The mafic alkaline volcanism of SW Madagascar (Ankililoaka, Tulear region): 40Ar/39Ar ages, geochemistry and tectonic setting

Abstract

High-precision40Ar/39Ar ages, major and trace element, and radiogenic isotope data are presented for the basanites and alkali basalts forming the southwesternmost monogenetic volcanic field in Madagascar. The volcanic rocks were erupted along fissure zones and aligned cones in a nearly flat area covered by the Cenozoic sediments of the Morondava basin. The high-precision40Ar/39Ar ages constrain the beginning of the magmatism in the Ankililoaka area to about 12 Ma, significantly earlier than suggested by previously published K/Ar ages. The Ankililoaka basanites include primitive compositions (MgO >10 wt%, Ni >200 ppm and Cr >400 ppm), whereas other basanites and alkali basalts experienced limited removal of olivine, chromiferous spinel and clinopyroxene. Initial Sr and Nd isotope ratios of the basanites are 0.70343–0.70445 and 0.51279–0.51282, respectively. The Pb isotope compositions are in the ranges206Pb/204Pb = 19.08–19.38,207Pb/204Pb = 15.61–15.64 and208Pb/204Pb = 39.1–39.4. The alkali basalts have similar87Sr/86Sr,143Nd/144Nd and207Pb/204Pb, but slightly lower206Pb/204Pb and208Pb/204Pb than the basanites. The isotopic composition of the Ankililoaka rocks partially overlaps with that of the Cenozoic volcanic mafic rocks of northern Madagascar, and differs significantly from that of the mafic volcanic rocks of central Madagascar, which have lower206Pb/204Pb and207Pb/204Pb, and higher87Sr/86Sr. Major and trace element systematics and geochemical modelling suggest that the Ankililoaka mafic alkaline rocks are low-degree melts of an incompatible element enriched peridotite source starting from depths where garnet is stable. Crustal contamination during ascent was insignificant. We argue that the genesis of the Ankililoaka alkaline magmas was triggered by melting an enriched, volatile-rich lithospheric mantle uplifted in the Cenozoic.