GEOCHEMICAL EVIDENCE FOR INCREMENTAL EMPLACEMENT OF THE CRETACEOUS HALF DOME GRANODIORITE, YOSEMITE NATIONAL PARK, CALIFORNIA

The Tuolumne Intrusive Suite (TIS) is a large nested plutonic complex in Yosemite National Park, California. U-Pb zircon data indicate that the complex was emplaced over a period of at least 5 m.y. (~93-88 Ma) by incremental addition of magma to growing plutons. New geochemical data for 52 samples collected across the full exposed width and depth of the complex support this hypothesis and reveal a surprising decoupling of major elements from trace elements and Sr isotope ratios.

Major-element data show linear correlations although individual plutons show considerable scatter, especially for more felsic compositions. In contrast, trace elements correlate poorly with everything. Initial Sr ratios correlate positively with silica but are scattered in the more felsic rocks. Surprisingly, there is no correlation of major or trace elements with elevation over 2.5 km of vertical exposure within the Half Dome pluton, nor is there a correlation with distance from its outer margin. These observations are in contrast to previous studies which show correlations between geochemistry and depth in some plutons (e.g., Sawka et al., 1989). Half Dome U-Pb zircon ages decrease discontinuously from the margins inward (92.7-89.5 Ma), but show no correlation with chemistry. The porphyritic facies of the Half Dome pluton is identical in isotopic and chemical characteristics to the Cathedral Peak pluton, as noted by Kistler and Fleck (1994).

Decoupling of major and trace elements and lack of spatial coherence is inconsistent with either 2-component mixing or crystal fractionation, but is consistent with incremental emplacement (e.g., by diking). Following Kistler et al. (1986), we suggest that the TIS formed by mixing of mafic magma with felsic magmas generated by partial melting of deeper crustal rocks. Partial melting of K-rich rocks of a variety of compositions yields granodiorite or granite liquids with consistent major-element trends; however, trace-element and isotopic compositions of such felsic melts will vary with source material, degree of melting, and residual mineralogy. Incremental addition of these melts to the growing pluton can account for the observed lack of geochemical and spatial coherence.