The shoshonitic association in the upper Mesozoic of Tasmania

Abstract

During the Late Mesozoic, igneous activity in Tasmania produced a number of small intrusive bodies and swarms of dykes. Typical examples of these rocks are found on the West Coast and around Cape Portland and Cygnet. The rocks belong to a shoshonitic association and they form part of a more extensive petrographic province that extends from Tasmania into eastern Australia.

It is proposed that the magmas from which these rocks were derived evolved at depth beneath the stabilised western margin of the New Zealand Geosyncline. The composition of this magma which formed in a relatively stable environment, contrasts with the more basic magma that erupted during the period of crustal tension that accompanied the break up of the Australian segment of Gondwanaland during the past 200 m.y.