Zircon U–Pb Geochronology and Hf–O Isotope Characteristics of Granitoids from the Capricorn Orogen, Western Australia

Abstract

The Capricorn Orogen, Western Australia, is a complex orogenic zone that records the convergence and collision of the Archaean Yilgarn and Pilbara cratons in forming the West Australian Craton (WAC), then over one billion years of subsequent intracontinental reworking. Granites associated with these tectonothermal events (the Dalgaringa, Bertibubba, Moorarie, Durlacher and Thirty Three supersuites) are exposed in the eastern part of the Capricorn Orogen. This study integrates radiogenic (U–Pb and Hf) and stable isotope (O) analysis of zircon grains from granitic rocks in the Capricorn Orogen to determine their ages and magmatic sources, including the relative contributions of mantle versus crustal material. Granites from the margin of the Yilgarn Craton record periods of crustal growth and reworking during the Archaean that influenced later Proterozoic magmatic events. Components of the Capricorn Orogen, collectively termed the Glenburgh Terrane, have previously been considered to be exotic to the adjacent Pilbara and Yilgarn cratons. However, new U–Pb zircon geochronology and Lu–Hf isotope compositions of basement rocks in the Glenburgh Terrane (the Halfway Gneiss) have similarities with some terranes of the Yilgarn Craton, and are interpreted to represent a reworked portion of the craton that was re-accreted during the Glenburgh Orogeny. Arc magmatism during the c. 2005–1950 Ma Glenburgh Orogeny resulted in a period of crustal growth, with magmas representing a mixture of 50–90% mantle-derived magmas and 50–10% magmas derived from an evolved crustal component with an isotopic composition equivalent to that of the Halfway Gneiss. Following assembly of the WAC, granite magmatism in the Capricorn Orogen records a significant change from one dominated by mantle-derived magmatism to one dominated by crustal melting and an increased contribution from metasedimentary material. This transition reflects a geodynamic evolution from subduction-accretion to collision and intracratonic reworking. The isotopic characteristics of granites from the c. 1820–1775 Ma Moorarie Supersuite indicate three distinct sources including: (1) a metasedimentary component; (2) an evolved crustal component, comparable to the Glenburgh Terrane, and (3) a mafic juvenile component. Following this, the Hf–O compositions of the Durlacher Supersuite indicate they were derived from reworking of the Moorarie Supersuite granites, and require no juvenile contribution or any additional sedimentary source. The isotopic compositions of the Thirty Three Supersuite pegmatites indicate that they were largely derived from reworking of the Moorarie and Durlacher supersuites.

Secular Evolution of Continents and the Earth System

“…The now adjoining margins of these two cratons are overlain by late Archean and early Paleoproterozoic basins recording cycles of extension and subsidence, consistent with continental rifting and passive margin thermal subsidence. The two cratons were juxtaposed along the intervening Capricorn Orogen with its record of foreland basin sequences, accreted continental fragments and magmatic arc successions and associated tectonothermal pulses, including the 2 and 1.8 Ga Glenburgh and Capricorn orogenies (Cawood & Tyler, 2004; Jahn et al., 2021; Sheppard et al., 2004). The West Australian and North Australian cratons are also inferred to have undergone assembly at this time (Cawood & Korsch, 2008; L. Zhao et al., 2022).…”