The Pan-African Biotite-Muscovite Granite and Amphibole-Biotite Granite of Doua (Central Cameroon): Zircon Features, LA-MC-ICP-MS U-Pb Dating and Implication on Their Tectonic Setting

The Doua area belongs to the Adamawa-Yade domain (AYD) of the Central African Fold Belt (CAFB) in Cameroon. It is crossed by the Central Cameroonian Shear Zone (CCSZ). The purpose of this research is to determine the tectonic setting of granites of the studied area, based on their petrography, zircons feature and their ages. Petrographicaly, the Doua area is made up of plutonic rocks hosted in an ortho or paraderivative metamorphic basement. Amphibole-biotite granite (ABG) and biotite-muscovite granite form hills and crop out on the hill side, as flagstones in the valley or river bed, and as huge blocks. Amphibole-biotite granite is granular in texture and made up of amphibole, biotite, feldspar, accessory minerals (Sphene, zircon, and apatite) and secondary minerals (sericite). Biotite-muscovite granite (BMG) is granular to granular porphyritic in texture, made up of biotite, muscovite, plagioclase, K-feldspar, quartz, accessory minerals (zircon, opaques minerals) and secondary minerals (chlorite, sericite). ABG show elongated zircon grains with well-developed magmatic oscillatory zonation. LA-ICP-MS U-Pb data from these zircons define a concordia age of 607 ± 3 Ma and considered as crystallization age of the granite. BMG shows zircon with various shape and various internal structures (oscillatory zonation, sector zonation, blurred oscillatory zonation …) which are divided into two sets. The first set regarded as xenocryst from Paleoproterozoic granitoids emplaced at 2126 ±36 Ma or from sediments which detritus comes from such granitoids. The second set of zircons shows effects of recent lead lost and a subconcordant U-Pb age of 646 ± 39 Ma . ABG was emplaced during the D3 deformational phase, date at 607 ± 3 Ma , while BMG was trigger because of syn-D1 activity of the CCSZ at 647 ±46 Ma in association with the collisional process.


Introduction
The Central African Fold Belt (CAFB, Figure 1) is classicaly subdivided into three main lithostructural domains [16,18,32]. Between the West Cameroon Domain (WCD) and the Yaounde Domain (YD) lies the Adamawa Yade Domain (AYD). The AYD is characterized by the abundance of syn to late tectonic granitoidic rocks [1,9] and the regional transcurent shear zone among which the Tcholliré Banyo Shear Zone (TBSZ), the Central Cameroon Shear Zone (CCSZ), the Sanaga Shear Zone (SSZ). Petrological, geochemical, tectonical and geochronological investigation have been carry out in granitoids of central Cameroon, part of the Adamawa Yade domain [4,17,19,30]. Despite these numerous works, many areas still unexplored and it is difficult in the present state of knowledge to decipher the number of successive generation of granitoids during the Pan-African orogeny in central Cameroon. In the present paper, we present petrography, zircon feature and Th-U-Pb data of biotite-muscovite granite and associated amphibole-biotite granite. We then deduce the implication on their geodynamic setting.

Geological Setting
Doua area ( Figure 2) is part of the Adamawa Yade domain in central Cameroon, very close to the central cameroonian shear zone. Central Cameroon is characterized by widespread of granitoids, many in close association to the transcurrent shear zone [3,17]. The hosted rocks are ortho-and paraderivative gneisses, of various composition (Amphibole biotite gneiss, biotite gneiss), of Paleoproterozoic ages [1,20]. They have been considered as belonging to the West Central African Fold Belt and which have been reworking during the panafrican orogeny at Neoproterozoic ). The paleoproterozoic ages referred to igneous and contemporary metamorphic ages. Archean crustal inheritances have been demonstrated in these rocks [2,16]. In Ngaoundere, granitoids are amphibole biotite granite, biotite granite and biotite and muscovite granite of mantle and crustal origin respectively [30]. In the Meiganga area, granitoids are amphibole biotite granite, biotite muscovite granite, metadiorite with metaluminous to slightly peraluminous in character [3]. They show crustal, mantle, and/or metabasalt or metatonalite melt signature. Emplacement and cristallisation of metadiorite and pyroxene-amphibolebiotite granite was date at 614-619 Ma and 601 ± 1 Ma respectively (Lead evaporation ages, [1]). In Tibati area, diorite, tonalite, granodiorite and granite form four plutons, emplaced in close association with central Cameroon Shear Zone [17]. In Ngaoundere, granitoids are made up of hornblende-biotite granitoids, generated by magma from subcontinental lithospheric mantle, biotite granitoid and biotite -muscovite granitoid, both probably derived from melting of middle continental crust [30].

