An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad

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(1)NATIONAL TAIWAN NORMAL UNIVERSITY College of Science DEPARTMENT OF EARTH SCIENCES. MSc. Thesis. An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad. Student: Supervisor:. Chi Thi Pham Prof. John Gregory Shellnutt. July 2019.

(2) ACKNOWLEDGEMENT First of all, I would like to express my most enormous gratitude to my supervisor, Professor John Gregory Shellnutt, who gave me a chance to study at National Taiwan Normal University, for his patience, encouragement, guidance and immense knowledge. Besides, I am grateful to Associate Professor Ngo Xuan Thanh at Hanoi University of Mining and Geology for his influence and encouragement during my graduate study in Taiwan. My sincere thanks also go to all professors and staffs at the Department of Earth Sciences, National Taiwan Normal University, who taught and supported me during my time in the department. Without their precious help, I would not possibly be a better student as right now. I would like to thank Prof. Meng-Wan Yeh, Prof. Tung-Yi Lee, and CPC Taiwan for their assistance with fieldwork in Chad; Dr. Yoshiyuki Iizuka and his assistant for their laboratory assistance. I would also like to acknowledge the Ministry of Science and Technology Taiwan for financial support. I thank my fellow labmates (Carol, Thuy, Ha, Alice, Dieu, Andy, and Robert) for their help and kindness, especially, I would hereby like to send my dearest appreciation towards Dieu and Linh for stimulating discussions and for their help regarding grammar correction. A big thank to my classmates, roommates for their kindness and companion to make my time in Taiwan becoming colorful and enjoyable. Finally, I would like to thank my family for their absolute support in term of material and spiritually throughout my years of study. Chi Thi Pham i.

(3) Content ACKNOWLEDGEMENT ......................................................................................... i Content ...................................................................................................................... ii Abbreviations ............................................................................................................ v List of Figures .......................................................................................................... vi List of Pictures ........................................................................................................ xii List of Tables .......................................................................................................... xv Abstract ..................................................................................................................... 1 CHAPTER 1. INTRODUCTION ............................................................................. 3 1.1. General introduction ...................................................................................... 3 1.2. Gondwana supercontinent ............................................................................ 10 1.3. Pan-Africa orogeny ...................................................................................... 13 1.4. Domains of the Saharan Metacraton in Chad .............................................. 15 1.4.1.. Tibesti massif..................................................................................... 17. 1.4.2.. Ouaddaï massif .................................................................................. 18. 1.4.3. Guéra massif ...................................................................................... 18. 1.5. Domains of the CAOB in Chad ................................................................... 19 1.5.1.. Mayo Kebbi massif ........................................................................... 19. 1.5.2.. Yadé massif ....................................................................................... 20. 1.6. The purpose of the study .............................................................................. 21 ii.

(4) CHAPTER 2. GEOLOGICAL BACKGROUND .................................................. 23 2.1. Guéra massif ................................................................................................ 25 2.2. Lake Fitri region........................................................................................... 31 2.3. Phanerozoic sedimentary basins in Chad ..................................................... 33 CHAPTER 3. PETROGRAPHY ............................................................................ 35 3.1. Post-collisional granite ................................................................................. 35 3.2. Collisional granite ........................................................................................ 48 CHAPTER 4. METHODS ...................................................................................... 57 4.1. Principles of electron probe micro-analyzer (EPMA) ................................. 57 4.1.1.. X-ray generation and interaction volume .......................................... 57. 4.1.2.. Wavelength dispersion and focusing of characteristic X-ray............ 58. 4.2. Source of data and analytical method .......................................................... 60 CHAPTER 5. RESULTS ........................................................................................ 63 5.1. Rock classification ....................................................................................... 63 5.2. Mineral classification ................................................................................... 66 5.2.1. Classification of minerals from collisional granite ............................... 66 5.2.2. Classification of minerals from post-collisional granite ....................... 69 5.3. Mineral composition .................................................................................... 72 5.3.1. Post-collisional granite .......................................................................... 74 5.3.2. Collisional granite ................................................................................. 80 iii.

(5) CHAPTER 6. DISCUSSION .................................................................................. 85 6.1. Temperature estimates ................................................................................. 85 6.2. Pressure estimates ........................................................................................ 87 6.3. Oxygen fugacity estimates ........................................................................... 96 6.4. The change in magmatic conditions during magma course ....................... 101 6.5. Biotite composition and parental magma .................................................. 109 6.6.. Significance. of. biotite. composition. in. the. determination. of. tectonomagmatic setting ......................................................................................... 112 CHAPTER 7. CONCLUSION .............................................................................. 116 References ............................................................................................................. 118 Appendix ............................................................................................................... 149 Appendix 1 ........................................................................................................ 150 Appendix 2 ........................................................................................................ 157 Appendix 3 ........................................................................................................ 177 Appendix 4 ........................................................................................................ 216. iv.

(6) Abbreviations ANS: Arabian-Nubian Shield AYD: Adamawa-Yadé domain CAOB: Central African Orogenic Belts CASZ: Central African Shear Zone COLG: Collisional granite EDS: Energy Dispersive Spectrometer EPMA: Electron Probe Micro-Analyzer fO2: Oxygen fugacity GC: graphite-CO2 equilibrium buffer Hd Il: hedenbergite-ilmenite buffer HFSEs: High Field Strength Elements IW: iron-wustite LILEs: Large Ion Lithophile Elements MH: magnetite-hematite NNO: nickel-nickel oxide ORG: Ocean ridge granite Ox: Oxidizing QFM: quartz-fayalite-magnetite buffer QIF: quartz-iron-fayalite Rd: Reducing SCLM: Subcontinental Lithospheric Mantle syn-COLG: syn-collisional granite TMQ: titanite-magnetite-quartz buffer VAG: Volcanic arc granite WCARS: West and Central African Rifts System WDS: Wavelength Dispersive Spectroscopy WM: wustite-magnetite WPG: within plate granite WPL: Within plate v.

(7) List of Figures Figure 1.1. Classification of granites based on tectonic setting (modified from Pitcher, 1983). ................................................................................................................... 7 Figure 1.2. Gondwana supercontinent configuration after the termination of the East African Orogen (after Gray et al., 2007). .......................................................... 12 Figure 1.3. Geological sketch map of the Saharan Metacraton (after Küster et al. 2008). ................................................................................................................. 16 Figure 2.1. Geological overview of Chad (modified from Saleh, 1994) showing the study area. .......................................................................................................... 24 Figure 2.2. The simplified geological sketch map of Guéra massif and sample locations in this study (modified from Pham, 2018). ........................................ 28 Figure 2.3. The simplified geological sketch map of Mongo area showing doleritic dykes within Guéra massif (after Nkouandou et al., 2017). .............................. 29 Figure 2.4. (a) Simplified geological map of North-Central Africa, showing the location of this study area (modified from Abdelsalam et al., 2002). The sampling localities in Lake Fitri region (b, c after Shellnutt et al., 2018)......... 32 Figure 2.5. a. The location of Chad in Africa (Google Earth, 2018). b. Simplified map showing the location of Phanerozoic basins in Chad (modified from Genik, 1993). ................................................................................................................. 34 Figure 4.1. A sketch illustrating the imaginary Rowland circle. .................................. 59 Figure 5.1. Tectonic discrimination diagrams for intrusions in this study. (a) after Batchelor and Bowden (1985); (b) after Pearce et al. (1984), WPG: within plate granite, VAG: volcanic arc granite, ORG: oceanic ridge granite, syn-COLG: vi.

