ZIRCON GEOCHRONOLOGY OF ANATECTIC MELTS AND RESIDUES FROM A HIGHGRADEPELITIC ASSEMBLAGE AT IHOSY, SOUTHERN MADAGASCAR - EVIDENCE FOR PAN-AFRICAN GRANULITE METAMORPHISM

We report U-Pb and Pb-207/Pb-206 Zircon ages for a granulite facies gn eiss assemblage exposed in a large quarry at Ihosy, southern Madagasca r. The granulites are derived from pelitic to arkosic sediments and at tained equilibrium conditions at 650-700 degrees C and 4-5 kbar. Highe r P-T conditions of 750-800 degrees C and 6 kbar in the presence of lo w water activities have led to dehydration melting processes. The form ation of granitic melts, which (partly) moved away from their source r egion, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bea ring rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between similar to 720 and sim ilar to 1855 Ma, suggesting a chronologically heterogeneous source reg ion and a depositional age of less than similar to 720 Ma for these ro cks. High-grade metamorphism and anatexis are documented by zircon age s between 526 +/- 34 and 557 +/- 2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ih osy rocks are similar to those in the Wanni Complex of northwestern Sr i Lanka and in high-grade assemblages of southernmost India and suppor t the contention that all these terrains were part of the Mozambique b elt which formed as a result of collision of East and West Gondwana in latest Precambrian time.