Uncoupled O and Hf isotopic systems in zircon from the contrasting granite suites of the New England Orogen, eastern Australia: Implications for studies of Phanerozoic magma genesis

Abstract

The Permo-Triassic granites of the New England Orogen, eastern Australia, were emplaced into a volcanic arc complex accreted to the eastern Gondwana margin in the Late Devonian or Early Carboniferous. Zircon U-Pb dating shows that the S-type Hillgrove (~297Ma) and Bundarra (~287Ma) Supersuites predated intrusion of the I-type Moonbi Supersuite (~250Ma) by up to 50Ma. The high δ18Ozrn of the S-type granites (10.0-11.5‰), and range of U-Pb ages (~370-300Ma) and δ18Ozrn (~5-10‰) of their inherited zircon cores, show that their source rocks were predominantly weathered Carboniferous volcaniclastics, the youngest deposited<25Ma before the granites were emplaced. In contrast, the lower δ18Ozrn (6.9-7.8‰) and lack of inheritance in the I-type granites is consistent with a zircon poor, more juvenile source, probably a mafic igneous underplate mixed with a small amount of volcanogenic and/or oceanic sediment. Despite the differences in source materials, the εHf(t) values of all granites, both S- and I-type, are similar (+5.0±0.5 cf. +5.9±0.5), consistent with both the mafic and sedimentary components in the granite sources being relatively young and similar in Hf isotopic composition at the time of granite genesis. In young, isotopically juvenile orogens, the O isotopic composition of well-dated igneous and inherited zircon can be a much more sensitive indicator of petrogenetic processes than the zircon Hf isotopic compositions alone.