INTERPRETATION OF DOMAINAL GROUNDMASS TEXTURES IN BASALT LAVAS OF THESOUTHERN LAMINGTON VOLCANICS, EASTERN AUSTRALIA

The arrangement of crystals, vesicles, and glass in the matrix of a vo lcanic rock records information about the kinematics accompanying its formation. Simple rock textures involve homogeneous distributions of f eatures such as aligned crystals, whereas more complex rock textures i nvolve inhomogeneous distributions comprising subregions or domains of homogeneous textural types. In the groundmass of volcanic rocks, two types of domainal textures are defined by (1) differences in the degre e of alignment or direction of alignment of crystals and (2) different concentrations of crystals. A petrographic survey of basalt lavas of the Tertiary Lamington Volcanics in northeast New South Wales, Austral ia, reveals the presence of both crystal alignment and crystal concent ration domainal textures. Alignment domains are interpreted as shear z ones overprinting homogenous aligned textures during flow. The formati on of crystal concentration domains appears to involve multiple proces ses including(1) crystal clustering during flow, (2) complex overprint ing of dilatant groundmass shear zones, (3) melt segregations due to s tress, or (4) postvesiculation processes.