Metamorphic and isotopic characterisation of Proterozoic belts at the margins of the North and West Australian Cratons

Abstract

The tectonic evolution of the cratonic elements of Proterozoic Australia has been debated for over 20 years. There is a growing view that plate margin processes were involved in the tectonic evolution and growth of the pre-Cryogenian elements of Australia, however the timing, nature and configuration of cratonic amalgamation remains contentious. This study investigates the metamorphic, geochronological and isotopic evolution of key or debated areas of Proterozoic Australia, focusing on the proposed southern margin of the Archean to Paleoproterozoic North Australian Craton (NAC) in the Arunta Region, and eastern margin of the Archean to Paleoproterozoic West Australian Craton (WAC) in the Rudall Province. The overall aim of this study is to provide new constraints on Proterozoic tectonism in the Arunta Region and Rudall Province in order to better understand the timing and nature of Proterozoic Australia assembly. In the southern Aileron Province (Arunta Region), the Mount Hay area and Adla Domain occur close to the proposed Paleoproterozoic southern margin of the NAC. Pressure– temperature (P–T) constraints indicate the attainment of peak metamorphic conditions of ~8–10 kbar, ~850−900 °C for Mount Hay and the adjacent Capricorn Ridge, and ~7–10 kbar, ~850−900 °C for the Adla Domain fabrics. The granulite facies metamorphism postdates a period of extensive basin development in the Arunta Region between c. 1805−1780 Ma. This basin development was associated with magmatism and localised high temperature–low pressure (HTLP) metamorphism. Hf isotopic data on late Paleoproterozoic granitoids (c. 1650–1625 Ma) from the Aileron Province have isotopic compositions close to CHUR (ɛHf -6.2 to +1.5) and crustal model ages between 2200–2700 Ma. The granitoids are broadly contemporaneous with the c. 1640–1635 Ma Liebig Orogeny in the Warumpi Province, which involved coeval mafic magmatism, suggesting at least some component of extension. The Paleoproterozoic tectonic evolution of the Arunta Region (southern NAC) is considered to have involved a long-lived (>150 Ma) margin with an overall extensional character punctuated by comparatively localised and short lived periods of thickening. In the central Aileron Province, the tectonothermal evolution of the Anmatjira Range Province has been debated considerably over the last 20 years. The timing and metamorphic evolution of the Anmatjira Range was investigated using monazite U–Pb geochronology and P–T pseudosections calculated for high temperature granulite facies metapelites in the southeastern Anmatjira Range. Estimated peak conditions of ~870–920 °C and ~6.5–7.2 kbar were attained at c. 1580–1555 Ma, followed by a clockwise retrograde evolution. In the absence of concurrent magmatism, and lack of evidence of decompression from high-P conditions, the most probable driver for this metamorphism is heating largely driven by high-heat production from older granites (c. 1820–1760 Ma) in the region. To the west, the Rudall Province (eastern WAC) is one of the few localities of Proterozoic, Barrovian-style metamorphism in Australia. In several previous studies, the Rudall Province has been considered to record the collision of the WAC and NAC during the Yapungku Orogeny at c. 1780 Ma. However, prior to this study, medium-P assemblages interpreted to have grown during the Yapunkgu Orogeny (inferred thermal gradients of minimum ~60–80 °C/kbar) had not been directly age-constrained. Monazite age data on metasedimentary rocks from both medium-P and high temperature–low pressure (HTLP) assemblages, and zircon U–Pb age data from a medium-P, garnet-diopside bearing mafic amphibolite yield age populations between c. 1380 and 1275 Ma, with one monazite age population of c. 1665 Ma. No evidence for older c. 1780 Ma metamorphism was found in this study. The large age population range of c. 1380– 1275 Ma yielded in this study may be a response of a stage-wise tectonic evolution, involving the accretion of ribbons. If the Yapunkgu Orogeny does reflect the collision between the WAC and NAC, it most likely did not occur until the Mesoproterozoic, contemporaneous with initial breakup stages of supercontinent Nuna. The overall results of this work support a long-lived, retreating margin on the southern NAC during the late Paleoproterozoic, prior to the assembly of cratonic Australia in the Mesoproterozoic. The proposed Mesoproterozoic assembly negates the need for Australian cratons to be in close proximity in supercontinent Nuna reconstructions.