(8) syn-collisional granite; (c) after Eby (1992), A1, A2 are sub-groups of A-type granites. * and ** denotes data from Shellnutt et al. (2018) and Pham (2018), respectively. ....................................................................................................... 64 Figure 5.2. Compositional classification diagram of granite for this study. (a) Fe* vs. SiO2 showing the distinction between ferroan and magnesian composition (after Frost et al., 2001); (b) molar Al/(Na+K) vs. Al/(Ca+Na+K) showing the range of metaluminous and peraluminous for granite composition (after Shand, 1943); (c) (Na2O+K2O-CaO) vs. SiO2 showing the ranges of alkalic, alkalic calcic, calc-alkalic and calcic (after Frost et al., 2001). * and ** denotes data from Shellnutt et al. (2018) and Pham (2018), respectively. ............................ 65 Figure 5.3. Classification of biotite from collisional granites in Guéra massif. (a) The Fet/(Fet+Mg2+) versus total Al content diagram (ASEP diagram) after Rieder et al. (1998), and the tectonic discrimination line from Shabani et al. (2003), Fet = Fe2++ Fe3+; (b) (Aliv+Fe3++Ti)-Mg-(Fe2++Mn) diagram after Foster (1960); (c) (FeO+MnO)-10*TiO2-MgO diagram after Nachit et al. (2005) * denote data from Shellnutt et al. (2018). .............................................................................. 67 Figure 5.4. Classification of amphibole from collisional granites in Guéra massif. (a) Amphibole classification for calcic group recommended by the International Mineralogical Association (IMA) in the modified version of Hammarstrom and Zen (1986); (b) Amphibole classification follows the IMA recommendation (Leake, 1978) for the calcic group; (c) The plot of amphiboles in this study in terms of cations (Ca+Na+K) versus Si (apfu) diagram (after Giret et al., 1980). ........................................................................................................................... 68. vii.

(9) Figure 5.5. Classification of biotite from post-collisional granites in Guéra massif and Lake Fitri region. (a) The Fet/(Fet+Mg2+) versus total Al content diagram (ASEP diagram) after Rieder et al. (1998), and the tectonic discrimination line from Shabani et al. (2003), Fet = Fe2++ Fe3+; (b) (Aliv+Fe3++Ti)-Mg-(Fe2++Mn) diagram after Foster (1960); (c) (FeO+MnO)-10*TiO2-MgO diagram after Nachit et al. (2005) * denote data from Shellnutt et al. (2018). ........................ 70 Figure 5.6. Classification of amphibole from post-collisional granites in Guéra massif. (a) Amphibole classification for calcic group recommended by the IMA in the modified version of Hammarstrom and Zen (1986); (b) Amphibole classification follows the IMA recommendation (Leake, 1978) for the calcic group; (c) The plot of amphiboles in this study in terms of cations (Ca+Na+K) versus Si (apfu) diagram (after Giret et al., 1980). ........................................... 71 Figure 5.7. The major element composition of biotite in the post-collisional granite during magma evolving. apfu: atom formula unit. ............................................ 76 Figure 5.8. The major element composition of amphibole in the post-collisional during magma evolving. ............................................................................................... 79 Figure 5.9. The major element composition of biotite in the collisional granite during magma evolving. ............................................................................................... 82 Figure 5.10. The major elements composition of amphibole in the collisional granite during magma evolving. .................................................................................... 84 Figure 6.1. The average pressure inferred from amphibole composition including propagation error and its relative probability in respect of plutons in Guéra massif. ................................................................................................................ 90. viii.

(10) Figure 6.2. The average pressure inferred from biotite composition, including propagation error and its relative probability in respect of plutons in Guéra massif (this study) and Lake Fitri region (data from Shellnutt et al., 2018). .... 94 Figure 6.3. The plot of titanium oxide and octahedral aluminum content in biotite showing level crystallization of depth. * denotes data from Shellnutt et al. (2018). ............................................................................................................... 95 Figure 6.4. (a) Comparison of hornblende geobarometers, with calibration curves obtained by a sphalerite geobarometer (after Uchida et al., 2007); (b) Biotite geobarometer in this study within a calibration curve of biotite geobarometer for granitic rocks from Uchida et al. (2007). ..................................................... 95 Figure 6.5. Composition of biotite from granitic rocks of Guéra massif and Lake Fitri region in the Fe2+-Fe3+-Mg2+after Wones and Eugster (1965) along with the three common oxygen fugacity (fO2) buffer: hematite-magnetite (HM), NickelNickel oxide (NNO) and quartz-fayalite-magnetite (QFM). * denotes data from Shellnutt et al. (2018). ....................................................................................... 99 Figure 6.6. The correlation between estimation values of oxygen fugacity in log unit and temperature of the granitic rocks from Guéra massif ad Lake Fitri region (after Wones, 1981). The sample plotted areas represent for oxygen fugacities trend of crystallizing magmas. HM: hematite-magnetite buffer; TMQ: titanitemagnetite-quartz buffer; Hd Il: hedenbergite-ilmenite buffer; QFM: quartzfayalite-magnetite; GC: graphite-CO2 equilibrium. In which, HM is referred to the highest level of oxidizing condition, the direction from QFM toward HM is an oxidizing (Ox) trend of magma crystallization by the addition of oxygen or. ix.

(11) hydrogen loss during crustal emplacement; while the opposite direction is reducing (Rd) trend of magma by the addition of hydrogen or carbon. * denotes data from Shellnutt et al. (2018). ..................................................................... 100 Figure 6.7. The simplified location map showing the subdivision of biotites using the Fe-number, and the magmatic condition estimated for each intrusion. (a) modified from Pham (2018), (b, c) modified from Shellnutt et al. (2018). * denotes data from Shellnutt et al. (2018). ....................................................... 102 Figure 6.8. (a, b, c) the magmatic conditions of granitic rocks from the Guéra massif and Lake Fitri deduced from biotite composition. (d, e) The plot of biotite emplacement pressure and oxidation state of studied rock with biotite crystallization temperature, and (f) the plot of oxidation state and biotite emplacement pressure indicate a clear trend among Guéra and Lake Fitri granites, (g) the amphibole pressure of Guéra granites generally drops during rock formation. * denotes data from Shellnutt et al. (2018). .......................... 103 Figure 6.9. The positive correlation between biotite temperature crystallization and silica content. * denotes data from Shellnutt et al. (2018). ............................. 104 Figure 6.10. The concentration of Ti reflects the variation of the crystallization temperature of biotite and oxygen fugacity of magma from Guéra massif and Lake Fitri region. * denotes data from Shellnutt et al. (2018). ....................... 104 Figure 6.11. The variation of Fe2+ content records the transfer of the oxidation state in the course of magma during its evolution. (*) the data using from Shellnutt et al. (2018).......................................................................................................... 106. x.

(12) Figure. 6.12.. A. possible. tectomagmatic. evolution. suggesting. repeated. deformation/relaxation episodes in southern Chad (after Shellnutt et al. (2017). (a) Subduction-related magmatism and deformation stages (D1 and D2). (b) Transtentional deformation D3 is followed by a first (c) crustal relaxation stage at ~ 570 Ma. (d) The final deformation stages D4 stage is followed by a second (e) (~560 Ma) and (f) final crustal relaxation stage. SCLM: subcontinental lithospheric mantle. CAFB: Central African Fold Belt. .................................. 108 Figure 6.13. Alvi vs. Mg diagram for granites of Guéra massif and Lake Fitri region. * denotes data from Shellnutt et al. (2018). ....................................................... 111 Figure 6.14. Distribution of biotite composition on the FeO-MgO-Al2O3 discrimination tectonomagmatic diagram after Abdel-Ranhman (1994). FeOT= [FeO+(0.89981*Fe2O3)], (*) the data using from Shellnutt et al. (2018). ...... 114 Figure 6.15. A conceptual scenario for the crustal evolution of the Guéra massif, South-Central Chad. ........................................................................................ 115 Figure 1. The oxygen fugacity vs. temperature diagram shows the defined curve for common buffers. All buffers at a pressure of 1 bar (compiled by Frost, 1991). MH: magnetite-hematite; NNO: nickel-nickel oxide; QFM: quartz-fayalitemagnetite; WM: wustite-magnetite; IW: iron-wustite; QIF: quartz-iron-fayalite. ......................................................................................................................... 156. xi.

(13) List of Pictures Picture 2.1. (a) Biotite granite intruded by gabbro; (b) biotite granite including microgranitic enclaves. ............................................................................................... 29 Picture 2.2. Biotite granite recorded deformation evidence. ....................................... 30 Picture 2.3. An outcrop of (a) hornblende-biotite granite and (b) rhyolite within Guéra massif. ................................................................................................................ 30 Picture 2.4. An outcrop of biotite granite in (a) Ngoura and (b) Moyto domain. ....... 31 Picture 3.1. The thin section photograph of representative crystals characteristic for the sample 14ZA10. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Apt: anti-perthite; Mus: muscovite; Myr: myrmerkite. ....................................................................................................... 36 Picture 3.2. Some representative crystals for main mineral assemblage of the sample 14ZA18. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende ......................................................... 38 Picture 3.3. The thin section photograph of main mineral assemblage typical for the sample 14ZA19B (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: perthite; Myr: myrmerkite; Hbl: hornblende. ........................................................................................................ 40 Picture 3.4. The subhedral crystals of the main minerals observed in the sample of 14ZA20B. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende. ........................................................ 42 Picture 3.5. Microscope thin section photograph of the sample 14ZA21B, showing the euhedral to subhedral crystals of K-feldspar, plagioclase, quartz, and xii.

