Evolution and architecture of a large felsic Igneous Province in western Laurentia: The 1.6 Ga Gawler Range Volcanics, South Australia

摘要

The Gawler Range Volcanics (GRV) of South Australia are an intracontinental, subaerial Large Igneous Province (LIP) formed during supercontinent assembly in the Mesoproterozoic. Like many LIPs, the GRV are intraplate, were erupted over a geologically short time (~2 million years), and are dominated by voluminous lavas. However, the GRV are overwhelmingly dominated by felsic units. The GRV are essentially undeformed and have not been deeply buried so that their original textures are exceptionally well preserved. Furthermore, they are well exposed in very gently northward and eastward dipping sections up to 4 km thick, allowing assessment of the facies architecture and evolution of this felsic volcanic LIP. The evolution of the GRV can be clearly separated into two main stages. Initial eruptions at numerous volcanic centres produced small to moderate volume, geochemically distinct, felsic lavas and lava domes, together with ignimbrites and minor mafic and intermediate lavas, forming a sequence 0.5 to 3 km thick. Volcanic activity in this lower sequence varied from effusive to explosive and was not much different in style or products from Phanerozoic felsic volcanic provinces. However, the second stage produced at least three voluminous felsic units, each of which represents about 1000-3000 km~3 of magma. Moreover, the dominance of evenly porphyritic textures and lack of pyroclastic textures (pumice, shards, broken crystal, lithic clasts) in these units suggests that they were erupted effusively and flowed as lavas. Each of these felsic lavas are generally dominated by a single uniform composition, and commonly mingled with a subordinate and compositionally distinct lava.