HETEROGENEOUS KFS MEGACRYST SIZE AND ABUNDANCE VARIATION IN THE NORTHERN CATHEDRAL PEAK LOBE, TUOLUMNE BATHOLITH

Within unit compositional variations in the 95-85 Ma, normally zoned Tuolumne batholith in the Central Sierra Nevada are increasingly apparent as our detailed studies progress, although the significance of these variations remains poorly understood. Here, we focus on internal variations in the Cathedral Peak (CP) granodiorite lobe at the northern end of the batholith, which formed at a relatively late stage of chamber construction (88-86 Ma). Emplacement of ~400 km2, Kfs-megacrystic granodiorite resulted in Kfs megacryst sizes and abundances varying greatly.

The CP lobe has a normally zoned compositional pattern best preserved in the NW part of the lobe. The typical CP unit is composed of a medium grained granodiorite that contains 1-4 cm large Kfs megacrysts. The outer marginal unit contains more and larger Kfs (up to 7 cm) and Kfs cumulates, which might be due to effects of filter pressing at the boundary to older units. The central granite is free of Kfs megacrysts and contains large quartz-pools, representing a relatively fractionated melt pod. We furthermore mapped a 1-2 km wide, NW striking zone with abundant moderately to steeply dipping mafic schlieren layers that contain Kfs megacrysts, which we interpret to represent a zone of focused magma flow that potentially formed along the migrating margin of the magma chamber. As the northern Half Dome (HD) granodiorite is approached, the CP composition gradually changes into typical HD granodiorite with no Kfs megacrysts and up to 2 cm long euhedral hornblende and biotite. Here in the CP, we find the largest, up to 12 cm long, more stubby Kfs megacrysts.

The above observations and geochemical analyses suggest that fractionation crystallization within the CP and localized mixing processes within and between the CP and the HD units are largely responsible for the compositional variations in this part of the batholith. Kfs megacryst characteristics clearly indicate a magmatic origin although the heterogeneous Kfs megacryst size and distribution are observations we are still trying to explain. CA-TIMS U/Pb zircon geochronology is in progress and will further constrain our present hypothesis that the CP granodiorite most likely formed a large, melt interconnected magma chamber in which crystal-mush zones grew inward.