(14) hornblende. (+): crossed polarized light; (-) plane polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende. ............ 44 Picture 3.6. The mineral assemblage of the sample 14ZA01. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: Perthite; Ms: muscovite; Hbl: hornblende; Chl: chlorite; Opq:opaque; Zrn: zircon. ................................................................................................................ 46 Picture 3.7. Microscope thin section photograph of the sample 14ZA23, showing phenocryst of K-feldspar, quartz, and biotite. (+): crossed polarized light. Qz: quartz; Bt: biotite; Kfs: potassium feldspar. ...................................................... 47 Picture 3.8. The various crystal form of feldspar, quartz, biotite, and myrmerkite. (+): crossed polarized light, (-): plane polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Myr: myrmerkite. .................................... 49 Picture 3.9. (a) perthite with cross-hatched twinning, (b) the lamellar twinning of plagioclase is visible in the most of crystals, (c) a cluster of biotite performing the high pleochroism associated with exsolution lamellae of K-feldspar, (d) the anhedral crystals of quartz and myrmerkite enclose a discontinuous band of biotite. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Myr: myrmerkite; Hbl: hornblende. .................................. 51 Picture 3.10. Main mineral assemblage of the sample 14ZA02. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: Perthite; Mus: muscovite; Myr: myrmerkite. .................................................... 53 Picture 3.11. Representative crystals for the chiefly mineral assemblage of the sample 14ZA06. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite;. xiii.

(15) Kfs: potassium feldspar; Apt: anti-perthite; Mus: muscovite; Myr: myrmerkite; Hbl: hornblende. ................................................................................................ 55 Picture 3.12. The thin section photograph of the sample 14ZA25C. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Ap: apatite; Opq: opaque................................................................................................................ 56 Picture 4.1. Samples were prepared for biotite and amphibole analysis. (AB): hornblende-biotite-granite; (B): biotite-granite. ................................................ 62. xiv.

(16) List of Tables Table 6.1. The range of crystallization temperature of biotites from Guéra massiff and Lake Fitri region estimates based upon biotite composition. ............................ 86 Table 6.2. The range of amphibole pressure of Guéra granies obtained based on the amphibole composition. .................................................................................... 89 Table 6.3. The range of biotite pressure computed for granites from Guéra massiff and Lake Fitri region. ............................................................................................... 92 Table 6.4. The range of oxygen fugacity for granites from Guéra massif and Lake Fitri region estimated based on biotite composition. ................................................ 98. xv.

(17) Abstract The Guéra massif of South-Central Chad recorded the late stages of the PanAfrican orogeny related to the collision between the Congo Craton and the Saharan Metacraton. The granitic rocks of the Guéra massif are Neoproterozoic and were emplaced during three distinct intervals: ~590 Ma, ~570 Ma, and ~560 Ma. The younger granites from the Guéra massif may be related to the slightly younger granites (~550 Ma) that constitute the Lake Fitri inliers to the west. The oldest (≥ 590 Ma) rocks have geochemical characteristics of collisional granites whereas the younger (≤ 570 Ma) rocks are similar to post-collisional granites. Biotite and amphibole chemistry for both types of granites are studied to estimate the magmatic conditions of the host rock, subsequently, to distinguish the type of granites and interpret the relationship between them, hence, to better understand the geology of south-central Chad area. The biotite from the collisional granites have higher Al and Ti content, and low iron number (Fe# = Fe2+/(Fe2++Mg) = 0.40-0.96). Their average magmatic temperature is 639 ± 73oC, and average pressure is 1.9 ± 0.1 kbars, and the average redox state -18.3 ± 3.1 that lies between the nickel-nickel oxide and quartzfayalite-magnetite buffers. The biotites from the post-collisional granites are characterized by higher Fe# (≥0.7), and lower Al and Ti contents, as compared to collisional granites. All post-collisional biotites crystallized at lower temperatures (570 Ma = 620 ± 44oC, 560 Ma = 616 ± 31oC and 550 Ma = 613 ± 45oC). However, the Guéra granites are formed at pressures of 1.8 ± 0.1 kbars and 2.1 ± 0.1 kbars with the relative oxidation state transitioning from oxidizing to reducing environment (log fO2 = -19.5 ± 1.9 and -19.1 ± 1.6 for 570 Ma and 560 Ma, respectively) around the 1.

(18) quartz-fayalite-magnetite buffers and wüstite-magnetite buffers. In contrast, the Lake Fitri post-collisional granites were emplaced at higher pressure (4.1 ± 0.1 kbars) and higher relative oxidation state, similar to the Guéra massif collisional granites (log fO2= -18.3 ± 3.1). Amphiboles are more common in the post-collisional granites and are calcic with a low magnesian number (Mg#=Mg2+/(Mg2++Fe) ≤ 0.3) which is consistent with the lower oxidation state. It appears that the granites of the Guéra massif display distinct evolution from high to low in temperature, pressure, oxidation state over time. Moreover, the Guéra post-collisional granites had very different magmatic conditions than the post-collisional granites of Lake Fitri, suggesting there may be a terrane boundary between the two exposures.. 2.

(19) CHAPTER 1. INTRODUCTION 1.1. General introduction Continental crust is the outermost portion of the lithosphere that preserves the geological history of Earth. It consists of the main continents, their margins, and several submerged microcontinents, and occupies ~0.6% of the silicate mass of the Earth, with density ranging from 2.7-2.9 g/cm3 (Rudnick and Gao, 2003). The continental crust covers ~41% of the earth’s surface and occupies ~79% of the total crust volume (Coley, 1984). The topography of the continental crust is entirely rugged and has the largest elevation gradients between the base of the continental slope in deep water to the highest mountain peaks reaching 16-17 km (Sorokhtin et al., 2011). The structure of continental crust is variable and generally can be subdivided into three parts, including the upper crust, the middle crust, and the lower crust with a variation in lithological and geochemical features. The upper part has a granodioritic bulk composition, enriched in incompatible elements, for instance, U, Th, K, La, Cs, and Rb (Rudnick and Gao, 2003), which mostly comprises gneisses, granites, and granodiorites, and variable from 10 to 25 km thick (Rudnick and Fountain, 1995). The middle part is thinner (5 to 15 km), intermediate in bulk composition, and mainly comprises metamorphic rocks at the amphibole facies. The lower crust part consists of igneous and metamorphic rocks at the granulite facies (mainly diorites, gabbros, amphibolites, and granulites), which are lithologically heterogeneous (Rudnick and Fountain, 1995; Wedepohl, 1995). Continental crust first emerged during the early Archean (Bickford, 1988), in which, only 7% of the present-day crust is older than 2.5 Ga (Hawkesworth et al., 3.

(20) 2010) with the oldest fragments (~4.0 Ga) preserved in the Acasta Gneiss, Slave Province, NW Canada (Taylor and McLennan, 1995; Bowring and Williams, 1999). Many models are suggested for the formation of a new continental crust such as: melting of primitive sialic crust, fractional crystallization of basaltic magma and/or remelting a primitive sialic crust (Green and Ringwood, 1968), by contamination (mixing) of basaltic magma with crustal materials, or derived from the mantle (Carmichael et al., 1974). The evolution of continental crust is controlled by plate tectonics, that is derived from continental drift theory firstly suggested by Wegener (1912b). Accordingly, plates interact along their boundaries, where they converge, diverge, or slip past to another. In cases of continent-continent convergence, they collided and merged to form a vast of landmass, called supercontinent. Supercontinents exist for some periods of geological time and then are fragmented due to the movement of plate tectonics (Rogers et al., 2004), for example, Gondwana supercontinent was the most extensive piece of continental crust during Late Neoproterozoic to Jurassic (~500-180 Ma). A sequence of supercontinent formation and break-up occurred during middle Mesoproterozoic and Late Neoproterozoic period, including the amalgamation of Rodinia (~1300-950 Ma) and its separation (850-600 Ma) (Condie, 2003). This lead to the formation of a new supercontinent called Pannotia-Gondwana (680-550 Ma). Such tectonic events represent for the recycling of the continental crust, by which new continental crust will be formed and correspond with partly vanishing of the previous continental crusts. For example, the stage of 750-550 Ma produced at least ~50% of juvenile continental crust in ArabianNubian Shield as well as along the northern boundary between Amazonia and West. 4.

