Paleoproterozoic Synkinematic Magnesian High-K Magmatism from the Tamkoro-Bossangoa Massif, along the Bossangoa-Bossembele Shear Zone in North-Western Central African Republic

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This paper provides new data on the geology and geochemistry of representative intrusions from the groups of granites and discusses the geological significance and petrogenesis of orogenic Paleoproterozoic, high-K calc-alkaline magmas from the Northern-western part of the CAR in North-Equatorial Fold Belt; we also compare it with similar rock types in this region. The good exposure of apparently co-magmatic metaluminous, peraluminous and hyperpotassic rocks, spanning a continuous range of chemical compositions (with about 55 - 76 wt.-% SiO₂), make the Tamkoro-Bossangoa Massif a key site to study, the formation of orogenic metaluminous and peraluminous, calc-alkaline magmas in this part of the NEFB. We report the first and new SHRIMP U-Pb in zircon and titanite geochronology.

2. Regional Geological Setting

The CAR has been divided in three major structural units [16-24]^[16], from south to north:

1. Southern unit represent a Northern part of Congo Craton and consists of (i) micaschists and quartzites of Archean and Paleoproterozoic age ^[25]; (ii) metabasites (amphibolites, pyroxenites of Mbomou) of Archean age (2900 Ma; ^[18]); (iii) charnockites series and gneisses similar to those of the Congo Craton in Cameroon ^[21], and (iv) Archean greenstones (komatiites), itabirites, greywacke, rhyodacitic tuffs and granitoids ^[26]. (v) The central part of this domain is occupied by an intermediate rock series (2100 Ma, ^[18]), which consists of quartzites, amphibolites and orthogneisses.

2. The intermediate domain consists of Archean gneisses, metabasites, granites and Paleoproterozoic metasedimentary rocks and migmatites. According to Rolin ^[24] these rocks are separated from the Neoproterozoic gneisses by a ductile shear zone.

3. The Northern part is composed of granulites, orthogneiss and granite of Neoproterozoic age (833 ± 66 Ma ^[18]). It corresponds to the western extension of the Pan-African fold belt in . This domain is bordered in the south by a late Pan-African shear zone ^[27]. The Tamkoro-Bossangoa area (Figure 2) is affected in the north by a tectonic lineament more or less parallel to the Centrafricano-Tchadian border. This lineament extends from through the CAR to . The major lineament that crosses the Tamkoro-Bossangoa–Bossembele region from east to west is a mylonitic band that consists of pseudotachylites, blastomylonites and orthogneisses of the basement. This basement comprises also schist, micaschist, granulites and amphibolites ^[19]. The precise ages of rock units in the study area and the timing of movements along the Proterozoic structures in the CAR are unknown.

Figure 2. Geological map of the Tamkoro-Bossangoa massif

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3. Analytical Methods

Microprobe analyses were carried out at the with a CAMEBAX SX50. Counting times were 15-30 s per element on peak and 5-30 s on background depending on concentration. The accelerating voltage was 15 kV for a beam current of 10-20 nA, depending on the sample analyzed. The beam was rastered over a surface of 10 x 8 microns for feldspars and 3 x 2 microns for hydrated minerals. Natural silicates were used as standards. After the petrographic characterization of the rock units, 24 representative samples were selected for whole rock chemical analysis. Samples were crushed and subsequently reduced to a very fine powder by grinding in a tungsten carbide ring mill. Major and trace element concentrations of whole-rock from 24 samples were analyzed by X-ray fluorescence spectrometry and LA-ICP-MS, at the . Major elements (Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K, P, Cr and Ni) were measured on fused lithium borate glass disks using a Philips PW2400 X-ray fluorescence spectrometer. Trace elements were measured from pressed powder pellets on the same XRF spectrometer (Nb, Zr, Y, Sr, U, Rb, Th, Pb, Ga, Ni, Cr, V, Ce, Ba and La), and by LA-ICP-MS on glass disks (Be, Sc, Ti, V, Cr, Ni, Cu, Zn, Y, Zr, Nb, Cs, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th and U). Tests were made to assess the amount of trace element contamination, such as Ta and Nb, from the tungsten carbide mill. The samples analyses in this study are relatively rich in these elements, therefore contamination is considered negligible. Laser ablation measurements were made with a 193 nm Lambda Physik Excimer laser (Geolas 200M system) coupled to a Perkin-Elmer 6100 DRC ICP-MS. Laser settings were 27 kV with a 10 Hz repetition rate, yielding a fluorescence of about 12 J/cm² on the ablation site. Helium was used as carrier gas (1.1 l/mn) and NIST612 glass was used as the external standard, and Ca and Al as internal standards (on the basis of electron microprobe measurements on the ablation pit site). BCR2 basaltic glass was regularly used as monitor to check for reproducibility and accuracy of the system. Results were always within ± 10% range of the values reported by Witt-Eickschen et al. ^[28], while Rb, Cs, Y and especially Cr were sometimes of the ± 10% range of the USGS recommended values for BCR2. Analytical uncertainties are currently better than 1% for major elements and 5-10% for trace element concentrations higher and lower than 20%, respectively. Analytical precision for rare earth elements (REE) is estimated at 5% when concentrations are >10 times chondritic and at 10% when lower.

