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

Abstract

High-precision 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 206 Pb/ 204 Pb = 19.08–19.38, 207 Pb/ 204 Pb = 15.61–15.64 and 208 Pb/ 204 Pb = 39.1–39.4. The alkali basalts have similar 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and 207 Pb/ 204 Pb, but slightly lower 206 Pb/ 204 Pb and 208 Pb/ 204 Pb 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 206 Pb/ 204 Pb and 207 Pb/ 204 Pb, and higher 87 Sr/ 86 Sr. 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. Supplementary material: Analytical techniques, X-ray fluorescence whole-rock data, mineral compositions, 40 Ar/ 39 Ar dataset, mantle source modelling and figures showing volcanological features and thin sections are available at https://doi.org/10.6084/m9.figshare.c.4065743