THE HALF DOME GRANODIORITE ON THE EAST SIDE OF THE TUOLUMNE INTRUSIVE COMPLEX, YOSEMITE NATIONAL PARK, CA: BIG OR SMALL MAGMA CHAMBERS?

Coleman et al. (2012) suggest that the formation of the Half Dome granodiorite in the W lobe - and by inference, the entire Tuolumne Intrusive Complex (TIC) - occurred through stacking of km-scale sills. These outward, moderately dipping sills with sharp eastern contacts are composed of granodiorite that gradually becomes aplite westward due to local fractional crystallization. This sill-model contrasts with models of large magma mush bodies, which suggest that magma pulses amalgamate to form large magma bodies undergoing large-scale crystal fractionation and across-unit magma mixing (Paterson et al., 2016).

To test these models, field mapping, petrography and XRF whole rock element geochemistry was conducted in the equigranular Half Dome granodiorite (eHD) unit at Lyell Canyon to determine if ‘sills’ occur on the east side of the TIC as well. Field mapping determined gradational contacts between Kuna Crest and Half Dome units and hybrid mineralogical characteristics. The eHD granodiorite exhibits local, up to several m-wide aplite dikes with sharp contacts and varying orientations. Locally, eHD granodiorite is more leucocratic (LeHD) transitioning into more felsic granodiorite and granite at up to tens of m scales. Petrographic analyses of eHD rocks indicate major mineral abundances ranging from 27% qtz, 14% kfs, 45% plag, 5% hbl, and 9% bt to 47% qtz, 22% kfs, 25% plag, 5% bt and <1% hbl. Aplite samples consist of 40% qtz, 45% kfs, 15% plag and <1% bt and hbl. XRF analysis on nine eHD samples yielded a range of granodiorite to granite compositions. Harker diagrams indicate linear correlations of major oxides with increased SiO₂, and a decrease in mg # and compatible traces (e.g., Sr), and increasing incompatible elements with increasing SiO₂, which are permissive of crystal fractionation.

Our results suggest that the Coleman et al. (2012) sill-model proposed for the TIC cannot be confirmed for the eastern eHD exposures. The gradational contacts between TIC units, irregular shaped local variations of eHD compositions, randomly oriented local aplite dikes, petrography and geochemistry of eHD rocks are in accord with a large, interconnected magma mush with local compositional variations produced by crystal fractionation. The aplites interpreted as sills on the W side of the TIC are likely late crystallization fronts.