High temperature deformation and fluid enhanced zircon modification along an exhumed subduction megathrust

The San Emigdio and related Pelona, Orocopia, Rand, and Sierra de Salinas schists of southern California were underplated beneath the southern Sierra Nevada batholith and adjacent southern California batholith along a shallow segment of the subducting Farallon plate in Late Cretaceous - early Tertiary time. Inverted metamorphism, i.e., an upward increase in peak temperature, in the San Emigdio and related schists is attributed to tectonic assembly during progressive underplating. However, a compilation of new and published U-Pb geochronology of detrital zircons from the San Emigdio Schist reveal increasing maximum depositional age clusters from 94 Ma to 108 Ma with structural depth, opposite the trend expected for sequential accretion by tectonic underplating. The youngest age clusters from samples adjacent to the mylonitic contact with upper plate batholithic assemblages (the Rand fault) contain grains with high (~1750 - 3200 ppm) U concentration, suggesting significant Pb loss in these grains, compared to moderate (~ 500 ppm) concentrations away from the fault. The timing of this Pb loss event is thought to coincide with upper amphibolite facies peak metamorphism of the schist. Cathodoluminescence imaging of zircon supports this view, indicating a clear progression from simple oscillatory zoning at the deepest levels of exposure (~ 1000 m) to complex metamorphic zonation characterized by multiple crosscutting domains immediately beneath the Rand fault. Broadening of the main age peak in normalized probability plots with decreasing structural depth may also reflect Pb loss. Aside from this broadening effect, detrital zircon age spectra do not vary significantly with structural depth, with apparent age groups of 85 - 128 Ma (n = 197), 132 - 180 Ma (n = 22), 188 - 216 Ma (n = 5), and 263 - 2000 Ma (n = 19). Preliminary oxygen isotope results indicate higher δ18O in zircons exhibiting metamorphic domains than in magmatic grains, suggesting that infiltration of metamorphic fluids resulted in significant zircon isotopic resetting. The observations presented here indicate that the San Emigdio Schist was deposited within a narrow time interval after ca. 108 Ma and upper structural levels experienced a Pb loss event at ca. 94 Ma during underthrusting beneath the then recently active Late Cretaceous arc. Continued schist underplating led to refrigeration of the upper plate and relaxation of the initially high geothermal gradient, explaining the lack of evidence for Pb loss at deep structural levels. We postulate that some combination of high temperatures, deformation, and fluid infiltration adjacent to the Rand fault triggered this Pb loss episode.