RECRYSTALLIZATION OF APLITE—A FEATURE, NOT A BUG

Aplite dikes reflect extraction of silicic pore melt into fractures in the solid matrix that contained it. Multiple generations of silicic dikes are found in the Half Dome Granodiorite in Yosemite National Park. The youngest dikes are sugary aplite with sharp contacts, older dikes are progressively coarser-grained, and the oldest dikes have the same grain size as the host. Silicic dikes of all ages have saddle-shaped REE patterns and low Y/Sr ratios that reflect equilibration with titanite and that distinguish aplite from erupted high-silica rhyolite. We hypothesize that all of the silicic dikes formed as fine-grained aplite, and that the grain-size increase with relative age reflects recrystallization owing to prolonged residence at elevated, fluctuating temperatures as the pluton grew incrementally, and to infiltration by fluid released by crystallization of later magmatic increments. Close spatial association of hydrothermal pipes with felsic dikes suggests that the dikes became conduits for fluid released by crystallization of deeper magmatic increments.

Backscattered electron and cathodoluminescence imaging reveals complex histories in the dikes. Textures indicate that all of the samples examined, including sugary aplite, underwent subsolidus recrystallization. Evidence includes smoothly curved grain boundaries and 120° grain-boundary intersections which indicate recrystallization to minimize surface energy, and scalloped grain boundaries that reflect grain-boundary migration. Albite selvages separate intermediate plagioclase from alkali feldspar with contacts that are sharp at the micrometer scale, indicating that the rims did not form by diffusive re-equilibration but rather by dissolution and reprecipitation of the other feldspars. K-feldspar was persistently cracked and healed. Fluid infiltration along grain-boundary cracks would have enhanced grain-boundary mobility, thus promoting recrystallization, and also could have mediated reaction between higher-temperature feldspars to form the albite rims. We conclude that all the dikes, including the youngest, most sugary ones, have undergone extensive recrystallization, and that such re-equilibration to lower temperatures is a feature, not a bug, of granitic systems.