Petrology and Geochemical Characteristic of Granitoids From Guéra Massif in the Central Part of Chad: An Example of Mixing Magmas

The granitoids of Guéra Massif are composed of biotite-granite, amphibole-biotite granite and gabbro-diorite and commonly contain micro granular mafic enclaves which vary from monzogabbro to syenite composition. They are metaluminous, high-K calc-alkaline to shoshonitic series. Gabbro-diorite rocks are magnesian while amphibole-biotite granites are magnesian to ferroan, and biotite granites are ferroan. They are enriched in LREEs relative to HREE and display negative anomalies in Nb, Ta and Ti. Fields relationships, petrology and geochemistry indicate that mixing and mingling processes could be more relevant for the genesis of granitoids associated to fractional crystallization. Thus, the presence of mafic enclaves of gabbro-diorite composition in the granites, the resumption of alkaline feldspar xenocrystals in the gabbro-diorites, as well as the linear correlation between the granites and the gabbro-diorites and the intermediate position of the mafic enclaves between the two formations, enable us to propose magmatic mixing as the major process that presided over the evolution of the Guéra granitoids. The delamination of the continental lithosphere during the post-collisional phase of the Pan-African orogeny would have caused the partial melting of the subduction-modofied mantle and lower continental crust and thus produced the magmas of the Guéra granitoids.


Introduction
Granitoids form the major part of continents (Clarke, 1992). They show in addition, a very great diversity in their petrogenesis and geochemistry. This diversity was used to determine their source, origin and evolution in relation to their geodynamic environment of genesis and emplacement (Hutton, 1988;Barbarin, 1999). Another characteristic of these granites is their richness in hosted mafic enclave that may retain important information on crust-mantle interaction and thus are significant for study of crustal growth and differentiation. Furthermore, magma mixing and mingling between mantle-derived mafic magmas and crust-derived felsic magmas is one of the key processes underlying magma differentiation and have been used widely to explain the origin of granitoids (Collins, 1996;Elburg and Nicholls, 1995;Keay et al., 1997;Kemp et al., 2007). The Precambrian basement exposed in the Gué ra massif in central Chad is part of the Pan-African Central Africa Orogenic Belt (Fig. 1) separating the Congo craton to the south, the West African craton to the west and the Sahara metacraton to the north (Bessoles et Trompette, 1980;Black et Lié geois, 1993;Abdelsalam et al., 2002. It is made of Neoproterozoic juvenile crust composed of granitoids and enclaves (Kasser, 1995;Doumnang, 2006;Penaye et al., 2006;Isseini, 2011). Field relationships and geochemical trends have led many authors to argue that mixing between two end-member magmas underlies the evolution of granitoid (Collins, 1998;Gray, 1984;Gray and Kemp, 2009;Keay et al., 1997). Moreover, the heat sources for the production of its magma still remaining controversial (Nabelek and Liu, 2004).
Contrary to previous works that proposed magmatic mixing to explain isotope heterogeneities, this paper is essentially based on good field observations. Thus, this paper investigates magma mixing and mingling by linking better field observation and petrographic evidence with whole rock major and trace-element from granitoids of Gué ra massif in which mafic enclaves are commonly observed in order to better constraints their source nature and their mechanism for the formation, which in turn might have great significance in understanding of the Proterozoic crustal evolution in the southern of Sahara Metacraton. Indeed, in the locality of Bitkine outcrop large slabs and domes of granitoids that offer very good structures of magmatic breccias and swarms of microgranular basic enclaves in the granites reminiscent of incomplete mixing of magmas.  Castaing et al. (1994) and Abdelsalam et al. (2002), after

