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Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism

Published online by Cambridge University Press:  01 May 2009

A. Kröner ,
I. Braun  and
P. Jaeckel
A. Kröner
Affiliation:
Institut für Geowissenschaften, Universität Mainz, 55099 Mainz, Germany
I. Braun
Affiliation:
Mineralogisch-Petrologisches Institut, Universität Bonn, Poppelsdorfer Schloβ, 53115 Bonn, Germany
P. Jaeckel
Affiliation:
Institut für Geowissenschaften, Universität Mainz, 55099 Mainz, Germany Max-Planck-Institut für Chemie, Postfach 3060, 55060 Mainz, Germany
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Abstract

We report U—Pb and 207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. Higher P—T conditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages 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 Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.

Type
Articles
Information
Geological Magazine , Volume 133 , Issue 3 , May 1996 , pp. 311 - 323
Copyright
Copyright © Cambridge University Press 1996

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