ORIGIN OF CORONA TEXTURES IN DIKES NEAR THE MT. STUART BATHOLITH, NORTH CASCADES, WA, USA: X-RAY TOMOGRAPHY AND PSEUDOSECTION MODELING

Coronas or haloes on garnet are common to many locations in the North Cascades Core. Previously, we described and modeled several such rocks, including SP45 from a felsic dike near the Mt. Stuart batholith in the Nason Terrane. Garnet, in SP45, is surrounded by coronas of plagioclase + quartz, in a matrix of fine-grained biotite + plagioclase + quartz. The coronas are elongate, deformed, and complex which reduces confidence in mode estimation.

3-D visualization from X-ray tomography reveals that the coronas are much larger and more complex than is evident in thin section. Garnet occurs in clusters of up to a dozen grains and is surrounded by coronas that encompass several discrete garnet clusters. Multiple connections between coronas are apparent.

Prior to X-ray tomography, thermodynamic pseudosection modeling and mass balance analysis, based on electron microprobe data, were used to infer a prograde near-isobaric [ca. 6 kbar] P-T path for garnet growth and corona development. Tomographic results are generally consistent with this modeling but draw attention to possible garnet growth at temperatures just above the solidus. The extent of garnet intergrowth and intercorona connection evident in the 3-D imagery is consistent with the pseudosection's suggestion of rapid garnet growth at the onset of partial melting.

Mode calculation with X-ray tomography included biotite grains that were smaller (in one or more dimensions) than the nominal resolution of the scan. Because the signal attenuation attributable to biotite contrasts strongly with attenuation due to quartz and plagioclase, the average attenuation for a given volume is distinct even where biotite fills only part of the volume. The tomographic mode estimates support new modeling aimed at exploring the possibility that disequilibrium textures in SP45--particularly the persistence of high anorthite plagioclase compositions--may be magmatic rather than metamorphic in origin.