Geochronology and geochemical characteristics of Precambrian granite in the Main Islands, Republic of Seychelles

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(1)NATIONAL TAIWAN NORMAL UNIVERSITY DEPARTMENT OF EARTH SCIENCES. MSc. Thesis. GEOCHRONOLOGY AND GEOCHEMICAL CHARACTERISTICS OF PRECAMBRIAN GRANITE IN THE MAIN ISLANDS, REPUBLIC OF SEYCHELLES. Student Supervisor. : Nguyen Thi Dieu : Prof. John Gregory Shellnutt. January 2019.

(2) ACKNOWLEDGEMENT. The thesis has been completed at Department of Earth Sciences, National Taiwan Normal University under the guidance by Prof. John Gregory Shellnutt. I would like to express my sincere gratitude to my advisor for his patience and continuous support for my study. To individuals and organizations that supplied the instruments as well as assistance in analyzing samples for this project. I would like to thank Prof. Sun-Lin Chung and members of the NTU geochronology laboratory; Prof. Yoshiyuki Iizuka and Yu-Hsiang Wang for their help with CL image processing; Prof. Kuo-Long Wang and Wen-Yu Hsu for their help with the SrNd isotopic analysis; Prof. Yu-Ming Lai and Wen-Yu Hsia for thin section preparation; Prof. Mei-Fei Chu, Dr. Hao-Yang Lee and Fu-Long Lin for trace elements analysis. Additionally, I am grateful to teachers, staff members at Department of Earth Sciences, National Taiwan Normal University for their help, insightful comments and suggestions. I thank my labmates (Carol, Thuy, Alice, Ha, Chi and Andy) for their encouragement. Lastly, I would like to thank my family for encouraging and supporting me spiritually throughout my life. Nguyen Thi Dieu. ii.

(3) Table of contents ACKNOWLEDGEMENT .............................................................................................ii List of figures ................................................................................................................ vi List of pictures .............................................................................................................. ix List of tables ................................................................................................................... x Abbreviations ................................................................................................................ xi Abstract .......................................................................................................................... 1 CHAPTER 1. INTRODUCTION .................................................................................. 3 1.1. Continental crust ................................................................................................. 3 1.2. Super continental cycles ..................................................................................... 4 1.3. Relationship between Rodinia and Gondwana ................................................... 5 1.4. East African Orogen ........................................................................................... 8 1.5. The Seychelles microcontinent ......................................................................... 12 1.6. Purpose of the study ......................................................................................... 14 CHAPTER 2. GEOLOGICAL BACKGROUND ....................................................... 18 2.1. Mahé group island ............................................................................................ 19 2.2. The Praslin - La Digue island group ................................................................ 22 2.3. Sampling ........................................................................................................... 24 CHAPTER 3. PETROGRAPHY ................................................................................. 28 3.1. Mahé granites ................................................................................................... 28 3.2. Praslin granites ................................................................................................. 31 CHAPTER 4. METHODS ........................................................................................... 36 iii.

(4) 4.1. Zircon U-Pb dating by LA-ICPMS .................................................................. 36 4.1.1. Zircon preparation ...................................................................................... 36 4.1.2. Principles of zircon U-Pb geochronology .................................................. 37 4.1.3. Procedure of U-Pb geochronology............................................................. 38 4.2. Whole rock major elements by XRF ................................................................ 39 4.2.1. Principles of X-ray Fluorescence spectrometry ......................................... 39 4.2.2. Procedure ................................................................................................... 39 4.3. Whole rock trace elements by ICP-MS ............................................................ 41 4.3.1. Principles of ICP-MS ................................................................................. 41 4.3.2. Procedure ................................................................................................... 42 4.4. Whole rock Sr-Nd isotopes by TIMS ............................................................... 43 4.4.1. Principles of TIMS operation..................................................................... 43 4.4.2. Procedure ................................................................................................... 43 CHAPTER 5. RESULTS ............................................................................................. 46 5.1. Zircon geochronology ...................................................................................... 46 5.1.1. Zircons morphology ................................................................................... 46 5.1.2. Zircon U-Pb ages ....................................................................................... 49 5.2. Geochemistry .................................................................................................... 54 5.2.1. Major elements geochemistry .................................................................... 54 5.2.2. Trace element geochemistry ...................................................................... 58 5.2.3. Sr-Nd Isotopes ........................................................................................... 61. iv.

(5) CHAPTER 6. DISCUSSION ....................................................................................... 64 6.1. Geochronology of the Seychelles microcontinent ............................................ 64 6.1.1. Age of emplacement .................................................................................. 64 6.1.2. Age of inheritance ...................................................................................... 65 6.1.3. Regional temporal correlation.................................................................... 67 6.2. Petrogenesis ...................................................................................................... 68 6.3. Tectonic setting of the Seychelles microcontinent ........................................... 70 6.3.1. Tectonic setting .......................................................................................... 70 6.3.2. Regional tectonic correlation ..................................................................... 74 6.4. Tectonomagmatic evolution of the Seychelles microcontinent ....................... 76 CONCLUSIONS.......................................................................................................... 78 REFERENCES ............................................................................................................ 79 Appendix A .................................................................................................................. 92 Appendix B .................................................................................................................. 99. v.

(6) List of figures Figure 1.1. Reconstruction of Rodinia supercontinent by ca. 1000 Ma .................................... 5 Figure 1.2. Peri-Gondwanan terranes at ∼570 Ma .................................................................... 7 Figure 1.3. The reconstructed Gondwana supercontinent and East African Orogen................. 9 Figure 1.4. Simplified geological map of Madagascar ............................................................ 10 Figure 1.5. Simplified geological map of India ....................................................................... 11 Figure 1.6. Regional topography of Mascarene plateau and the western Indian Ocean, eastern Africa, Madagascar ...................................................................................................... 13 Figure 1.7. Simplified map of the Seychelles microcontinent ................................................. 13 Figure 1.8. Palaeogeographic reconstruction of Greater India, the Seychelles microcontinent and Madagascar ........................................................................................................... 15 Figure 1.9. Palaeomagnetic reconstruction of the Seychelles 16 Figure 2.1. Simplified geological map of Mahé islands .......................................................... 19 Figure 2.2. Simplifed geological map of Praslin, La Digue group .......................................... 23 Figure 3.1. Microscope thin section photograph of sample MH-01 ........................................ 29 Figure 3.2. Microscope thin section photograph of sample ..................................................... 30 Figure 3.3. Microscope thin section photograph of sample MH-02 ........................................ 31 Figure 3.4. Microscope thin section photograph of sample PL-02 .......................................... 32 Figure 3.5. Microscope thin section photograph of PL-01 and PL-02 .................................... 32 Figure 3.6. Microscope thin section photograph of sample PL-03 .......................................... 33 Figure 3.7. Microscope thin section photograph of sample, from left to right corresponding to photo taken under PPL and CPL: (a),(b), (e), (f) PL-01; (c),(d) PL-03; (g),(h) PL-02 35 Figure 4.1. Composite representative image of zircon row: secondary electron (top) and cathodoluminescence images (bottom) of zircon crystals. .......................................... 36 Figure 4.2. Weighted mean U-Pb ages of 91500 and Plešovice standard ............................... 39. vi.

(7) Figure 4.3. Chondrite normalized plots of 4 USGS standard.) ................................................ 42 Figure 4.4. Column chemistry separation for Sr and Nd isotopic analysis .............................. 44 Figure 5.1. The CL images of several selected zircons of studied samples............................. 48 Figure 5.2. Concordia plots of zircon LA-ICPMS data for the studied samples ..................... 50 Figure 5.3. Inherited ages from (a) Mahé and (b) Praslin zircons ........................................... 53 Figure 5.4. Plots of all inheritance zircon ages from Seychelles granite ................................. 53 Figure 5.5. Haker diagram showing variations of major element oxides with silica (wt.%) ... 55 Figure 5.6. Diagram of Fe* showing the boundary between ferroan and magnesian ............ 57 Figure 5.7. Diagram of molar Al/(Na+K) vs Al/(Ca+Na+K) showing metaluminous to peraluminous character of Seychelles granite.............................................................. 57 Figure 5.8. Plot of Na2O + K2O-CaO versus SiO2 (wt.%) variation showing the ranges for the alkalic, alkali calcic, calc-alkalic ................................................................................. 58 Figure 5.9. Harker diagram showing variation of race elements (in ppm) with silica content (wt.%) for the Seychelles granite ................................................................................. 59 Figure 5.10. Chondrite-normalized REE plots for the Seychelles granite. .............................. 60 Figure 5.11. Primitive mantle normalized multi-element diagram showing trace element concentrations of the Seychelles granite ...................................................................... 60 Figure 5.12. Sm-Nd and Rb-Sr isochron diagram for Seychelles granite ............................... 63 Figure 6.1. Diagram of all available U-Pb zircon ages for Seychelles granitoids, including data from this study and Tucker et al. (2001). 65. Figure 6.2. Relative probability of ~750 Ma zircons and inherited zircons ............................ 66 Figure 6.3. εNd(t) and ISr plot for Precambrian rock from the Seychelles .............................. 69 Figure 6.4. Tectonic discrimination diagrams of Maniar and Piccoli (1989) for the Seychelles rocks with fields of granitoids of IAG: island arcs, CAG: continental arcs, CCG:. vii.

