Abstract
The Antampombato-Ambatovy complex is the largest intrusion in the central-eastern part of the Cretaceous flood basalt province of Madagascar, with an exposed surface area of about 80 km (super 2) . It has an (super 40) Ar/ (super 39) Ar incremental heating age of 89.9+ or -0.4 Ma and a U-Pb age of 90+ or -2 Ma. The outcropping plutonic rocks range from dunite and wehrlite, through clinopyroxenite and gabbro, to sodic syenite. A dyke swarm cross-cutting some of the above lithologies (and the nearby Precambrian basement rocks) is formed of picritic basalts, alkali to transitional basalts, benmoreites and rhyolites; some of the latter are peralkaline. A few basaltic dykes have cumulate olivine textures, with up to 26 wt % MgO and 1200 ppm Ni, whereas others have characteristics more akin to those of primitive liquids (9 wt % MgO; Mg-number 0.61; 500 ppm Cr; 200 ppm Ni). These basalts have relatively high TiO (sub 2) (2.2 wt %) and total iron (14 wt % as Fe (sub 2) O (sub 3) ), and moderate contents of Nb (10-11 ppm) and Zr (c. 100 ppm). Initial (at 90 Ma) Sr- and Nd-isotope ratios of the clinopyroxenites and basalt dykes are 0.7030-0.7037 and 0.51290-0.51283, respectively. Syenites and peralkaline rhyolites have Sr- and Nd-isotope ratios of 0.7037-0.7039 and 0.51271-0.51274, respectively. The data suggest derivation of the parental magmas from a time-integrated depleted mantle source, combined with small amounts of crustal contamination in the petrogenesis of the more evolved magmas. The isotopic compositions of the mafic-ultramafic rocks are most similar to those of the mid-ocean ridge basalt (MORB)-like igneous rocks of eastern Madagascar, and suggest the existence of an isotopically "depleted" component in the source of the entire Madagascar province, even though the Antampombato basalts are chemically unlike the lavas and dykes with the same depleted isotopic signature found in western Madagascar. If this depleted component is plume-related, this suggests that the plume has a broadly MORB-source mantle composition. The existence of isotopically more enriched magma types in the Madagascan province has several possible petrogenetic explanations, one of which could be the interaction of plume-related melts with the deep lithospheric mantle beneath the island.