THE SIGNIFICANCE OF PLAGIOCLASE-DOMINANT CORONAS ON GARNET, WENATCHEE BLOCK, NORTHERN CASCADES, WASHINGTON, U.S.A.

摘要

Coronas on garnet, reported from numerous localities throughout the North Cascades Crystalline Core, have been interpreted to record rapid exhumation, associated with regional late Cretaceous and early Paleogene unroofing. Granodiorite dikes adjacent to tonalite and gabbro bodies, in the southern and central Cascades Core, contain fine-grained plagioclase-dominant coronas cored by garnet. Locally, these coronas were strongly deformed into ellipsoids with aspect ratios of 1:1.8:3.1. The matrix assemblage around coronas is biotite + plagioclase + quartz + ilmenite + apatite + zircon + - allanite + - muscovite + - sillimanite. Garnet is strongly zoned with simple or complex patterns. Simply zoned garnet shows a core-to-rim variation: 70-81 percent almandine, 7-10 percent pyrope, 10-2 percent spessartine, 14-8 percent grossular, with no reversals in zoning near the rim (e.g., no Mn increase), and is interpreted as metamorphic growth-zoning. Examples of complexly zoned garnet show core-to-rim zoning indicative of diffusive re-equili-bration of central grains, and overgrowth of younger grains. Plagioclase in all samples is mostly anhedral and unzoned, but many grains include small An_(60-70) cores with oscillatory zoning that are interpreted as relict phenocrysts. Plagioclase composition and texture. and phase-equilibria constraints on mineral reactions, are most compatible with garnet growth by replacement of igneous high-An plagioclase plus chlorite by "new" lower-Ca (An_(40)) plagioclase and garnet. Incomplete progress of the reaction, as indicated by persistence of coronas and "relics" of primary plagioclase. may be due to an abrupt decrease in temperature, with possible loss of fluid. The textural and mineral-chemical data support the interpretation that these plagioclase-dominant coronas around garnet from the central Cascades Core developed during prograde m_3 metamorphism, after intrusion of the Mount Stuart batholith, not late decompression.