SKARN-GARNET ARCHIVES OF METASOMATIC AND HYDROTHERMAL CONDITIONS IN THE MINERAL KING ROOF PENDANT, SOUTH-CENTRAL SIERRA NEVADA

Grossular-andradite garnets from skarns of the Mineral King metamorphic roof pendant, south-central Sierra Nevada batholith, California, record grain-scale effects of varying fluid composition that aid in understanding fluid source and conditions during metasomatic skarn reactions. Oxygen isotope values in conjunction with microscale textures and major/trace element compositions of garnet provide information on conditions of garnet growth in Cretaceous skarns at White Chief canyon (ca. 136 Ma, Sisson & Moore 2013, USGS-OFR) and Empire Mountain (ca. 109 Ma, D'Errico et al. 2012, Geology). Internal zoning of O isotope ratios (δ¹⁸O in ‰, VSMOW) was measured along transects from core to rim of individual garnets by SIMS (10 µm spot size); SIMS mass fractionation was calibrated against grossular-andradite standards (Page et al. 2010, ChemGeol). Trace element (including REE) concentrations measured by LA-ICP-MS (for domains >30 µm) along the same transects from core to rim help to delineate zones.

Results reveal distinct fluid-controlled domains and chemical zoning of individual garnets at both localities. Values of δ¹⁸O measured by SIMS vary within single garnets (−9.0 to −2.2‰ at White Chief; −4.5 to 3.6‰ at Empire Mountain) as do chondrite-normalized REE patterns. In garnet cores from White Chief, negative Eu anomalies correspond to low δ¹⁸O values (−7.5 to −9‰); whereas, in garnet rims, positive Eu anomalies are accompanied by depletions in HREE and higher δ¹⁸O values (−2 to −4‰). These associations indicate pulses of garnet growth and changes in metasomatic fluid, such as its source and redox conditions, which match morphological instabilities observed in the garnets. Negative values indicate infiltration of meteoric water into both hydrothermal systems that variably diluted magmatic fluids. A near surface environment at 136 Ma is supported by voluminous ca. 134-136 Ma metarhyolite tuffs in the Mineral King pendant (Sisson & Moore 2013; Klemetti et al. 2013, Geosphere).

The White Chief skarn formed with a greater meteoric component than did Empire Mountain, by as much as 20 percent, raising the possibility that the skarn environment deepened over ca. 20 million years before the region was invaded by younger plutons at ca. 98 Ma during thickening of the continental arc, diminishing infiltration of meteoric water.