(21) Africa, ~20% is in Pan-African orogens within Amazonia and ~16% in the Adamastor ocean (between Africa and South America) and West African orogens (Condie, 2003). Meanwhile, only ~11% of the older continental crust that formed during 800 Ma period is preserved (Condie, 2003). The succession process of the breakup of one supercontinent and the development of another (i.e., pieces of pre-supercontinent reorganize elsewhere in an entirely new configuration as a supercontinent) taking place on a global scale is referred to as the supercontinent cycle. The supercontinent cycle is reflected by (1) area of the largest continent and number of continents; (2) abundance and timing of passive margins; (3) granites and detrital zircons; (4) isotopic composition of seawater strontium; and several secular trends such as (5) greenstone belts deformation events; (6) eclogiteand granulite-facies metamorphism; (7) carbonatites; (8) emplacement of large igneous provinces (e.g., Worsley et al., 1986; Nance et al., 1986; Barley and Groves, 1992; Condie, 2005; Condie et al., 2009; Hawkesworth et al., 2010; Bradley, 2011). The supercontinent cycle results in the opening and closing of ocean basins, which effect on many portions of the earth system and also control the formation and distribution of continental crust that likely affects the spatial and temporal distribution of metal deposits (Barley and Groves, 1992). The initial stages of the breakup of a vast continent are usually related to anorogenic and continental deposits (e.g., Sn-W, Ba, F, REE, U, Th, Nb). Meanwhile, orogenic deposits (e.g., Cu-Au-Zn-Pb-Ag deposits) related to convergent tectonics commonly occur during periods of rapid plate movement and supercontinent combination (Bradley, 2011).. 5.

(22) Granitic rocks are vital components to form the continental crust. They intrude into pre-existing rocks and emplace at different depths with various size ranging from large (batholiths) to small (stocks) or even to dykes (Clarke, 1992; Rollinson, 2015). The emplacement of granitic magma develops in four stages: partial melting, segregation, ascent, and emplacement. The granitic rocks are commonly found in Archean cratons, for instance, the Slave Province, the Superior Province (Canadian Shield). (Hoffman,. 1988),. Dharwar. craton,. Bastar. craton. (Indian. shield). (Subrahmanyam and Verma, 1982; Sharma, 2009), thus, they are able to record historical information of the Earth’s crust including its formation, tectonic settings, and geodynamic evolution. Granites are also the main magmatic results of collision events and maybe tectonically subdivided to the type of collisional setting (e.g., continent-continent, continent-arc, and arc-arc), and temporally to the deformation event (syn- or postcollision) (Figure 1.1, Pearce et al., 1984). In essence, the collisional granites are mostly I-type granites that generated in the arc-related environment (e.g., subduction) under higher water content (Naney, 1983) and higher oxidation state (Ewart, 1979; Zhang et al., 2017) than post-collisional granites (A-type) which are produced at extensional settings.. 6.

(23) Figure 1.1. Classification of granites based on tectonic setting (modified from Pitcher, 1983).. In case of Phanerozoic continent-continent collision zones, intermediate to felsic rocks can be classified into four groups, corresponding to stage in tectonic evolution of collision zone: (1) pre-collision calc-alkaline (volcanic-arc) intrusions, which are typically selective enrichment in large ion lithophile elements (LILEs) and crystallize from mantle-derived magma mixed with subduction materials; (2) Syn-collision peraluminous intrusions (leucogranites) which have high Rb-Sr, Ta-Nb and low K-Rb ratios, and may formed by magma derived from hydrated bases of continental thrust sheets; (3) Late or post-collision calc-alkaline intrusions, which are characteristic of higher Ta/Hf and Ta/Zr ratios, as compared to pre-collision type and originated from magma mantle source that was contaminated by crustal components; (4) Post-collision alkaline intrusions having high LILEs and high field strength elements (HFSEs) contents, which are generated by magma from mantle lithosphere after collision (Harris et al., 1986). Many orogenic belts are characterized by associations of alkaline and calc-alkaline granites centers showing a close linked in space and time, and the tectonic environment rapidly switches from orogenic to anorogenic geodynamic conditions. In 7.

(24) this case, alkaline granites can be subdivided into two groups based on geological signatures: (1) The post-orogenic granites are ‘red granites’ bearing Mg-rich mafic minerals, enriched in Ba and Sr, which are peralkaline or peraluminous in composition, high Mn content. They are characteristic of high oxygen fugacity and the subsolvus crystallization of alkali feldspars due to water-rich fluids. (2) The early anorogenic granites include pale-green to off-white hypersolvus granites, bearing mafic minerals that are enriched in Fe, deficient in the content of Ba and Sr, less abundant of Mg/F-rich aqueous fluids and the hydrothermal alteration only depending on late-stage of oxidation. The magmatic centers are significantly younger than the post-orogenic group (Bernard, 1990). Presently, oxygen fugacity is considered as one of the most critical parameters in the petrology of igneous and metamorphic rocks as it relates to the relative stability of sulfides, silicates, oxides, and carbonates (Lindsley, 1991). It has strong effects on physicochemical processes such as rheological creep, diffusion, and electrical conductivity, magma genesis, recycling of oceanic lithosphere, metasomatism, and chemical differentiation (Arculus, 1985; Ryerson et al., 1989; Dai et al., 2014). Interestingly, numerous recent studies have pointed out the importance of oxygen fugacity in relation to magmatic-hydrothermal mineralization (Richards, 2009, 2015; Mengason et al., 2011; Trail et al., 2012; Zhang et al., 2013; Qiu et al., 2013, Sun et al., 2013, 2015), for instance, high oxygen fugacity generally plays an essential role in controlling of the formation of porphyry Cu-Au and epithermal Au-Cu deposits (Wyborn and Sun, 1994; Ballard et al., 2002; Liang et al., 2009; Dilles et al., 2015; Shen et al., 2015; Richard, 2015; Sun et al., 2015). Furthermore, the depth of. 8.

(25) emplacement is closely related to the type of mineral deposit, for example, intrusions solidified at a relatively shallow depth generally produce porphyry copper (±gold) deposits (Chiaradia et al., 2012; Richards et al., 2012), but the relatively deep intrusions are exclusively related to skarn copper (±gold) deposits (Burt, 1998; Meinert, 1998) as well as gold deposits (Lang and Baker, 2001; Thorne et al., 2002; Yang et al., 2008). Thus, determining the magmatic conditions of granitic intrusions provides a critical understanding of regional tectonic and constraint on mineral exploration.. 9.

(26) 1.2. Gondwana supercontinent The name “Gondwana” was first used by Medlicott (1872) for a sequence of nonmarine sedimentary rocks and also used in a report by Medlicott and Blandford (1879). The concept of Gondwana was further advanced by Suess (1885), Neumayr (1887), Wegener (1912a, 1915). At present, the term Gondwana (originally Gondwanaland) is used to convey the structure and contiguity of an ancient supercontinent that existed in the southern hemisphere throughout the Late Neoproterozoic and Paleozoic. The Gondwana supercontinent was the largest continent on earth with an area of about 132 million km2 (Smith, 1999), and its fragments occupy 64% of all land area today (Torsvik and Cocks, 2013; Svensen et al., 2018). Gondwana fragmented to produce the current continents of Africa, Arabia, South America, often called West Gondwana, and India, Antarctica, and Australia called East Gondwana (Torsvik and Cocks, 2013; Nance et al., 2014) (Figure 1.2). Gondwana was probably an integral part of the Mesoproterozoic Rodinia supercontinent which subsequently broke up at ~800 Ma. Gondwana was unified from thirteen Precambrian cratons belonging to Africa, Australia, South America, East Antarctica, and India (Baranov and Borov, 2018) by the subduction of ancient oceans and numerous collisional events that ended diachronously (Clifford, 1968). The final stages of Gondwana unification occurred during the Late Neoproterozoic Pan-African Orogeny and coeval orogenies such as the Pampean and Brasiliano orogenies (South America), the Kuungan orogeny (between East Antarctica and India), and the Cadomian orogeny (North Africa and Southern Europe) (Torsvik and Cocks, 2013). 10.

