The evolution of Archaean greenstone terrains, Eastern Goldfields Province, Western Australia

Abstract

Abstract Available petrological, structural and geochronological data suggest that metamorphism and deformation of greenstone sequences and the evolution of intrusive granitoids in the Eastern Goldfields Province, Yilgarn Block, were related to a widespread and integrated tectonic event in the time interval 2700-2600 m.y. Polyphase deformation of the greenstone sequences involved the superimposition of a series of upright folds and related subvertical foliations on earlier macroscopic recumbent folds. Metamorphism was imposed rapidly on these previously deformed but relatively unaltered greenstone sequences, synchronously with a third phase of deformation. Static-style metamorphic recrystallization at very low to medium grades occurred over most of the province, but contemporaneous high grade recrystallization of dynamic style was restricted to elongate narrow zones which were also the sites of synkinematic granitoid diapirism. These zones commonly mark the present margins of greenstone belts. The extensive areas between greenstone belts are dominated by outcrops of post-kinematic granitoids whose abundance may be overestimated because of the limited exposure. Their emplacement caused only minor contact metamorphic overprinting on the pre-existing metamorphic patterns. Also present are banded gneisses interpreted as modified basement to the greenstone sequences. These gneisses are enclosed in post-kinematic granitoid batholiths or occur as remnants in synkinematic diapirs within the dynamic domains. All major granitoid groups, including gneisses, are geochemically similar and show parallel but limited variations. Both field and chemical evidence points to the gneisses being parental to intrusive granitoids derived by both anatectic and solid-state processes. The data provide important constraints on any model for greenstone belt evolution. Our preferred model involves a widespread disturbance resembling the kind currently referred to as a “mantle plume”, which initially led to extrusion of mafic and ultramafic magmas via tensional fractures in a sialic crust, then subsequently caused their deformation and metamorphism and generated the intrusive granitoids by widespread reactivation of the basement. The dynamic metamorphic domains may reflect pre-greenstone crustal lineaments that controlled the initial vulcanism. The evolution of Archean greenstone terrains proposed here appears distinct from that of subsequent Proterozoic and Phanerozoic tectonic belts.