Paper No. 39-5
Presentation Time: 9:00 AM-5:30 PM
THE ORIGIN AND GROWTH OF K-FELDSPAR MEGACRYSTS IN GRANODIORITE, TUOLUMNE INTRUSIVE SUITE, CALIFORNIA
K-feldspar megacrysts are common in granitoid rocks, but the crystal growth mechanisms and the physical evolution of the systems remain unclear. Two opposing hypotheses describe subsequent crystallization after the K-feldspar nucleates in an ~50% crystal-by-volume environment as predicted by phase equilibria. The first hypothesis predicts crystal growth in a system where the melt and crystals are free to physically mix and mingle due to melt replenishments. The second hypothesizes that K-feldspar crystals grow in a system rendered immobile due to rheological lock-up, with melt replenishments travelling through a static crystal matrix, resulting in crystals growing in their respective melt pockets with only small scale equilibrium between crystals. Barium zoning resulting from the coupled substitution of BaAl5+ for KSi5+ in K-feldspar is characteristic of the megacrysts found in the Tuolumne Intrusive Suite (TIS) and likely relates to the chemisty of the melt from which the zones crystallized. X-ray computed tomography (Micro CT) of megacryst interiors, where celsian-rich zones are resolvable, aids in understanding the chemical zoning. An initial study of 10 megacrysts from the Cathedral Peak Granodiorite, TIS, reveals uncorrelated core-to-rim Ba zoning patterns between individual megacrysts and from the different crystal faces of the same megacryst. This implies that each crystal formed in a unique melt pocket with minimal communication with other megacrysts in the static crystal matrix. Dissolution features and zoning patterns on the same megacrysts also imply that dissolution-reprecipitation assisted in crystal coarsening both above and below the solidus. Two new populations of megacrysts from the TIS will be tested using similar methods to determine if these results are unique to individual groups of megacrysts or if they are consistent throughout a zoned pluton.