U-PB DATING OF SKARNS IN THE SIERRA NEVADA AND MOJAVE SECTIONS OF CALIFORNIA'S MESOZOIC ARC

Coupled U-Pb geochronology and oxygen isotope analysis of skarn garnet is emerging as a powerful way to trace fluid flow in arcs. U-Pb dating of skarn garnet is especially valuable to deconvolute unclear age relations between skarns and plutons (Gevedon et al. 2018,EPSL). Oxygen isotopes establish if fluids sources were strictly magmatic, or in the case of shallow hydrothermal systems, include surface fluids which can change ore endowments (e.g. Ryan-Davis et al. 2019, CMP). Here we summarize new garnet U-Pb data collected by LA-ICP-MS at Pomona College. We find that ages of most Sierran skarns are within uncertainty of ages on associated plutons. For example, skarns of the Minarets Mining district yield a 99.4 ± 4 Ma age, similar to local plutonic and volcanic rocks. High U (16–60 ppm) garnets in skarn west of Big Pine, yielding a U-Pb age of 148.8 ± 3.6 Ma, are adjacent to Independence Dikes. Andradite-rich garnet from the well-known Garnet Hill skarns in Calaveras County gives a U-Pb age 105.9 ± 3.0 Ma where no pluton has been dated.

Values of δ¹⁸O of most Sierran skarns are 5.0–7.0‰, consistent with mostly magmatic-derived fluid, however Jurassic arc skarns in the Sidewinder-Rodman-Fry Mountains of the Mojave Desert, include many low-δ¹⁸O skarns (to –9.8‰) that require involvement of significant meteoric water in this section of the arc by the late Jurassic based on their ages (~162–148 Ma; Gevedon et al. 2018). A subset of skarns east of the Johnson Valley (Bessemer Mine & Galway Lake) have higher values of δ¹⁸O garnet (1.9–5.6‰). U-Pb ages of these higher δ¹⁸O skarn garnets reveal unexpected Late Cretaceous ages (77–70 Ma). Such ages overlap 78–77 Ma U-Pb zircon ages on granites in the area that were emplaced during the eastward sweep of magmatism of the Laramide Orogeny. Petrographically, younger garnet ages may be localized to specific garnet populations or epitaxial growths. Broadly, the data highlight a regional hydrothermal pulse in this part of the Mojave in the Late Cretaceous. Higher-δ¹⁸O in the skarn garnets is consistent with diminished meteoric fluid input during Cretaceous growth. Viewed practically, these data show that skarn garnets offer a new way to identify the poly-metamorphic and poly-metasomatic events that will accompany arc magmatic flare-ups.