U-Pb, Lu-Hf and O isotopes in zircon from late palaeozoic granites across orogens, Southeastern Australia

Abstract

Southeastern Australia, part of the Phanerozoic Tasmanides, is a unique region where large amounts of granite (~30% of the surface rocks) with a very wide range of compositions (S-, I- and A-types) were intruded in the Ordovician to Triassic.

The distinctive Carboniferous granites in the Lachlan Fold Belt (LFB) are transitional in time and space between the major magmatic episodes of the LFB and New England Orogen (NEO). There was a contemporaneous continental-arc developed in the NEO, products of which became the dominant source for the NEO Early Permian S-type granites. The Carboniferous granites in the LFB have characteristic compositions (K, Sr and LILE-enrichment and Y-depletion) similar to the Late Permian I-type granites (and to the S-type granites in part) in the NEO. The in situ microanalyses of zircon (both melt-precipitated and inherited; over 1280 grains analysed from 31 samples) from those granites, using SIMS and LA-MC-ICPMS, show that the Carboniferous magmatism in the LFB was closely related to the tectonic movement of the NEO, and that the granites with similar compositions which transect the two contrasting orogens had similar source compositions.

From the isotopic compositions of zircon from the Carboniferous granites in the LFB, it is evident that the granites are distributed in zones of different ages and Hf and O isotopic compositions. The zones are approximately parallel to the NEO boundary. It is likely that tectonic activity related to the NEO triggered the production of the Carboniferous magmas and their source rocks. The new data demonstrate, however, that the Carboniferous granites were not directly related to the contemporaneous arc volcanism in the NEO. The source rocks of the Carboniferous granites in each zone consisted of different mixtures between a mantle-like underplate and relatively mature pre-existing lower crust during the Devonian.

The distinctive O isotopic compositions of inherited zircon from the NEO S-type granites demonstrate that the source rock of the granites was a Carboniferous arc-related volcanogenic sedimentary pile which included increased amounts of Devonian volcanogenic sediments with time. The age difference between the inherited and melt-precipitated zircon indicates rapid crustal recycling to produce the peraluminous magmas within 15 Ma of source rock deposition. The I-type Moonbi Supersuite granites were generated from the underplate with a minor crustal contribution. As all the studied NEO granites have remarkably similar initial Hf isotopic compositions, the components comprising most of the NEO, including sediments, are similarly radiogenic, probably due to the geologic processes operating in the young accretionary orogen.

Combining all the results of U-Pb dating and Hf and O isotopic analyses of zircon from the Carboniferous granites in the LFB and the Permian I- and S-type granites in the NEO, it is also inferred that the source rocks of the studied granites had similar isotopic compositions. This suggests a potential petrogenetic link between the granites across the two orogens. From a source composition similar to that of the most primitive LCG, three igneous components have evolved in different ways depending on the nature of the crustal materials incorporated into each component.