THE PANAFRICAN STRATOID GRANITES OF MADAGASCAR - ALKALINE MAGMATISM IN A POST COLLISIONAL EXTENSIONAL SETTING

A major alkali province of late Panafrican age occupies central Madaga scar and takes the form of a thick sequence of 'stratoid'(sheet-like) granites emplaced in a mid-crustal gneissic basement. This alkaline ma gmatism 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 gr anites and syenites. Major and trace element analyses have typical A-t ype characteristics. Two distinct magmatic suites are recognized: a mi ldly alkaline suite including all the subsolvus granites and a strongl y alkaline suite including the hypersolvus alkaline granites and the s yenites. We propose that the mildly alkaline suite was derived from a granodioritic crustal protolith. Some of the strongly alkaline granite s and the quartz syenites display low delta(18)O isotopic signatures o f around +6 parts per thousand. The parental magmas for this suite are most probably of mantle derivation. The more evolved compositions are consistent with crystal fractionation processes. Contemporaneous alka line silicic plutonism occurs in many parts of the Panafrican belt of Eastern Africa; however, sheet-like intrusions have rarely been descri bed. As a large-scale province, the nearest analogues of the stratoid granites of Madagascar are the rapakivi granites of earlier Proterozoi c age in Scandinavia and Greenland.