The Panafrican Stratoid Granites of Madagascar: Alkaline Magmatism in a Post-Collisional Extensional Setting |
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Authors: | NDLEC ANNE; STEPHENS EDRYD W; FALLICK ANTHONY E |
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Institution: | 1URA 67 CNRS, PTROPHYSIQUE ET TECTONIQUE, UNIVERSIT PAUL-SABATIER 38 RUE DES TRENTE-SIX-PONTS, F-31400 TOULOUSE, FRANCE
2DEPARTMENT OF GEOLOGY, UNIVERSITY OF ST ANDREWS ST ANDREWS KY16 9ST, UK
3ISOTOPE GEOSCIENCES UNIT, SCOTTISH UNIVERSITIES RESEARCHAND REACTOR CENTRE EAST KILBRIDE, GLASGOW G75 0QU, UK |
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Abstract: | A major alkali province of late Panafrican age occupies centralMadagascar and takes the form of a thick sequence of stratoid(sheet-like)granites emplaced in a mid-crustal gneissic basement This alkalinemagmatism has been interpreted as a consequence of extensionaltectonics accompanying the collapse of the Mozambique belt.The rocks belong to three petrographic types: subsolvus granites,hypersolvus alkaline granites and syenites. Major and traceelement analyses have typical A-type characteristics. Two distinctmagmatic suites are recognized: a mildly alkaline suite includingall the subsolvus granites and a strongly alkaline suite includingthe hypersolvus alkaline granites and the syenites. We proposethat the mildly alkaline suite was derived from a granodioriticcrustal protolith. Some of the strongly alkaline granites andthe quartz syenites display low 18O isotopic signatures of around+6.The parental magmas for this suite are most probably of mantlederivation. The more evolved compositions are consistent withcrystal fractionation processes. Contemporaneous alkaline silicicplutonismoccurs in many parts of the Panafrican belt of Eastern Africa;however, sheet-like intrusions have rarely been described. Asa large-scale province, the nearest analogues of the stratoidgranites of Madagascar are the rapakivi granites of earlierProterozoic age in Scandinavia and Greenland. KEY WORDS: alkaline granite; Madagascar; Panafrican; pastcollisional magmatism
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