QUANTIFYING THE VOLUME OF INHERITED ANTECRYSTIC ZIRCON ACROSS THE CATHEDRAL PEAK GRANODIORITE OF THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA, USA

The evolution and emplacement of granitic rocks in convergent margin magmatic arcs have broad implications for the formation of continental crust. The Late Cretaceous Tuolumne Intrusive Suite (TIS) is an archetypical arc pluton that formed over the period of 10 million years in the Sierra Nevada batholith, and consists of geochemically and temporally zoned units that are younger and increasingly felsic toward the interior of the body. Two competing models used to explain its formation alternately call for 3–4 large intrusions that created kilometer-scale upper crustal magma chambers capable of crustal mixing and differentiation, or alternatively, for the sequential emplacement of numerous sub-kilometer scale intrusions that were not capable of mixing in the upper crust with magma that was previously differentiated in the lower crust.

To test these models, we have collected samples of Cathedral Peak granodiorite across a transect from its outer contact with the older and less evolved portions of the TIS to its interior contact with the younger, more evolved units. In addition, we have collected samples of Cathedral Peak granodiorite from its northernmost extent where it cuts across all older TIS units and is direct contact with Early Cretaceous metavolcanic rocks. Previous work has shown that zircon from each of the TIS units has characteristic dates and geochemical signatures that can be used to differentiate zircon from each of the TIS units. Our future work on samples collected for this study will use zircon geochronology and geochemistry measured by laser ablation split stream ICP-MS to identify zircon inherited from older TIS units, and together with detailed CL imagery, used as a proxy to estimate the volume of inherited material mixed into different parts of the Cathedral Peak granodiorite. We expect that if widespread mixing occurred in the upper crust, the volume of material inherited from older TIS units will be greatest near exterior contacts with these units. In contrast, if magma evolution occurred in the lower crust and mixing was limited in the upper crust, we expect the volume of inherited material across the Cathedral Peak granodiorite to be relatively homogeneous.