(27) (Figure 1.2). Afterward, Gondwana collided with continents of North America, Europe, and Siberia to form Pangea during the Late Paleozoic (Reeves, 2014). The breakup of Gondwana began during the middle Mesozoic and ended by the Early Paleogene. North America separated from Africa and South America corresponding to the creation of the south-central part of Atlantic Ocean at about 195 Ma; India rifted from Antarctica and Australia at the same time. Thus, the center of the Indian Ocean was opened (Torsvik and Cocks, 2013). The separation of East and West Gondwana begun during the Middle Jurassic (~180 Ma) as rifting between Antarctica and southern Africa began (McLoughlin, 2001). At ~50 Ma, India collided with Eurasia to form the Himalayan mountains, and the Australia plate was moving northward and collided along the southern margin of Southeast Asia that is still underway today (Yoshida and Hamano, 2015). The dispersal of Gondwana was accompanied by mantle plume-related magmatism that occurred within and between the major cratons (Storey, 1995, Riley and Knight, 2001; Bardintzeff et al., 2010; Buiter and Torsvik, 2014) that led to one of the most significant periods of large igneous province formation on Earth (Torsvik and Cocks, 2013, Svensen et al., 2018).. 11.

(28) Figure 1.2. Gondwana supercontinent configuration after the termination of the East African Orogen (after Gray et al., 2007).. 12.

(29) 1.3. Pan-Africa orogeny Collectively, the Pan-African orogeny was initially considered to be a series of orogenic events occurring at about 500 million years ago, which recorded lithotectonic processes that lead to the formation of Gondwana and Pennotia. The timing of the Pan-African events was based on K-Ar and Rb-Sr mineral ages by Kennedy (1964), but a more detailed evaluation indicated that it developed during different episodes (Clifford, 1967). Currently, the term “Pan-African” is described as tectonic, magmatic and metamorphic activities in Africa during the Neoproterozoic and earliest Paleozoic, and particularly the crustal portions that comprise Gondwana (Kröner, 1979, 2004). The Pan-Africa domains include two main types of mobile belts or orogenic that can be recognized. In general, two types of belts are similar but formed at different crustal levels associated with a collision and/or accretion systems. One type contains Neoproterozoic supracrustal, magmatic assemblages, and numerous juvenile (mantlederived) terranes. These belts are equivalent to upper to middle crustal levels and are comprised of ophiolites, subduction- or collision-related granitoids, island-arc or passive continental margin assemblages and exotic terranes, e.g., the Lufilian Arc and Damara-Kaoko-Gariep belt of south-central and south-western Africa, the TransSahara belt of West Africa, and the Arabian-Nubian Shield (ANS) of Arabia and north-east Africa, the West Congo belt of Angola and Congo Republic, the Rokelide and Mauretanian belts along the western part of the West African Craton. The other type is mobile belts containing polydeformed high-grade metamorphic rocks, corresponding to the middle to lower crust levels, which differ in the formation and 13.

(30) structural evolution (Kröner, 2004). This type is representative for the deeply eroded part of the collisional orogeny as it forms lower crustal levels of collisional systems. They are the Mozambique, Zambezi belt and Central African Orogenic Belt (CAOB) also called the Öubanguides belt.. 14.

(31) 1.4. Domains of the Saharan Metacraton in Chad The Saharan Metacraton is located in the north-central part of Africa and covers an area of ~5 million km2. It is enclosed by the Tuareg Shield to the west, the ArabianNubian Shield to the east, the Congo craton to the south, and the northern African continental margin in the south of Egypt and Libya (Figure 1.3). The boundaries of Saharan Metacraton are the result of suturing processes in lithospheric-scale that caused by Neoproterozoic collision events together with the surrounding terranes. The eastern boundary is considered to be the north-trending Keraf-Kabus-Sekerr Shear Zone which is an arc-continental suture that separates the Saharan Metacraton from the ANS (Abdelsalam and Dawoud, 1991; Stern, 1994; Abdelsalam et al., 1996). The north-trending Raghane Shear Zone has referred its boundary to the west, which is a collision zone between the Tuareg Shield and the Saharan Metacraton (Liégeois et al., 1994, 2003; Henry et al., 2009). This boundary is capped by Neoproterozoic juvenile terranes and ancient molasse sediments (Abdelsalam et al., 2002). To the south, the boundary is not well-defined but is considered to be the northern border of the Öubangides orogenic belt (CAOB) in the southwest and the Aswa Shear Zone further east. The east-trending Öubanguides Orogenic belt separates the Saharan Metacraton from the Congo Craton (Pin and Poidevin, 1987; Poidevin, 1994; Toteu et al., 2006a). To the north, the boundary of the Saharan Metacraton is located in the south of Egypt and Libya and covered by thick Phanerozoic sediments derived from the northern part of the African continent margin (Abdelsalam et al., 2002) (Figure 1.3). The Saharan Metacraton contains medium to high-grade metamorphic rocks including gneiss, metasediments, migmatites and granulites as well as rarer low-grade volcano15.

(32) sedimentary, which are intruded by Neoproterozoic granitic rocks during 750 to 550 period (Klerkx and Deutsch, 1977; Pin and Poidevin, 1987; Sultan et al., 1994; Stern et al., 1994; Abdelsalam et al., 2002; Shang et al., 2010). Isotopic and geochronological data of rocks in the Saharan Metacraton are variable in a wide range of 3100 to 500 Ma (Abdelsalam et al., 2002). Nevertheless, the Rb-Sr and U-Pb isotopic data report the main magmatic phase in the late Neoproterozoic occurred from 650 to 550 Ma (Abdelsalam et al., 2002, 2011; Shellnutt et al., 2017, 2018; Pham, 2018).. Figure 1.3. Geological sketch map of the Saharan Metacraton (after Küster et al. 2008).. 16.

(33) The Saharan Metacraton is exposed in Chad as the Tibesti, Ouaddaï, and Guéra Precambrian massifs.. 1.4.1. Tibesti massif The Tibesti massif covers the northernmost area of Chad and expanding into southern Libya with a small domain, and its areal extent is ~100,000 km2. This massif is one of six primary Precambrian crystalline complexes exposed in the north of Africa. All the volcanic units are now located in the extreme north of Chad, covering approximately 33% of massif’s total area, which refers to the Tibesti Volcanic Province (Permenter and Oppenheimer, 2007). The Tibesti massif is made up of intrusive and metamorphic rocks in the core that is enclosed by Paleozoic and younger sedimentary sequences, which are partly covered by Tertiary volcanic rocks (Ghuma and Rogers, 1978). Suayah (2006) considered that the Tibesti region was sitting in an ocean basin which had advanced during the late Neoproterozoic and closed during subduction underneath the continental crust to the west. The basement rocks of Tibesti massif are divided into two units by degree of metamorphism and distinguished by an unconformity overlain by a thick section of the conglomerate. The older unit is a 900 Ma high-grade metamorphic rocks that called Lower Tibestian (El-Makhrouf, 1988) and intruded by syn-orogenic granodiorites ranging between 600 and 550 Ma (Pegram et al., 1976; El-Makhrouf, 1988); the younger unit consists of low-grade metamorphic rocks that are referred to as Upper Tibestian (Suayah, 2006).. 17.

