Continental ca 1.7 – 1.69 Ga Fe-rich metatholeiites in the Curnamona Province, Australia: a record of melting of a heterogeneous, subduction-modified lithospheric mantle

Abstract

Mg-rich and Fe-rich metatholeiites intruded the Willyama Supergroup of the southern Australian Curnamona Province in the Late Palaeoproterozoic at ca 1700 Ma and 1685 Ma, respectively. Intrusion of the Fe-rich metatholeiites occurred during a period of punctuated extension in the Willyama basin. Major-element concentrations are variable (SiO₂ 45.4 – 56.5 wt%; Fe₂O₃∗ 8.5 – 20.7; TiO₂ 0.46 – 2.52 wt%; Mg# 70.5 – 29.1) and, in conjunction with trace-element data, support near-closed-system fractionation of a mantle-derived melt with little or no replenishment. Fractionation produced progressively Fe-rich derivative melts. Crystallising phases were dominated by clinopyroxene and olivine, whereas Fe – (Ti) oxide crystallisation was hindered. Primitive mantle-normalised immobile trace elements are characterised by variable Th, Nb, Sr, P and Ti anomalies. Chondrite-normalised rare-earth element patterns for the most primitive, Mg-rich samples from the western Broken Hill Domain have La_N/Sm_N < 1, whereas the most evolved Fe-rich samples from the Olary Domain have ratios of La_N/Sm_N > 1. Initial εNd values range between – 2.2 and + 2.7 for the majority of the samples, with the isotopic compositions showing no correlation with differentiation or assimilation. The combined geochemical and isotopic data suggest that the southern Curnamona Province metatholeiites were extracted from a depleted mantle in the western Broken Hill Domain, and a variably enriched, heterogeneous subcontinental lithospheric mantle in the Olary Domain. Magmatism most likely occurred in a backarc basin or intracontinental setting. It is speculated that the geochemically enriched mantle component was derived from subduction-related processes, probably related to pre-Willyama basin accretionary processes along the southern and eastern margins of the North Australian Craton.

Key words:

• Curnamona Province
• geochemistry
• Proterozoic
• radiogenic isotopes
• tectonic setting
• tholeiites

Acknowledgements

We thank: Robin Offler and Roland Maas for thoughtful and thorough reviews that greatly improved this manuscript; Wolfgang Preiss and Nick Direen for reviews of an earlier version of this manuscript; Mike Szpunar, Alistair Crooks, Colin Conor, Wolfgang Preiss, Andy Burtt and Stuart Robertson from the Minerals Resources Group, Primary Industries and Resources of South Australia for their help with fieldwork and discussion on the geology of the southern Curnamona Province; Massimo Raveggi for discussions on mafic magmatism in the Broken Hill region. This work was supported by an ARC Linkage grant LP0347342 to MH, KB and JF.