Some petrographic and mineralogical diagnostics of sandstone intrusions

Sandstone intrusions in giant injection complexes are characterised by texturally immature sand with common micro-fractured framework grains. Individual micro-fractures are distinctive in geometry and unaligned within or between grains thus differentiating them from micro-fractures formed by shock metamorphism or tectonics. Individual grains preserve histories of multiple impacts during sand injection. Micro-fracture geometry and their textural association makes them diagnostic of high energy inter-granular collisions during sand injection. Mudstone clasts have sand propped micro-fractures associated with hydraulic fracturing and individual sand grains embedded in clasts by corrasion, which is diagnostic of high grain-velocity. Heavy mineral assemblages record abrasion of apatite and hydrodynamic segregation of zircon (both relative to abundance of tourmaline) upward through the injection complex. Granular abrasion and hydrodynamic segregation are consistent with turbulent flow during sand injection. Collectively the petrographic and mineralogical data support the interaction of high velocity grains in turbulent flow during sand injection in which the granular content is likely to be dilute.