(8) continental-collision settings, POG: post-orogenic, RRG: rift-related, CEUG: continental epiorogenic uplifts ..................................................................................... 71 Figure 6.5. The R1-R2 multicationic diagram, in which R1=4Si -11(Na+K) - 2(Fe+Ti); R2= 6Ca+2Mg+Al ............................................................................................................... 72 Figure 6.6. Tectonic discrimination diagram for the Seychelles granite ................................. 73 Figure 6.7. Representative triangular plots for distinguishing between A1 and A2 granitoids ...................................................................................................................................... 73 Figure 6.8. Conceptual tectonic model of transition from compression to post-collision in the Seychelles .................................................................................................................... 77. viii.

(9) List of pictures Picture 2.1. The image show samples location in northern coast of Mahé island ................... 25 Picture 2.2. The image show samples location in southwest of Praslin island ........................ 26 Picture 2.3. Rounded mafic enclaves, sample MH-02, within grey granite boulder ............... 26 Picture 2.4. Sample PL-01, show distinctive moderate-grained pink granite .......................... 26 Picture 2.5. Sample MH-01 showing dominant of grey color ................................................. 27 Picture 2.6. Pink granite, sample PL-01, with transparent quartz and dark mineral ............... 27. ix.

(10) List of tables Table 2.1. GPS coordinates of studied samples ........................................................... 24 Table 4.1. The major elements analyzing result of the standard ................................. 39 Table 5.1. Major element geochemistry from the Seychelles microcontinent ............ 54 Table 5.2. Sr and Nd isotopic data of the Seychelles samples ..................................... 60. x.

(11) Abbreviations. Am : Amphibole. ANS: Arabian-Nubian Shield. Ap : Apatite. EAO: East African Orogen. Bt. HFSE: high field strength elements. : Biotite. Fig. : Figure. LA-ICPMS: Laser Ablation Inductively Coupled Plasma. FTO: Fe-Ti oxides. Mass Spectrometer. Hbl : Hornblende. LILE(s): large ion lithophile element(s). i.e. : example. MIS: Malani Igneous Suite. K-fs: Potassium feldspar. REE(s): rare earth element(s). Mc : Microcline. TIMS: Thermal Ionization Mass Spectrometer. Pl : Plagioclase. XRF: X-ray Fluorescence. Qz : Quartz. CL: Cathodoluminescence. Zrn : Zircon. xi.

(12) Abstract The Seychelles microcontinent located in western Indian Ocean consists of a large number of granitic islands. The oldest known rocks of the Seychelles microcontinent are Neoproterozoic and comprise the Main Islands (Mahé and Praslin group). Previous studies demonstrated that the emplacement age of the Neoproterozoic granites is ~750 Ma although the age of Île aux Récifs is ~50 million years older (808.8±1.9 Ma). New weighted-mean zircon 238. U/206Pb ages for rocks collected from northern Mahé (750.6±4.1 Ma, 756.9±4.5 Ma,. 756.4±5.1 Ma) and western Praslin (761±12 Ma, 754.2±6.9 Ma, 753.8±4.9 Ma) are reported in this study. The new radioisotopic ages overlap with previous results but a significant number of inherited zircons were identified that fall within two major groups: ~790 Ma to ~820 Ma and ~850 Ma to 880 Ma. Geochemically, the Seychelles granites are characterized by high SiO2 and are metaluminous to peraluminous, alkali-calcic to calc-alkalic, and ferroan. The negative correlation of SiO2 with Al2O3, Fe2O3, CaO, TiO2, MgO and P2O5 indicates that the rock experienced crystal fractionation either during or after emplacement. They also show the enrichment in light rare earth elements compared to heavy rare earth elements with distinct negative Eu anomalies (0.1-0.84). Mahé samples illustrate positive εNd(t) = +1.85 to +2.83, and ISr values from 0.7022 to 0.7031 while those of Praslin shows negative εNd(t) = -1.52 to 1.29 and extremely low ISr values. Consequently, it appears that the Mahé and Praslin granites are derived from different sources. Tectonomagmatic discrimination of the granites is compatible with within-plate or post-orogenic environment. The ~50 million year gap between the oldest and youngest rocks, and their compositional (within-plate vs. volcanic-arc) dichotomy are difficult to reconcile. It is likely that Neoproterozoic granites of the Seychelles microcontinent were not associated with a volcanic-arc but rather related to post-collision extension or rifting. Furthermore, it is possible that the age of Île aux Récifs may be indicative of inherited zircon and not the emplacement age. 1.

(13) Keywords:. Seychelles,. ferroan. granite,. geochronology. 2. post-collisonal. extension,. zircon.

(14) CHAPTER 1. INTRODUCTION. 1.1. Continental crust Continental crust is a unique feature of the Earth and together with oceanic crust makes up the outermost layer of the planet. Its density ranges from 2.7-2.9g/cm3 and extends from the surface to the Mohorovicic discontinuity which marks the crust–mantle boundary (Rudnick and Gao, 2003). It covers ~41% of the Earth surface area and constitutes ~70% of Earth’s crustal volume (Cogley, 1984). The continental crust thickness varies between 20 and 70 km with averaging of 35-40 km. The lateral extent of the continent crust is marked by continental shelf, of which 31% is submerged beneath the oceans (Cogley, 1984; Mooney et al., 1998). The continental crust is ancient as it preserves a record of the Earth's physical and chemical evolution. Present-day continental crust mostly formed at the end of the Archean and the beginning of the Proterozoic, between 3.2 to 2.0 Ga (Taylor and McLennan, 1995). The oldest rock within the continental crust is the 4.0 Ga Acasta Gneisses, Slave Province of NW Canada (Taylor and McLennan, 1995). The oldest detrital zircons are found in the Jack Hills area in western Australia and reported to be 4.4 billion years old (Wilde et al., 2001). In contrast, the oldest oceanic crust is ~200 Ma (Cloos, 1993). The structure of the continental crust consists of an upper layer composed mainly of sedimentary rock and granite to granodiorite, a middle layer of amphibolite facies and granulite facies, and a lower layered consisting of granulitefacies country (Rudnick and Fountain, 1995; Wedepohl, 1995). Continental crust formation is a continuous process with at least 80% produced at a convergent margin setting (Barth et al., 2000; Cawood, 2013). The formation of continental crust is more or less related to convection within the mantle that is caused by hot mantle material rising upwards, cooling, and followed by the subduction of cooler oceanic crust back into the mantle (Plank and Langmuir, 1998).. 3.

(15) 1.2. Super continental cycles Generally, the crust of the Earth is comparable to an egg shell that has been repeatedly cracked where the cracks represent plate boundaries. The majority of crust recycling occurs at plate boundaries as well as the formation of new crust. Plate interactions occur in three principal ways: 1) collision (convergent), 2) transform, and 3) spreading (divergent). The current paradigm of plate tectonics is the modern version of continental drift theory (Wegener, 1912) that suggests the continents have moved to their current positions. The significant concept derived from plate tectonics theory is that continents have grown through time and have experienced a cycle or multiple cycles of assembly and break-up (Umbgrove, 1940). A supercontinent is regarded as an inevitable consequence of plate tectonics, and is defined as the assembly of most or all of Earth's continental blocks or cratons to form a single large landmass (Rogers and Santosh, 2004). Sutton (1963) was amongst the first to suggest there were multiple (four) supercontinent cycles during Earth’s history. One supercontinent cycle is said to take 300 to 500 million years (Worsley et al., 1985). Currently, it is thought there were five pre-Pangean supercontinents at supercontinents at ca. 2.6 Ga, 2.0 Ga, 1.8 Ga to 1.6 Ga and 1.1 Ga that correspond to the amalgamation of Sclavia and Superia (Kenorland), Columbia (or Nuna), Rodinia and the later fifth, Pangaea (~600 Ma) which broke up into Laurasia to become North America and Eurasia and Gondwana (Suess, 1885). The geological evidence is more compelling for the younger supercontinents (Pangaea and Gondwana) as the rock record is better preserved compared to older lithologies. Consequently, older supercontinents are less constrained than the younger supercontinents.. 4.