Geological Background
The Precambrian basement in Chad is comprised of four major massifs. The Tibesti massif (located in the north), the Ouaddaï massif (located in the east), the Gué ra massif in the centre and the Mayo Kebbi massifs in the south and southwest (Fig. 1). The Gué ra Massif, one of the smallest massifs within the Saharan Metacraton, is composed about of 90% of magmatic and metamorphic rocks (Fig. 2) and inferred Neoproterozoic age buried under a sedimentary cover dominated by Quaternary alluvium. In addition, the Gué ra Massif is comprised of volcanic and intrusive facies and meta-sedimentary (schist, marbles, quartzite, paragneiss and orthogneiss) and meta-volcanic (amphibolite, meta-andesite) sequences that are partially buried under Quaternary alluvium ( Fig. 2; Isseini et al., 2013). They are composed of two types of granite (older granite and younger ones).The older granites (595 ± 8 Ma, 589 ± 6 Ma, method U-Pb on insitu zircons) are metaluminous to peraluminous, magnesian and alkali-calcic to calc-alkalic in composition . They are volcanic-arc-type, indicating the subduction zone and the subsequent collision between the southern Saharan Metacraton and the Congo Craton continued in the east after collision occurred in the west (Cameroon). The younger ones can be separated into two groups based on their emplacement age (570 Ma or ≤560 Ma). They are peraluminous, ferroan and calc-alkalic. Whole rock compositions and petrological modeling indicate that these granites were derived by melting of crustal materials but the whole rock are characteristic of a mantle source. The granitic rocks of the Gué ra Massif range in age from 595 ± 8 Ma to 556 ± 7 Ma (method U-Pb on insitu zircons) indicating they are related to tectonomagmatic processes associated with the collisional (> 590 Ma) and post collisional (< 590 Ma) stages of the Central African Fold Belt (Pin and Poidevin, 1987;Kusnir and Moutaye, 1997;Abdelsalam et al., 2002;Toteu et al., 2004;Pouclet et al., 2006;Ngako et al., 2008;Shellnutt et al., 2017). Based on geophysical studies, Louis (1970) suggests the existence of a suture zone in the region, as evidenced by Bouger anomalies positive Ounianga-Massenya. The contrasting inheritance-rich nature of the granites with a juvenile Nd isotopic signature is likely due to mixing between magmas derived from juvenile (Neoproterozoic) arc-related crust and asthenospheric magmas .

Fields Relationship and Petrography
In the Gué ra massif, the main rocks type (crop out as flagstone or bowls) consists of biotite-granite (BG), amphibole-biotite granite (ABG) and gabbro-diorite (GD) (Fig. 2). The contacts between GD and ABG are brecciated (Fig. 3a). ABG are crosscut by sinistral N-S trending mafic dykes (GD) and E-W aplitic dykes (consisted of BG), which can exhibit cooling rims (Fig. 3d). Textures of granitoid vary from preserved igneous fabric with K-feldspar defining strongly magmatic fabric in the porphyritic sample to strongly foliated (Fig. 3e). Rapakivi feldspars occurring in the ABG (Fig. 3f). Microgranular mafic enclaves (MMEs) are common in ABG and in GD, but scarce in the BG. The MMEs are mostly more mafic than their host-rocks. They are rounded lens shaped or angular and brecciated and centimeter to decimeter in size. Some of them are elongated-ellipsoidal shaped. Size of long axis of enclaves is ranging from centimeter to meter and defined N-S magma foliation (Fig. 3b). The boundaries between mafic enclaves and host rock commonly have sharp; seldom gradually. The sharp contacts with granitoid are sometimes locate and commonly underlined by finer-grain-rize. A mafic intruded and mingled with the host granite which were partially crystallized because phenocryst of host granite was transferred to microgranular enclaves as xenocryst (Fig.3b). Plutonic dykes often disrupted into enclave swarms exchange of crystals (xenocrysts) of K-feldspar were observed (Fig. 3f).

