Intrusive relationships of granite and dolerite at Lagunta Creek, Freycinet Peninsula

Abstract

Coastal exposures immediately south of the mouth of Lagunta Creek, Freycinet Peninsula, eastern Tasmania, illustrate the temporal and spatial relationships between various Devonian granites and a dolerite dyke. The oldest granite, an equigranular grey hornblende-biotite adamellite containing abundant mafic enclaves, was intruded by a more felsic, equigranular to variably porphyritic or seriate pink alkali feldspar granite, and associated aplite, leucogranite and pegmatite dykes. Field relationships indicate brittle fracture and stoping of the adamellite during dilational emplacement of the pink granite. Minor bodies of granite porphyry are also younger than the adamellite. A dolerite dyke, of hawaiitic composition and containing plagioclase megacrysts, intrudes the adamellite and also crosscuts the aplite and leucogranite dykes, and is the youngest phase. The granites are metaluminous to very weakly peraluminous, and are classified as I-types on both mineralogical and geochemical criteria. The adamellite is unfractionated and chemically reduced, and, together with similar rocks at Wineglass Bay and Cape Tourville, constitutes the Bluestone Bay Suite, which is geochemically distinct from, and younger than, other eastern Tasmanian granodiorite/ adamellites. In contrast, both the pink granite and, to a lesser extent, the granite porphyry are fractionated and locally magnetite-bearing. Fractionation has resulted in high Rb, Y, Th and Pb and low P, Sr and Ba. The dolerite dyke, despite its alkalic major element chemistry, is depleted in Nb like some other granite-associated mafic rocks in the southern Lachlan Fold Belt.