Geochemistry of Cenozoic mafic potassic and sodic volcanic rocks in southwestern Madagascar: Long-lived lithospheric mantle heterogeneities in an extensional tectonic setting

Cenozoic (≥ 43 Ma) silica undersaturated (potassic) trachybasalts and trachyandesites in southwestern Madagascar (Tsianihy-Manja, southern Morondava Basin) form a small monogenetic volcanic field emplaced above Paleogene detritic sedimentary rocks, along a NE-SW-trending fault system. These olivine-chromite±clinopyroxene-phyric primitive lavas (Mg# = 69; MgO = 10–11 wt%; Cr = 450 ppm, Ni = 200 ppm; K₂O = 3–4 wt%) have highly peculiar trace element and isotopic composition (e.g., Ba/Nb = 18.4; ⁸⁷Sr/⁸⁶Sr_i = 0.70529–0.70555, ¹⁴³Nd/¹⁴⁴Nd_i = 0.51262–0.51263, ²⁰⁶Pb/²⁰⁴Pb_m = 18.415–18.424, ²⁰⁷Pb/²⁰⁴Pb_m = 15.576–15.579, ²⁰⁸Pb/²⁰⁴Pb_m = 38.799–38.813). A hitherto undescribed plug of primitive (sodic) basanite of the 11–12 Ma-old Ankililoaka district south of Tsianihy-Manja (hosting spinel lherzolite mantle xenoliths) has noticeable different geochemistry (Ba/Nb = 8–9.2; ⁸⁷Sr/⁸⁶Sr_i = 0.70346–0.7036, ¹⁴³Nd/¹⁴⁴Nd_i = 0.51281–0.51282, ²⁰⁶Pb/²⁰⁴Pb_m = 19.079–19.374, ²⁰⁷Pb/²⁰⁴Pb_m = 15.621–15.645, ²⁰⁸Pb/²⁰⁴Pb_m = 39.115–39.424).

The relatively low CaO, Sc, V, Fe₂O_3t, MnO at high MgO, Cr and Ni, and the potassic affinity of the Tsianihy-Manja trachybasalts, all indicate that the mantle source is relatively clinopyroxene-poor (i.e., depleted by previous melt extractions), in the same way as the source of lamproitic (or boninitic) magmas, but the primitive nature, the concentration of high field strength elements, the incompatible element patterns and their isotopic ratios indicate their unequivocal within-plate setting and indicate a derivation by low-degree partial melting of an incompatible element-enriched mantle and insignificant role for crustal contamination. In terms of incompatible element concentrations, and thus also Sr-Nd-Pb-isotopic composition, we find no evidence in favour of a mid-ocean ridge basalt (MORB)-mantle component, or for a MORB-mantle strongly enriched by ocean island basalt-like components, to form the mantle source regions of the Tsianihy-Manja and Ankililoaka mafic alkaline rocks. The significant isotopic change from the northernmost Cenozoic volcanic rocks of Madagascar and those in the central and southern part of the island (which range in composition from sodic to potassic, and from tholeiitic basalt to olivine melilitite) implicates a distinct source heterogeneity, and ultimately assess the role of the old continental lithospheric mantle as source region.