Samples of 1-2 kg were subjected by routine heavy mineral separation. Zircon and titanite were hand-picked, mounted in epoxy, and polished. Individual zircon grains were subjected to U-Pb isotopic analysis using the Sensitive High-Resolution Ion Microprobe (SHRIMP) at the Research School of Earth Sciences at the , . Detailed SHRIMP analytical procedures have been reported by Compston et al. ^[29] and Williams and Claesson ^[30]. The technique focuses a primary beam of negative oxygen ions in vacuo onto the zircon and titanite surface from which a small area (25-30μm diameter) of sputtered positive secondary ions is extracted. Secondary ions, which include Zr, Th, U and Pb from the zircon, are passed through a curvilinear flight path in a strong magnetic field and then counted at a mass resolution of 6500 on a single collector using cyclic magnetic stepping. Isotopic ratios and inter-element fractionation are monitored by continuous reference to a standard Sri Lankan zircon (SL13), fragments of which are mounted with each sample. Progressive changes in the Pb/U ionic ratio during sputtering were corrected using an empirical quadratic relationship between Pb⁺/U⁺ and UO⁺/U⁺determined for the standard zircon ^[31]. A radiogenic ²⁰⁶Pb/²³⁸U ratio of 0.0928 for the standard zircon, corresponding to an age of 572 Ma, is obtained through standard isotope dilution analysis. Initial Pb isotope compositions of the analyzed zircons are assumed to be similar to that of model-derived average crustal Pb of similar age according to Cumming and Richards ^[32]. The analytical precision of Pb isotope ratios is controlled by machine counting statistics whereas the precision of Pb/U ratios is affected by uncertainties in the recommended Steiger and Jäger ^[33] decay constants. Weighted mean ²⁰⁷Pb/²⁰⁶Pb ages were obtained for zircons exhibiting an obvious clustering and indistinguishable ²⁰⁷Pb/²⁰⁶Pb ages at a 95% confidence level. Individual analyses in the data tables and Concordia diagrams are presented at 1σ. Errors associated with scatter within the cluster are obtained by standard statistical techniques. Dispersion within the cluster is attributed to modern Pb loss.

4. Results

4.1. Petrography

We use the P = K - (Na + Ca) vs. Q = Si/3 – (K+Na+2Ca/3) multi-cationic diagram (Figure 3) of Debon and Le Fort ^[34] for chemical classification of the different petrographic types. Orthogneiss plot in the quartz-monzodiorite, quartz-diorite and granite fields.

