectonic granitoids (ca. 640–580 Ma) and LCT pegmatites (ca. 489 Ma) challenges earlier interpretations that both were derived from a single late Pan-African melt system. Rather, the age disparity implies that these systems are genetically unrelated. Recent work by Cao et al. (2024) further refines this temporal framework, distinguishing between three types of mineralized pegmatites in the north-central basement: cassiterite-rich, beryllium-rich, and lithium-rich pegmatites. These pegmatites crystallized between ca. 570–550 Ma, within the post-kinematic time window, further highlighting the diversity and temporal complexity of pegmatite magmatism across the Benin-Nigeria Shield. Rare-metal pegmatite mineralization in this study area is predominantly of the lithium–cesium–tantalum (LCT) type, and is enriched in tin (Sn), niobium–tantalum (Nb–Ta), lithium (Li), beryllium (Be), and zinc (Zn), among other elements (Olade, 1980; Akoh et al., 2015; Kolawole et al., 2021). Trace element geochemistry of previous studies (Vincent et al., 2023; Cao et al., 2024) indicates that Sn, Nb, Ta, Zr, and Hf are of magmatic origin. Hafnium isotope data from cassiterite and zircon in Nb–Ta- and Be-rich pegmatites (Cao et al., 2024) display consistent negative εHf(t) values ranging from −19.8 to −11.7, with corresponding two-stage depleted mantle model age (T_DM2) between 2.7 and 2.2 Ga for Nb–Ta pegmatites. Be-rich pegmatites record εHf(t) values between −20.9 and −12.8 and T_DM2 ages between 2.3 and 2.8 Ga. In contrast, Li-rich pegmatites show slightly less negative εHf(t) values (−16.6 to −8.3) with T_DM2 ages between 2.5 and 2.0 Ga. These Hf isotope compositions indicate derivation from ancient crustal reservoirs that have undergone extensive reworking and partial melting, likely enriched by mantle input. Such signatures are consistent with Pan-African granitic systems throughout the Benin-Nigeria Shield (Ukaegbu & Ekwueme, 2006; Adetunji & Ocan, 2010; Bute et al., 2020, 2022). Additionally, Bute et al. (2020, 2022) have proposed lithospheric delamination at ca. 580 Ma as a significant geodynamic process driving widespread magmatism within the shield. The results of this study provide compelling support for the lithospheric delamination model. Garnet Lu–Hf geochronology combined with pressure–temperature (P–T) modeling of metapelitic rocks from the Ife–Ilesha Schist Belt indicates peak metamorphic conditions at ca. 540 Ma. This metamorphism was accompanied by a significant increase in geothermal gradient, from approximately 1040 °C/GPa to 1160 °C/GPa. Such thermal conditions are interpreted to reflect heating induced by lithospheric delamination or slab breakoff, which would have facilitated the upwelling of asthenospheric material beneath a thinned lithosphere, thereby enhancing crustal reworking and partial melting. This interpretation 77 is consistent with the strongly negative εHf(t) values and ancient two-stage Hf model ages (T_DM2 = 2.8–2.0 Ga) recorded in cassiterite and zircon from LCT pegmatites in the region (Cao et al., 2024). In orogenic settings, LCT pegmatites commonly form through the partial melting of pre-existing igneous rocks or the anatexis of metasedimentary sequences (Koopman et al., 2023). The Benin-Nigeria shield is extensively underlain by migmatitic gneisses and granitic bodies, which provide clear field and petrographic evidence for widespread partial melting and crustal reworking throughout multiple orogenic episodes. Observations from this study confirm that partial melting of both granitoids and surrounding country rocks is pervasive. We report a new case of LCT pegmatite mineralization that crystallized at ca. 489 Ma, during the post-peak thermal relaxation of the orogenic system. This crystallization age postdates all previously known cooling ages from the Benin-Nigeria Shield, suggesting a distinct and