QUANTIFYING THE VOLUME OF INHERITED ZIRCON IN LAYERED GRANITIC ROCKS OF THE TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CALIFORNIA, USA

Granitic plutons provide an opportunity to investigate the evolution of magma in volcanic arcs with implications for the formation of continental crust and the storage of magma beneath volcanic centers. Although the relative size and duration of granitic magma bodies in the upper crust is debated, insight into their emplacement can be preserved in magmatic structures. For example, layered granitic rocks (schlieren), characterized by alternating layers of darker rock that grade into lighter, more felsic layers, are alternately interpreted to have formed through 1) crystal gravity flows in large, dynamic upper crustal magma chambers capable of physical and chemical mixing between older intrusive units and younger magmas, or 2) crystal settling or exsolution of immiscible liquids in dikes cutting static upper crustal magma mushes and rocks.

To test these different models, we will use zircon geochronology and geochemistry to estimate the volume of inherited material in layered granites from the Cathedral Peak granodiorite (CPg) within the Tuolumne Intrusive Suite (TIS) of the Sierra Nevada batholith. Petrographic observations from two samples of CPg layered granite—collected near the center of the TIS and in its northernmost extent where it cuts older metavolcanic rocks— reveal granodioritic lighter layers and darker layers rich with biotite and opaques, and amphibole that is abundant in the central sample but conspicuously absent in the northern sample. Previous studies have demonstrated that zircon from different units within the TIS can be distinguished by zircon geochronology and geochemistry. Our future work this spring will use LA-SS-ICP-MS to characterize zircon date–geochemistry zoning correlated to CL images to quantify the volume of inherited zircon in lighter and darker layers, respectively. These results will place new constraints on the degree of mixing at the level of CPg emplacement: if layered granites are crystal gravity flows in dynamic upper crustal magma chambers, we expect to observe abundant inherited antecrystic zircon in darker layers, whereas if layered granites are dikes cutting static upper crustal rocks, we expect the volume of antecrystic zircon to be smaller and approximately constant in darker and lighter layers.