(16) 1.3. Relationship between Rodinia and Gondwana Rodinia supercontinent The name “Rodinia” (Fig. 1.1) was first used in McMenamin and McMenamin (1990). Rodinia is a Mesoproterozoic supercontinent formed after the breakup of an older supercontinent, i.e. Columbia (Zhao et al., 2002) and was surrounded by the superocean called Mirovic Ocean. The evidence for the existence of Rodinia was found during the 1970s by the discovery of orogenic events from 1.23 Ga to 1.0 Ga that exist on virtually all cratons (i.e. the Grenville orogeny in America, the Dalslandian orogeny in Europe; Dewey and Burke, 1973). Since then, based on the orogens correlation on different cratons, a number of configurations have been proposed.. Figure 1.1. Reconstruction of Rodinia supercontinent by ca. 1000 Ma [modified after Meert and Torsvik, 2003)]. In 2009, International Geological Correlation Project 440 named “Rodinia Assembly and Breakup” recognized that from 825 to 550 Ma, Rodinia broke up in four different stages to form Gondwana. They are:. 5.

(17) . 825 to 800 Ma: correlative evidence (crustal arching, coeval magmatism, and. accumulation of thick rift-type sedimentary) of a super plume event recorded in the South Australia, South China, Tarim, Kalahari, India, and the Arabian - Nubian cratons. . 800 to 750 Ma: rifting proceeded within the same cratons and spread to Laurentia and. Siberia; . 750 to 700 Ma: a new pulse of magmatism and rifting continued along most margins of. Laurentia; . 650 to 550 Ma: the Pan-African orogenies, opening of the Iapetus Ocean, and closure. of the Mozambique ocean all resulted in the formation of Gondwana. The break-up of Rodinia led to the formation of smaller continental blocks that eventually amalgamated into Gondwana (Meredith et al., 2016). Most of evidence for this plate reconfiguration is found within the African and neighbouring (South America, Antarctica, India and Australia) cratonic domains as a number of orogenic belts developed from ~750 Ma until ~500 Ma. Of particular note is the north-south trending East African Orogen (EAO) that formed as a consequence of closure of the Mozambique Ocean collision with India (Collins and Pisarevsky, 2005; Fritz et al., 2013).. Gondwana supercontinent Gondwana (Fig. 1.2), named by Suess (1885), was a supercontinent that fully assembled during the Neoproterozoic (~550 Ma) and the first stage of its breakup began during Carboniferous (~320 Ma). The remnants of Gondwana make up about two-thirds of today's continental area, including Africa, Antarctica, South America, Australia, and India. The formation of Gondwana was initiated from 800 to 650 Ma and completed by 600 to 530 Ma with the collision of India with eastern Africa and South America with western Africa (Meert and Van Der Voo, 1997). The break-up of Gondwana occurred in a number of stages. During 6.

(18) Mesozoic, supercontinent Pangaea started to break-up and separation of Gondwana into different individual continents (Australia, Antarctica, Africa, Madagascar, South America and India). Beginning in the middle Jurassic, around 167 Ma, recorded the separation of Gondwana into East Gondwana (comprising Antarctica, Madagascar, India and Australia) and West Gondwana (comprising Africa and South America). The next stage, Africa separated from South America to form the South Atlantic and India move northward from Antarctica. Between 95 and 84 Ma, rifting separated India from Madagascar. Subsequently, oceanic crust began to form within the Mascarene basin causing a rotation of the Seychelles/India land mass. This continued until ca. 66 Ma when formation of the currently active Carlsberg Ridge severed the Seychelles from India (Plummer and Belle, 1995; Torsvik et al., 1998). At that time, India collided with Asia, resulting in the formation of the Himalayas and Antarctica separated from Australia.. Figure 1.2. Peri-Gondwanan terranes at ∼570 Ma [modified from Linnemann et al. (2007)] (Mad: Madagascar, Sey: Seychelles). 7.

(19) 1.4. East African Orogen The Pan-African Orogeny (sensu lato) formed by collisional events resulting from the convergence of lithospheric microplates created by the break-up of Rodinia supercontinent. The term ‘Pan-African’ is widely used to denote to the principal part of the global orogeny that led to the formation of the supercontinent Gondwana (Kennedy, 1964). There are two types of orogens within the associated with the Pan-African belts. The first type consists of Neoproterozoic supracrustal and magmatic assemblages, which have mantle-derived origin, and contain ophiolites as a diagnostic feature. Such belts include the Arabian-Nubian Shield (ANS) of Arabia and NE Africa. This type exposes rocks from the upper to middle crust. The second type of mobile belt generally contains polydeformed high-grade metamorphic assemblages, which expose rocks from the middle to lower crust. The assemblages consist of Mesoproterozoic to Archaean continental crust that was strongly reworked during Neoproterozoic. One of the best examples of the high-grade belts is the Mozambique Belt of East Africa. Together, the thin-skinned ANS and the thick-skinned high-grade Mozambique Belt comprise the EAO (Stern, 1994; Stern, 2002). This orogen is about 6,000 km long, stretches from ANS through Tanzania and northern-central Madagascar into the southern India (Dharwar craton), Sri Lanka and eastern Antarctica (Fig. 1.3). The EAO is made up of deformed and metamorphosed crust from Neoproterozoic to Cambrian. The tectonic evolution of the EAO started with rifting and break-up at ~900-850 Ma. Rifting is followed by seafloor spreading (870 to 690 Ma) that led to arc and back-arc basin formation. It is followed by continent-continent collision from 630 to 600 Ma and ended by further crustal shortening (orogenic collapse, extension) that will eventually be exploited during the Mesozoic break-up of Gondwana (Stern, 1994).. 8.

(20) Arabian - Nubian Shield The ANS is an exposure of Precambrian rocks and the largest section of predominantly juvenile Neoproterozoic crust in Africa (Kröner and Stern, 2004). The evolution history of the shield is intimately linked with two supercontinent cycles, the amalgamation of Gondwana, results from the fragmentation of Rodinia and the closure of the Mozambique Ocean. The ANS was fully assembled by 550 Ma after undergoing ~300 Ma growth that included terrane suturing and island arc convergence starting at 780 Ma.. Figure 1.3. The reconstructed Gondwana supercontinent and East African Orogen [modified from Zhou et al. (2018)]. Madagascar Madagascar (Fig. 1.4) is an ideal area to study EAO, especially the central and northern parts. There are five principal tectonic units identified in that area including the Archean Antongil-Masora block, the Antananarivo block, Tsaratanana complex, the Neoproterozoic Bemarivo Belt and the Proterozoic metasedimentary rock of Itremo group (Collins et al., 2001). The Antongil-Masora block of eastern Madagascar is comprised of 3.2 Ga gneiss intruded by 9.

(21) ~2.5 Ga granites that have only experienced low-grade metamorphism since ~2.5 Ga. The Antananarivo block, which underlies the central highland area, is comprised of 2.5 Ga gneiss interlayered with 820 to 740 Ma granitoids and gabbros. It is overlain by Itremo group on the central-west that contains metasedimentary rocks of the Mesoproterozoic to early Neoproterozoic Itremo Group thrust over, and imbricated with, rocks of the Antananarivo block (Cox et al., 1998). The Tsaratanana complex overlies the northern Antananarivo, dominated by Neoarchean orthogneiss (2.7 to 2.5 Ga) and cut by 800 to 760 Ma gabbro intrusions. The northern Madagascar, Bemarivo Belt, juxtaposed the Antongil and Antananarivo, consists of amphibolite-granulite gneiss on the southern and granitic massif on the northern (Collins, 2000; Kröner, 2000; Collins and Windley, 2002).. Figure 1.4. Simplified geological map of Madagascar [modified from Collins and Windley (2002)]. 10.