prolonged magmatic episode. Pegmatites formed from magmatic fractionation or extreme crystal fractionation are expected to be closely related in age to the regional magmatism. From this study, we observed that our pegmatites postdate the regional magmatism between 90–130 Ma, suggesting that they were formed from the remelting of felsic fractionated intrusions. The pegmatite formation is interpreted to result from remelting of granites that were previously enriched by fractional crystallization, with rare metal components becoming concentrated in structurally conducive zones during the waning stages of the Pan-African orogeny (Fig. 5.2). 78 Figure 5. 2. Tectonic model of the southern Benin–Nigeria Shield illustrating a three-stage evolution associated with the Pan-African orogeny. Stage I corresponds to the magmatic emplacement of peraluminous granites (~621–600 Ma). Stage II is characterized by slab break-off, fractional crystallization of peraluminous granites, a static increase in geothermal gradient, and peak regional metamorphism (~550–530 Ma). Stage III represents post-orogenic thermal cooling and the subsequent emplacement and mineralization of LCT pegmatites (~530–480 Ma). 79 Chapter Six Conclusions Based on comprehensive field investigations across the east–west transect of the southern Benin-Nigeria Shield, supported by detailed petrological and microstructural analyses, pressure–temperature modeling, and a robust geochronological dataset derived from in-situ Rb–Sr dating of mica, Lu–Hf dating of garnet, and U–Pb dating of monazite and titanite in granites, LCT pegmatites, and metapelites from three key locations, the following conclusions can be drawn: 1. Magmatic emplacement of peraluminous granites occurred between ca. 623–600 Ma, marking the timing of significant crustal reworking during the Pan-African orogeny. 2. Slab breakoff and/or lithospheric delamination led to peak crustal metamorphism of the current exposure level during the interval of ca. 550–530 Ma, representing a phase of static isobaric heating. 3. Post-orogenic cooling and emplacement of LCT pegmatites occurred between ca. 530–480 Ma, marking the late stages of the Pan-African thermal regime. 4. The LCT pegmatites are interpreted to have formed through the remelting of highly fractionated Pan-African peraluminous granites, induced by continued post-orogenic thermal evolution and crustal maturation within the Benin-Nigeria Shield. 5. Garnet–staurolite schists from the region document a static increase in geothermal gradient, rising from approximately 1040 °C/GPa to 1160 °C/GPa, consistent with thermal input from asthenospheric upwelling following lithospheric thinning. 80 Chapter Seven Limitations and Recommendations for Future Studies 7.1 Limitations One of the primary limitations encountered was the challenge of safely accessing key outcrop locations. The ongoing rise in kidnapping for ransom across parts of West Africa significantly limited fieldwork opportunities, particularly in remote highland and lowland regions of the Benin-Nigerian Shield. Additionally, dense vegetation in the eastern sector and extensive overburden in the far western areas impeded the identification and examination of suitable outcrops. The initial project objectives were reoriented to focus on LCT pegmatites; however, only a single LCT pegmatite body was sampled and analyzed, which limits the representativeness of the findings across the entire study area. This constraint was further compounded by the fact that preliminary fieldwork had already been conducted before the commencement of the graduate program, and logistical limitations prevented a return to Nigeria for additional sampling. In the eastern study area (Oban Massif), the absence of suitable mineral assemblages precluded the ability to constrain peak metamorphic conditions or estimate pressure–temperature parameters. Similarly, the analyzed LCT pegmatite lacked monazite and apatite, mineral phases that could have provided independent age constraints to corroborate the Rb–Sr mica geochronology. Moreover, samples from the Igarra Schist Belt lacked essential minerals such as staurolite, muscovite, sillimanite, and kyanite, making thermobarometric analyses unfeasible. 