Petrogenesis of the Proterozoic Gawler Range Volcanics, South Australia

Abstract

The post-orogenic, bimodal Gawler Range volcano-plutonic province developed in South Australia between 1600 and 1500 Ma ago. Vast felsic ash-flow tuff units and intercalated, volumetrically minor basalt flows comprise a thick, comparatively undeformed volcanic pile that is intruded in many places by the extensive, comagmatic Hiltaba Granite. The geochemical and petrographic features of the felsic volcanic rocks point to an origin in high temperature, dry partial melting of a comparatively refractory, mafic-intermediate granulitic sialic crustal source. Heat was probably supplied during a period of crustal rifting and underplating associated with an episode of mantle diapirism, which also generated the primary basic magmas for the basaltic rock suite. The primary felsic magmas differentiated along broadly calc-alkaline trends to yield a variety of products, many of which do not conform to a single line of liquid descent owing to appreciable incorporation of an accumulative crystal component in differentiated, high K 2O residual liquids. The Gawler Range Volcanics have comparable geochemical/petrographic characteristics and field settings to several other proterozoic volcanic suites in Australia, thus implying a similar origin. This analogy extends to the familiar Proterozoic rapakivi granite suite of the Northern Hemisphere, suggesting development in response to a unique globe-encircling tectonic regime that was fundamentally different to that which has spawned modern calc-alkaline volcanic arcs.