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The geochemistry of Archaean plagioclase-rich granites as a marker of source enrichment and depth of melting

Published online by Cambridge University Press:  01 March 2009

J.-F. Moyen
Affiliation:
Department of Geology, Geography and Environmental Studies, University of Stellenbosch, PVT Bag X1, 7602 Matieland, South Africa Email: moyen@sun.ac.za Current address: Département de Géologie, Université Jean-Monnet & CNRS 23, rue du Docteur Michelon, 42023 Saint-Etienne cedex, France Email: jean.francois.moyen@univ-st-etienne.fr
D. Champion
Affiliation:
Geoscience Australia, Canberra, Australia Email: David.Champion@ga.gov.au
R. H. Smithies
Affiliation:
GSWA, Perth, W. Australia Email: hugh.smithies@mp.wa.gov.au

Abstract

In geochemical diagrams, granitoids define ‘trends’ that reflect increasing differentiation or melting degree. The position of an individual sample in such a trend, whilst linked to the temperature of equilibration, is difficult to interpret. On the other hand, the positions of the trends within the geochemical space (and not the position of a sample within a trend) carry important genetic information, as they reflect the nature of the source (degree of enrichment) and the depth of melting. This paper discusses the interpretation of geochemical trends, to extract information relating to the sources of granitoid magmas and the depth of melting.

%Applying this approach to mid-Archaean granitoids from both the Barberton granite–greenstone terrane (South Africa) and the Pilbara Craton (Australia) reveals two features. The first is the diversity of the group generally referred to as ‘TTGs’ (tonalites, trondhjemites and granodiorites). These appear to be composed of at least three distinct sub-series, one resulting from deep melting of relatively depleted sources, the second from shallower melting of depleted sources, and the third from shallow melting of enriched sources. The second feature is the contrast between the (spatial as well as temporal) distributions and associations of the granites in both cratons.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 2010

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