OXYGEN ISOTOPES OF CALIFORNIA SKARN GARNETS: MONITORS OF FLUID INFILTRATION AND DECARBONATION IN MESOZOIC ARCS

Studies of skarn mineralization allow better understanding of decarbonation reactions and volatile budgets in continental magmatic arcs (Lee et al. Geosphere, 2013). Numerous skarn localities in the Cordilleran arcs of California represent ideal targets to study how the interplay of magma and wallrock type and crustal depth control mixtures of volatiles that accompany skarn formation and ultimately control the extent of decarbonation reaction progress. Oxygen isotopes are powerful tracers of fluid and wallrock reservoirs. Garnet in skarns is the ideal mineral to study because it retains the δ¹⁸O value of the primary skarn-forming fluids and preserves major and trace element as well as δ¹⁸O zoning that reflects changing fluid flow conditions (e.g., D’Errico et al., Geology, 2012). Here, we report new δ¹⁸O analyses of skarn garnets from several localities in the Peninsular Ranges, Mojave Desert, and Owens Valley of California that span key periods (Triassic to Cretaceous) of skarn formation in the Mesozoic arcs. Analyses of single crystals and mixtures of crystals yield the following δ¹⁸O(Garnet) values: Riverside area: Crestmore/Jensen Quarries (4.5–6.9‰), Old City Quarry (7.2–11.1‰); Mojave area: White Knob quarry (5.8–10.8‰) and Sidewinder Mountain (2.4–2.9‰); Tungsten Hills and Darwin area (4.1–5.2‰). When compared with values of δ¹⁸O of skarn garnet elsewhere in the Cordillera, skarn volume and reaction progress show distinct patterns: Skarns that show limited reaction progress, typically thin, cm-thick veneers between plutons and carbonate wallrock, are commonly buffered by metamorphic fluids. Because such skarns are often found adjacent to large-scale plutons in the middle crust, we hypothesize that emplacement of early increments of magma stifle continued reaction in such skarns as plutons are constructed in the middle crust. Massive, >10 m thick, skarns formed in the presence of fluids dominated by magmatic to meteoric water. Ingress of meteoric water provides an especially potent driver of decarbonation reactions in shallow levels (<3 km) of continental magmatic arcs.