Petrology of the Guenfalabo ring-complex: An example of a complete series along the Cameroon Volcanic Line (CVL), Cameroon

In the Guenfalabo ring-complex (GRC), two non-comagmatic rock suites have been identified as a result of two volcanic episodes: Suite 1 (68.8 ± 1.7 Ma by K/Ar on trachyte) of peralkaline trachytes and pantellerites cogenetic with alkaline syenites, granites and rhyolitic flows and tuffs; Suite 2 (62 ± 2 Ma by K/Ar on basalt), a bimodal and complete series of alkali olivine basalts and associated microgabbro dykes, diorites, syenites and granites, cross-cutting the former. Kaersutite in the trachytes of Suite 1 has mantle-derived signatures: TiO2 > 4%, MgO < 15%, FeO > 8%, Ti = 0.63 c.p.f.u. and Al = 2, characteristic of kaersutites of HP and HT origin: 13–23 kbar, 1100–1220 °C. The trachytes are probably products of FC of a basaltic parent that did not attain higher crustal levels. The Suite 1 rocks are enriched in Rb, K, Zr, Nb, LREE, alkalis, and (Ce/Yb)N = 7–15 probably due to some effect of metasomatism during the magma ascension. Fe–Ti enrichment is corroborated by the presence of ferropseudobrookite–ilmenite–ulvospinel in the syenites and ilmenite in the pantellerite. The Rb/Ba > 1 in the trachytes (2.44, 26.7), pantellerite (6.33), alkaline granites (0.63–1.8) and the 87Sr/86Sr in the alkaline granites (=0.74060) depict the role of AFC. The ankaramites of the Suite 2 rocks are olivine-phyric (25%), Fo85–88, have 50–52% clinopyroxene (salite), 5% plagioclase (An55–36) and 7% Fe–Ti oxides. Trace element modeling indicates an origin from a basaltic magma of about 25% PM of spinel lherzolite mixed with a magma from <1% PM of garnet lherzolite (3–4% garnet) in a proportion of 1:4. The cogenetic alkali basalts and the microgabbro–diorite–syenite–granite that constitute the Suite 2 rocks, with a Daly gap of 54% > SiO2 < 58%, result from this Early Cenozoic magmatic event. The basalts have: Zr = 225–253, Nb = 98–111, Y = 33–56, typical of FOZO, a HIMU-type OIB related magmas (Sr/Sri = 0.70202–0.7034; Nd/Nd = 0.51282–0.512545; 206Pb/204Pb = 19.13, 207Pb/204Pb = 15.59 and 208Pb/204Pb = 38.64. The (La/Yb)N ratios (9–20) indicate a high REE fractionation and garnet-lherzolite source. Elemental ratios as La/Nb = 0.49–0.81, Nb/Y = 1.00–2.04 suggest crustal contamination of minor importance. The (La/Sm)N ratios (2–3.5) and LREE enrichment corroborate the contribution of a subcontinental lithospheric mantle (SCLM). REE spectra and minor element spidergrams are concordant for each rock suite.