Archaean and Proterozoic crustal evolution in the Eastern Succession of the Mt Isa district, Australia: U – Pb and Hf-isotope studies of detrital zircons*

Abstract

Over 500 zircon grains separated from modern sediments in 10 drainages covering the Eastern Succession of the Mt Isa Inlier have been analysed for U – Pb ages, Hf isotopes, and trace elements, using in situ LAM-ICPMS techniques, to evaluate the efficacy of this approach in characterising large-scale crustal evolution. U – Pb age spectra are used to estimate the timing of terrane-scale events, primarily magmatic episodes; Hf isotopes provide information on the relative contributions of juvenile material and reworked older crust at each stage of crustal evolution; trace-element patterns of zircons are used to characterise original magma types. The integration of these data for individual zircon grains produces an event signature that provides more information than that gained from U – Pb dating alone. The data define four major stages of crustal evolution in the area: 2550 – 2330 Ma, 1950 – 1825 Ma, 1800 – 1600 Ma, and 1590 – 1420 Ma. Each stage, except the last, involved crustal extension, and ended with a period of crustal homogenisation, which is reflected in the isotopic composition of magmatic rocks generated by crustal reworking in the succeeding stage. Reworking of Neoarchaean crust contributed significantly to crustal magmatism throughout the Proterozoic history. A major input of juvenile mafic material around 1625 Ma (interpreted as the magmatic age of the Toole Creek Volcanics) is poorly represented in the database of conventional geochronology but is prominent in the detrital zircon record. The major late-stage granitic magmatism of the Williams and Naraku Batholiths (1520 – 1490 Ma) was generated almost entirely from older crust, with little juvenile input. The study demonstrates that sampling of carefully selected modern drainages and the analysis of statistically large numbers of detrital zircons can provide insights additional to those gained from conventional analysis of U – Pb and Sm – Nd systematics in selected rock samples. The integrated use of U – Pb age, Hf-isotope composition and trace-element patterns in detrital zircons is a powerful and relatively inexpensive tool for the analysis of terrane-scale crustal evolution, and for the correlation of terranes.