Guéra massif - Domains of the Saharan Metacraton in Chad

CHAPTER 1. INTRODUCTION

1.4. Domains of the Saharan Metacraton in Chad

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).

19 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)

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 meta-sediments and orthogneisses bearing signatures of Archean crust recorded by the inherited zircon (Toteu et al., 2001; 2004); (2) Neoproterozoic metavolcanics-metasedimentary rocks are formed under low- to medium-grade metamorphic conditions during Pan-African times (Toteu et al., 2006a); (3) commonly syn- to late-tectonic 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

(660-585 Ma) that correspond to the emplacement of pre- and syn-tectonic calc-alkaline 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 South-Central 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;

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.

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 south-central 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).

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

25 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 fine-grained 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).

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 ENE-WSW 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

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 calc-alkalic, 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 calc-alkalic, 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

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).

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.

Picture 2.2. Biotite granite recorded deformation evidence.

Picture 2.3. An outcrop of (a) hornblende-biotite granite and (b) rhyolite within Guéra massif.

31 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 post-collisional 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.

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).

33 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 central-northwestern 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).

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).

CHAPTER 3. PETROGRAPHY

In this study, 12 thin sections that correspond to the collisional granite (14ZA02, 06, 25C, 12D, 16A) and postcollisional 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

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.

37 14ZA18

Sample 14ZA18 comprises plagioclase (8-10 vol. %), biotite ( 5-10 vol. %), K-feldspar (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 (reddish-brown to (reddish-brown and yellow-(reddish-brown to dark (reddish-brown), flaky, and perfect cleavage, and parallel extinction. The reddish-brown to brown biotite crystals are larger than the tan-brown to dark tan-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 120^o to 60^o. 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.

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

39 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.

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.

41 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,

在文檔中 An assessment of the magmatic conditions of Late Neoproterozoic collisional and post-collisional granites from the Guéra Massif, South-Central Chad (頁 34-0)