Amphibole-Biotite Granite (ABG)
ABG outcrop as bowls and flagstone at Doli, Ambasera, Banala, Bokio and Morgue localities ( Fig. 2 and 3d). In the field, they are grey to dark-grey in colour and made up of K-feldspar megacryst (up to 2 cm), quartz, plagioclase, amphibole and biotite as major minerals with porphyritic texture while zircon, apatite and oxides occurred as accessory minerals. The preferred orientation of those K-feldspar megacryst define a magmatic foliation. Some crystals display micro-fractures filled with quartz. Under the petrographic microscope, K-feldspar (40%) is euhedral to subhedral and contains inclusions of grains of quartz. Quartz (25%) is anhedral and present as large crystals with homogeneous extinction. Plagioclase (18%) forms automorphic tabular crystals. Some crystals are zoned and altered to sericite (Fig. 4a). Plagioclase phenocrysts were rim resorbed. Sometimes it develops pertithic texture. Amphibole (15%) is euhedral and subhedral and is partially chloritized. Biotite (6%) forms small dark brown crystals dispersed in the rock. Its abundance is variable, giving to this granite a mesocratic to leucocratic aspect. Biotite occurs as flakes of various dimensions that may be clustered.

Biotite Granites (BG)
BG are common in the Arangha and Mataya localities in the north part of the pluton (Fig. 2). They are small grain size (40 x 60 cm) and outcrop as vein oriented E-W, showing clear contact with the biotite orthogneiss and gabbro-diorite ( Fig. 2  and 3d). Rocks are grey to grey-dark in color medium-to fine-grained, displaying granular textures. They are also composed of quartz (25-30%), K-feldspars (45-50%), plagioclase (15-20%) and biotite (3%). as ABG, but differ by the absence of amphibole (Fig. 4b). The accessory minerals are represented by zircon, apatite and oxides. Microscopic observation shows that, quartz occurs as anhedral to subhedral with homogeneous extinction and a late interstitial phase crystals with homogeneous extinction. Biotite is pleochroic from brown to yellow and partially chloritized. Some plagioclase grains contain small anhedral K-feldspath crystals. K-Alkali feldspars are large, anhedral to subhedral.

Gabbro-Diorite (GD)
Gabbro-diorite outcrop as elongated bands along the north and south part of the pluton ( Fig. 2 and 3c-d). Apart from the presence of clinopyroxene, these rocks contains a similar mineralogical assemblage with ABG. The rocks are dark-grey in color and display fine to coarse grain texture. Plagioclase (30-40%) occurs mainly as medium to coarse, tabular, subautomorphic grains forming aggregates with other groundmass minerals, mainly quartz, biotite and sometimes amphibole. Some of the plagioclase grains contain inclusions of quartz and biotite while small plagioclase is observed in inclusions in larger crystals. Amphibole (20-25%) is yellowish green to brownish green or dark green and occurs as xenomorphic to subautomorphic, medium-to coarse grains, commonly associated with clinopyroxene and biotite (Fig. 4c).

