NON-SYMMETRIC ZONING AND LACK OF CORRELATION AMONG NEIGHBORING K-FELDSPAR MEGACRYSTS IN GRANODIORITE

K-feldspar megacrysts are characterized by strong zoning of Ba. Two main hypotheses describe the origin of this zoning. In one, high-Ba zones reflect magma influxes into a melt-dominated system with efficient mixing and homogenization of new Ba throughout. In the second, megacrysts grow in a static, crystal-dominated system during the latter half of a magma’s crystallization interval, and zoning results from both thermal cycling that transfers mass preferentially from small to large crystals via dissolution-reprecipitation, and from advective flow through the crystal framework. The first hypothesis predicts that zones should correlate from crystal to crystal, whereas the second predicts that zoning depends on the number and cm-scale proximity of smaller crystals consumed during thermal cycling and thus should not necessarily correlate from crystal to crystal.

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.