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Oxford University Press (OUP), Journal of Petrology, 3(38), p. 371-391

DOI: 10.1093/petroj/38.3.371

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Characterization and Origin of Aluminous A-type Granites from the Lachlan Fold Belt, Southeastern Australia

Journal article published in 1997 by P. L. King ORCID, A. J. R. White, B. W. Chappell, C. M. Allen
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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Abstract

The metaluminous to weakly peraluminous A-type granites of the Lachlan Fold Belt are a distinctive group of igneous rocks, on the basis of chemical and mineralogical criteria. Those granites that contain ∼65–72% SiO 2 can be distinguished from other types on the basis of higher abundances of Fe total /(Fe total + Mg), high field strength elements, trivalent rare earth elements, Ga and Zn. Mineralogically, they contain Fe-rich hydrous mafic minerals and primary ilmenite, and hence are reduced relative to the Ni–NiO buffer. However, the extremely felsic A-type granites (SiO 2 > ∼72%) have the same chemical and mineralogical characteristics as felsic, fractionated I-type granites. Recent analyses indicate that the Lachlan Fold Belt A-type granites have Sc, F, alkali element, trace transition element and H 2 O contents similar to those of other unfractionated I-type granites. Rb–Sr and Nd–Sm isotopic compositions are highly variable, probably reflecting source region heterogeneity. The metaluminous to weakly peraluminous A-type granites of the Lachlan Fold Belt are distinct from peralkaline rocks in terms of chemical composition, petrography and field associations, although these rocks have been grouped together as a single type in current classification schemes. We propose that the metaluminous to weakly peraluminous A-type granites, such as those of the Lachlan Fold Belt, should be defined as ‘aluminous A-type’ granites and should not be grouped with peralkaline granites. The Lachlan Fold Belt aluminous A-type granites have relatively high calculated zircon saturation temperatures. We suggest that these granites were produced by high-temperature partial melting of a felsic infracrustal source.