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

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ID: 296622
1997
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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% SiO2 can be distinguished from other types on the basis of higher abundances of Fetotal/(Fetotal + 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 (SiO2> ∼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 H2O 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.
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Authors P. L. King, A. J. R. White, B. W. Chappell, Charlotte M. Allen
Journal journal of petrology
Year 1997
DOI
10.1093/petroj/38.3.371
URL
https://academic.oup.com/petrology/article-pdf/38/3/371/4456830/petrology-38-371.pdf
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