Enclaves (MMEs)
The enclaves have black to dark gray color and are melanocratic and fine grained. They have generally microgranular but sometimes porphyritic textures due to the presence of plagioclase phenocrystal (Fig. 4e). Enclaves range in composition from monzogabbro in host gabbro-diorite, syenite in amphibole-granite and gabbro-diorite in biotite-granite. Most of the enclaves are microgranular and mafic than the host rock and are composed of plagioclase (40-45%), amphibole (25-30%), biotite (10-15%), K-feldspar (>10%), clinopyroxene (10-15%) and quartz (>5%). Zircon, acicular apatite ( Fig. 4f) and oxides are accessory minerals. Chlorite is only secondary mineral which resulted from the hornblende alteration. K-feldspar is as anhedral to subhedral crystals and sometimes contains mafic mineral inclusions. Alkali feldspar phenocrysts were rim resorbed (Fig. 4e). Plagioclase occurs mainly as medium to coarse, tabular, subautomorphic grains forming aggregates with other groundmass minerals, mainly quartz and biotite, occasionally hornblende. Some of the plagioclase grains contain inclusions of subrounded quartz and biotite. Amphibole is yellowish green to brownish green or dark green in color and occurs as subautomorphic to xenomorphic medium to coarse grains and commonly associated with pyroxene and biotite. Clinoyroxene occurs as phenocryst euhedral to subhedral. It is generally rimmed by amphibole. Some aggregates of hornblende and biotite interpreted to represent pseudomorphose after clinopyroxene are observed (Fig. 4g). Quartz is rounded and every so often occurs as ovoid grains (Fig. 4h). Biotite is yellow-brown and occurs as medium grained size. It appears as late mineral sometimes result of alteration of amphibole or pyroxene (Fig. 4h). The mineral assemblages of enclaves show that their chemical composition is similar to gabbro-diorite. They specific rocks are gabbroic-diorite and amphibole-biotite and biotite-granite. All the microgranular studied are enriched biotite and contain abundant green hornblende and small amount quartz. Monzogabbroic-and dioritic enclaves have I-type mineral assemblages that are broadly similar to those in the host granitoids except for the greater abundance of mafic minerals, such as biotite. They are mostly more mafic than their host rocks.

Analytical Methods
Twenty-five samples were collected from different petrographic types including (5 samples) for gabbro-diorite, (5 samples) for biotite granite, (5 samples) for amphibole biotite granite and (10 samples) for enclaves. Seventeen representative samples were analyse in the commercial ACME Analytical Laboratories Ltd, Vancouver, Canada, for whole-rock chemical analysises and results are in Table 1. Representative Major and trace elements data of granitoids from the Gué ra Massif are listed in Table 1. Whole rock analyses for major and trace elements were carried out by Inductively-Coupled Plasma Mass Spectrometry (ICP-MS) from pulps. 0.2g of rock powder was fused with 1.5g LiBO 2 and then dissolved in 100 mm 3 5%HNO 3 . The REE contents were determined by ICP-MS from pulps after 0.25g rock-powder was dissolved with 4 acid digestions. Analytical precisions vary from 0.1% to 0.04% for major elements; from 0.1 to 0.5ppm for trace elements and from 0.01 to 0.5 ppm for rare earth elements. More information on the procedure, precision and accuracy of ACTLABS FUS-ICP and FUS-MS analyses could be found at www.actlabs.com.

Trace and Rare Earth Elements
They are highly variable from a group to another and even within each group. Primitive-normalized REE spider diagram (McDonough and Sun, 1995) show that (Fig. 8) (1995) shows a more pronounced enrichment in Large Ion Lithophile Elements (LILE) than high field strength elements (HFSE) relatively to the primitive mantle (Fig. 8). Enclaves display strongly positive anomaly in Ba and Pb while gabbro-diorite rocks show slightly positive anomaly in Ba and negative anomaly in P and Zr correlated to apatite and zircon crystallization. Both granites display negative anomalies in Ba associated to positive anomaly in Pb. In addition, all analyzed samples show negative anomalies in Nb-Ta, P, Ti that could been related to their tectonic environment.

