Citation: | Aliou Mamouda, Sylvestre Ganno, Guy Tchoupe Takam Bertin, Arnold Mbita Motto Steven, Hermann Donald Fossi, Jean Paul Nzenti, Joseph Mvondo Ondoa, 2022. Origin and intraplate tectonic setting of mafic magmatic enclaves from the Ngaoundal area, Adamawa-Cameroon: Insights from petrography and geochemistry, China Geology, 5, 579-594. doi: 10.31035/cg2022041 |
In this contribution, detailed field descriptions together with petrographic and bulk-rock major, trace and rare earth elements (REE) data are used to constrain the origin and geodynamic setting of the mafic magmatic enclaves (MMEs) recently discovered within the Pan-African Ngaoundal pluton, Adamawa area, central Cameroon. The investigated MMEs are dark-colored with chilled margins, and display medium to coarse-grain igneous textures. The mineral assemblage is either dominated by K-feldspar and carbonate (group I), or by amphibole and plagioclase (group II), though the overall mineral phases made of amphibole, plagioclase, K-feldspar, and biotite are similar to that of their host syenite but in different proportions. The MMEs in Ngaoundal area are foid-gabbro in composition with SiO2 contents ranging between 41.52% and 43.74% and are contiguous with their host granitoids of intermediate composition (SiO2=57.52% to 58.98%). The host granitoid rocks are metaluminous, and belong to the shoshonitic series. Petrographic and geochemical data have revealed that the Ngaoundal MMEs derived from rapid cooling of hot injected lithospheric mantle-derived magma within cooler host granitoids magma and were emplaced in the intraplate geodynamic setting.
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Location of the Ngaoundal area (a) in the pre-drift reconstruction of Pan-African and Brasiliano terranes (after Castaing C et al., 1994) and (b) the geological map of Cameroon. BOSZ–Betare-Oya shear zone; CCSZ–Central Cameroon shear zone; KCSZ–Kribi-Campo shear zone; MSZ–Meiganga shear zone; SSZ–Sanaga shear zone; TBSZ–Tcholliré-Banyo shear zone. YD–Yaounde domain; AYD–Adamawa-Yade domain; WCD–Western Cameroon domain.
Representative outcrop photos of the Ngaooundal MMEs and their syenite host. a–elliptical and lenticular shapes; b–Group I MMEs showing rounded shape; c–Group II enclave contains phenocrysts of amphibole and feldspar.
Microphotographs of syenite (a) group I (b), and group II (c, d) MMEs. Mineral abbreviations after Whitney DL and Evans BW (2010). Amp–amphibole; Cb–carbonate; Chl–chlorite; Ep–epidote; Fsp–feldspar. XPL–cross-polarized light; PPL–plane-polarized light.
Total Akali Silica (TAS) discrimination diagram of the Ngaoundal MMEs.
K2O-SiO2 diagram (a) and A/NK-A/CNK diagram (b) of the host rocks and the MMEs.
Harker diagrams of major elements (Symbols as in Fig. 4).
Primitive mantle-normalized trace element spider diagram (a) and chondrite-normalized REE pattern (b) for the host rocks and the MMEs. Normalized values are from McDonough WF and Sun SS (1995).
Harker diagrams of trace elements (Symbols as in Fig. 4).
Zr/Hf vs. Y/Ho diagram with the CHARAC field from Bau M (1996), showing the chondritic compositions of the Ngaoundal MME samples (Symbols as in Fig. 4).
a–La/Yb vs. Nb/La (Smith EI et al., 1999). Values of N-MORB, E-MORB and OIB according to Sun SS and McDonough WF (1989). b–(La/Sm)PM vs. (Tb/Yb)PM diagram illustrating very low degree (ca. 2%) of partial melting in non-modal fractional melting models (after Melluso L et al., 2016). The values along curves correspond to the degrees of partial melting. The composition of the source is the lithospheric mantle median of McDonough WF (1990). Symbols as in Fig. 4
a–DF1 vs. DF2 tectonic discriminant diagrams (after Agrawal S et al., 2008). IAB–island arc basalt, CRB–continental rift basalt, MORB–mid-ocean ridge basalt, OIB–ocean island basalt. DF1= 0.3518 ln(La/Th)+0.6013 ln(Sm/Th)-1.3450 ln (Yb/Th)+2.1056 ln (Nb/Th)-5.4763; DF2= −0.3050 ln(La/Th)-1.1801 ln(Sm/Th)+1.6189 ln(Yb/Th)+1.2260 ln(Nb/Th)-0.9944. b–Ti−V diagram (Shervais JW, 1982) indicating OFB (Ocean Flood Basalt) signature of the studied MME samples. Symbols as in Fig. 4.