(22) India - Dharwar craton The Indian Shield as shown in Fig. 1.5., is comprised of four main cratons that formed and stabilized during Archean to Paleoproterozoic and Meso- to Neoproterozoic mobile belts (Meert and Pandit, 2015; Saha and Mazumder, 2012). The four cratons of the Indian Shield include: the Dhawar craton, the Bastar craton, the Singhbhum craton, and the AravalliBundelkhand craton (Srivastava, 2008).. Figure 1.5. Simplified geological map of India [modified from Srivastava (2008)] (MIS: Malani Igneous Suite). The Dharwar craton of southern India (Fig. 1.5) is comprised of four major units: the Archean tonalite-trondhjemite-granodiorite gneiss, the supracrustal belts, the Proterozoic metasedimentary basin and the late potassic granite, such as Closepet granite. The Chitradurga schist belts divided the craton into two cratonic domains named the Western Dharwar (WDC, ca. 3.3 to 2.7 Ga) and the Eastern Dharwar (EDC, ca. 3.0 to 2.5 Ga) (Meert and Pandit, 2015). The WDC contains most of tonalite-trondhjemite-granodiorite gneiss while the EDC is a suite of granitic rocks, intrusive volcanics, and middle Proterozoic to recently sedimentary basins. 11.

(23) The emplacement of Closepet granite during ~2.5 Ga indicated the amalgamation of WDC and EDC as well (Saha and Mazumder, 2012; Chadwick et al., 2000). India - MIS The term MIS was first used by Murthy at al. (1961). MIS is located in northwestern Indian Shield and covers an area of ~50,000 km2. The magmatic activities in MIS began with mafic and felsic flows and the accumulation of conglomerate. Next followed stage is dominant by the felsic flows. At this stages, granite plutonism occurred mostly at central MIS, especially Siwana and Jalore. The end of Malani magmatism was marked by felsic and basic dyke intrusions (Sharma, 2005). The Siwana is a large ring structure, around 30 km diameter, and consists of mostly intruded peralkaline granite. The Jalore shows a variety range from quartz syenite to granite (Eby and Kochhar, 1990).. 1.5.The Seychelles microcontinent The elliptical-shaped block located at the northern end of the Mascarene plateau in the Indian Ocean is known as the Seychelles microcontinent. The surface exposure is a series of granitic islands that are ~1,000 km from Madagascar and East Africa (Fig. 1.6). The Mascarene plateau is the second largest submarine plateau in the Indian Ocean. It is a shallow (8 m to 150 m deep), extensive rise that forms a crescent shape from Mauritius in the south to the Seychelles in the north. The Mascarene plateau extends nearly 2,000 km and covers an area of ~115,000 km2.. 12.

(24) Figure 1.6. Regional topography of Mascarene plateau and the western Indian Ocean, eastern Africa, Madagascar. Figure 1.7. Simplified map of the Seychelles microcontinent [modified after Baker (1963) and Suwa et al. (1983)]. There are two distinctive regions in the Seychelles, the granitic islands and the coralline outer islands. The granitic group is concentrated within a 56 km radius and consists of 45 covering a total area of 247.2 km2. The main islands of this group are Mahé, Praslin, Silhouette, 13.

(25) La Digue, and North Island. The outer islands contain five groups of coralline islands, comprise 211.3 km2 (46% of the Seychelles). The coral islands show elevated coral reefs at different stages of formation in flat terranes. The Main Islands of the Seychelles are only islands in the world that are entirely composed of granite and they are also the oldest islands in the world as they have a cluster of zircon concordant ages in the range of 750-760 Ma although Tucker et al. (2001) reported one (Île aux Récifs) high-precision U-Pb zircon age of 808.8±1.9 Ma, interpreted as magmatic crystallization ages. The Seychelles is a key region as it was located at the centre of the EAO and preserved rocks that recorded the Rodinia and Gondwana supercontinent cycle. There are three main types of granite that comprise the Main Islands of the Seychelles and include: the dominant grey granite on Mahé, the dominant pink granite on Praslin-La Digue and the younger syenite on Silhouette and North Island (Fig. 1.7). Both Wegener (1924) and Du Toit (1937) interpreted the Seychelles microcontinent as a continental fragment left behind during Gondwana breakup. Once sandwiched between Africa and India as part of Gondwanaland, the Seychelles split away from India during the Early Paleogene (Plummer, 1995; Shellnutt et al., 2015).. 1.6. Purpose of the study The granitic islands of the Seychelles have been considered as a geological curiosity for a long time. The interest in the Seychelles is inspired by their unique geologic history with a great deal of research on its geology, especially the debate regarding their tectonomagmatic evolution as well as their cratonic affinity. Currently, there are a number of first order issues regarding the Seychelles that have yet to be resolved. Specifically, whether the Seychelles represents a fragment of ancient crust derived from India or Africa. Moreover, there is debate concerning the tectonomagmatic evolution of the Neoproterozoic granites vis-à-vis subduction-related vs. extension-related.. 14.

(26) Weis and Deutsch (1984) measured the Nd and Pb isotopes and suggested Precambrian Seychelles rocks are alkaline anorogenic and mantle-derived with slight upper-crust interaction. Taylor (1974) published the oxygen isotopes and suggested an extension, hotspot- or riftgenerated magmatism. Plummer (1995) illustrated the rocks are characteristics of mildly alkaline and concluded three emplacement phases, i.e. hotspot- and rift-related, within-plate magmatism. Alternatively, Tucker et al. (2001) suggested the Precambrian Seychelles magmatism is consistent with a continental arc setting than an extensional environment. He also suggested 100 Myr of magmatic activity in Seychelles based on U-Pb zircon ages of 703 Ma and his U-Pb age of 808 Ma. Based on chemical and isotopic data, Ashwal et al. (2002) supported the continental arc setting for the Precambrian Seychelles magmatism and emphasized that they were likely formed at an Andean-type arc.. Figure 1.8. Palaeogeographic reconstruction of Greater India, the Seychelles microcontinent and Madagascar [modified from Torsvik et al. (2001b)]. 15.

(27) Figure 1.9. Palaeomagnetic reconstruction of the Seychelles [modified after Suwa et al. (1994)]. Palinspastic reconstructions by Torsvik et al. (2001a, b) show the spatial contiguity at ~750 Ma of the Seychelles and northwestern India. The new Seychelles - India fitting proposes that the Seychelles and the MIS, northwestern India were separated by ~600 km. The age and geochemistry of Seychelles igneous rocks by Tucker et al. (2001) are also correlative with the volcanic and plutonic rocks in the MIS. In contrast, others consider the Seychelles to be a fragment of Africa (Fig. 1.9). Rabinowitz et al. (1982) showed that Madagascar was located off the east coast of Africa bordering southern Somalia, Kenya and Tanzania before parting Gondwana. Additionally, paleomagnetic reconstruction of Seychelles suggests that it rotated by 10o clockwise relative to Madagascar and ~17o clockwise to Africa (Suwa et al., 1994). The Seychelles pre-drift position consequently is thought to be near the eastern end of the Horn of Africa (Hargraves and Duncan, 1990). Reconstruction of the Seychelles, therefore, first occupied the eastern of the Horn of Africa and then drifted to the currently position. Suwa et. 16.

(28) al. (1983, 1994) showed petrological, geological, geochronological and geochemical data to conclude that Neporoterozoic granitic rock of the Seychelles are well correlated with and similar to those of ANS. In this report, we used in situ zircon U-Pb geochronology and whole rock geochemistry in order to: 1) determine age of emplacement of Mahé and Praslin granites, 2) identify recycled components (inherited zircon clusters) within the granites, 3) used whole rock geochemical data and Sr-Nd isotopic data to address the origin of the Seychelles microcontinent regarding the India-Africa dichotomy and 4) address the tectonomagmatic evolution of the Mahé and Praslin granites.. 17.

(29) CHAPTER 2. GEOLOGICAL BACKGROUND. The seminal work on the geology of the Seychelles was written by Baker (1963). Baker’s field observations and maps provided a reference frame for all subsequent studies. The bedrock geology of the Seychelles is primarily comprised of late Precambrian granitic rocks (Mahé and Praslin groups) that are cross-cut by doleritic dykes. There are 42 granitic islands within the the Seychelles, in descendant order of size: Mahé, Praslin, Silhouette, La Digue, Curieuse, Félicité, Frégate, Ste. Anne, North, Cerf, Marianne, Grand Sœur, Thérèse, Aride, Conception, Petite Sœur, Cousin, Cousine, Long, Récif, Round (Praslin), Anonyme, Mamelles, Moyenne, Ile aux Vaches Marines, L'Islette, Beacon (Ile Sèche), Cachée, Cocos, Round (Mahé), L'Ilot Frégate, Booby, Chauve Souris (Mahé), Chauve Souris (Praslin), Ile La Fouche, Hodoul, L'Ilot, Rat, Souris, St. Pierre (Praslin), Zavé, Harrison Rocks (Grand Rocher). Lithified sedimentary and metamorphic rocks are absent but there is a minor volume of volcanic and volcaniclastic rocks. The youngest igneous rocks are the syenites, diorites, and rhyolites from North Island and Silhouette that yielded Early Palæogene ages (Shellnutt et al., 2017). The oldest rocks of the Seychelles are the Precambrian granites that form the principal island groups – the Mahé and Praslin groups. The granites of Mahé and Praslin groups were first suggested to be Neoproterozoic (663±17 Ma) by Miller and Mudie (1961).. 18.