Figure 3. Position of the Tamkoro-Bossangoa granitoids in the Q = Si/3 – (K+Na+2Ca/3) vs P = K – (Na+Ca) multi-cationic diagram of Debon and LeFort [34]. (Mzdq = quartz monzodiorite; Dq = quartz-diorite; Gr = granite. Grey filled square = quartz monzodiorite; Open square = quartz diorite; Open circle = granite)

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4.1.1. Quartz Monzodiorite

The rock is grey-dark in colour, coarse grained, showing granoblastic or mylonitic microstructure (Figure 4a and Figure 4b). The samples are rich in ferromagnesian minerals, and the size of the grains vary from 1 cm to 8 cm and composed of quartz (2-8 vol.-%), plagioclase (20-30 vol.-%), K-feldspar (10-20 vol.-%), biotite (15-20 vol.-%), amphibole (30-45 vol.-%). Accessory phases are titanite (≤ 3 vol.-%), allanite (≤1 vol.-%) and zircon (≤1 vol.-%). Quartz forms elongated polycrystalline ribbons (≤ 10 mm in the length). Perthitic K-feldspar (Or_95-97) exhibits sigmoid (10 x 15 mm) or almond shape (35 x 20 mm), shows perthitic exsolutions of albite and also often show extensive development of myrmekite at the grain peripheries. The crystals contain small inclusions of opaque, apatite, biotite flakes or quartz grains. Plagioclase (An_20-25) occurs as almond shape crystals that may be aligned sub parallel to the schistosity and may show undulatory extinction, or sigmoid shape preferentially oriented and show sinistral asymmetric (Figure 4c) or deformational twinning. Some crystals are locally sheared and have microfractures filled by biotite flakes and quartz grains. Large crystals are kinked. All are rimmed by quartz aggregates. Biotite (X_Mg = 0.50-0.56) occurs as flakes of various dimensions that may be clustered, most of which derive from destabilization of amphibole. Amphibole is a green hornblende (ferroedenite) with almond shape (Figure 4d). Amphibole contains small euhedral inclusions of biotite, titanite, apatite, zircon and ferriferous oxides.

4.1.2. Quartz-Diorite

The Quartz-diorite (Figure 4e) is dark grey medium-grained rocks with mylonitic microstructure (Figure 4f). They are composed of quartz (≤ 5 vol.-%), K-feldspar (15-20 vol.-%), plagioclase (25-30 vol.-%; An_30-31), biotite (30-35 vol.-%; X_Mg = 0.62), amphiboles (20-35 vol.-%) and titaniferous oxide (≤3 vol.-%); accessory minerals including titanite, allanite, zircon and apatite. Quartz forms elongated polycrystalline ribbons with undulatory extinction. K-feldspar (Or_93-95) shows perthitic exsolutions of albite and inclusions of hornblendes. The crystals have sigmoid or almond shape (5 to 2 mm long axis). Some sections are fractured and filled by quartz grains and titaniferous oxide. Plagioclase occurs as almond shape crystals (3 x 2 mm to 2 x 5 mm. Some crystals display exsolutions of quartz (myrmekite) and others are zoned. Inclusions of apatite, zircon, biotite, opaque and titanite occur frequently in plagioclase. Biotite occurs as flakes of various dimensions that may be clustered, most of which have corroded margins, or forms euhedral inclusions in feldspars. Amphibole is a green hornblende with anhedral, or sigmoid shape Amphibole contains small euhedral inclusions of titanite, opaque and zircon.

Figure 4. Field occurrence and photomicrographs of the Tamkoro-Bossangoa granitoids. a: Field photograph of quartz monzodiorite; b: Mylonitic microstructure (quartz monzodiorite); c: Almond shape K-feldspar megacrystal in quartz monzodiorite; d: Almond shape amphibole in quartz monzodiorite. f. Field photograph of quartz diorite; f: Mylonitic microstructure of quartz diorite; g: Field photograph of granite; h: Sheared K-feldspar crystals showing sinistral shear movement in granite

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4.1.3. Granite

The granite is medium- to fine-grained rocks and display mylonitic microstructure (Figure 4g and Figure 4h). They are composed of quartz (20-30 vol.-%), K-feldspar (15-20 vol.-%), plagioclase (30-40 vol.-%), biotite (5-10 vol.-%); accessory minerals including titanite, zircon and apatite. K-feldspar shows perthitic exsolutions of albite. Some crystals are almond shape; more rarely rounded and contain small inclusions of biotite flakes, euhedral allanite. Plagioclase occurs as elongated, subhedral or rounded crystals that may be orientated and may show undulatory extinction. Biotite occurs as flakes of various dimensions, most of which have corroded margins, or forms euhedral inclusions in feldspars.