Mineral Separation
Zircons were extracted by conventional mineral separation using crushing, sieving, shaking table, heavy liquid and manual separation. Highest quality zircons were mounted in 1-inch epoxy resin discs, grinded and polished to median sections and investigated by SEM-CL imaging to reveal internal chemical zonation features.

Laser-ablation MC-ICP-MS
The LA-ICP-MS (laser-ablation microprobe -inductively coupled plasma -mass spectrometry) analytical work was performed at the Laboratory of Technische Universität Graz, Austria. Analytical procedures were identical to the methodology outlined in [8]: Zircon 206 Pb/ 238 U and 207 Pb/ 206 Pb ages were determined using a 193nm solid state Nd-YAG laser (NewWave UP193 Excimer) coupled to a multi-collector ICP-MS (Nu Instruments; Nu Plasma II). Ablation using He as carrier gas was raster-wise according to the CL zonation pattern of the zircons. Line widths for rastering were 10 µm with a rastering speed of 5 µm/sec. Energy densities were 5 -8 J/cm2 with a repetition rate of 10 Hz. The He carrier gas was mixed with the Ar carrier gas flow prior to the ICP plasma torch. Ablation duration was 60 to 120 sec with a 30 sec gas and Hg blank count rate measurement preceding ablation. Ablation count rates were corrected accordingly offline. Remaining counts on mass 204 were interpreted as representing 204 Pb. Static mass spectrometer analysis was as follows: 238 U in a Faraday detector, 207 Pb, 206 Pb, and 204 (Pb+Hg) were in ion counter detectors. 208 Pb was not analysed. An integration time of 1 sec was used for all measurements. The ion counter -Faraday and inter-ion counter gain factors were determined before the analytical session using reference zircon Plesovice [27]. Sensitivity for 206 Pb on reference zircon Plesovice was c. 30'000 cps/ppm Pb. For 238 U the corresponding value was c. 35'000 cps/ppm U.
Mass and elemental bias and mass spectrometer drift of both U/Pb and Pb/Pb ratios, respectively, were corrected using a multi-step approach: first-order mass bias is corrected using a dried 233 U-205 Tl-203 Tl spike solution which was aspirated continuously in Ar and mixed to the He carrier gas coming from the laser before entering the plasma. This corrects for bias effects stemming from the mass spectrometer. The strongly time-dependent elemental fractionation stemming from the ablation process is corrected for using the "intercept method" of [29]. The calculated 206 Pb/ 238 U and 207 Pb/ 206 Pb intercept values are corrected for mass discrimination from analyses of reference zircon Plesovice measured during the analytical session using a standard bracketing method [7]. The correction utilizes regression of standard measurements by a quadratic function. A common Pb correction was applied to the final data using the apparent 207 Pb/ 206 Pb age and the Stacey and Kramers [28] Pb evolution model. Final age calculations were performed with Isoplot© 3.0 [11]. All errors reported for LA data are at the 2-sigma level.

Petrography of the Study Samples
The studied rocks are located in the Doua area ( Figure 2). Amphibole-bioite granite (ABG) crop out on the hills west of Doua village (Figure 3a). It shows frequently amphibolite enclave ( Figure 3b). The rock is granular in texture ( Figure 3c) and made up of amphibole, biotite, Feldspar, quartz, accessory minerals (sphen, zircon, apatite), opaque mineral and secondary minerals (cericite). Amphibole is a green hornblende of variable size (1 to 6 mm long), with inclusions of zircon, apatite, opaques (Figure 3d), and portrays sometime Carlsbad twin system. Amphibole is associated to brownish biotite which is sometime transformed into chlorite and opaque mineral. Feldspar is constituted of plagioclase and K-feldspar. Plagioclase displays regularly combined albite and Carlsbad twining. Albite twin is sometime beveled in the mineral. Some crystals are affected by ceritization. Plagioclase is associated to quartz and K-feldspar. K-feldspar is perthitic, with film exsolutions sub perpendicular to the elongation of the crystal. Bourgeons of myrmeckite usually growth at the border of the Kfeldspar.  Biotite-muscovite granite (BMG) crops out at the top of the Baggo hill, as flat stones surmounted by huge decametric blocks (Figure 4a). It shows granular to granular porphyritic in texture (Figure 4b) made up of biotite, muscovite, plagioclase, K-feldspar, quartz, accessory minerals (zircon, opaque minerals), and secondary minerals (chlorite, sericite). The rock shows fain schistosity with the aligment of biotite. Biotite forms brownish flake of 0.8 to 1.2 mm long, associated to feldspar, muscovite of the same size ( Figure 4c) and opaque minerals. Quartz is subhedral with ondulose extinction and sometime grouped into many crystals. Plagioclase usually combines albite and carlbad twins. It is associated to Kfeldspar and is being transformed into sericite. K-feldspar is made up of perthitic orthoclase with flaming albitic exsolution, and microcline. K-feldspar regularly shows myrmeckitic intergrowth at its border ( Figure 4d). Opaque minerals are associated to other minerals of the rocks and sometine in inclusion in feldspar.

