Geochemical and Nd isotopic systematics of granites from the Arunta Inlier, central Australia: implications for Proterozoic crustal evolution

Abstract

Geochemical and SmNd isotopic results are reported for granites from the Proterozoic Arunta Inlier of central Australia. These, combined with new geochronological data for the granites (Zhao and Bennett, 1995), allow important constraints to be placed on Proterozoic tectonic and crustal evolution in central Australia. Granites from the Arunta Inlier can be divided into three major geochemical groups, a Calcalkaline-trondhjemitic Group (CAT), a High-heat-production Group (HHP), and a volumetrically significant Main Group. The CAT Group, which occurs only in the southern margin of the inlier, is characterised by high Na 2O, Na K , Sr, K Rb and Sr Y , and relatively low K 2O, Rb, Rb Sr , Th, U, REE, Nb and Y, analogous to calc-alkaline suites occurring in modern convergent plate margins. The HHP Group, which occurs mainly in the interior of the Arunta Inlier and is spatially associated with the Main Group, is characterised by high K, Rb, Th, U, Rb Sr and Rb Zr , and relatively low Sr, Ba, Na K , K Rb , Ba Rb , MgO, Cr and Ni. The Main Group, which occurs throughout the Arunta Inlier, is geochemically intermediate between the CAT and HHP groups. It is geochemically analogous to the 1880-1850 Ma old Barramundi Igneous Association recognised in other Proterozoic terrains of central Australia, and can be further subdivided into four age subgroups, 1820, 1780-1750, 1650 and 1615-1600 Ma, respectively. All three groups show similar negative Nb anomalies on the trace-element-normalized diagrams. Despite the geochemical diversity of the Arunta granites, no correlations between the Nd isotopic and geochemical signatures are observed. The ranges of initial ϵ Nd values and Nd-depleted mantle model ages ( T Nd DM)) for the three groups of granites overlap with each other. Overall, there are two groups of T Nd DM ages, the most common ranging from 2.33 to 1.96 Ga, and the other from 1.83 to 1.72 Ga. Although initial ϵ Nd values of the granites show large variations, there is a general trend for the ϵ Nd values to increase (i.e., become less negative) with decreasing crystallization ages. The large geochemical and isotopic variations of the Arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform, 2.3-2.1 Ga old Barramundi-type underplate source for the origin of the granites. The combined geochemical and Nd isotopic data suggest the sources of the granites contain at least two main components, an Palaeoproterozoic mantle-derived component and an older crustal component. The older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism. The increasing ϵ Nd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island-arc materials being incorporated in the subducted sediments. Alternatively, a juvenile mantle-derived component may have been added to the source region of the younger granites. It is apparent that the Nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two-stage protolith models can as yet satisfactorily explain the observations. The petrogenesis of the three geochemical groups of granites can be interpreted in a plate-tectonics scenario. It is considered that the CAT Group was formed by fractionation of arc-type magmas and/or partial melting of arc-related intrusions or underplates in a subduction-related continental margin setting, whilst the formation of the Main Group may involve partial melting of fractionated and/or modified arc-related underplates, but not necessarily in a subduction environment. The younger HHP Group was probably generated by remelting of the coexisting Main Group during subsequent tectonothermal events.