Origin and sources of recent continental alkaline volcanism in the Southwest Ankaratra volcanic field, Central Madagascar: Insights from major and trace element and Sr, Nd, Pb, Hf, Os isotopes

Abstract

We conducted 40Ar/39Ar, major element, trace element and Sr, Nd, Pb, Hf, Os isotopic studies of basaltic lavas from the Southwest Ankaratra volcanic field, in order to assess the origin and sources of recent monogenetic volcanism in the central highlands of Madagascar. All samples in this study yielded ages less than 110 ka, providing further evidence for Pleistocene volcanism in the central highlands of Madagascar including the Southwest Ankaratra volcanic field. The Southwest Ankaratra magmas are characterized by significant chemical and isotopic heterogeneity across the volcanic field and within an individual eruptive center (Iavoko cinder cone), similar to that observed in other monogenetic volcanic fields. The data suggest two distinct geochemical groups within the Quaternary Ankaratra magmas: trachybasalts and basanites. All of the samples have trace element abundances and patterns similar to those observed in ocean island basalts (OIB). The trachybasalts are characterized by more evolved compositions, yet display lower trace element abundances, than the basanites. Furthermore, the trachybasalts are characterized by more radiogenic 87Sr/86Sr and lower 143Nd/144Nd than the basanites, and are displaced to slightly lower 206Pb/204Pb, but similar 207Pb/204Pb and 208Pb/204Pb, compared to the basanites.Samples from the Southwest Ankaratra volcanic field display suprachondritic 187Os/188Os values that can be explained by minor crustal assimilation, but their trace element and Sr-Nd-Pb-Hf isotopic characteristics are incompatible with crustal assimilation, and instead represent features of their mantle sources. Our data show that the magmas are compositionally distinct from those expected for subcontinental lithospheric mantle (SCLM)-derived basalts, and likely derive from an asthenospheric mantle source polluted by plume materials and delaminated SCLM. Chemical and isotopic studies of Pleistocene alkaline basalts from SW Ankaratra suggest the involvement of the African superplume C in their origins, and further suggest that superplume materials must be widely distributed in the upper mantle beneath the entire EARS from the Red Sea to Madagascar, both near to and distal from the low seismic velocity mantle regions.