Volume 38: January - December 1997

Issue 3: March 1997

Abstract

Characterization and origin of aluminous A-type granites from the Lachlan Fold Belt, Southeastern Australia

PL. King, AJR. White², BW. Chappell¹ and CM. Allen¹ ¹Department of Geology, Australian National University, Canberra, A.C.T. 0200, Australia, ²VIEPS, Department of Earth Sciences, Melbourne University, Parkville, VIC. 3052, Australia and Corresponding author. Present address: Department of Geology, Box 87-1404, Arizona State University, Tempe, AZ 85287-1404, USA
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.

Keywords: A-type granite; aluminous A-type; peralkaline; Australia

Pages: 371 - 391

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