(30) 2.1. Mahé group island. Figure 2.1. Simplified geological map of Mahé islands [modified from Suwa et al. (1983)]. Mahé is the largest and tallest island in the Seychelles at 145 km2 (~27 km long, maximum width of ~11 km). The island is mountainous with a central line of peak up to 910 m elevation. The central mountain rises steeply from the sea and from the localized coastal plateau. Mahé is almost entirely composed of coarse grained granular hornblende granite with minor porphyries. Xenoliths, enclaves, and associated hybrid rocks of tonalitic, dioritic and gabbroic composition are found in localized zones on the coast. The granitic massif is crosscut by dolerite and basalt dykes. It is necessary to mention of the variability of exposure on Mahé. The coastal exposures are excellent and tend to form low cliff and have wave-beaten surfaces. However, away from the coastline there is a slope of the red-brown soils with granite 19.

(31) boulders. The boulders are likely derived from the hinterland and were transported by landslips and soil liquefaction, or by the progressive lowering of the surface by erosion. Deeply weathered granite areas and deep-seated surface blocks conceal much of the solid geology (Baker, 1963). The Mahé granite The Mahé granite is the basis of the greater part of the island and is considered as a single intrusive. That granite has medium grain size with the grains averaging between 5 and 7 millimeters. Color variations of hand specimens depend on the degree of sericitization of the alkali feldspar; normally show from the pale mottled grey to the buff of the fresh rock and pale grey brown shades of the slightly weathered rock. The quartz grains frequently occur in aggregates and have glassy and slightly smoky color. In some weathered surfaces, the breakdown of ferro-magnesian led to the formation of limonite and goethite, which are the reasons for the grading to red-brown observed within the boulders. Xenoliths are common within the Mahé granite with many along the NW coast exhibiting flow alignment. Generally, Mahé consists of three type of Precambrian granitic rocks series, named (i) gneissose granodiorite gneissose tonalite and amphibolite - leucogranite series, (ii) grey granite series and (iii) porphyritic granite series. (i). Gneissose granodiorite - gneissose tonalite and amphibolite - leucogranite series That rock series occurs mainly in the northern part of Mahé. The gneissose granodiorite. - gneissose tonalite have grey color and occur near the boundary the amphibolite - leucogranite series. Along the west coast of north Mahé, the gneissose granodiorite - gneissose tonalite are intruded discordantly by the amphibolite - leucogranite series and is the likely reason why these rocks appear as angular blocks and agmatitic. The amphibolite - leucogranite has white-grey color, distributed along the west and northwest coast of Mahé. From field observation, that. 20.

(32) rocks are considered to be an intrusive body after emplacement of gneissose granodiorite gneissose tonalite. (ii). Grey granite series This series including gneissose grey granite is volumetrically minor with insignificant. distribution, in such small islet lying on the east. The grey granite forms the main part of Mahé Island and is coarse-grained and massive in structure. The boundary between grey granite and gneissose granodiorite - gneissose tonalite and amphibolite - leucogranite series on the north is not so sharp and may be more gradational. (iii). Porphyritic granite series. Porphyritic granodiorite and microgranite occupy in a substantial area western coast of Mahé between Port Glaud, Port Launay and Mare Cochons, including Therese and Conception islands. In general appearances, these rocks are similar to the Grey granite but have porphyritic texture with phenocrysts of feldspar and a fine-grained groundmass. Therefore the porphyritic granite is viewed as a facies of the Mahé grey granite rather than a separate intrusive (Baker, 1963). St. Anne Island group granite There are slight petrographic differences between the granite of the five islands to the east of Victoria and the grey granite. The distinction between the gneissose granodiorite or grey granite is quite difficult on the field. Petrographically, the rocks are distinguished by their hypiodiomorphic inequigranular texture and rarity of plagioclase. Quartz grain shows the rounded shape. The rocks also display a mortar texture suggesting they have undergone recrystallization and granulation. The St. Anne Island group granite are slightly deformed alkali-granites, therefore may consider representing a marginal facies of the grey granite (Baker, 1963).. 21.

(33) Dolerite Dykes Dolerite dykes cut the granite and range in thickness from 0.2 m to 10 m. Some mafic xenoliths and veins may be correlative with the dykes as well. The Mahé dolerites are distinguished by their low silica and high potasium percentage. The emplacement of those parallel dolerite dykes is considered to be the last event of magmatic activity recorded in Mahé island (Baker, 1963). Coastal sediments Coastal plateau is made up of calcareous beach sand which forms some of the best beaches in the world. The sand plateau is derived from the erosion of the fringing reefs and consists of broken up coral, mollusk and algae. The structure of plateau is affected by the erosion of the mountainous hinterland. The streams play the vital role for the deposition of quartz and formation of the swamp and tidal zones where the detritus is deposited (Baker, 1963).. 2.2. The Praslin - La Digue island group The Praslin - La Digue island group lies to the NE of Mahé and consists of two major islands, Praslin and La Digue. After Mahé, the Praslin is the second largest island and it comprised mainly of pink granite. Praslin is hilly and steep-sided island rising directly from the sea in which the central hill rise extends to 400 m in elevation. Marianne Island, ~10 km east of La Digue, is unusual as it has a porphyritic texture rather than granular texture. The Praslin granite The Praslin granite is relatively homogenous, showing only slight local variations in texture and composition. It is typically pale reddish grey in color owing to the color of alkali feldspar. The rocks are somewhat similar to the Grey granites of Mahé with the primary difference being the color. Subhedral feldspar is more abundant than tabular feldspar phenocryst. Quartz grains are found in individual or aggregate in clear, glassy and rounded. 22.

(34) shape. There are narrow medium-grained pure quartz veinlets as well as plane quartz vein with coarser texture (Baker, 1963).. Figure 2.2. Simplifed geological map of Praslin, La Digue group [modified from Suwa et al. (1983)]. Marianne granite porphyry Marianne differs slightly from the other smaller islands of the Praslin – La Digue group in structure. The island is 2 km in length and has a conical peak at 128 m elevation but is porphyritic. The rocks are distinguished by their pale pinkish buff tabular feldspar and rounded quartz phenocrysts in a groundmass of fine-grained pale reddish grey aplitic with cluster of ferromagnesian minerals. This porphyry is remarkable uniform over the island. In a north-west direction, there is observation of jointed metadolerite, which have speckled green brownish color with 1 mm in grain size (Baker, 1963). Dolerite dykes Dolerite dykes are observed throughout the Praslin – La Digue group, but are especially numerous on Felicite Island. They are slightly altered and have medium- to fined-grain size 23.

(35) and are thought to be syn-plutonic or slightly younger. The dykes are generally trend to the NW and are frequently found in valleys and gullies. Most of the dolerites have grey color or are slightly greenish grey and granular. Metadolerites in which substantial alteration has taken place and in which the grain size is greater than the unaltered dolerites were found on Marianne Island (Baker, 1963). Coastal deposits On the coasts of the Praslin – La Digue group, there are a number of places that have limestones, calcareous conglomerates and sands containing coral and algal pebbles which were observed adhering to granite blocks at altitudes between sea-level and 30 feet above sea level. In comparison, the coastal plateau of the Praslin – La Digue group is similar in appearance, age and structure to those of Mahé. On the western side of Praslin, there are intensive plateau composed of calcareous beach sands and are furrowed by swamps and tidal inlets. Furthermore, northern Marianne has brown earthy rocks with cavernous and pitted weathering (Baker, 1963). Superficial deposits A basic mixture of calcium carbonate and humus is the main components of alkaline soils which overland coastal plateau. Soils are rarely seen but are particularly numerous in small sheltered patches in valley bottoms. Owing to soil erosion, some top-soil of huge areas is missing, exposed the surface of gravelly sub-soil or bare rock. Curieuse Island has the most serious soil erosion. It is also difficult to see the red-brown, yellow-brown La Digue granite soils (Baker, 1963). 2.3. Sampling In this studied, three samples were collected from each group island. Detailed GPS coordinates is showing in table below: Table 2.1. GPS coordinates of studied samples. Mahé island MH-01. GPS o. Praslin island ’. ’’. 4 34 0.44 S. PL-01. 24. GPS 4o17’59.73’’S.

