Contact structures at a granodiorite intrusion, Piccaninny Point, north-east Tasmania

Abstract

At Piccaninny Point, structures developed at the contact between the Piccaninny Creek Pluton and the Mathinna Beds have, a bearing on the mode of emplacement of this body and other granodiorite plutons in north-eastern Tasmania. The contact has both concordant and irregularly discordant segments, and a dilational mode of intrusion is indicated. During the late crystallisation stage flattening of partially crystallised magma against the contact produced a secondary cataclastic foliation and flattened xenoliths, and caused injection of cross-cutting leucocratic dykes. Magmatic pressure on the wall continued after the marginal granodiorite had crystallised, producing conjugate faults and quartz gash-veining. The Piccaninny Point contact has features analogous to regional features of the granodiorite plutons of the Blue Tier Batholith to the north. The granodioritic plutons appear to have been emplaced by fracture controlled dilation, with upwelling of magma in the centre and lateral spreading against the walls.