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Control of redox state and Sr isotopic composition of granitic magmas: a critical evaluation of the role of source rocks

Published online by Cambridge University Press:  03 November 2011

Michel Pichavant
Affiliation:
M. Pichavant & B. Scaillet. Centre de Recherche sur la Synthèse et la Chimie des Minéraux, CNRS, 1A rue de la Férollerie, 45071, Orléans, France
Tahar Hammouda
Affiliation:
T. Hammouda, Geophysical Laboratory, 5251 Broad Branch Road. N. W., Washington, DC 20015-1305, U.S.A.
Bruno Scaillet
Affiliation:
M. Pichavant & B. Scaillet. Centre de Recherche sur la Synthèse et la Chimie des Minéraux, CNRS, 1A rue de la Férollerie, 45071, Orléans, France

Abstract:

The current underlying assumption in most geochemical studies of granitic rocks is that granitic magmas reflect their source regions. However, the mechanisms by which source rocks control the intensive and compositional parameters of the magmas remain poorly known. Recent experimental data are used to evaluate the ‘source rock model’ and to discuss controls of (1) redox states and (2) the Sr isotopic compositions of granitic magmas.

Experimental studies have been performed in parallel on biotite-muscovite and tourmaline-muscovite leucogranites from the High Himalayas. Results under reducing conditions ( = FMQ – 0·5) at 4 kbar and variable suggest that the tourmaline-muscovite granite evolved under progressively more oxidising conditions during crystallisation, up to values more than four log units above the FMQ buffer. Leucogranite magmas thus provide an example of the control of redox conditions by post-segregation rather than by partial melting processes.

Other experiments designed to test the mechanisms of isotopic equilibration of Sr during partial melting of a model crustal assemblage show that kinetic factors can dominate the isotopic signature in the case of source rocks not previously homogenised during an earlier metamorphic event. The possibility is therefore raised that partial melts may not necessarily reflect the Sr isotopic composition of their sources, weakening in a fundamental way the source rock model.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 1996

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