VARIATION OF SKARN METASOMATIC FLUID SOURCES WITHIN THE SIERRA NEVADA ARC
Values of δ18O data (LF and ion probe [SIMS]) from multiple skarns from the Jurassic Mojave segment of the arc vary from −9.6‰ to +5.5‰ VSMOW, and are interpreted to reflect mixing of meteoric and magmatic-derived hydrothermal fluid sources. SIMS analysis of two Mojave skarn garnets with oscillatory zoning reveals cores with δ18O values of −9.6‰, internal variations of −9.4‰ to −3.3‰, and rims of −2.2‰ and −2.9‰ (precision ±0.3; 2σ). In general, δ18O values do not correlate with Fe in the mostly andraditic garnet.
The preponderance and magnitude of negative δ18O values from Jurassic Mojave skarn garnets preclude skarn formation below the Jurassic sea and corroborate the presence of a large, shallow, sub-aerial hydrothermal system as documented by Solomon and Taylor (1991) and Battles and Barton (1995). In contrast, Jurassic skarns of the eastern Sierra Nevada generally lack low-δ18O values, and may indicate formation at greater crustal depths below the depth at which brittle deformation in hydrothermal systems would allow infiltration of surface water. Instead δ18O values of the Jurassic eastern California skarns suggest a component of sedimentary-derived fluid.
New methods for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb dating of grossular-andradite garnet provide the ability to determine the absolute timing and longevity of skarn formation, and to monitor regional changes in the fluid composition through time. Garnet U-Pb dates from Black Rock skarn (203±7 Ma) match the age of the causative pluton (~196 Ma) within in error. Skarn grossular-andradite is typically uranium-rich (1 – 100ppm); common Pb is highly heterogeneous at the micron scale, which allows for the construction of robust Discordia lines in Tera-Wasserburg space with relatively few analyses over a small area.