Clarence River Supersuite: Primitive I-Type Granites of Eastern Australia

Colleen J. Bryant, R.J. Arculus and B. W. Chappell, GEMOC, Department of Geology, Australian National University, ACT 0200, AUSTRALIA

The Clarence River Supersuite is one of three late Permian I-type granites supersuites in the New England Batholith of the southern New England Orogen, eastern Australia. It is composed of eleven small (usually <100 km2) plutons that occur at the northeastern and southern extensions of the batholith, and collectively intrude five tectonically distinct tectonostratigraphic terranes. The intrusions are compositionally very diverse, ranging from gabbro through to monzogranite, but tonalite, granodiorite and diorite are the most abundant rock types. With the exception of the Omadale Brook and Barrington River plutons (Barrington Tops) which contain minimal amounts of hydrous phases, hornblende and biotite are the most abundant ferromagnesian minerals. However, most intrusions do contain significant amounts of clinopyroxene +/- orthopyroxene, or their alteration products. The granites have a marked depletion of the high field strength elements relative to other incompatible elements, a characteristic feature of arc-related granites. They are also characterised by relatively low abundances of the alkalis, P, Nb, Ba LREE, Pb, Th and U, thereby distinguishing them from other I-type supersuites in the New England Batholith. With the exception of one sample from the Dumbudgery Creek Granodiorite, initial 87Sr/86Sr and eNd vary between 0.7031-0.7042 and +1.8-+6.1, respectively; constituting the most primitive isotopic compositions reported for Palaeozoic granites in eastern Australia.

These granites are geochemically distinct from the Devonian I-type granites of the Lachlan Fold Belt having lower abundances of K, high field strength elements, particularly Nb, and LREE, as well as being more isotopically primitive. The broad mineralogical, geochemical and isotopic characteristics of the Clarence River Supersuite granites are very similar to the tonalitic association in the American Cordillera, such as the Peninsular Ranges batholith. However, with an age of approximately 250 Ma, they are distinctly older than those granites.

The mineralogical and geochemical characteristics of the granites of the Clarence River Supersuite, are consistent with their derivation by dehydration-melting of amphibolite in the lower or middle crust at pressures generally less than 0.8 GPa and temperatures of at least 1000° C, leaving a granulitic residue of clinopyroxene, plagioclase and Fe-Ti oxides +/- orthopyroxene. Very steep REE patterns and the absence of Eu anomalies in one intrusion, the Duncans Creek Trondjhemite, imply higher pressures of origin (>0.8 Ga). Hornblende barometry indicates the magmas only became water-saturated at shallow depths (pressures < 0.24 GPa).

The Clarence River Supersuite granites were derived from relatively young, isotopically primitive, but heterogeneous arc-related materials. At least three distinct isotopic components are required in the genesis of the Clarence River Supersuite granites, including both more isotopically primitive lower crustal and LILE-enriched and isotopically evolved upper crustal sources. However, the importance of individual components varies from one intrusion to another. The chemical and isotopic heterogeneity appears to be unrelated to the host tectonostratigraphic terrane.