AN OVERVIEW OF THE NATURE, DISTRIBUTION AND INFERRED TECTONIC SETTING OF GRANITOIDS IN THE LATE-ARCHEAN NORSEMAN-WILUNA BELT

The highly mineralized late-Archaean Norseman-Wiluna Belt, Yilgarn Block, Western Australia has lithostratigraphic associations and a structural style and history that has recently been interpreted in terms of a subduction-related tectonic setting. It is important to investigate whether the geochemistry and petrology of the granitoids in the belt are compatible with such a model. The granitoid suite within the belt can be subdivided into: Association 1-large, composite batholiths of calc-alkaline affinity spatially and temporally associated with calc-alkaline volcanic centres; Association 2-plutons ranging from quartz diorite and tonalite to monzogranite and syenogranite, spatially related to dike swarms of felsic- to intermediate-porphyries and calc-alkaline lamprophyres; and Association 3-peralkaline to alkaline batholiths and stocks which are spatially and temporally associated with rhyolitic volcanic centres and major lineaments. The calc-alkaline Associations 1 and 2 have characteristics comparable with suites of variable fractionated I-type granitoids in Phanerozoic continental-margin arc settings, including: (1) a wide range in SiO2; (2) meta- to peraluminous compositions; (3) a progression from granodiorite to granite, with plutons becoming more K2O-rich and hornblende-poor with time; (4) LILE enrichment and relative depletion of Ta, Nb and Ti. In contrast to most Phanerozoic granitoids, the Norseman-Wiluna Belt granitoids have highly fractionated REE patterns and low contents of Yb and Y with respect to average post-Archaean equivalents. REE data and trace element characteristics suggest derivation from a mafic source, leaving hornblende and/or garnet as residual phases. Peralkaline granitoids of Association 3 exhibit enrichments on SiO2, Na2O + K2O, Zr, Y, Nb and Ta with respect to the calc-alkaline granitoids. They post-date Associations 1 and 2, and are consistent with the concept that they represent a continuation of magmatism in a post-orogenic environment. Thus, the nature and distribution of the granitoids of the Norseman-Wiluna Belt are consistent with interpretations that the belt formed in a subduction-related convergent margin setting.