Textures recording transient porosity in synkinematic quartz veins, South Coast, New South Wales, Australia

Abstract

The textures of synkinematic veins involve a complex interplay of crystal growth and deformation. Quartz veins in Devonian sandstone and altered felsic volcanic rocks of the South Coast region of New South Wales, Australia, display a range of microstructures recording complex vein growth mechanisms. The synkinematic veins have not suffered significant later imposed deformation and all deformational features within the veins are associated with the kinematics of vein formation. The veins are commonly fibrous but growth patterns are not restricted to simple margin-parallel types (for example, antitaxial and syntaxial). Mineral fibres and blocky cavity fillings developed concurrently throughout all stages of vein growth. Dilation and pressure-solution of vein textures is recorded by transgranular, intergranular, and intragranular deformation microstructures. Textures of progressively deformed synkinematic veins, such as arrays of sigmoidal vein and shear veins related to folding, contain microscopic folds and faults of fibrous quartz and the development of crystallographically controlled microcrack arrays within crystals. Small displacements of the microcrack arrays locally rotate the crystal lattice and at low power magnification are indistinguishable from non-brittle shear bands. Where arrays of veins have been linked to form through-going fault veins, intense deformation microstructures combine with a variety of cavity growth features including dilation of weaknesses such as micaceous seams.