Cordilleran Section - 111th Annual Meeting (11–13 May 2015)

Paper No. 4
Presentation Time: 1:30 PM-5:30 PM


GEVEDON, Michelle L.1, SEMAN, Spencer1, RYAN-DAVIS, Juliet2, BARNES, Jaime D.1, LACKEY, Jade Star2, STOCKLI, Daniel F.3, KITAJIMA, Kouki4 and VALLEY, John W.5, (1)Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78712, (2)Geology Department, Pomona College, Claremont, CA 91711, (3)Jackson School of Geosciences, The University of Texas at Austin, 2305 Speedway, Stop C1160, Austin, TX 78712, (4)Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, (5)WiscSIMS, Department of Geoscience, University of Wisconsin-Madison, 1215, W. Dayton St, Madison, WI 53706,

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.