7.2 Recommendation for future studies In light of the limitations outlined above and recognizing that this research is not exhaustive, the following recommendations are proposed to guide future investigations: 1. Garnet Lu–Hf geochronology should be applied to high-grade metamorphic rocks from the Oban Massif to better constrain the timing of peak metamorphism within the granulite-facies terranes. 2. 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The effect of reaction overstep on garnet microtextures in metapelitic rocks of the Ilesha Schist Belt, SW Nigeria. Journal of Petrology, 44, 967–994. 97 Appendices Appendix A Thin Section Scans and Phase Maps Figure A1. Thin section scan of sample Igarra 1A showing pre/post kinematic fabrics Figure A2. Thin section scan of sample Igarra 02 showing pre/post kinematic fabrics 98 Figure A3. Thin section scan of sample ND003 showing pre/post kinematic fabrics Figure A4. Thin section scan of sample ND003-2 showing pre/post kinematic fabrics 99 Figure A5. Thin section scan of sample ND004 showing pre/post kinematic fabrics Figure A6. a) phase map of mylonite (specimen ND004) showing major mineral phases; b) photomicrograph in plane and cross polar showing fractured orthoclase boudinage; c) shows cross polar photomicrograph of biotite rapping around orthoclase boudin and it also defining the foliation of the specimen by it other parallel crystals which are intercalated between plagioclase. 100 Figure A7. Thin section scan of sample ND021A showing pre/post kinematic fabrics Figure A8. Thin section scan of sample AJ2 101 Figure A9. a) phase map of specimen AJL2 showing constituent mineral phases; b) cross polar photomicrograph showing quartz forming boudins structure; c) photomicrograph in cross polar showing gneissose texture form by alternating quartz and biotite matrix. Figure A10. Thin section scan of sample Igarra 1C 102 Figure A11. Thin section scan of sample Igarra 1E Figure A12. Thin section scan of sample Igarra 07 103 Figure A13. a) phase map of specimen L7 showing different mineral phases; b, c and d) shows cross polar photomicrograph of zoned feldspars. Figure A14. Thin section scan of sample Igarra 08 104 Figure A15. a) phase map of specimen Igarra 08 showing different mineral phases; a) cross polar photomicrograph showing sericitized plagioclase zonation in a plagioclase porphyroblast; b) plane polar photomicrograph of plagioclase porphyry showing zonation. Figure A16. Thin section scan of sample Igarra 09 105 Figure A17. Thin section scan of sample ND002 Figure A18. a) Phase map of sample ND002 expressing the different mineral phases in the Amphibolite rock; b) Photomicrograph of specimen ND002 viewed under cross polar, showing amphibole, apatite, hornblende, and calcite; C) Cross polar micrograph, showing hornblende porphyroblast. 106 Figure A19. Thin section scan of sample ND007A Figure A20. Thin section scan of sample ND007B 107 Figure A21. Thin section scan of sample ND009 Figure A22. a) phase map of specimen ND009 showing different mineral phases; b) photomicrograph in cross polar showing sillimanite forming mainly along the edges of coarse quartz grains; c) photomicrograph in cross polar showing sillimanite fibrous texture nucleated upon muscovite surrounded with very coarse quartz grains. 