GEMOC ARC National Key Centre

COMPOSITIONAL VARIATION IN THE S-TYPE GRANITES OF THE LACHLAN FOLD BELT AND ITS CAUSES

B.W. Chappell¹ and A.J.R. White²

1. GEMOC ANU
2. Victorian Institute of Earth and Planetary Sciences (VIEPS), School of Earth Sciences, University of Melbourne, Parkville, Vic 3052

S-type granites of the Lachlan Fold Belt are always peraluminous or oversaturated in Al. This implies that they were derived from a similarly Al-oversaturated source, which therefore must have been through at least one cycle of weathering.

The S-type granites range from mafic cordierite- and biotite-rich granodiorites and tonalites, to very felsic, and sometimes strongly fractionated, monzogranites. The variation from mafic to felsic compositions cannot be due to the direct mixing of felsic crust-derived melts with mafic melts, because suites of S-type granites become more peraluminous as they become more mafic.

Compositional variation within the S-type granites results from a variety of factors and processes:

(a) degree of weathering of the source rocks. Rocks of the Cooma Supersuite are of restricted occurrence and were derived from the most strongly weathered materials and are therefore lower in Na and Ca than the other S-type granites. The component suites of the Bullenbalong and Koetong supersuites, for example, were derived from less-weathered source rocks, while source rocks of the Wyangala Supersuite were only mildly weathered. Granites that were derived from a particular source composition are placed in suites; differences between source rocks (and suites) are related to weathering rather than the addition of mafic rock, as some have suggested.

(b) retention of variable source-rock compositions. The more mafic rocks of the Bullenbalong Suite have increasing Zr contents with increasing SiO2. In magmatic rocks saturated in Zr, that can only be due to the retention of a corresponding compositional feature from the source sedimentary rocks, so that the igneous rocks represent sedimentary rock compositions that were mobilised by partial melting.

(c) restite fractionation. In the more felsic rocks of the Bullenbalong Suite, and for the mafic rocks (<69 % SiO2) of the Koetong Suite, for example, variation resulted from the separation of varying amounts of restite from a more felsic melt.

(d) cumulate rocks formed by fractional crystallisation. For several reasons, the most mafic S-type granites cannot be cumulates. However, there are rocks in which the accumulation of feldspar has resulted in Sr contents significantly higher than other rocks in that suite. Such rocks are complementary to (e).

(e) rocks formed from liquids produced by fractional crystallisation. More felsic granites produced by the equilibrium crystallisation of liquids resulting from the fractional crystallisation of precipitated minerals, are found in the Koetong Suite and some other felsic suites. These include rocks produced by extreme fractional crystallisation, such as the Interview Suite of western Tasmania.

(f) late stage hydrothermal alteration. Almost all of the most felsic S-type granites have compositions close to the Tuttle & Bowen minimum, so that crystal-liquid equilibrium, either during partial melting or fractional crystallisation, dominated in their formation. However in some rocks, e.g. the Ardlethan Granite, there is sufficient departure from such compositions to show that some hydrothermal alteration was superimposed on dominant fractional crystallisation.

These mechanisms of producing variation have some important implications. For example, many mafic S-type granites are very close to source-rock compositions, which enables the development of that source to be modelled, and the fractionation of the granites from that composition to be examined in detail. At the other extreme of composition, the most highly evolved felsic granites are the result of extended fractional crystallisation ± hydrothermal alteration. This is important for understanding the origin of felsic granites in areas where more mafic granites are not exposed, such as Tasmania and Cornwall.

FOR AGC Session 3B: Magmatism in the Tasman Orogenic Zone