(36) MH-02 MH-03. 55o27’18.26’’E 4o34’0.45’’S 55o27’17.34’’E 4o34’26.9’’S 55o27’42.23’’E. PL-02 PL-03. 55o40’39.21’’E 4o17’37.10’’S 55o40’54.51’’E 4o17’53.29’’S 55o40’57.04’’E. The Mahé samples were collected near the coast. The outcrops and adjacent scattered boulders are exposed along the beach up to a level of 1 to 2 m above the mean sea-level. The distance between MH-01, MH-02 and MH-03 is ~1 km. The Praslin samples were collected along the shore (PL-01) as well as inland (PL-02 and PL-03). All samples are fresh although sample PL-02 is more weathered. The overall color of Mahé fresh rock is light gray, which is thought to be due to quartz whereas the Praslin samples are pink color probably due to their abundance of potassium feldspar. All rocks contain biotite and amphibole.. Picture 2.1. The image show samples location in northern coast of Mahé island. 25.

(37) Picture 2.2. The image show samples location in southwest of Praslin island. Picture 2.3. Rounded mafic enclaves, sample MH02, within grey granite boulder. Picture 2.4. Sample PL-01, show distinctive moderate-grained pink granite. 26.

(38) Picture 2.5. Sample MH-01 showing dominant of grey color. Picture 2.6. Pink granite, sample PL-01, with transparent quartz and dark mineral. 27.

(39) CHAPTER 3. PETROGRAPHY. The polished section preparation was conducted and then observed by a Carl Zeiss Axioplan 7082 optical microscope at Department of Earth Sciences, National Taiwan Normal University. The photomicrographs are placed from the left to the right, corresponding to the photo taken under taken in plane polarized light (PPL) and cross polarized light (CPL). 3.1. Mahé granites Quartz is the primary mineral observed in the Mahé samples, with around 30% of mode. Grain size varies from 0.2×0.3 mm to over 1×2 mm. Grains are most commonly anhedral, subhedral to euhedral grains are uncommon. Quartz has low birefringence and lacks cleavage, twinning and alteration, and commonly shows undulose extinction. Usually the shape of the quartz is controlled by the space between the crystals suggesting is crystallized after feldspar. In some cases the quartz are distributed along the fringes of feldspar. The feldspar in the Mahé granite includes two types. Type I is plagioclase that has polysynthetic twinning, fresh, varies in size, from 0.2×0.4 mm, and has a scattered distribution in the groundmass (Fig. 3.1c). Generation II is alkali feldspar with perthite exsolution (Fig. 3.2d). The extinction angle of the alkali feldspar is inclined. In some cases, there is replacement of plagioclase by microcline (Fig. 3.1d). Microcline is easily identified as is displays distinguishing tartan twinning, which consists of two nearly perpendicular directions of microtwins, according to the albite and pericline law. Microcline occurs as subhedral to anhedral crystal, or cloudy on account of incipient alteration. The colors observed are light to dark grey, brownish black. K-feldspar is usually anhedral although some are long prismatic forms. Many crystals are over 1 mm in width. Most K-feldspar shows kaolinization whereas some larger crystals have carlsbad twining (Fig. 3.2f).. 28.

(40) Figure 3.1. Microscope thin section photograph of sample MH-01, showing hornblende, quartz and feldspar. The most abundant mafic mineral is hornblende (Fig. 3.1a) which ranges in size but is typically greater than 0.1×0.3 mm. Most hornblendes are randomly distributed and have irregular boundaries. The pleochroism of hornblende is pale yellow-green (Fig. 3.2a) and many crystals are euhedrad and show two perfect cleavages at 120o and 60o (Fig. 3.2e). In some cases (Fig. 3.2a), under PPL, the hornblende shows blue-green pleochoism. Oxyhorblende occurs as a subhedral phenocrysts with distinctly reddish brown color and strong pleochroism (Fig. 3.3). Zoning is not clearly shown but the inner part has darker color, corresponding to more oxidization to compare with the outer part. They may be up to 0.5 mm in width, associated with Fe-Ti oxides.. 29.

(41) Figure 3.2. Microscope thin section photograph of sample (a) MH-01, under PPL; (b) MH-01, under CPL (c) MH-03, under PPL; (d) MH-03, under CPL (e) MH-01, under CPL and (f) MH-02, under CPL. Biotite crystals observed in Mahé samples have lath or tabular shapes and form micaceous masses and groupings. Biotite is randomly distributed and represents ~8-10% abundance of the mineral mode. The size of the biotite in the sample varies greatly but has a 30.

(42) maximum size of 1×1.5 mm. Under PPL, biotite shows pleochroism from pale yellowishbrown to deep brown, elongate crystals darkest brown. Most crystals are platy and resemble prisms with a single cleavage (Fig. 3.2a) or shows scaly, cluster develop along the edge of hornblende (Fig. 3.1a).. Figure 3.3. Microscope thin section photograph of sample MH-02, showing oxyhornblende. Apatite is an accessory mineral that is most often observed with an acicular texture. The small elongated crystals stand out in relief against K-feldspar and other colourless minerals. The photograph under CPL, shows that the birefringence of apatite is colorless to first order gray (Fig. 3.2a). Zircon is a fairly common accessory mineral with short and stubby crystals as well as prismatic shapes. The crystals are small but have strikingly high relief and third to fourth order birefringence colors. Fe-Ti oxides are also observed but they are not abundant (< 5 vol.%).. 3.2. Praslin granites Quartz is the most common mineral in the Praslin samples. The PPL photographs show clear quartz phenocrysts (Fig. 3.4) that have interstitial groundmass. Although relatively uncommon, this texture is considered to be related to resorption of older crystals or may be due to rapid crystallization. 31.

(43) Figure 3.4. Microscope thin section photograph of sample PL-02, showing quartz phenocryst. Additionally, the intergrowth of K-feldspar and quartz with a vermicular texture is clearly seen in the view under CPL. Abundant throughout the granite are intergrowths between K-feldspar and vermicular quartz, known as myrmekite. Quartz of myrmekite intergrowths has been referred to as leaf-like, flower-like or plumose. The origin of the vermicular development of quartz in intergrowths with plagioclase has never been satisfactorily explained by replacement or simultaneous crystallization from a liquid. Even in PPL, the intergrowth is visible because of the difference in birefringence colour of the two minerals, the K-feldspar is pale brown whereas the areas of quartz are clear.. Figure 3.5. Microscope thin section photograph of PL-01 and PL-02, showing distinctive mymerkite. 32.

(44) Feldspar in Praslin rocks is more abundant than in Mahé, and comprised 30-40% of the mineral mode. The feldspars are subhedral to anhedral. In addition to mymerkite, weathering of plagioclase is apparent in this sample. Outlines of and interiors of plagioclase crystals are clearly distinguished from the surrounding granite by the dusty grey brown color of their alteration products, and altered cores. The feldspar in the Praslin granites shows an increase in dusty brown alteration in comparison with Mahé.. Figure 3.6. Microscope thin section photograph of sample PL-03, showing tabular biotite. Fibrous amphibole is the most abundant mafic minerals and frequently associated with biotite and Fe-Ti oxides. The amphibole crystals are subhedral to anhedral, have irregular grain boundaries, and tend to be fractured. Figure 3.7a and b shows the association between amphibole and biotite. The biotite are elongate with average dimensions of ~0.3x0.5 mm. Some sections show the one good cleavage of biotite. The small black globules of Fe-Ti oxide minerals are uniformly distributed within the biotite but are more common near the crystal rim (Fig. 3.7b and d). In some cases the biotite is altering to chlorite (Fig. 3.7e and f). The granoblastic amoeboid of sample PL-01 is also shown in Fig. 3.7e and f in which the grains have irregular outlines. Accessory minerals comprise apatite, zircon and Fe-Ti oxide minerals. Apatite crystals are common tabular with a length of ~90 to 200 μm, high-relief, and first order birefringence colors (Fig. 3.7g and h). Under CPL, zircon shows elongate grains with parallel 33.

(45) extinction. There distinct pleochroic radiation halos observed in the zircon which is probably related to the uranium and thorium content of the crystals.. 34.

(46) Figure 3.7. Microscope thin section photograph of sample, from left to right corresponding to photo taken under PPL and CPL: (a),(b), (e), (f) PL-01; (c),(d) PL-03; (g),(h) PL-02. 35.

