GEOCHEMICAL AND ND ISOTOPIC SYSTEMATICS OF GRANITES FROM THE ARUNTA-INLIER, CENTRAL AUSTRALIA - IMPLICATIONS FOR PROTEROZOIC CRUSTAL EVOLUTION

Geochemical and Sm-Nd isotopic results are reported for granites from the Proterozoic Arunta Inlier of central Australia. These, combined wi th 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 Inli er can be divided into three major geochemical groups, a Calcalkaline- trondhjemitic Group (CAT), a High-heat-production Group (HHP), and a v olumetrically significant Main Group. The CAT Group, which occurs only in the southern margin of the inlier, is characterised by high Na2O, Na/K, Sr, K/Rb and Sr/Y, and relatively low K2O, Rb, Rb/Sr, Th, U, REE , Nb and Y, analogous to calc-alkaline suites occurring in modern conv ergent plate margins. The HHP Group, which occurs mainly in the interi or of the Arunta Inlier and is spatially associated with the Main Grou p, is characterised by high K, Rb, Th, U, Rb/Sr and Rb/Zr, and relativ ely low Sr, Ba, Na/K, K/Rb, Ba/Rb, MgO, Cr and Ni. The Main Group, whi ch occurs throughout the Arunta Inlier, is geochemically intermediate between the CAT and HHP groups. It is geochemically analogous to the 1 880-1850 Ma old Barramundi Igneous Association recognised in other Pro terozoic terrains of central Australia, and can be further subdivided into four age subgroups, 1820, 1780-1750, 1650 and 1615-1600 Ma, respe ctively. All three groups show similar negative Nh anomalies on the tr ace-element-normalized diagrams. Despite the geochemical diversity of the Arunta granites, no correlations between the Nd isotopic and geoch emical signatures are observed. The ranges of initial epsilon(Nd) valu es and Nd-depleted mantle model ages (T-DM(Nd))) for the three groups of granites overlap with each other. Overall, there are two groups of T-DM(Nd), ages, the most common ranging from 2.33 to 1.96 Ga, and the other from 1.83 to 1.72 Ga. Although initial epsilon(Nd) values of the granites show large variations, there is a general trend for the epsi lon(Nd) values to increase (i.e., become less negative) with decreasin g crystallization ages. The large geochemical and isotopic variations of the Arunta granites reflect considerable heterogeneity in the sourc es of the granites and do not support a uniform, 2.3-2.1 Ga old Barram undi-type underplate source for the origin of the granites. The combin ed geochemical and Nd isotopic data suggest the sources of the granite s contain at least two main components, an Palaeoproterozoic mantle-de rived component and an older crustal component. The older crustal comp onent was incorporated into the source region of the granites either t hrough magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatis m. The increasing epsilon(Nd) values with decreasing crystallization a ges may be a result of progressively increasing proportions of newly a ccreted island-are materials being incorporated in the subducted sedim ents. Alternatively, a juvenile mantle-derived component may have been added to the source region of the younger granites. It is apparent th at the Nd isotopic signatures in the granites has resulted from comple x processes and neither simple mixing nor simple two-stage protolith m odels can as yet satisfactorily explain the observations. The petrogen esis of the three geochemical groups of granites can be interpreted in a plate-tectonics scenario. It is considered that the CAT Group was f ormed by fractionation of are-type magmas and/or partial melting of ar c-related intrusions or underplates in a subduction-related continenta l margin setting, whilst the formation of the Main Group may involve p artial melting of fractionated and/or modified are-related underplates , but not necessarily in a subduction environment. The younger HHP Gro up was probably generated by remelting of the coexisting Main Group du ring subsequent tectonothermal events.