Static recrystallization of vein quartz pebbles in a high-pressure-low-temperature metamorphic conglomerate

摘要

Microfabrics of quartz pebbles in HP-LT (c. 1 GPa, c. 350 ℃) metamorphic conglomerates are investigated. The conglomerate was deformed by dissolution-precipitation creep, while the interior of the pebbles remained undeformed. The different pebbles display a wide variety of quartz microstructures imported from the source rocks. One type of pebble is derived from quartz veins; it shows old grains with numerous fluid inclusions, subgrains, and undulatory extinction, which are partly replaced by new grains devoid of inclusions and substructure. Free dislocation densities are on the order of 10~(12) m~(-2) in both grains. We conclude that: (1) the quartz vein underwent inhomogeneous crystal-plastic deformation in the source rock; (2) recrystallization took place by strain-induced grain boundary migration starting from small crystalline volumes poor in defects; (3) recrystallization was purely static and commenced during re-burial of the conglomerate; which (4) was simultaneously deformed by dissolution-precipitation creep at low differential stress, insufficient for crystal-plastic deformation of quartz; (5) fluid inclusions within old grains were eliminated and their fluid content was drained along the migrating high angle grain boundaries; and (6) strain-induced grain boundary migration ceased once the driving force became too low by static recovery (concurrent to recrystallization) within the deformed old grains.