CLARENCE RIVER SUPERSUITE - 250-MA CORDILLERAN TONALITIC I-TYPE INTRUSIONS IN EASTERN AUSTRALIA

The Clarence River Supersuite (CRS) is one of three late Permian I-typ e supersuites in the southern New England Orogen of eastern Australia. It comprises 12 small (mostly < 100 km(2)) intrusions that occur at t he northeastern and southern extremities of the batholith. The intrusi ons are compositionally diverse, ranging from gabbro to monzogranite, but are dominated by tonalite, granodiorite and diorite. They have low abundances of alkalis, large ion lithophile elements, high field stre ngth elements, and light rare earth elements (LREE) relative to granod ioritic I-type intrusions. They are also amongst the most isotopically primitive plutonic rocks in eastern Australia, typically having initi al Sr-87/Sr-86 ratios of 0.7031 - 0.7042 and epsilon(Nd) values of +6. 2 to +1.6. In these aspects they are similar to the Mesozoic tonalitic association in the American Cordillera, and in particular to the west ern Peninsular Ranges batholith. Considerable chemical and isotopic di versity within the CRS points to variable conditions of formation and the involvement of multiple sources. Most intrusions are characterized by LREE enrichments, moderate negative Eu anomalies and relatively co nstant chondrite-normalized middle to heavy rare earth element (MREE t o HREE) abundances. Such REE patterns and the presence of early formed pyroxenes are consistent with formation involving high degrees of deh ydration melting of amphibolitic source rocks at pressures < 0.8 GP al pha, producing a melt in equilibrium with a granulitic residuum. Despi te similar pressures of formation, the MREE depletions and absence of negative Eu anomalies in the high-Si Kaloe Granodiorite group indicate the stabilization of amphibole, during either partial melting or crys tallization, under conditions of higher f(H2O). In contrast, the Dunca ns Creek Trondhjemite has steep REE patterns and small positive Eu ano malies indicating the stabilization of garnet at depths > 26 km. Both higher f(H2O) and higher pressure led to the destabilization of plagio clase, generating magmas with higher abundances of Al, Ga and Sr. At l east three isotopically distinct sources were involved in the petrogen esis of the CRS, but the extent to which they contribute varies betwee n plutons. Most intrusions have incorporated an isotopically primitive component that may represent either young isotopically primitive crus t or mantle-derived magma. The other sources include granulitic materi als with very low initial Sr-87/Sr-86 but more evolved epsilon(Nd), an d isotopically evolved upper-crustal material.