(47) CHAPTER 4. METHODS. 4.1. Zircon U-Pb dating by LA-ICPMS 4.1.1. Zircon preparation Zircons separated from samples were prepared as following those steps: . Preparing the mount plate with double-sided tape. . Cutting out mount surface and preparing row markers. . Mounting grains on the tape. . Preparing the mount plate for epoxy. . Preparing and pouring the epoxy. . Trimming and labelling the mount. . Polishing the mount. . Imaging the mount. Figure 4.1. Composite representative image of zircon row: secondary electron (top) and cathodoluminescence images (bottom) of zircon crystals.. 36.

(48) The mount is imaged using cathodoluminescence (CL) and secondary electron (SE) detectors in a Scanning Electron Microscope (SEM). In this study, the CL images were taken in Electron Probe Microanalyzer Laboratory, Institute of Earth Sciences, Academia Sinica.. 4.1.2. Principles of zircon U-Pb geochronology Uranium (U) and Thorium (Th) belong to the Actinides series, both of them are radioactive elements. Their decay to stable isotopes of lead (Pb) is the basis for several methods of dating. U and Th are concentrated in residual silicate melts as they are highly incompatible during partial melting and fractional crystallization. Therefore, granitic rocks tend to have high concentrations of U and Th compared to basaltic or ultramafic rocks. The three naturally occurring isotopes of U are 232. 238. U,. 235. U and. 234. U. Thorium has only one radioactive isotope,. Th. In addition, there are five other radioactive isotopes of Th which were short-lived. intermediate daughters and produced by decaying. 238. U, 235U and. 232. Th. Each of three are the. parent of a chain of radioactive daughters completion with stable isotopes of Pb (Steiger and Jäger, 1977): 238 92𝑈. →. 206 82𝑃𝑏. + 8 42𝐻𝑒 + 6𝛽 − + 𝑄238 (𝜆238 = 1.55125 × 10−10 𝑦 −1 , 𝑡1/2 = 4.468 × 109 𝑦). 232 90𝑈. →. 208 82𝑃𝑏. + 6 42𝐻𝑒 + 4𝛽 − + 𝑄232 (𝜆232 = 4.9475 × 10−11 𝑦 −1 , 𝑡1/2 = 14.010 × 109 𝑦). 235 92𝑈. →. 207 82𝑃𝑏. + 7 42𝐻𝑒 + 4𝛽 − + 𝑄235 (𝜆235 = 9.8485 × 10−10 𝑦 −1 , 𝑡1/2 = 0.7038 × 109 𝑦). where 𝑄238 = 47.4 Mev/atom; 𝑄232 = 39.8 Mev/atom; 𝑄235 = 45.2 Mev/atom (Wetherill, 1966). The decay of 238U to 206Pb and 235U to 207Pb as a function of time is expressed as: 206 82𝑃𝑏 ( 238 ) = 𝑒 𝜆238 𝑡 − 1 92𝑈 207 82𝑃𝑏 ( 235 ) 92𝑈. = 𝑒 𝜆235 𝑡 − 1. 37.

(49) Two above equations are the parametric equations of a curve that is locus of all concordant U-Pb systems, called the “Concordia” (Wetherill, 1956). The points that do not fall on the curve are referred to as “discordant”. By extrapolating the discordia, two intersection points are obtained. The lower point is the time elapsed since closure and the higher point is the time elapsed since original crystallization of the rocks.. 4.1.3. Procedure of U-Pb geochronology Zircon U-Pb isotopic analyses were conducted by laser ablation - inductively coupled plasma mass spectrometry (LA-ICPMS) technique using an Agilent 7500s ICPMS and a Photon MachinesAnalyte G2 193 nm laser ablation systems set up at Department of Geosciences, National Taiwan University (Chiu et al., 2009). A spot size of 35 μm with 7 Hz repetition rate of laser was applied to all analyses. Calibration was accomplished by using the reference zircon standards GJ-1 (Jackson et al., 2004), as a primary standard, and 91500 (Wiedenbeck et al., 1995), Plešovice (Sláma et al., 2008) as secondary reference materials. During each analytical run, the reference zircons were analysed as unknown together with the samples for data quality control. GLITTER 4.4.4 software was used to calculate the measured U-Th-Pb isotopic ratios (Griffin et al., 2008). Common lead was corrected by using the common lead correction function proposed by Andersen (2002). The weighted-mean 206Pb/238U ages were calculated and Concordia plots, probability density plot and weighted average were created using Isoplot v.4.15 (Ludwig, 2012).. 38.

(50) Figure 4.2. Weighted mean U-Pb ages of 91500 and Plešovice standard. 4.2. Whole rock major elements by XRF 4.2.1. Principles of X-ray Fluorescence spectrometry Wavelength Dispersive X-ray Fluorescence (WDXRF) is one of two general types of XRF instrumentation used for elemental analysis applications. The atoms in the samples are excited by X-ray emitted from X-ray tubes. WDXRF spectrometer uses a semiconductor material (an X-ray detector) to convert characteristic X-ray into electrical signals. The spectrometer's electronics digitize the signals produced by the detector, and send this information to a PC or internal electronics for display and analysis. The X-ray can be detected, displayed as a spectrum of intensity against energy. The positions of the peaks identify which elements are present in the sample (qualitative analysis) and in counting and comparing the number of energies at the same energy level reaching the detector (peak heights), we can determine percentages and so identify how much of each element is present in the sample (quantitative analysis).. 4.2.2. Procedure . Loss on ignition (L.O.I.) 39.

(51) The empty ceramic crucibles weighted around 10 grams were used to put 3 or 4 grams of sample powders. Then the weight of crucibles (n1) was recorded and dried at low temperature, around 1050C in the oven around three hours to release ambient water. The following step was made up by weighting the crucibles (n2) and drying it at higher temperature, 9000C for five to six hours. At this step, the samples was oxidized and freed of molecular water. L.O.I. determination completed by cooling crucible in desiccator and reweighting the crucibles (n3). L.O.I. value can be determine by using following equation: L.O.I. (weight percent) =. 𝑛2 −𝑛3 𝑛2 −𝑛1. x. 100. The L.O.I. is made of contributions from volatile compounds (H2O, CO2, F, Cl, S) and some added compounds (O2 (oxidation, e.g. FeO to Fe2O3), later CO2 (CaO to CaCO3). . Glass bead preparation. The amount of 0.600±0.005 gram of each samples powder after L.O.I. measurement was mixed with 6.000±0.005 gram of Claisse lithium borates and lithium bromide flux powder, which contains 49.75% Li2B4O7, 49.75% LiBO2 and 0.5% LiBr. This mixture was fuse state at 12000C in platinum crucibles to make a glass bead by using Claisse M4 Fluxer. Finally, the glass beads were loaded and the analyses were carried out by Panalytical Axios mAX WDXRF at XRF Laboratory, Department of Earth Sciences, National Taiwan Normal University. The measurement was accomplished with four USGS standard reference materials, BIR-1a and SDC-1 after every five samples running to detect the accuracy and stability of the machine. The precision for BIR-1a standard reference material is±0.5% on all elements. The precision for SDC-1 standard reference material is±0.5 % on all elements except Al2O3, Fe2O3 and Na2O, which are±2%. In this study, the oxides selected for analyzing in weight percent (wt.%) were SiO2, TiO2, Fe2O3, Al2O3, MnO, MgO, CaO, Na2O, K2O and P2O5. Table 4.1. The major elements analyzing result of the standard. 40.

(52) Sample. SiO2. TiO2. Al2O3. Fe2O3. MnO. MgO. CaO. Na2O. K2O. P2O5. L.O.I.. TOTAL. BIR-1a(2016). 47.97. 0.98. 15.65. 11.20. 0.17. 9.67. 13.31. 1.83. 0.03. 0.02. -0.24. 100.60. Recommended 47.96. 0.96. 15.50. 11.3. 0.17. 97. 13.3. 1.82. 0.03. 0.021. -. -. value SDC-1(2016). 66.46. 1.01. 16.11. 6.81. 0.12. 1.70. 1.45. 2.12. 3.21. 0.15. 1.41. 100.55. Recommended. 65.8. 1.01. 15.8. 6.32. -. 1.69. 1.4. 2.05. 3.28. 0.16. -. -. value. 4.3. Whole rock trace elements by ICP-MS 4.3.1. Principles of ICP-MS Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) is a powerful analytical technique used for elemental determinations. ICP-MS combines two technologies into one: an ICP attached to MS for generation of ions. The trace element was analyzed at Mass Spectrometer Lab, Institute of Earth Sciences, Academia Sinica by an Aglient 7500cx spectrometer. The precision and accuracy of the ICP-MS was generally better than 5% for most trace elements and have been tested against known international standards including four USGS rock references materials (AGV-2, BCR-2, BHVO-2, and DNC-1a). A high-temperature ICP source with ion source of Ar plasma in converts the atoms of the sample into positive ions of different elements. These ions are then separated and detected by a mass spectrometer. Normally the positive ions are formed by the ICP discharge. Once the ions enter the mass spectrometer, they are separated by their mass-to-charge ratio.. 41.

