The Panafrican Stratoid Granites of Madagascar: Alkaline Magmatism in a Post-Collisional Extensional Setting

Abstract

A major alkali province of late Panafrican age occupies central Madagascar and takes the form of a thick sequence of ‘stratoid’(sheet-like) granites emplaced in a mid-crustal gneissic basement This alkaline magmatism has been interpreted as a consequence of extensional tectonics accompanying the collapse of the Mozambique belt. The rocks belong to three petrographic types: subsolvus granites, hypersolvus alkaline granites and syenites. Major and trace element analyses have typical A-type characteristics. Two distinct magmatic suites are recognized: a mildly alkaline suite including all the subsolvus granites and a strongly alkaline suite including the hypersolvus alkaline granites and the syenites. We propose that the mildly alkaline suite was derived from a granodioritic crustal protolith. Some of the strongly alkaline granites and the quartz syenites display low δ18O isotopic signatures of around +6‰.The parental magmas for this suite are most probably of mantle derivation. The more evolved compositions are consistent with crystal fractionation processes. Contemporaneous alkaline silicicplutonism occurs in many parts of the Panafrican belt of Eastern Africa; however, sheet-like intrusions have rarely been described. As a large-scale province, the nearest analogues of the stratoid granites of Madagascar are the rapakivi granites of earlier Proterozoic age in Scandinavia and Greenland.