Geochemistry of theVolcan de l’Androy Basalt^Rhyolite Complex, Madagascar Cretaceous Igneous Province

The 4000 km2 Androy massif in southeastern Madagascar is a 42000 m thick sequence of interbedded basalt and rhyolite erupted during a widespread Cretaceous episode of predominantly basaltic volcanism. Two geochemically different groups of basalt, tholeiitic group B1 and mildly alkalic B2, are present, as are two different groups of rhyolite, R1 and R2. Both the basalts and rhyolites appear to have issued from relatively nearby feeders, as compositionally equivalent intrusions are exposed in the vicinity. The R2 rhyolites define a whole-rock Rb^Sr isochron of 840 24Ma (2), the same, within error, as an 40Ar^39Ar sanidine age reported by earlier workers. Plate reconstructions suggest that the area was near the Marion hotspot at this time. Some involvement of hotspot mantle is allowed, but not required, by Nd^Pb^Sr isotope data for the basalts. The two types of basalt may have formed by different amounts of melting of the same mantle source, which remains rather poorly specified, but group B1was affected much more than B2 by contami-nation with continental material, probably Archean crust. The R1 rhyolites are petrogenetically related to the B1 basalts, with which they are interbedded.The R2 rhyolites may be derived from melts of frozen high-"Nd B1 basalt coupled with fractionation and assimila-tion of relatively small amounts of crust. Alternatively, although these rhyolites were erupted significantly later than the B2 basalts, they may have formed through advanced crystal fractionation of B2-type magma and relatively small amounts of crustal assimila-tion. Separate magmatic plumbing systems appear to have existed more or less contemporaneously in the Androy area. KEY WORDS: large igneous province; rhyolite; mantle plume; basalt