Structural analysis of flow-related textures in lavas

The textures of coherent volcanic rocks, including lavas and volcanic intrusives, commonly contain features that are attributed to flowage. Previous applications of structural analysis to volcanic rocks are expanded here to provide a framework for analysis, Textures, defined as the crystallinity, granularity and shapes and arrangernents of the components (crystals, glass and voids) of a rock, together with structures, defined as individual features composed of the disposition, attitude, arrangement or relative positions of the components of a rock, are first described. Second, the spatial fabrics (shapes and arrangements of the components of a rock and the orientation of textures and structures) are identified. Third, textures, structures and fabrics are placed in the spatial and temporal geological context. Finally, detailed interpretations of the kinematics and Rheology of structures and fabrics is made, leading to an integrated flow history of the rock. Illustrative case studies include rhyolite from the basal part of the Tertiary Minyon Falls dome, northeastern New South Wales, Australia, which has a texture comprising planar domains of differing crystal abundance (flow bands), multiple folds of these domains, relatively homogeneous crystal alignment parallel to the fold axes and microfolding of these domains in the zone of interaction between phenocrysts and matrix, including retrorotation of phenocrysts on short limbs of inequant folds. Trachyte dykes on Fraser Island, Queensland, Australia have a texture comprising crystal alignment, planar concentration domains (banding), two planar domains of crystal alignment interpreted to be conjugate shear zones. Phenocrysts influence the spacing and distribution of the domains and interacted with shear zones by undergoing small amounts of rotation. The shear zones overprinted a homogeneous crystal alignment during the last stage of flow before solidification as a result of dilatant granular interactions. Lava from Miocene submarine-banded silicic-mafic domes on Katsurajima on the coast of the Shimane Peninsula southwestern Japan has a texture comprising crystal alignment, irregular domains (interpreted as the result of quenching), local internal deformation of crystals, including brittle boudinage, and elongate vesicles and amygdales. Dacite from the late stages of the 1991-1995 eruption of Mount Unzen in western Kyushu, Japan has a texture comprising crystal alignment, deformation of phenocrysts and their surrounding matrix producing voids by mechanical means with the possible involvement of vapour phases. The pervasive mechanical failure of the flowing magma may have triggered frontal collapse and the ensuing pyroclastic eruptions. A spectrum of increasingly intense flow-related textures can be recognised from simple planar alignments of crystals and vesicles to overprinting effects and domainal textures, such as banding and shear zones, and deformed domains. Pervasive ductile tearing and ultimately brittle fracture record flow during the transition from ductile magma to brittle rock. The development of these flow-related textures is controlled by the interplay of 'driving forces' (magma pressure, gravity) and 'resisting forces' (viscosity). Where 'driving forces' are strong flow-related textures are well developed. When viscous resistance exceeds the 'driving forces', flow and the development of flow-related textures ceases. (C) 2002 Elsevier Science B.V All rights reserved.