THE NATURE OF TEXTURAL AND COMPOSITIONAL VARIABILITY IN THE LAKE VERNON GRANODIORITE IN WESTERN YOSEMITE NATIONAL PARK, CA

Granodioritic Sierran plutons are often characterized by within-unit, local to regional textural and compositional (modal) variability, too subtle for separation into mapping units. These within-unit changes might be due to differences in cooling histories, magma mixing, internal fractionation, and crystal accumulation. This study uses cathodoluminescence (CL) and petrography on plagioclase and its abundance in 7 samples from across the 97-95 Ma Lake Vernon Granodiorite (Klv), Jack Main Canyon intrusive suite, western Yosemite National Park, CA, to determine the nature of this variability.

Three plagioclase populations were identified. In order of abundance they are: (1) grains (150-3000 µm) that exhibit oscillatory and patchy zoning in both crossed-polarized light (XPL) and CL, 95% of grains have albitic overgrowth; (2) grains (150-2000 µm) that have no zoning (XPL and CL); and (3) grains (350-3000 µm) with oscillatory zoning characterized by thicker rims and inclusions of hornblende and opaque minerals (XPL). All samples display cumulate textures of populations 1 and 2 over 5-25% of the thin section area. K-feldspar crystallized interstitially adjacent to the plagioclase cumulates in 4 of 7 samples; the other 3 have K-feldspar grown in interstitial pockets throughout the thin section. The samples with the highest degree of plagioclase accumulation (15-25%) are located at the NE gradational contact with an older plutonic unit. The samples with the highest abundance of population 2 are found at the gradational contacts with an older plutonic unit (NE), and a younger plutonic unit (SW). Population 3 has no spatial preference.

Our results suggest that 3 petrographically distinct plagioclase populations likely originated in an earlier Klv magma or from adjacent units and concluded their magmatic histories together in the Klv. Cumulate textures suggest that melt migrated at least locally. More abundant plagioclase accumulation at Klv boundaries suggests boundary effects led to greater interstitial melt loss. These findings suggest Klv variability was caused by changing conditions throughout crystallization and/or magma mixing and accumulation. The presence of 3 populations in all samples across the Klv suggests that the Klv was melt interconnected across the pluton and underwent thorough crystal mixing.