Palaeozoic high-grade metamorphism within the Centralian Superbasin, Harts Range region, central Australia

Abstract

Dating of remnant detrital zircon from high-grade metasediments of the Harts Range Group (HRG) in central Australia shows that the sedimentary protoliths were deposited during the Neoproterozoic to Cambrian, demonstrably younger than Palaeoproterozoic metamorphic rocks of the surrounding Arunta Inlier. The inferred depositional age of the HRG indicates that it was deposited at the same time as sedimentary rocks of the former Centralian Superbasin, now represented by the Amadeus and Georgina structural basins adjacent to the Harts Range. Detrital zircon data from the sedimentary rocks in these basins show that both the unmetamorphosed and high-grade metamorphic sequences shared common source regions and display similar provenance changes with time. These similarities imply that the HRG is the high-grade metamorphic equivalent of the Centralian Superbasin, meaning that the well-studied patterns of sedimentation in the basin can be used to constrain tectonism that occurred at mid- to lower-crustallevels in the Harts Range. The detrital zircon data indicate that the HRG extends at least 100 km east of the Harts Range, possibly grading eastwards beneath cover into unmetamorphosed sedimentary rocks of the Warburton Basin. Granitoids from the lower part of the HRG have Early Cambrian crystallisation ages of - 520 Ma, around 45 million years older than metamorphism recorded by metamorphic zircon, which has ages between -475-460 Ma (the Larapinta Event). The granites appear to have been derived from partial melting of their Early Cambrian host rocks and were coeval with mafic magmas, forming a bimodal igneous complex. During the Early Cambrian, deposition in the Centralian Superbasin adjacent to the Harts Range was clastic-poor and was accompanied by a marine transgression in the southern part of the Georgina Basin, implying that the Harts Range region was actively subsiding. Deeperwater pelitic sedimentation in the Harts Range area at this time and the presence of bimodal magmatism are consistent with an extensional setting for Early Cambrian partial melting and magmatism, here termed the Stanovos Event. Continued extension and subsidence resulted in the formation of a shallow marine seaway across central Australia in the Early Ordovician, below which granulite-facies metamorphism of the HRG took place at -10-12 kbar (-30-35 km). This metamorphism was accompanied by the formation of a pervasive layer-parallel foliation and the intrusion of syn-tectonic mafic dykes. Rare metamorphic and igneous zircon ages at -475 Ma possibly date peak metamorphism of the Larapinta Event, while widespread metamorphic zircon overgrowths at -460 Ma are probably related to retrograde metamorphism. Burial of the HRG to lower crustal levels is interpreted to have taken place in a rift or transtensional setting, implying that burial took place primarily by sediment loading within an actively subsiding basin (the Irindina sub-basin). The -30-35 km depth of metamorphism indicated by thermobarometric data imply that the Irindina sub-basin was deeper than any other known basin in Earth history. Potential field modelling of magnetic and gravity data was unable to distinguish whether a prominent linear gravity high in the Harts Range region is due to a preserved thick remnant of the Irindina Sub-basin or a large mafic body in the lower crust. However, the intensity of the anomaly indicates that a large accumulation of mafie material is present at depth, consistent with the interpreted rift setting for both the Stanovos and Larapinta Events. U-Pb zircon dating of the Entia Gneiss Complex and adjacent Strangways Metamorphic Complex shows that Larapinta Event had little effect on the Palaeoproterozoic basement adjacent to the Irindina sub-basin, with evidence limited to rare Early Ordovician isotopic disturbance. This is consistent with the interpretation that the Larapinta Event took place within the lower part of a deep sub-basin rather than as a result of a contractional event that would have affected both the basement and cover sequences. Basin inversion and uplift closely followed the retrograde phase of the Larapinta Event, culminating in the Alice Springs Orogeny at --400-300 Ma. The HRG was exhumed at this time and thrust over Palaeoproterozoic basement of the Entia Gneiss Complex along a major crustal detachment. Metamorphic zircon overgrowths between -~360-330 Main both the basement and cover sequences, and granitoid intrusions in the HRG at -360 Ma confirm that the Alice Springs Orogeny was a major tectonothermal event in the Harts Range region. U-Pb dating of monazite indicates that the Entia Gneiss Complex was pervasively reworked by a flat-lying kyanite-grade foliation at - 336 Ma, which was subsequently deformed into a complex domal culmination, the Entia Dome. The flat-lying foliation and doming possibly reflecting extensional collapse towards the end of the Alice Springs Orogeny, following a prolonged period of N-S to NNE-SSW directed contraction.