Evidences of Mixing and Mingling
Field observations such as those in Figure 3a and b are quite significant of the coexistence of two magmatic liquids, basic and acidic. Brecciated structures and swarms of microgranular mafic enclaves in the granite are clear evidence of magmatic mixing. These types of structures are well documented by several authors (Rollinson 1993;Castro et al., 1990;1991a;Didier, 1987;Wiebe, 1996;Tobisch et al., 1997;Poli and Tommasini, 1999;Vernon, 1983;Wyllie, 1984;Barbarin and Didier, 1992). The presence of the syn-plutonic mafic dyke and enclaves in granitoids indicate contemporaneous presence of mafic and felsic magmas and magma mingling and mixing processes (Fig.3b, c and d). The presence of K-feldspar megacrysts in the mafic enclave may imply that they had a similar origin (Vernon, 1991), because K-feldspar esr.ccsenet.org Vol. 9, No. 2; megacryst-bearing microgranular enclaves are common in K-feldspar megacrystic granites. K-feldspar forms in the early stages of crystallization; moreover, the mechanical transfer of K-feldspar megacrysts from a more felsic magma into mafic magma has been suggested as a likely process (Fig.3f). The rimming of K-feldspars by plagioclase (Fig. 3f) is usually a result of magma mixing (Silva et al., 2000). Wiebe (1991 and and Hibbard (1991) have believed that chilled margin of mafic enclaves is a mingling evidence. Field relations show that the enclaves and foliation defined by aligned feldspars have similar orientations, suggesting either magmatic flow or crystal settling within the magma chamber (Fig.3b). The granitoid of Gué ra massif have a mineralogical compositions dominated by abundant biotite, K-feldspar, subordinate amphibole and clinopyroxene. Therefore they may be compared to I-types series (Chappell and White, 1992) and more precisely to the KCG (K-rich and feldspar porphyritic calc-alkaline granitoids) or ACG (Amphibole-bearing calc-alkaline Granitoids) that result from mixing mantle and crustal magmas (Barbarin, 1999). One of the most important petrographic features observed in these samples is the presence of abundant resorbed crystals of plagioclase, alkali-feldspar and quartz (Fig.4). The zoning of plagioclase phenocryts in amphibole-biotite granite suggest involvement mixing process (Barbarin and Didier, 1992). The swarms of mafic enclaves in the granite and the clinopyroxene cores in the hornblende indicate that the hybridization process was not complete (Fig.3b). The presence of amphibole-rich enclaves within amphibole-biotite granite and gabbro-diorite indicate that fractional crystallization played a role during the early stages of the granitoid petrogenesis. Acicular apatite in mafic enclaves has also been identified as a mixing/mingling texture (Fig.4f).
Evidence for mixing between basic and acidic melts can be seen on both the outcrop and microscopic scales, and is also supported by linear and curvilinear differentiation trends on variation diagrams. A considerable good rectilinear trend has been obtained in the TAS plot for the granitoid of Gué ra, strongly marking mixing of magma (Fig.5b). In Harker's diagrams the two lines of evolution converge towards the granites, thus attesting that the latter result from the mixing of two very different magma sources. This is best expressed in the SiO 2 vs P 2 O 5 diagram (Fig.7) where the two granites and some enclave samples are aligned on the mixing line and the gabbro-diorites follow the line of unsaturated apatite rocks.
In addition, plots of MgO vs. Fe 2 O 3 t diagram (Fig.9) after Zorpi et al. (1989) suggest that magma mixing was a major mechanism in the formation of the Gué ra granitoids according to Langmuir et al. (1978). This magma mixing is further suggested by linear array of plots on the Al 2 O 3 /TiO 2 vs. CaO/Na 2 O (Fig.9a) and Na 2 O/CaO vs. Al 2 O 3 /CaO (Fig.9b), White, 1992 andSylvester, 1998) diagrams (Fig.9). The enrichment in large ion lithophile elements and depletion in high field strength elements (e.g., Nb, Ta, and Ti) of granitoid that imply a significant amount of crust material ( Fig.8 and Fig.10c), was involved in their genesis most probably as a result of magma mixing (Langmuir et al., 1978;Pearce et al., 1984). The mafic enclaves are characterized by higher REE contents and a distribution profile comparable to that of the host rocks. Such geochemical similarity between mafic enclave and host rock is very common and testifies to mixing between the two magmas (Tindle, 1991). All of these data indicate significant magma mixing and mingling processes during the history of the Gué ra granitoids.