(34) 1.4.2. Ouaddaï massif The Precambrian Ouaddaï massif is the largest crystalline massif of Chad and spreads from Chad to the Darfur province of Sudan (Guiraud and Maurin, 1992; Schlüter, 2008). This massif comprises high-grade regionally metamorphosed rocks. The schists and gneisses are intruded by late-orogenic granites, which yielded ages of 590-570 Ma (Schlüter, 2008). This massif became a stable region after the Pan-Africa orogeny sequences, then a small amount of marine transgressive sequences deposited during the Paleozoic and Mesozoic (Guiraud and Maurin, 1992).. 1.4.3 Guéra massif The Guéra massif in South-Central Chad is primarily comprised of granitic rocks that are cross-cut by dolerite dykes (Nkouandou et al., 2017). The chief lithologic and structural characteristics are described by Isseini et al. (2013), and the geochronology and geochemistry are documented by Pham (2018). The granite intrusions were emplaced during and after the collision between Saharan Metacraton and Congo craton. Pham (2018) proposed that there are two sets of granites: (1) the oldest granites (≥590 Ma) are considered as collisional granites, (2) the younger granites (570 Ma to 560 Ma) are post-collisional granites. Furthermore, younger (~550 Ma) post-collisional granites are located ~100 km west of Lake Fitri, have similar geochemical features with the Guéra post-collisional granites, thus, they could be related to a third post-collisional period in south-central Chad, and signify an extension of the Guéra massif to the west (Shellnutt et al., 2017, 2018).. 18.

(35) 1.5. Domains of the CAOB in Chad The CAOB (Öubanguides Belt or Pan-African Belt in center of Africa continent) extends from the Atlantic coastline in Cameroon to southwest Sudan and is considered to be the border of the Saharan Metacraton to the south (Abdelsalam et al., 2002). The evolution of this belt is the consequence of the collision among the Saharan Metacraton, the West African craton, and the São Francisco-Congo craton (Castaing et al., 1994). This orogenic event preserves both Paleoproterozoic and Neoproterozoic juvenile accretion and represents one of the essential suturing orogens of Gondwana. Thus, its evolutional investigation provides essential information to reconstruct the Gondwana supercontinent (Pouclet et al., 2006; Penaye et al., 2006; Isseini, 2011). In Chad, the CAOB is exposed as the Mayo Kebbi massif and Yadé massif (Isseini et al., 2012).. 1.5.1. Mayo Kebbi massif The Mayo Kebbi massif is located in southwestern Chad, belonging to the northern portion of the Central African Pan-African Fold Belt and (Bessoles and Trompette, 1980; Toteu et al., 2004), surrounded by the Congo craton to the south, the West African craton in the west, and between Paleoproterozoic Adamawa-Yade domain and the Pan-African reworked Archean in the east. Recently, this massif is thought to have formed during the Neoproterozoic and was an active margin tectonic setting (Pouclet et al., 2006; Penaye et al., 2006; Isseini, 2012). The Mayo Kebbi massif consists of a metavolcanics-metasedimentary sequence intruded by Neoproterozoic calc-alkaline granitic suites which can be arranged into three distinct groups of rocks: (1) 19.

(36) metamorphic rocks derived from plutonic and volcanic rocks accompanied with metasediments and meta-volcaniclastics of the Zalbi and Goueyoudoun groups; (2) metamorphic rocks originate from mafic to intermediate volcanic-plutonic associations; (3) mafic and intermediate rocks intruded by Neoproterozoic granitic rocks (Kasser, 1995; Doumnang, 2006; Penaye et al., 2006; Pouclet et al., 2006). The Precambrian basement of this region is mostly overlaid by Cretaceous platform sediments (the Lame series) (Schneider and Wolff, 1992; Kasser, 1995; Penaye et al., 2006). Penaye et al. (2006) reported the major magmatic event recorded in this area varying between 737 Ma and 640 Ma. The Mayo Kebbi A-type granites belonging to the post-collisional Pan-Africa sequence were emplaced at ca. 567 Ma that probably represents the last stage of Pan-African orogeny in this domain (Isseini et al., 2012).. 1.5.2. Yadé massif The Yadé (Yaoundé) massif is very poorly studied of the basement massif of Chad but is an extension of the Adamawa-Yadé domain (AYD) in Cameroon (Toteu et al., 2001, 2004; Seguem et al., 2014). The AYD comprises: (1) Paleoproterozoic metasediments and orthogneisses bearing signatures of Archean crust recorded by the inherited zircon (Toteu et al., 2001; 2004); (2) Neoproterozoic metavolcanicsmetasedimentary rocks are formed under low- to medium-grade metamorphic conditions during Pan-African times (Toteu et al., 2006a); (3) commonly syn- to latetectonic granitoids which have typically transitional composition or crustal-derived origin (Toteu et al., 2001 and references therein; Njanko et al., 2006; Nzenti et al., 2006). The northern portion of this domain exposed Pan-African magmatic rocks 20.

(37) (660-585 Ma) that correspond to the emplacement of pre- and syn-tectonic calcalkaline granites as well as 585-540 Ma late-tectonic granites which intrude into intermediate to high-grade Neoproterozoic schists, metabases, and gneisses (Toteu et al., 2001; Dawaï et al., 2013).. 1.6. The purpose of the study At present, the geology of South-Central Chad is poorly constrained, and the only information that is known as measured by remote sensing, field observations (Isseni et al., 2013), petrography, and geochemistry studies from the Mongo area (Nkouandou et al., 2017). Investigations of the timing and petrogenesis for the main granite bodies of the Guéra massif were undertaken by Pham (2018) (see section 2.1). Additional studies on two granitic inliers to the west of the Guéra massif near Lake Fitri were completed by Shellnutt et al. (2018) (see section 2.2). These studies have partly interpreted the petrogenesis and geological tectonic setting of this area, hence, to reconstruct its formation and development. However, the tectonic evolution of SouthCentral Chad is still complicated and poorly constrained in part due to the uncertainty of the southern boundary of Saharan Metacraton. Mafic minerals (e.g., amphibole, biotite) are a fundamental component of granitic rocks. A majority of previous studies demonstrated that the composition of these minerals reflects both the magmatic conditions and the nature of parental magma during the emplacement of intrusive rocks (Zen, 1988; Abdel-Rahamen, 1994; Shabani et al., 2003). In general, composition of biotite is utilized (1) to track the oxygen fugacity of melt from which is crystallized (Wones and Eugster, 1965; 21.

(38) Burkhard, 1991, 1993) or (2) to discriminate the nature of geotectonic settings (Nachit et al., 1985; Abdel-Rahamen, 1994; Hecht, 1994; Shabani et al., 2003), or (3) to estimate crystallization temperature (Luhr et al., 1984) and (4) emplacement pressure of host rocks (Uchida et al., 2007) to constrain their formation and evolution. Similarly, amphibole also utilized as an indicator of magmtic conditions of granitic host rocks (Wones, 1981) due to the variation of amphibole composition with temperature, oxygen fugacity, pressure, and bulk composition. In this study, the mafic mineral chemistry of collisional and post-collisional granites from Guéra massif and Lake Fitri region carried out to: (1) describe the mineral-chemical evolution of the mafic minerals from Guéra mafic; (2) estimate prevailing magmatic conditions (temperature of crystallization and pressure, and oxidation state) through the magma crystallizing; (3) distinguish the collisional and the post-collisional granites from Guéra massif; (4) interpret the relationship of original formation of the Guéra and Lake Fitri post-collisional granites; (5) discuss the implications of the data on tectonomagmatic interpretation of studied plutons. Subsequently, the crustal evolution of Guéra massif is reconstructed with the expectation is that an understanding of the evolution of Guéra massif will be better constrained (i.e., crystallization conditions including depth of emplacement) and that more insight into the geology of south-central Chad will be gained.. 22.

(39) CHAPTER 2. GEOLOGICAL BACKGROUND Chad occupies an area of 1,284,000 km2 in north-central Africa. Generally, the geology of Chad is characterized by the exposure of Precambrian massifs such as Tibesti massif in the north, Ouaddaï massif in the east, Guéra massif in the southcentral part, Mayo Kebi, and Yadé massifs in the southern part and surrounded by younger sedimentary rocks. The sedimentary rocks include Lower Paleozoic sandstone sequences, Lower Cretaceous continental clastic rocks and Upper Cretaceous marine sediments in the north and northeast, Tertiary continental sediments in the south and the Neogene lacustrine sediments (known as Chad Formation) overlying mostly Chad basin (Figure 2.1).. 23.

(40) Figure 2.1. Geological overview of Chad (modified from Saleh, 1994) showing the study area.. 24.