Zircon Feature
The amphibole-biotite granite has zircon grains characterized with few exceptions by length/wide ratio of approximately 2.5/1. The internal structure of representative grains is shown in Figure 5. They show well developed magmatic oscillatory zonation. Some zircons have elongated to sub roundish inclusions (e.g. LAO-08, LAO1-20).

U-Pb isotopic Analysis of Zircons
The 206 Pb/ 238 U and 207 Pb/ 206 Pb apparent ages vary from 2326 ± 2 Ma (Siderian) to 565 ± 4 Ma (Ediacaran), and 2146 ±0.5 Ma (Rhyacian) to 585 ± 6 Ma (Ediacaran). A concordia plot of all zircon grains data of the BMG sample (BAG2) portrays a Discordia line with an upper intercept at 2123 ± 52 Ma (Rhyacian) and lower intercept at 610 ±57 Ma (Ediacaran, Figure 8). With respect to 206 Pb/ 238 U ratio, two groups of data are distinguish: data with ratio superior to 0.3 and data with ratio inferior to 0.3. In the Concordia plot, the first group of data defines a Discordia line (Figure 9) with Rhyacian upper intercept (2126 ± 36 Ma) and Cryogenian lower intercept (667 ± 46 Ma); zircon grain BAG2-18 is in a reverse discordancy, which is difficult to understand in a local geological context. Nevertheless, we do not discard it because at the error limit, it has no influence on the result. The second group defines a discordia line with upper intercept at 647 ± 46 Ma (Cryogenian) and lower intercept at 6 ± 530 Ma (plot not schown). We consider the lower intercept as being due to recent lead loss; therefore, we anchored the lower intercept at a more realistic and better constrained age, 1 ±0.5 Ma and therefore, the upper intercept becomes 646 ±39 Ma ( Figure 10). The data of the first group are obtain in a rastering line located in the inner part of the grain (BAG2-18, BAG2-38, BAG2-48) except the data of one zircon (BAG2-70). Data of the second group are obtained mainly from the outer domains of zircon.

