Microstructural evidence for convection in high-silica granite

Abstract

High-silica (>70 wt% SiO2) granites (HSGs) are critical carriers of tin, copper, and other melt-incompatible elements, yet much remains unknown about the mechanisms responsible for their formation. One of the key issues is the apparent lack of evidence for crystal-melt segregation (e.g., modal layering), without which little can be inferred about the dynamics (or lack thereof) of crystallizing HSGs. We examined the crystallographic orientation relationships of clustered quartz crystals from the 300-m-thick Bobbejaankop sill, Bushveld Complex, South Africa. We report an inward increase in the number density and size of quartz clusters toward the central horizon of the sill, coinciding with a significant increase in concentrations of tin, copper, and tungsten. The majority of crystal pairs within each cluster exhibit coincident-site lattice orientation relationships, representing low grain-boundary energy configurations. These clusters must have formed by synneusis in a magmatic environment where crystals could have moved freely, rotating into low-energy orientations on contact. We argue that this not only demonstrates that 100-m-scale crystal-poor and liquid-rich regions can be present in bodies of HSG, but also that such bodies can undergo long-lived convection during crystallization, driven by downwards movement of crystal-rich plumes at the roof, without significant crystal-melt segregation. This dynamic behavior provides a mechanism to homogenize major-element distribution across HSGs and to concentrate highly incompatible and economic elements into central mineralized horizons.