GEOCHEMICAL CHARACTERIZATION OF TUOLUMNE INTRUSIVE SUITE ZIRCON, SIERRA NEVADA BATHOLITH, CALIFORNIA

Granitic plutonic rocks provide direct insight into the first-order petrogenetic processes associated with arc magma genesis, evolution, migration and storage, all of which have implications the formation of continental crust. The Late Cretaceous Tuolumne Intrusive Suite (TIS) in the eastern Sierra Nevada batholith has received extensive study given its areal extent, excellent exposure, and well-defined textural and geochemical zoning. Numerous textural, structural, and geochemical investigations have led to competing models for its magma evolution prior to and during emplacement in the upper crust. Here, we present coupled date–trace element maps of individual detrital zircon grains sampled from modern streams to further constrain the evolution of magma during intrusion of the different TIS units.

Our results identify key differences between zircon from earlier and later TIS units. 95–92 Ma zircon from the Kuna Crest granodiorite (KC) sampled from Tuolumne Meadows and 92–90 Ma zircon from the equigranular Half Dome granodiorite (eHD) sampled from Tenaya Canyon display progressively increasing Hf, U/Yb, and decreasing Ti with date, consistent with crystallization of a progressively fractionated magma over a temperature range of ~830–675°C (assuming a[TiO₂] = 0.7). In contrast, 90–86 Ma zircon sampled from Rafferty Creek, potentially derived from porphyritic Half Dome granodiorite, Cathedral Peak granodiorite, and/or Johnson Peak granite, are characterized by higher U/Yb and lower Ti, indicative of crystallization at 675–625°C, but with lower Hf relative to eHD. Many grains from Rafferty Creek sand display core–rim textures, with core dates and geochemistry characteristic of older KC or eHD zircon. A small subset of the Rafferty zircon have ~86 Ma cores with low Hf, low U/Yb, and high Ti, suggesting initial crystallization in hot (800–750°C), less evolved magma. Higher U/Yb and distinctly lower Hf in zircon younger than 90 Ma requires processes capable of enriching the Zr/Hf of TIS magmas at near-solidus temperatures. Together with previously published whole-rock geochemistry, these results place new constraints on the evolution of TIS magmas, and provide a foundation for future zircon geochemistry investigations designed to refine models for the formation and emplacement of the TIS.