Discussions and Conclusion
The amphibole-biotite granite is characterized by granular texture made up of amphibole, biotite, feldspar, quartz, sphene, apatite and secondary mineral (chlorite, sericite). The brownish color of the biotite and the presence of sphene are significant of Ti in the original magma. ABG is characterized by zircon with well-developed oscillatory zonation. This zonation is inherited from the synmagmatic crystallization of the mineral. It expresses the cyclic accumulation of trace elements during the growth of the mineral in a melt [25]. The relative uniformity in the oscillatory zonation of the zircons means that they crystalize all in the same magmatic environment. Thus, the concordia age 607 ± 3 Ma from zircons of the LAO1 sample represents the crystallization age of the amphibole biotite granite. This value is at the error limit close to the mean age (601±1 Ma) obtained in the pyroxene-amphibole-biotite granite to the east of Meiganga and considered as dating its emplacement [3,5]. It is not unlikely that amphibole-biotite granite of Doua and pyroxene-amphibole-biotie granite of Meiganga are contemporaineous of the D3 deformational phase as mentioned by [1]. Contrary to the pyroxene-amphibolebiotite granite of Meiganga and Baibokoum with pyroxene, the amphibole-biotite granite under study has no pyroxene; it is important to note that the presence of pyroxene in such granite has been interpreted as xenocryst of the metamorphic basement [1,13]. One can deduce that the mineralogy hosted rock in the Doua area has no pyroxene.
The biotite-muscovite granite shows a granular to granular porphyritic texture with biotite, muscovite, plagiclas, K-feldspar, quartz, zircon, opaque minerals. The presence of two micas in the rock is indicative of its crustal origin. The faint schistosity observed in the BMG portrays its syn-tectonic character. Zircons of the biotite-muscovite granite show two domains, indicating that the evolution of the rock or minerals went through two different environments. The fact that the outer part is transgressing the inner part of the zircons (e.g. BAG2-12, BAG2-48) means that the outer part is formed by transformation of the preexisting zircon. Vavra et al., 1996, show that preexisting zircon can be transform during metamorphism. Th/U ratios of the zircons range from 0.22 to 1.07 except one value at 0.02 (BAG2-72); these values are typical of the magamatic zircon with respect to the lower limit (0.1) of the Th/U ratio define by Rubatto et al. [24,26]. The lower Th/U ration of zircon BAG2-72 can be explain by recrystallization during metamorphism as demonstrated by Hosking an black (2000). It is known that granitoids of central Cameroon are hosted in paleoproterozoic ortho-or para-derivative gneiss [1,22]. It is likely that the zircon grains which give ages of 2126 ±36 Ma (Upper intercept, cf Figure 10 ) and 667 ±46 Ma (Lower intercept, cf Figure 10) constitute xenocryst from the basement; may be from a paleoproterozoic granitoid emplaced at 2126 ±36 Ma or from sediments which detritus comes from such granitoids. The subconcordant zircons group with age of 646 ±39 Ma represent mineral of the biotite muscovite granite and thus date the crystallization of the magmatic body. The sub concordant character of these data could be linked to the recent lead lost as can be seen with the lower Anchored intercept at 1 ± 0.5 Ma (Cf Figure 10). While considering four zircon grains (BAG2-02, BAG2-06, BAG2-72, BAG2-74) of this group, one obtains a discordia with an upper intercept at 657±57 Ma, the lower intercept being anchored at 1±0.5 Ma. The other three zircons (BAG2-14, BAG2-40 and BAG2-60) give concordant ages of 577±17Ma. It is not unlikely that the two ages correspond to two distinct tectonothermal events. The age 657±57 Ma is obtain in the inner domain of zircon with dominantly sector zonation (e.g. BAG2-72, cf. Figure 10). This value is at the error limit similar to the crystallization age of the amphibolebiotite granite LAO1, 607 ±3 Ma. It can therefore be regarded as dating the generation of the biotite-muscovite granite. The concordant age at 577±17 Ma is obtained at the border of the zircons (e.g. BAG2-14, cf. Figure 10) showing blurred oscillatory zonation. This feature means that the original magmatic oscillatory zonation was later blurred during a tectonic event. The tectonic event may be the latest activity of the CCSZ, characterized by dextral slip and which were though not older than 558±2 Ma ( 207 Pb/ 206 Pb zircon evaporation age) by Ganwa et al. ([3]). The crystallization age of the granite is at the error limit similar to the lower intercept of the xenocrystal zircons which corresponds to the age of the metamorphism they suffer later. It is likely that biotite and muscovite granite is a syn metamorphic granite. This granite may originate from the anatectic melt of crustal material. The age 646 ±39 Ma may date not only the crystallization of the granite, but also the ultimate stage of metamorphism in the area with intense migmatization. Two micas granite is generally peraluminous rocks which occur in a variety of tectonic environment: closing back-arc basin environments, continental belts inboard of active arcs, active accretionary prism, continent-continent collision, large-scale shear zones associated with continental collision [10,12,23,33].
In fact, [4] associated the sinistral movement of the CCSZ to syn-D2 magmagenesis of the Meiganga metadiorite in the meantime of 614-619 Ma ( 207 Pb/ 206 Pb zircon evaporation age), while the right lateral wrench movement was bracketed between 585 and 540 Ma [14], which is younger than the age of the granite. In Cameroon, the early tectonic evolution of the central Africa Fold Belt involve thrusting and shortening that leads to the crustal thickening between 630-620 Ma [14]. The age of the biotite-muscovite granite is at the error limit similar to the age of the thrusting event date at 630 ±5 Ma in the Poli micaschist [31], and at 620 ±10 Ma in the Yaoundé granulite [21]. It is not unlikely to consider that the studied biotite-muscovite granite was trigger because of the pre-D2 (perhaps syn-D1) activity of the CCSZ at 647 ±46 Ma in association with the collisional process. The natures of the movement of the CCSZ at that time, as well as the imprints of the movement in the basement need to be target.