Evaluation of west-east migration in the Cretaceous Sierra Nevada batholith

Existing interpretation of geochronologic data for subduction-related plutonic rocks in the western United States suggests that the locus of magmatism migrated from west to east during the Cretaceous, perhaps in response to flattening of the subducting slab. However, Sierran rocks generally lack the high-precision geochronology necessary for a rigorous evaluation of arc migration. New U-Pb ages for the Tuolumne and John Muir intrusive suites in the eastern Sierra Nevada call into question ages determined by old, low-precision U-Pb zircon and K-Ar biotite and hornblende methods. We present new data for intrusive rocks of the western Sierra Nevada batholith (the Bass Lake Tonalite and Dinkey Creek and Mt. Givens granodiorites) combined with existing detailed geochronology for plutonic rocks of the Tuolumne and John Muir intrusive suites, and suggest that there is no simple pattern of arc migration preserved in the Cretaceous of the central Sierra Nevada. In the central Sierra Nevada (~37-38 N) Cretaceous plutonic rocks (from west to east) include the ~2000 km2 Bass Lake Tonalite, the ~600 km2 Dinkey Creek Granodiorite, the 1500 km2 Mt. Givens Granodiorite, and the John Muir and Tuolumne intrusive suites. Existing low-precision U-Pb zircon ages for the western tonalites and granodiorites suggest a range of ~124 to 90 Ma, and have been used to calculate an eastern migration of the arc of ~2.7 mm a-1; however the age data are of questionable reliability and are sometimes at odds with field relations. We are obtaining new U-Pb zircon ages for samples of the Bass Lake, Dinkey Creek and Mt. Givens plutons, and will combine them with our data from the Tuolumne and John Muir intrusive suites to test the hypothesis of eastward arc migration. Field relations indicate that the Bass Lake Tonalite and Mt. Givens Granodiorite are composite intrusions with as much or more petrologic diversity as the well-documented intrusive suites in the eastern Sierra Nevada. Preliminary data for the Mt. Givens Granodiorite indicates an age range of at least 6 Ma, with younger rocks in the center of the pluton. Dating of the Bass Lake and Dinkey Creek plutons is currently underway. Age maps using only modern, high precision U-Pb zircon data, show that intrusive rock age does not decrease on a strictly west-east path and that arc migration rates are inconsistent throughout the batholith. Concentrically zoned plutons and intrusive suites with significant age variation (6-10 Ma) such as the Mt. Givens Granodiorite and Tuolumne Intrusive Suite introduce acute problems when evaluating west to east migration rates and suggest that accumulation rates of the batholithic rocks in a vertical column will complicate calculation of horizontal migration rates.