VARIATION OF SKARN METASOMATIC FLUID SOURCES WITHIN THE SIERRA NEVADA ARC

Oxygen isotope compositions of skarn garnet can be used to trace hydrothermal fluid sources; regional and small-scale (inter-skarn and intra-grain) variation in δ¹⁸O values may potentially resolve magmatic flare-ups, fluid pulses, and relative depths of emplacement. Laser fluorination (LF) garnet δ¹⁸O data from 19 skarns (n = 136) associated with the Sierra Nevada arc vary by ~20‰. These data provide insight to the depth and dominant fluid regime associated with Sierra Nevada plutonism through time and space.

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.