Interpretation of domainal groundmass textures in basalt lavas of the southern Lamington Volcanics, eastern Australia

Abstract

The arrangement of crystals, vesicles, and glass in the matrix of a volcanic rock records information about the kinematics accompanying its formation. Simple rock textures involve homogeneous distributions of features such as aligned crystals, whereas more complex rock textures involve 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 degree 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, Australia, reveals the presence of both crystal alignment and crystal concentration domainal textures. Alignment domains are interpreted as shear zones overprinting homogenous aligned textures during flow. The formation of crystal concentration domains appears to involve multiple processes including (1) crystal clustering during flow, (2) complex overprinting of dilatant groundmass shear zones, (3) melt segregations due to stress, or (4) postvesiculation processes.