Incipient charnockitisation due to carbonic fluid transfer related to late Pan-African transcurrent tectonics in Madagascar; implications for mobility of Fe, Ti, REE and other elements

Abstract

Incipient charnockitisation of an A-type granite protolith has been recognized in a spectacular outcrop 35 km north of Antananarivo. The charnockites form structurally-controlled dark greenish patches suggesting channelized fluid transfer, during and after partial reworking of the granite coeval with the Antananarivo virgation zone (ca 560 Ma) and/or the Angavo shear zone of late-Panafrican age (ca 550 Ma). They are characterized by a significant increase of the bulk magnetic susceptibility. The granitic protolith contains quartz, perthitic alkali feldspar, high-Ti hastingsitand accessory minerals (apatite, allanite, magnetite and zircon). The charnockitic granite contains quartz, perthitic alkali feldspar and ghost (altered) orthopyroxene crystals, as well as secondary low-Ti hastingsite surrounding orthopyroxene. The large increase of magnetic susceptibility magnitudes is related to the formation of pockets of secondary magnetite, spatially associated with quartz and other...