4.2. Geochemistry
4.2.1 Major Elements

Figure 5. Harker diagrams of selected major elements. Symbols as in

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Major element concentrations of the Tamkoro-Bossangoa rocks are presented in Table 1. The rocks are silica-rich (with a total range of 55.75 - 75.65 wt. -% SiO₂). In major element variation diagrams (Figure 5), the three rock types form well defined clusters and, taken together define more or less well-defined trends, where Al₂O₃, CaO, Fe₂O₃^tot, MgO, Na₂O, TiO₂ and P₂O₅ concentrations decrease monotonously with increasing amount of SiO₂, whereas K₂O increase with increasing of the SiO₂ content. The total alkali concentrations are uniform within each rock unit, i.e., 7.10 - 7.23 wt. -% in quartz monzodiorite, 5.81 - 6.0 wt. -% in quartz diorite and 8.24 - 9.44 wt.-% in granite. The granitoids are K-rich and display characteristics of shoshonite to high-K calc-alkaline series (Figure 6). High alkalis contents relative to CaO are characteristic of calc-alkalic to alkalic cordilleran granitoids according to the classification of Frost et al. ^[35]. When using the Al saturation index A/CNK = molar (Al₂O₃/ (CaO+Na₂O+K₂O), the quartz monzodiorite and quartz diorite are metaluminous, whereas granite is moderately peraluminous, and all conform to I-type (Figure 7) granitoids ^[36]. To discriminate between ferriferous granitoids and granitoids, we use the classification of Frost et al. ^[35], nearly all the granitoids of the Tamkoro-Bossangoa Massif plot in the magnesian field (Figure 8). These granitoids are similar to the other granites in the central domain of NEFB in [e.g. [8,37-42]]. These granitoids are also geochemically similar to those from the Eastern Nigeria ^{[43, 44]}, and Borborema Province, NE Brazil ^[45]: all the rocks are metaluminous to weakly peraluminous (Figure 7b), high-K calc-alkaline and shoshonitic type (Figure 6b), and they are enriched in most incompatible elements (especially for K, Rb, Ba).

Figure 6. (a) SiO₂ vs. K₂O diagram showing high-K calc-alkaline affinity of the Tamkoro-Bossangoa rocks. (b) SiO₂ vs. K₂O diagram for the comparison between Tamkoro-Bossangoa, eastern and NE Brazil granitoids. Symbols as in Figure 3

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Table 1. Representative chemical analyses of Tamkoro-Bossangoa plutonic rocks (wt. %)

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Table 1 (Continued)

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Table 1 (ended)

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Figure 7. Alumina index diagram (Molar Al₂O₃/ (CaO + Na₂O+K₂O) vs Al₂O₃/ (Na₂O + K₂O) for the Tamkoro-Bossangoa granites (a) and (b) comparison between Tamkoro-Bossangoa, eastern and NE Brazil granitoids. Symbols as in Figure 3. Boundary between I- and S-type according to Chappell and White [36]

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Figure 8. Position of the Tamkoro-Bossangoa granites in the SiO₂ vs. FeOt/(FeOt + MgO) diagram, axis units are wt%. Symbols as in Figure 3

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4.2.2. Trace Elements

Trace element concentrations of the Tamkoro-Bossangoa rocks are presented in Table 1. Selected elements are plotted against SiO₂ content in Figure 9. Ni, Sr concentrations decrease with decreasing SiO₂ content. Other concentrations are in general scattered on variation diagrams. Zr/Y ratios range between 0.01 and 8.05 (average 3.97) and higher than Nb/Y ratios (0.005-0.4) <1. These values are similar to those observed in continental calc-alkaline igneous suites ^[46]. Chondrite-normalized rare earth element (REE) patterns of the Tamkoro-Bossangoa suite (Figure 10) resemble each other, in general with LREE enrichment with Ce_N/Sm_N ratios of 0.68 - 1.8 and Gd_N/Yb_N ratios of 1-3, significant Positive Eu* anomalies (Eu/Eu* = 14-25) in the Quartz monzodiorite, Ce_N/Sm_N=1.28- 1.74 and Gd_N/Yb_N=1.68 -2.29, with strong positive Eu* anomalies (Eu/Eu* = 14-17) in the quartz diorite and Ce_N/Sm_N=0.29- 0.69, Gd_N/Yb_N=0.57 -3.74, positive Eu* anomalies (Eu/Eu* = 3.1-5.1) in the granite. This anomaly underlines the role of the plagioclase accumulation in the formation of these rocks.