(41) 2.1. Guéra massif The Guéra massif also known as massif Central Tchadien (Figure 2.2, Figure 2.4a) comprises magmatic and metamorphic rocks that are considered to be a part of the Pan-Africa orogeny (Black, 1992; Kusnir, 1995; Kusnir and Moutaye, 1997; Schlüter, 2008). This massif consists of four distinct groups of rocks including (1) gabbro, (2) hornblende-biotite granites (granite I), granodiorites and, diorites (3) two-mica granites (granite II), and (4) biotite granites (granite III). At least four deformation phases are identified in the Guéra massif (Isseini et al., 2013). The relative chronology of these rocks is distinguished based on field investigations, and remote sensing as gabbro, diorite, and granite are the main petrographic types. The hornblende-biotite granites (granite I) and granodiorites crystallized at the same time and are intruded by biotite granites (granite III); whereas the two micas granites (granite II) intrude the diorites. The rocks are concluded to be Neoproterozoic age and were subsequently buried under a sedimentary that predominately covered by Quaternary alluvium (Kusnir, 1995; Pham, 2018; Shellnutt et al., 2018). Structurally, four deformation phases are identified within the Guéra massif. The first and second deformation phases (D1 and D2) affected granite I and II by the preferential orientation of plagioclase and biotite crystals together with a compositional banding and a mineral lineation. The third deformation (D3) affected group II granites without the finegrained biotite granites; this phase is related to the development of brittle to ductile shear zones and was not affected by folding. The fourth deformation phase (D4) effected on all four groups of rocks and created several vertical brittle structures, including diaclases, faults, and dykes (Isseini et al., 2013). 25.

(42) Locally, there are many exposures of dolerite dykes in the Mongo area within Guéra massif that is oriented by major Pan-African NNE-SSW, NE-SW, and ENEWSW faults (Figure 2.3). They are described as vertical to sub-vertical mafic dyke occurring in several kilometers in length, 1.1 to 100 m in thickness and less than 3 m in width. The Mongo doleritic dyke swarm was injected into pre-existing fractures by basalt and basaltic trachyandesite magma which may record the signature of either the final stage of stabilization of ancient continental crust or the beginning of tectonic activity related to Pan-African mobile belts (Nkouandou et al., 2017). The differences in lithology facies and the complication in the historical evolution of Guéra massif were also observed by fieldwork; in particular, the biotite granites mainly expose in the northwestern domain, which is intruded by gabbro (Picture 2.1a) and commonly enclose the micro-granitic enclaves (Picture 2.1b) bearing deformation evidence (Picture 2.2). Meanwhile, biotite hornblende granites (Picture 2.4a), as well as hornblende granites and its grained equivalent (Picture 2.4b) distribute in the rest parts of the massif. As mentioned by the previous study, the petrographical investigation of granitic rocks from Guéra massif has shown that (1) the biotite granites are coarse-grained and principally comprise of plagioclase, potassium feldspar, biotite, quartz, and less amphibole. They perform an undulate extinction and the grid of fractures of the quartz and the progressively increasing in plagioclase proportion from 0 to 15% for the West to the East, indicating for the existence of deformation influence. (2) Similarly, the hornblende granites have the same felsic components, but amphibole is a mainly mafic mineral while biotite is an either absent or extremely low proportion. (3) The. 26.

(43) hornblende-biotite granites are charactered by the approximately same portion of hornblende and biotite in total volumetric of mineral assemblage in comparison to other groups. (4) The occurrence of hornblende-rich-rhyolite associated with hornblende granites in the northeastern part of the massif suggested a shallow granitic intrusion model for this subgroup of granites (Pham, 2018). In this study, a suite of twelve samples corresponding to biotite granite and hornblende-biotite granite which have geochronological, isotopic, and whole-rock chemistry is selected to investigate the biotite and amphibole compositions (Figure 2.2). Recently, the Guéra massif is considered to be built upon older continental crust or that the crust was overprinted by Neoproterozoic silicic magmatism. The granites from this massif are also known as collisional granites and post-collisional granites that were emplaced during and after the collision between the Saharan Metacraton and Congo Craton during the late stages of Pan-African orogeny (Shellnutt et al., 2017; Pham, 2018). The collisional granites are mostly biotite-granite, alkali calcic to calcalkalic, magnesian to slightly ferroan and metaluminous to peraluminous in compositions. They were emplaced at ~595-590 Ma and are related to a transitional subduction setting. In contrast, the post-collisional granites include biotite-granite and hornblende-biotite-granite, peraluminous to metaluminous, alkali calcic to calcalkalic, high K and ferroan in compositions. The ferroan rocks were emplaced after collisions at ~570 Ma and ~560 Ma (Pham, 2018). Therefore, the geological history of the Guéra massif, as well as south-central Chad, is only partially known. This study focuses on the biotite and amphibole mineral chemistry from granites of the Guéra massif along with biotite chemistry of six samples from the Lake Fitri inliers to. 27.

(44) constrain their emplacement conditions (Pham, 2018; Shellnutt et al., 2018). The new data are used to constrain the evolution of magmatism as it changes from a collisional setting to a post-collisional regime. Moreover, the results provide further constraints on the nature of crustal building processes within Central African Orogenic Belts and the Saharan Metacraton.. Figure 2.2. The simplified geological sketch map of Guéra massif and sample locations in this study (modified from Pham, 2018).. 28.

(45) Figure 2.3. The simplified geological sketch map of Mongo area showing doleritic dykes within Guéra massif (after Nkouandou et al., 2017).. Picture 2.1. (a) Biotite granite intruded by gabbro; (b) biotite granite including micro-granitic enclaves.. 29.

(46) Picture 2.2. Biotite granite recorded deformation evidence.. Picture 2.3. An outcrop of (a) hornblende-biotite granite and (b) rhyolite within Guéra massif.. 30.

(47) 2.2. Lake Fitri region Lake Fitri inliers were emplaced farther to the northwest of the Guéra massif are composed of fresh and coarse-grained biotite-bearing granites with some pegmatic patches and fine-grained biotite-poor granites (Picture 2.4). They are exposed as lenticular to ellipsoidal granitic hills up to several kilometers in length and less than 1 km in wide near or between the communities of Ngoura and Moyto (Figure 2.4) (Shellnutt et al., 2018). Generally, the Lake Fitri granites have similar texture and composition in terms of mineralogy (biotite, quartz, K-feldspar), whole-rock chemistry (peraluminous to metaluminous, alkali calcic to calc-alkalic, high potassium and ferroan) and Nd isotopes. Moreover, they have a similar composition as the postcollisional granites of Guéra massif but are slightly younger (~545 Ma and ~554 Ma for Moyto and Ngoura granite, respectively). Therefore, they could indicate a third period of post-collisional magmatism in south-central Chad that was related to the Congo Craton and Saharan Metacraton collision (Shellnutt et al., 2017).. Picture 2.4. An outcrop of biotite granite in (a) Ngoura and (b) Moyto domain.. 31.

(48) Figure 2.4. (a) Simplified geological map of North-Central Africa, showing the location of this study area (modified from Abdelsalam et al., 2002). The sampling localities in Lake Fitri region (b, c after Shellnutt et al., 2018).. 32.

(49) 2.3. Phanerozoic sedimentary basins in Chad The major sedimentary basins in Chad contain coeval rift basins in the West and Central African Rift System (WCARS) that formed during the early Cretaceous to the early Tertiary and include: Bongor, Doba, Doseo and Salamat basins in the south; the Lake Chad basin in the west and the Erdis basin (also known as the Al Kufra basin in Lybia) in the north that also expands into Libya (Figure 2.5) (Genik, 1992, 1993). Their formation is closely related to the Pan-African crustal consolidation in centralnorthwestern Africa, this tectonic event produced faults and fractures that influenced the orientation of these Cretaceous-Tertiary rifts in Chad (Black and Girod, 1970; Benkhelil, 1988; Daly et al., 1989).. 33.

(50) Figure 2.5. a. The location of Chad in Africa (Google Earth, 2018). b. Simplified map showing the location of Phanerozoic basins in Chad (modified from Genik, 1993).. 34.

