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Journal of Petrology Volume 42 Number 8 Pages 1429-1448 2001
© Oxford University Press 2001

U–Pb Zircon Studies of Mid-crustal Metasedimentary Enclaves from the S-type Deddick Granodiorite, Lachlan Fold Belt, SE Australia

R. MAAS1,*, I. A. NICHOLLS2, A. GREIG2,{dagger} and A. NEMCHIN3

1DEPARTMENT OF EARTH SCIENCES, LA TROBE UNIVERSITY, BUNDOORA, VIC. 3086, AUSTRALIA
2DEPARTMENT OF EARTH SCIENCES, MONASH UNIVERSITY, CLAYTON, VIC. 3168, AUSTRALIA
3SCHOOL OF EARTH SCIENCES, CURTIN UNIVERSITY, PERTH, W.A. 6001, AUSTRALIA

High-grade metasedimentary enclaves in granites can be used to directly characterize the lower and mid crust. Enclaves in the Silurian S-type Deddick Granodiorite, SE Australia, have chemical and Nd–Sr isotopic compositions broadly similar to Ordovician–Silurian clastic sedimentary rocks throughout the fold belt. SHRIMP U–Pb ages of detrital zircons in the enclaves indicate Early Ordovician maximum depositional ages for their psammitic–pelitic precursors. Patterns of zircon inheritance show the same ~500 and 1100–1200 Ma peaks commonly observed in Palaeozoic Gondwana margin sedimentary sequences in eastern Australia, New Zealand and East Antarctica. U–Pb ages for metamorphic zircon rims and Sm–Nd dating of garnet indicate that metamorphism in enclave precursors was coeval with granitic magmatism at ~430 Ma. The enclaves represent metamorphic equivalents of the voluminous Lachlan Fold Belt Palaeozoic turbidites. After tectonic transport to the mid crust and attendant amphibolite-facies metamorphism during the Benambran Orogeny (440–430 Ma), these Palaeozoic metasediments became the sources for extensive Silurian magmatism. S-type granite magmas in the eastern Lachlan Fold Belt are therefore derived from Ordovician turbidites with possible contributions from Cambrian volcano-sedimentary (‘greenstones’) sequences and from contemporaneous, mantle-derived magmas.

KEY WORDS: enclaves; granites; magma source; zircon U–Pb ages


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