PLAGIOCLASE CORONAS AROUND GARNET IN THE WASHINGTON NORTH CASCADES: PRELIMINARY INTERPRETATION FOR CORONAS FROM THE DINKELMAN DETACHMENT

Coronas of plagioclase ± hornblende ± biotite ± sphene ± cordierite ± quartz around garnet have been described in numerous localities from the Nason, Chelan Mountains, and Swakane terranes in the North Cascades metamorphic core. These textures have been interpreted to result from rapid exhumation, that produced high-temperature metamorphic changes while pressure decreased. We report preliminary results from garnet corona textures in the Dinkelman detachment near Entiat. The detachment is a high temperature ductile shear zone that separates the Swakane (underlying) and Chelan Mountains (overlying) terranes. Interlayered amphibolite and tonalite gneiss are abundant lithologies in the detachment near Dinkelman Ridge (Miller, Paterson, DeBari, & Whitney, 2000). Abundant garnet in the amphibolite has grains that vary from < 0.1 to 2.5 cm; small grains occur mixed with large porphyroblasts or in fine-grained (< 0.1 cm) lenses and layers parallel to foliation. Thermobarometry (Valley, 2001) indicates metamorphic conditions of 630-730º C at 9.5-11.5 kbar. Coronas appear to be best-developed around < 0.1 cm garnet grains; numerous larger grains have no appreciable coronas. Corona mineralogy varies from nearly pure plagioclase to plagioclase + hornblende ± sphene. Small garnet is weakly zoned, with Alm 62% (rim) to Alm 60% (core); Grs (26%), Prp (12%), and Sps (1%) are essentially unzoned. Alm may decrease slightly and Grs increase slightly, ca. 1% at the extreme outer rim adjacent to the plagioclase corona. The plagioclase corona is concentrically zoned; the central third is An 35%, decreasing to An 26% toward garnet (inside) and hornblende (outside). Corona textures and garnet zoning are compatible with high temperature diffusional re-equilibration of garnet during metamorphism. Our preliminary data do not allow us to distinguish between corona formation at high temperature and peak pressure and high temperature decompression.