Evidences of Fractional Crystallization
Whereas we have demonstrated above mixing and mingling magmas were important processes affecting the geochemical evolution of the granitoids, the inverse variation of MgO, Fe 2 O 3 t, and CaO with silica suggests that the primary minerals observed in thin section (plagioclase, biotite, and hornblende) accumulated or fractionated to produce the observed chemical variation (Fig.7, Fig. 9d and Fig. 10 a and b). General negative Nb, Ta, P, Ti anomalies in gabbro-diorites and granites attested the crystallization of accessory minerals like apatite, Ti-Fe oxide associated to the essential's minerals (biotite, hornblende; Fig.8). It is evident that fractional crystallization controlled much of the evolution of the magmas in the Gué ra massif. The behaviour of Al 2 O 3 compared to the behaviour of CaO, MgO (Fig. 7), suggests that clinopyroxene controls the first phase of fractional crystallization followed by plagioclase (Fig. 10 a and b). Indeed, Al 2 O 3 content increases in Gabbro-Diorites and microgranular mafic enclaves and decreases in both granites with increasing SiO 2 content.

Source Rock Characteristics
The investigated granitoids of Gué ra Massif exhibit petrographical and chemical compositions characteristic of high-K calc-alkaline to shoshonitic I-type granitoids White, 1992 and1974) derived from partial melting of igneous protoliths. From the several studies carried out to constrain the generation of high-K magmas in convergent tectonic settings, two main processes are commonly recognized (Altherr et al., 2000): (i) in continental arc settings, where mantle melts can be enriched by slab-derived fluids and further may become contaminated with crustal material during ascent (De Paolo, 1981); (ii) in syn-to post-collisional settings, crustal source rocks may melt as a consequence of decompression following delamination of the lithospheric root or slab break off (e.g. Roberts and Clemens, 1993;Liegeois et al.,1994;kwé kam et al., 2013).
The high-K calc-alkaline to shoshonite nature of granitoids of Gué ra massif suggests that the most suitable protholiths esr.ccsenet.org Earth Science Research Vol. 9, No. 2; could be ancient rocks (Roberts and Clemens, 1993). Compositional differences of melts produced by partial melting of different source rocks, such as amphibolites, tonalitic gneisses, metapelites and metagreywackes, under variable melting conditions can be visualized in terms of molar CaO/(MgO + FeOt) vs. molar Al 2 O 3 /(MgO + FeOt) (Altherr et al.,2000;Fig.11a). In this diagram, most of the samples of the granitoids plot in the field of partial melting from metabasaltic to metatonalitic sources. Few samples of biotite granite and amphibole-biotite granite plot in the field of partial melts from metagreywackes (Fig.11a). This distribution could portray the contribution of the crustal material to its source. Such granite of crustal origin and the contribution of crustal material to the source of granitoids have been demonstrated in the Cameroonian portion of the Adamawa-Yade Domain Ganwa, 2005). Many I-type granitoids e.g. Ngondo, Batié and Fomopé a (Talla, 1995;Kwé kam et al., 2010 and of west Cameroon show the same characteristics. Intermediate to felsic composition of granitoid is consistent with a derivation from the ancient source of tonalitic to granodioritic composition (Singh and Johannes, 1996). These source rocks are predominantly found in the lower part of the continental crust and we suggest that the source for the biotite granite and amphibole biotite granite was lower amphibolite continental crustal. The abundance of hydrated minerals (amphibole and biotite) in the plutonic rocks suggests that the melting of the protolith took place under hydrous conditions. This source can correspond either to a juvenile crust composed of high-K andesites (Roberts and Clemens, 1993) or to phlogopite-K-richterite enriched lithospheric mantle. In both cases, the main point is that the generation of the high K calc-alkaline magma needs the former existence of an important subduction phase to generate its source. Gabbro-diorites and amphibole-biotite granites show a moderate negative Zr anomaly that would suggest a mantle signature. In addition, the position of these rocks in the diagram in figure 8 leads to propose a mantle source for the gabbro-diorites. Highly potassic magnesian characteristics underline their shoshonite affinity. Shoshonites generally result from related subduction enriched lithospheric mantle. This hypothesis is supported by the positions of the Gué ra granitoids in the Ce/Pb versus Pb and La/Ba versus La/Nb diagrams in Figure 10 c and d (Langmuir et al. 1978). In Figure 10d, especially all samples are plotted in the modified-subduction sub-continental lithospheric mantle domain. This domain is the MORB domain enriched by the subduction trench sediments. In addition, the negative Nb-Ta and Ti anomalies and the positive Pb anomalies (Fig.8), indicate that this mantle source has been modified by the addition of material related to subduction.