Figure 9. Harker diagrams of selected trace elements. Symbols as in Figure 3

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Figure 10. Chondrite-normalized REE and multi-element patterns for the Tamkoro-Bossangoa granites

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The multi-element spectra (Figure 10) of all these rocks show a distinctive depletion in K, Nb, Sr and Zr relative to other trace elements (characteristic of calc-alkaline granites of crustal origin ^[47], and enriched in LILE (large ion lithophile elements) and display negative anomalies in K, Nb, Sr, Zr, Y, Th and Ti. These rocks display less pronounced negative anomalies in Sr and Ti, and lower Y and Yb values resulting in more "fractionated" trace element distribution patterns, characteristic of calc-alkaline arc granitoids.

4.3. Geochronology
4.3.1. Zircon Morphology and Internal Structure

Zircon grains from the quartz monzodiorite have distinct morphologies and internal structures. In sample TAM 1b, most grains are brownish and elongated with the original euhedral to subhedral shape clear visible despite of rounded terminations. Oscillatory zoning, typical of magmatic growth, is common (Figure 11). In CL images grain show thick oscillatory growth- and sector zonation and thin external zone due to a metamorphic growth type ^[48]. The Th/U ratios vary from 0.84-1.10 in the core to 0.33-0.78 in the zonation sector and 0.40 in the external zone (Table 2).

Table 2. Analytical data for zircons, from the selected sample, from Tamkoro-Bossangoa plutonic rocks

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Figure 11. Cathodoluminescence (CL) images of typical zircons from the quartz monzodiorite (Tam 1b). Note the thick oscillatory growth and sector zonation (typical of magmatic growth) and thin external zone due to metamorphic growth.

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4.3.2. U-Pb Results

On the Concordia diagram (Figure 12a) zircon fraction Tam 1b define a discordia line with upper and lower intercepts of 2069 ± 9.6 Ma and 500 ± 40 Ma (MSWD= 4.6). On the basis of the zircon morphology, structure and Th/U ratios, this age is interpreted as the magmatic age for the Tamkoro-Bossangoa massif. The remaining analyses (points 1.2 and 18.1) were of zircon have inherited core (as shown in CL images) giving a minimum ²⁰⁷Pb/²⁰⁶Pb age of 2012 Ma (Figure 12a). The lower intercept corresponds to a Neoproterozoic tectono-metamorphic history.

An important quantity of the good quality of dark brown titanite has been collected to heavy minerals separation. This titanite has high U contents that allow making U-Pb analysis. Six points were analyzed on titanite from sample Tam1b (Table 3). Titanite fraction define a Discordia line (Figure 12b) with upper and lower intercept of 2063 ± 28 Ma and 597 Ma (MSWD = 0.25) respectively. This result is quite similar to those obtained in zircons.

Figure 12. Concordia diagram showing all SHRIMP data points for zircon (a) and titanite (b) from the quartz monzodiorite (sample Tam 1b)

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Table 3. Analytical data for titanite, from the selected sample, from Tamkoro-Bossangoa plutonic rocks

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5. Discussion

5.1. Conditions of Emplacement

We applied Zr saturation geothermometer ^{[49, 50]} to Tamkoro-Bossangoa granitoids to estimate the minimum temperatures of the melts (assuming that there is no residual zircon inherited from the source). The calculated temperatures (Table 4) range between 756 - 846°C for the quartz monzodiorite, 722 - 758°C for the quartz diorite, 638 - 658°C for the granite. These temperatures are consistent with those of crustal granites ^{[40, 50, 51]}.

