Journal of Petrology | Volume 38 | Number 8 | Pages 975-1001 | 1997
© Oxford University Press 1997
Clarence River Supersuite: 250 Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia
Key Centre for the Geochemical Evolution and Metallogeny of Continents (CEMOC), Department of Geology, The Australian National University Canberra, A.C.T. 0200, Australia
Received April 3, 1995; Revised typescript accepted March 14, 1997
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Abstract |
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The Clarence River Supersuite (CRS) is one of the three late Permian I-type supersuites in the southern New England Orogen of eastern Australia. It comprises 12 small (mostly <100 km2) intrusions that occur at the northeastern and southern extremities of the batholith. The intrusions 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 strength elements, and light rare earth elements (LREE) relative to granodioritic I-type intrusions. They are also amongst the most isotopically primitive plutonic rocks in eastern Australia, typically having initial 87Sr/86Sr ratios of 0.7031–0.7042 and 
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 western Peninsular Ranges batholith. Considerable chemical and isotopic diversity 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 constant chondrite-normalized middle to heavy rare earth element (MREE to HREE) abundances. Such REE patterns and the presence of early formed pyroxenes are consistent with formation involving high degrees of dehydration melting of amphibolitic source rocks at pressures <0.8 GPa, producing a melt in equilibrium with a granulitic residuum. Despite 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 crystallization, under conditions of higher f(H2O). In contrast, the Duncans Creek Trondhjemite has steep REE patterns and small positive Eu anomalies indicating the stabilization of garnet at depths >26km. Both higher f(H2O) and higher pressure led to the destabilization of plagioclase, generating magmas with higher abundances of Al, Ga and Sr. At least three isotopically distinct sources were involved in the petrogenesis of the CRS, but the extent to which they contribute varies between plutons. Most intrusions have incorporated an isotopically primitive component that may represent either young isotopically primitive crust or mantle-derived magma. The other sources include granulitic materials with very low initial 87Sr/86Sr but more evolved Nd and isotopically evolved upper-crustal material.
KEY WORDS: Eastern Australia; Cordilleran I-type; isotopically primitive; southern New England Orogen; tonalite
* Corresponding author. Telephone: 61-6-2492056. Fax: 61-6-2495544. e-mail: colleen.bryant{at}anu.edu.au
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