(53) Figure 4.3. Chondrite normalized plots of 4 USGS standard. Chondrites value are used from Sun and McDonough (1989). 4.3.2. Procedure XRF-glass bead rock samples (~40±5 mg) were decomposed in screw-top Savillex Teflon beakers containing 30 and 25 drops mixture of 1:1 acid nitric (HNO 3) and acid fluoric (HF). The beakers are placed on a hot plate to dissolve reaction at 1400C overnight. After drying of the solution at 1600C around 2 hours, 1.5 ml HNO3 are replenished to the dried residua. The beakers once again are put on a hot plate at 1400C overnight. The solution was diluted to x1500 times. The samples solution first was diluted x500 times with 2% HNO3 into 50ml bottles. The solutions were then dilute x3 times with 2% HNO3 and internal standard 10 ppb (Rh+Bi) into 12ml tubes, already for ICP-MS trace element analysis.. 42.

(54) 4.4. Whole rock Sr-Nd isotopes by TIMS 4.4.1. Principles of TIMS operation Thermal Ionization Mass Spectrometry (TIMS) uses temperature to thermally ionize sample for isotopic analysis. A current passed through a conducting metal filament (Tantalum, Tungsten or Rhenium), on which the sample is on, to temperatures often exceeding 1000°C. Single heated filaments of varying materials can be used, dependent on the element to be isotopically analyzed together with single free filament as oxidation filament. The samples are loaded on the filament degassed in the filament oven, and then the filaments are set inside the ion source of the mass spectrometer. The ions that are formed on the filament are accelerated by a high voltage across an electrical potential gradient, and focused into a beam via a series of slits and electrostatically charged plates. The ion beam passes through a magnetic field and it is separated on the basis of their mass/charge ratios. Each separated isotope beam is collected simultaneously in an array of Faraday cups detector and the isotope ratios can be calculated from the relative intensities.. 4.4.2. Procedure Normally, ~100 mg of sample powder was added in to Teflon beaker with 50 drops of conc. HNO3 and conc. HF. Then it is heated at 1000C for two days before dried. The results is filled with 2 ml 6N HCl and dried. After repeating this step twice, the cooling samples were stored in 2 ml 1N HCl and centrifuge for 10 minutes. Separation of solid and liquid components. took place in this step lead to easily congregate supernatant. Those steps can reiterate of heating and dissolving until getting a clear, homogenous solution in preparation for chromatographic separation. Chromatographic/column separation was used in order to isolate Strontium (Sr) and Neodymium (Nd) from all other elements. Sr and Nd for TIMS were purified by three steps 43.

(55) column separation procedure. In the first column, Sr and REEs were separated using a polyethylene column (23 mm length × 5 mm in diameter) packed with 2.5 ml of cation exchange resin (sulfonated polystyrene resin, BioRad AG 50W-X8, Analytical Grade, 100-200 mesh). In the Sr column, Sr was further puriﬁed by passing through a cation exchange resin bed polyethylene column (23 mm length × 5 mm in diameter) filled with 1 ml of AG50W-X8. In the second column, a two layered resin bed polyethylene column (31 mm length × 3.5 mm in diameter) packed with 1 ml Ln resin, 100–150 µm, Eichrom Technologies, Inc., USA (the bottom layer) and 0.1 ml of Dowex 1-X8, 100-200 mesh (the upper layer) was used to isolated Nd from other REEs.. Figure 4.4. Column chemistry separation for Sr and Nd isotopic analysis. For the isotopic measurement, Sr and Nd were loaded with H3PO4 on treated filaments, a single Rhenium ﬁlament. 143. 87. Sr/86Sr ratios were normalized to. 86. Sr/88Sr = 0.1194 whereas. Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219. The 87Sr/86Sr isotopic ratios were. measured using a Finnigan MAT-262 TIMS, whereas the. 143. Nd/144Nd isotopic ratios were. measured using a Finnigan Triton TIMS in the Mass Spectrometry Lab, Institute of Earth Sciences, Academia Sinica, Taipei. The 2σ external precision (2σm) values for all samples were 44.

(56) expected to less than 0.000010 for both 87Sr/86Sr and. 143. Nd/144Nd in most case. The certified. standard NBS987 and JMC Nd were used to ensure quality both as an internal standard and as a check for instrumental functionality. Analyses of NBS987 and JMC Nd standard run during the study yielded 87Sr/86Sr of 0.710248±21 and 143Nd/144Nd of 0.511816±11.. 45.

(57) CHAPTER 5. RESULTS. 5.1. Zircon geochronology Zircons separated from six samples were subjected to in situ laser ablation U-Pb geochronology. Around 500 analyses were analyzed, including 94 analyses from MH-01, 96 analyses from MH-02, 84 analyses from MH-03, 60 analyses from PL-01, 95 analyses from PL-02 and 110 analyses from PL-03. 5.1.1. Zircons morphology Zircon morphology, including shape, size, aspect ratio, and zoning pattern, provide important clues to magmatic process during the lifetime of grains. The features listed below occur in both Mahé and Praslin samples, but archetypical characteristics are described below. Mahé granite (MH-01) Zircons in this sample are quite simple and two main populations were identified. The first is characterized by prismatic crystal (50 to 100 μm in width, 100 to 200 μm in length) with longitudinal banded zoning. The second is entirely comprised of subhedral to euhedral grains, but also some tetragonal shape. Many zircons have internal structures with bright cores and homogeneous features or oscillatory zoning that is surrounded by a thin growth rim. The remaining crystals are stubby with average length less than 70 μm. Mahé granite (MH-02) MH-02 has the smallest zircon size. Zircon selected for dating in this samples are euhedral to subhedral in shape, and have a low aspect ratio from 1.0 to 1.5 (with average of 100 μm in length). In general, the most populated grains revealed the concentric oscillatory growth zoning core, with color ranging from bright luminescence to dark-gray nonluminescence. Some grains show a core with smooth-edges, spherical, irregular or angular shape. Longitudinal banded zircons also occurred with rounded terminations as well.. 46.

(58) Mahé granite (MH-03) Most of zircon grains separated from the sample generally have the anhedral to euhedral shapes with length ranging from 150 to 300 μm. Morphologically, they can be divided into two groups. The first group is represented by sub-prismatic crystal and crystal fragments with high aspect ratio and parallel banded zonation. The second group is comprised of crystals with corerim structure. They all reveal bright luminescence to low luminescence in the core compared to the thinner rim. In some cases, high luminescence patches occurred obscure the primary texture in the external rims. Praslin granite (PL-01) CL images of this sample illustrated the zircons with subhedral to anhedral shape or in form as fragments of longer crystals. Zircons are both simple (~ 80%) and composite. Two types of simple zircon were recognized: (1) crystals with low aspect ratio (80 to 140 μm in length), which typically demonstrate concentric oscillatory zoning; (2) anhedral zircons that have gray to dark color, non-luminescence sometimes accompanied with bright regions and irregular, broken or rough margin, with no visible zoning. Praslin granite (PL-02) One of the most distinctive features of the studied zircons in sample PL-02 is the prismatic forms and subhedral or fractures shapes. The average zircon size is around 70×160 μm, but range up to a maximum of 110×300 μm. CL images illustrate that a significant amount of zircons have composite growth zoning. Some of them have zoned cores with concentric growth zoning of external rims or conversely or both. Also, some grains have parallel banded zonation and low luminescence cores. The zircons with well-developed oscillatory zoning from core to rim also observed. CL image also revealed some homogenous grains. Praslin granite (PL-03). 47.

(59) Most of zircons in this sample have subhedral shapes with a variety of lengths from 70 to 200 μm and have prismatic habit. The majority of zircons have complex growth zoning; those crystals can be concentric or missing parts but show the aspect ratio of 1.5 to 2. The cores that have low or high luminescence were also encountered while the rims display typical oscillatory zoning. Some of crystals have a high aspect ratio and parallel polysynthetic banded. The crystals with longitudinal banded and concentric zoning also were found. Numerous separated zircons appear to be fragment or longer crystals with chevron-shaped grew in one direction.. Figure 5.1. The CL images of several selected zircons of studied samples. 48.