Tectonic Settings
The Gué ra massif is composed of high-K calc-alkaline granitoids that show a volcanic arc geochemical signature (Pearce et al., 1984;Fig. 11b) and are classified as magnesian to ferroan granites. Ferroan granitoids reflect a close affinity to relatively anhydrous, reduced magma and source region. Such conditions are common in extensional environments. Because, these magmas also generally hotter, they are likely to undergo extensive fractionation towards iron-rich, alkali compositions (Forts and Forts, 1997). In contrast, a magnesium series reflects a close affinity to relatively hydrous, oxidizing magmas and region source (Forts and Lindsley, 1991), which is consistent with origins that are broadly subduction related. High-K calc-alkaline magmatism extensively occurs at post-collisional stages (e.g Barbarin,1999, Lié geois et al.,1998 but it also occurs at active continental margins (e.g Barbarin,1999;Wilson, 1989). Strongly magnesian to little ferroan transitional character of granitoid suggests their emplacement during a shift in tectonic regime from a compressive regime to an extensional regime commonly seen in the post-collisional setting of an orogeny (Bonin, 1990). So we can summarize that the granitoids of present study emplaced in the post-collisional setting (Fig. 11b). Futhermore, granitoids of Gué ra massif were formed at post-orogenic tectonic environment. Post-orogenic extension is always triggered by the delamination of the subducted slab (Sylvester, 1998;Kwé kam et al., 2013;Shellnutt et al., 2017). Thus, delamination inducing an asthenospheric upwelling. Such an upwelling could have caused partial melting of the lithospheric mantle previously metasomatized by subducted materials (Kwé kam et al., 2013), forming basaltic magma enriched in LREE, LILE, H 2 O but depleted in Nb, Ta and Ti. The underplating of such basic magma under the lower crust could have triggered partial melting of the mafic lower crust, forming a silicic magma. Mixing and mingling between the basic magma and the silicic magma, followed by crystal fractionation of mafic minerals and accessory minerals, may have led to the formation and evolution of the granitoids of the present area. Figure 11. (a) molar CaO/(MgO + FeOt) vs molar Al 2 O 3 /(MgO + FeOt) diagram (Altherr et al., 2000) showing probable magma sources of granitoids, (b) tectonic discrimination diagram (Pearce et al.,1984). (Square: Enclaves, Solid square: gabbro-diorites, solid circle: amphibole-biotite granites and circle: biotite granites)

Conclusion
Considering the field and petrographic data obtained for the Gué ra massif, three groups of granitoid can be distinguished: gabbro-diorite, amphibole-biotite granite and the biotite granite. The main mineral phases that characterize these rocks are alkaline feldspar, quartz, plagioclase, clinopyroxene, with subordinated hornblende and biotite, besides of accessory apatite, zircon and opaque minerals. Whole-rock geochemical data reveal a sub-alkaline affinity for the rocks of the Gué ra massif, which belongs to high-K calc-alkaline to shoshonitic series. In combination, the field, petrographic and geochemical evidence presented here suggests that mingling and mixing magma processes could be more relevant for the genesis of granitoids of Gué ra massif with minor fractionation of plagioclase, clinopyroxene. Although the rocks of the massif appear to have been emplaced in a post-collision context, their magmas have the characteristics of a mantle source modified by the subduction-material. This modified source would have undergone partial melting during the lithospheric delamination of the post-collisional phase of the Pan-African orogeny in Chad.