(51) CHAPTER 3. PETROGRAPHY In this study, 12 thin sections that correspond to the collisional granite (14ZA02, 06, -25C, -12D, 16A) and post-collisional granite (14ZA10, -18, -19B, -01, -20B, 21B, -23) of the Guéra massif were examined using a Carl Zeiss Axioplan 7082 Polarizing Optical Microscope. Their typical mineral assemblage consists of plagioclase, K-feldspar, biotite, quartz, and amphibole, which are identified by the mineral properties under the microscope.. 3.1. Post-collisional granite 14ZA10 Sample 14ZA10 is coarse-grained biotite-granite that is comprised of plagioclase, K-feldspar, biotite, and quartz with accessory amounts of opaques and muscovite (<2 vol. %). This sample is unique amongst the post-collisional granites as it does not have amphibole. Plagioclase (8-10 vol. %) mostly forms subhedral or prismatic crystals (0.4-0.6mm) in the habit, showing parallel extinction as albite twinning. The plagioclase displays dark grey interference color of the first order. Euhedral to subhedral biotite (10-20 vol. %) indicates parallel or extremely low-inclined extinction, flaky, and one set of good cleavage with brown to yellowish interference color. Some interstitial anhedral biotites are enclosed within plagioclase or K-feldspar are also observed. K-feldspar is euhedral to subhedral, prismatic crystals ranging from 0.6 to 1 mm in grain size (45-50 vol. %) that have low pleochroism. They exhibit a poikiolitic texture. Subhedral quartz (25-30 vol. %) is randomly oriented with grey to dark grey or pale brown interference color of the first order. This sample is generally 35.

(52) fresh, but some of the quartz and K-feldspar are cloudy. The texture is characteristic of coarse grain size and the occurrence of antiperthite (in which microcline forms first as a host while plagioclase is internally formed).. Picture 3.1. The thin section photograph of representative crystals characteristic for the sample 14ZA10. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Apt: anti-perthite; Mus: muscovite; Myr: myrmerkite.. 36.

(53) 14ZA18 Sample 14ZA18 comprises plagioclase (8-10 vol. %), biotite ( 5-10 vol. %), Kfeldspar (35-40 vol. %), quartz (30-35 vol. %) and hornblende (2-4 vol. %), and opaques (<1 vol. %). Plagioclase is one of the main constituting minerals in this sample. The euhedral plagioclase (>1.5mm) displays parallel extinction, polysynthetic twinning with low pleochroism from grey to black color and sharp boundary. Biotite is euhedral to subhedral, highly pleochroic, has third-order interference color (reddishbrown to brown and yellow-brown to dark brown), flaky, and perfect cleavage, and parallel extinction. The reddish-brown to brown biotite crystals are larger than the tanbrown to dark brown crystals in grain size, and their typical color is probably due to the high content of Ti regardless of Fe content (Hall, 1941); while the tan-brown to dark brown biotites contain quartz inclusions. K-feldspar is euhedral (>1mm), with the characteristic of Carlsbad twinning and exsolution lamellae showing dark grey blackish color. There are mineral inclusions within K-feldspar of quartz and biotite. Hornblende is typically subhedral in shape with two sets of good cleavage having an angle of nearly 120o to 60o. They are observed as perfect pleochroism (pale green), high birefringence (purple to dark brown). Quartz is yellow-grey, grey and dark grey of the first order of interference color, varying in grain size (coarse to fine) in shape of subhedral and rounded. The quartz has low relief, absence of cleavage planes, good oscillatory/wavy extinction. The main alteration is hornblende converting to biotite whereas some feldspar altered to clay minerals.. 37.

(54) Picture 3.2. Some representative crystals for main mineral assemblage of the sample 14ZA18. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende. 38.

(55) 14ZA19B Sample 14ZA19B comprises euhedral plagioclase (8-10 vol. %), euhedral to subhedral biotite (2-3 vol. %), euhedral to subhedral K-feldspar (45-50 vol. %), anhedral quartz (25-30 vol. %), and subhedral to euhedral amphibole (4-6 vol. %), with minor opaque (<1 vol. %). Plagioclase is variable in size from 0.5 to 2.25 mm, having low birefringent (grey to dark-grey of the first order of interference color). It occasionally encloses hornblende. Biotite has mostly third-order brown interference color, good cleavage, and appears to be weakly foliated. It sometimes occurs as minute inclusions trapped in interstitial grain position. K-feldspar commonly occurs as perthite (subsolidus exsolution) with the first order black-grey interference colors, a poikilitic relation with biotite, quartz, and probably hornblende. Quartz exhibits a fine to medium-grained size in euhedral crystal habit, low relief, and wavy extinction. It also occurs as megacrystal mymerkite (intergrowth of quartz in plagioclase). Amphibole (hornblende) has deep-green, and red-brown pleochroism, high relief, and two sets of cleavage. It is commonly euhedral to subhedral crystal, associated with biotite, ranges from fine to coarse-grained size and contains mineral inclusions of biotite. Locally, some amphibole crystals embay the plagioclase. Fractured crystals such as quartz or feldspar are more abundant than other samples in the same group. Some feldspar altered to clay minerals.. 39.

(56) Picture 3.3. The thin section photograph of main mineral assemblage typical for the sample 14ZA19B (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Pt: perthite; Myr: myrmerkite; Hbl: hornblende.. 40.

(57) 14ZA20B Sample 14ZA20B is coarse-grained and comprises subhedral to euhedral plagioclase (8-10 vol. %), subhedral to euhedral biotite (5-7 vol. %), subhedral to euhedral K-feldspar (30-35 vol. %), anhedral quartz (20-25 vol. %), subhedral to euhedral amphibole (15-20 vol. %) and accessory amounts of opaque minerals (1-3 vol. %). Plagioclase crystals have albite and Carlsbad twinning and display parallel extinction. It is commonly intergrowth with K-feldspar to create the mesoperthitic texture. Greenish and reddish-brown biotite exist both primary magmatic crystal and as inclusions. It is generally associated with amphibole, showing perfect cleavage, parallel to very low extinction. K-feldspar is the dominant mafic mineral, medium to coarse-grained size with grey to dark grey interference color of the first order, and has inclusions of biotite and quartz. The feldspars have microcline twinning, but there are crystals with perthite exsolution. Quartz is variable in grain size, showing grey-yellow and dark-grey in interference color, and low relief. It is often interstitial to other minerals or occurs as an inclusion. Fractured crystal of quartz also observed in the thin section. The amphiboles are strongly pleochroic and medium to coarse-grained. The minerals show the characteristic high relief and two sets of cleavage with an angle of approximately 120o to 60o are visible under the microscope. Some feldspar crystals altered to clay minerals.. 41.

(58) Picture 3.4. The subhedral crystals of the main minerals observed in the sample of 14ZA20B. (+): crossed polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende.. 42.

(59) 14ZA21B Sample 14ZA21B comprises plagioclase (10-15 vol. %), biotite (2-5 vol. %), Kfeldspar (25-30 vol. %), quartz (35-40 vol. %), amphibole (5-7 vol. %) and accessory amounts of zircon and opaques (1-3 vol. %). The plagioclase crystals are subhedral to euhedral and display albite twinning. Intergrowth between plagioclase and K-feldspar is often observed. The amount of biotite in this sample is lower in comparison to other post-collisional granites, more anhedral to subhedral, and mainly is interstitial to other minerals. However, it can still be identified based on its pleochroism (light to dark brown), parallel extinction, massive. K-feldspar is subhedral to euhedral, mediumgrained size (0.75-1mm). The characteristic perthitic texture is frequently visible. Fine- to medium-grained quartz is observed in the thin section with anhedral shapes, and low relief. Some coarse crystals are also present in a cluster or showing a poikilitic relation to other crystals. Inclusion of quartz is occasionally found. Most of the amphibole crystals are subhedral, fine- to medium-grained size, high relief, two sets of good cleavage, deep-green to deep red-brown pleochroism, and show a poikilitic texture. Accordingly, such amphiboles are likely hornblende. In some cases, the deep-green hornblendes are breaking down to oxide minerals that have a redbrown color, due to the change in the oxidation state of iron from Fe2+ to Fe3+. Murphy et al. (2000) considered that is opacitization can proceed via hornblende oxidation during the growth of an extrusion dome, but, the possibility that alteration was caused by the weathering cannot be ruled out.. 43.

(60) Picture 3.5. Microscope thin section photograph of the sample 14ZA21B, showing the euhedral to subhedral crystals of K-feldspar, plagioclase, quartz, and hornblende. (+): crossed polarized light; () plane polarized light. Qz: quartz; Pl: plagioclase; Bt: biotite; Kfs: potassium feldspar; Hbl: hornblende.. 44.