108 Figure A23. Thin section scan of ND021B 109 Table 1. Structural measurements and sample coordinates S/N Latitude (N) Longitude (E) Elevation (meters) Structural measurement Rock type Strike and dip Foliation plane Fracture plane ND001 7° 13΄ 33 ̎ 4 36΄ 6 ̎ 312 - - - Amphibolite ND002 7° 29΄ 58 ̎ 4° 35΄ 27 ̎ 316 - - - Amphibolite ND003 7° 35΄ 17 ̎ 4° 38΄ 44 ̎ 344 179° N-S; 54°E, 44° E - - Garnet-Staurolite Schist ND004 7° 37΄ 6 ̎ 4° 48΄ 51 ̎ 417 180 N-S Strike 008°, dip 68° Strike 168°, dip 65° Mylonite ND005 7° 29΄ 25 ̎ 4° 31΄ 25 ̎ 266 - - - Ultramafic talc unit ND009 7° 42΄ 20 ̎ 4° 54΄ 16 ̎ 447 150°, 58° 88° EW, dip 58° Quartzite ND006 7° 32΄ 20 ̎ 4° 34΄ 14 ̎ 271 - - - Granite ND007 7° 33΄ 24 ̎ 4° 36΄ 31 ̎ 317 - - - Granite ND008 7° 36΄ 44 ̎ 4° 49΄ 2 ̎ 406 - - 126° ESW, dip 22°, 124° ESE, dip 40°, 28° N, dip 42° Augen gneiss ND010 7° 37΄ 2 ̎ 4° 47΄ 12 ̎ 427 058°, dip 21° S - - Talc schist ND011 7° 31΄ 16 ̎ 4° 31΄ 9 ̎ 289 - - - Banded Gneiss ND012 7° 31΄ 24 ̎ 4° 31΄ 33 ̎ 275 - - - Banded Gneiss ND013 6° 44΄ 50 ̎ 4° 55΄ 32 ̎ 89 - - - Granite ND014 6° 45΄ 57 ̎ 4° 58΄ 16 ̎ 93 - - - Granite-gneiss ND015 6° 46΄ 48 ̎ 5° 01΄ 36 ̎ 97 - - - Biotite Granite 110 S/N Latitude (N) Longitude (E) Elevation (meters) Structural measurement Rock type Rock type Strike and dip Foliation plane Fracture plane ND001 7° 13΄ 33 ̎ 4 36΄ 6 ̎ 312 - - - Amphibolite ND017 6° 45΄ 56 ̎ 5° 5΄ 15 ̎ 107 - - - Pegmatite ND018 6° 45΄ 53 ̎ 5° 5΄ 27 ̎ 110 - - - Migmatite-gneiss ND019 6° 44΄ 50 ̎ 5° 9΄ 50 ̎ 90 - - - Migmatite-gneiss ND020 6° 45΄ 52 ̎ 5° 5΄ 3 ̎ 104 - - - Migmatite-gneiss ND021 5° 13΄ 6 ̎ 8° 20΄ 53 ̎ 55 50° ENE - - Granite, Amphibolite, Schist ND022 5° 18΄ 21 ̎ 8° 21΄ 57 ̎ 100 - - - Pegmatite ND023 5° 20΄ 36 ̎ 8° 19΄ 15 ̎ 67 - - - Granite ND024 5° 22΄ 18 ̎ 8° 16΄ 18 ̎ 100 - - - Granodiorite ND025 5° 28΄ 3 ̎ 8° 8΄ 21 ̎ 114 - - - Granodiorite ND026 5° 42΄ 30 ̎ 8° 3΄ 9 ̎ 32 - - - Phyllite 1C 7° 8΄ 20 ̎ 6° 12΄ 38 ̎ 158.79 - - - LCT pegmatite 1E 7° 8΄ 3 ̎ 6° 12΄ 33 ̎ - - - Calc-gneiss L7 7° 17΄ 5 ̎ 6° 9΄ 3 ̎ 278 - - - Leuco Granite L8 7° 17΄ 1 ̎ 6° 8΄ 35 ̎ 262 - - - Granite L9 7° 16΄ 36 ̎ 6° 7΄ 39 ̎ 275 - - - Biotite Granite Igarra 1A 7° 27΄ 1 ̎ 6° 9΄ 6 ̎ 301.18 35°NE, dip 39° SW - - Biotite-garnet schist 111 S/N Latitude (N) Longitude (E) Elevation (meters) Structural measurement Rock type Rock type Strike and dip Foliation plane Fracture plane Igarra 02 7° 18΄ 36 ̎ 6° 12΄ 2 ̎ 300 2°N, dip 60° SW - - Biotite-garnet schist AJL2 7° 29΄ 56 ̎ 6° 40΄ 16 ̎ 85 - - - Gneissic schist 112 Appendix B Electron Microprobe Backscatter Images 113 114 Figure B1. Backscatter electron images of sample ND003. a–c) electron microprobe spots on biotite; d–h) electron microprobe spots on chlorite; i–k) electron microprobe spots on garnet; l–n) electron microprobe spots on muscovite; o–q) electron microprobe spots on plagioclase; r–t) electron microprobe spots on staurolite. 115 Appendix C Location of analyzed grains Figure C1. Thin section scan of sample ND003 showing locations of probed grains Figure C2. Backscatter electron image of sample Igarra 1A showing the location of monazite grains analyzed 116 Appendix D Supplementary data file Description: All analytical files can be accessed through the Open Science Framework online data repository through the following link https://osf.io/wsv56/files/osfstorage The files are labelled as: 1. EMPA files 2024 08 08 Dieudonne ND003 Mineral Quants Bt Grt Pl NwekeD.xlsx 2. In-situ garnet Lu-Hf 2024-03-11-LuHf_Dieu_Hogsbo1052p_1083a.xlsx 3. Run 1 in-situ mica Rb-Sr 2023 02 22 RbSr Norm_1B Sr88 MG analogue normalized 447 pulse 520.xlsx 4. Run 1 in-situ Titanite U-PB 2024-07-05-Dieudonne-Titanite.xlsx 5. Run 2 in-situ mica Rb_Sr Result_2_with_corrected_Tanco.xlsx 6. Run 2 in-situ Monazite U-PB 2024-07-05-Dieudonne-Monazite.xlsx
Thesis/Dissertation
eng
Earth and Environmental Sciences
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Dyck, Brendan
Crustal evolution of the Southern Benin-Nigerian shield : insights from late post-orogenic lithium-cesium-tantalum (LCT) pegmatite mineralization
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