NON-SYMMETRIC ZONING AND LACK OF CORRELATION AMONG NEIGHBORING K-FELDSPAR MEGACRYSTS IN GRANODIORITE
To better characterize zoning patterns, we collected whole, euhedral megacrysts that weathered out of a dome summit in the Cathedral Peak Granodiorite. These were analyzed by X-ray CT (voxel size ~20 μm), which reveals intricate density zoning reflecting Ba concentration and inclusion shells. Density variation and inclusion abundance analyses along various radial transects of 10 megacrysts reveal several important features of megacryst zoning: (1) there is little correlation of zoning patterns among the crystals, despite having grown near one another in the same, relatively few m3 volume; (2) apparent dissolution features are well-imaged; (3) inclusion shells contain hornblende and titanite with little biotite, unlike the groundmass which contains all three minerals; (4) zoning patterns are not symmetrical about a crystal's center.
Megacryst zoning that is unique to each crystal implies that zoning is not solely controlled by influx of new magma in a melt-dominated system. Conversely, in a crystal-dominated system, anisotropic advective flow through the framework and dissolution-reprecipitation contributes to the lack of correlation between megacryst zones. We conclude that megacryst zoning revealed by CT is inconsistent with formation due to melt recharge in a melt-dominated system, and is consistent with late megacryst growth that responds to the local environment.