5.2. Source of the Tamkoro-Bossangoa Magmas

The nature of the igneous source can be constrained using the geochemical signatures of the plutonic rocks. The investigated plutonic rocks exhibit petrographical and chemical compositions characteristic of high-K to shoshonitic I-type granitoids ^[52] derived from partial melting of igneous rocks. The geochemical compositions of Tamkoro-Bossangoa granitoids as shown on Harker´s diagrams did not indicate continuous compositional variation from one group to another, but little internal compositional variation and overlap between some groups. This suggests that the compositional variability of these granitoids does not appear to be generated mainly by fractionation processes, but testify an evolution by accumulation as main process in the Tamkoro-Bossangoa granitoids. The High-K calc alkaline magmatism of these rocks is typical of those of collisional to post collisional orogenic domain [e.g. ^{[6, 8, 20, 38, 40, 53, 54]}], and also indicate an active margin environment.

Table 4. Calculated temperatures using Zr saturation geothermometer of Harrison and Watson [49], and Watson and Harrison [50]

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High Ba/Sr (1.5 -6.8) and Ba/Rb (1 - 1.5) ratios are typical for granites of crustal origin (Harris and Inger, 1992). The enrichment of Ba (465-980ppm) relative to Sr (110-200ppm) and the high K₂O (1.85-6.33%) suggest a pelitic parent ^[55].

The strong positive Europium anomaly (Eu/Eu=2-22) underline the role of the plagioclase accumulation in the formation of the granitoids. In fact, the plagioclase accumulation contribute to enrich the magma on Europium during the differentiation, this create the positive anomaly in that element. The LREE high content is certainly due to the abundance of amphibole and accessories phases such as titanite, allanite and apatite. The HREE significant content (10 to 50 times Chondrite values) is due to a relative abundance of Zircon.

The REE and multi-elements patterns (Figure 10) suggest genetic processes involving garnet. The high contents of LREE in all rocks could be related either to the enrichment of their source materials in LREE, or to a low degree of partial melting of source protoliths with garnet and amphibole among the residual phases. However, their concentrations in HREE (more or less 10 times the Chondrite values) do not support the assumption of a source containing garnet. The spider diagrams characteristically display negative anomalies for Sr, Ba and Ti. These anomalies result either from the low content of these elements in the source, or their retention in the residue during partial melting. The geochemistry and mineralogy of the granitic rocks reflect not only the nature of the protoliths from which they were derived, but also the dynamic conditions under which magmas were formed, evolved and eventually solidified ^[56]. 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 visualised in terms of molar CaO/(MgO + FeO_total) vs. molar Al₂O₃/(MgO + FeO_total) of Altherr et al. ^[57] (Figure 13). In that diagram, the samples plot in the field of partial melts from metagreywacke and metabasaltic sources. These source rocks are predominantly found in the low and upper part of the continental crust and we suggest that the source for the Tamkoro-Bossangoa plutonic rocks was a metagreywacke and a metabasalt. Such source protolith with significant proportions of metasedimentary rocks was indicated for the high-K calc-alkaline plutons of the Borborema province in the NE Brazil ^{[45, 58, 59, 60]} and eastern Nigeria ^{[43, 61, 62]}. The high-K calc-alkaline to shoshonitic and metaluminous nature of the Tamkoro-Bossangoa plutonic rocks require a metaluminous and relatively K-rich source ^{[63, 64]}. The differences observed in granites can be explained by differences in melting conditions and/or minor variation in source compositions. The low Rb/Sr and the enrichment of LREE of the rocks are probably inherited from the source. Thus, the magmatism of the Tamkoro-Bossangoa plutonic rocks may have involved remelting of a composite acid metagreywackes and metabasalt protoliths in the lower to upper crust. The large-scale melting of the source rocks could have been favoured by high heat flow during Paleoproterozoic orogenesis or underplating of mantle-derived magmas into the crust.

Figure 13. Molar CaO/ (MgO + FeOt) vs Al₂O₃/ (MgO + FeOt) for the Tamkoro-Bossangoa granites. Symbols as in Figure 3

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