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

High-precision Ar-40/Ar-39 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- precision Ar-40/Ar-39 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 ranges Pb-206/Pb-204 = 19.08-19.38, Pb-207/Pb-204 = 15.61-15.64 and Pb-208/Pb-204 = 39.1-39.4. The alkali basalts have similar Sr-87/Sr-86, Nd-143/Nd-144 and Pb-207/Pb-204, but slightly lower Pb-206/Pb-204 and Pb-208/Pb-204 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 lower Pb-206/Pb-204 and Pb-207/Pb-204, and higher Sr-87/Sr-86. 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.