CONSTRAINING P-T HISTORY OF DEEP LITHOSPHERE BENEATH CONTINENTAL ARCS USING MANTLE XENOLITHS – A CASE STUDY FROM THE SIERRA NEVADA, CALIFORNIA

Some hypotheses regarding continental crust formation and modification in magmatic arc settings involve mechanisms such as underplating by basaltic recharge, intracrustal intrusion and crustal inflation, and tectonic thickening. Knowledge of the P-T history may help to evaluate whether these mechanisms indeed play significant roles in continental arc evolution. One way to investigate this problem is to examine sub-arc mantle xenoliths. In this study, we examine peridotite xenoliths from the Sierra Nevada, California, which is one of the few places in the world where genuine arc mantle xenoliths are available. Textural features such as garnet exsolution in pyroxene and garnet-rimmed spinel indicate that these peridotites experienced cooling. Although as a whole, the peridotites show extensive textural disequilibrium, based on petrographic spatial relationships we can select individual mineral pairs that are most likely to be in local equilibrium with each other. Using mineral pairs (garnet-orthopyroxene, garnet-clinopyroxene, orthopyroxene-clinopyroxene, garnet-olivine), we calculate final temperatures in the range of ~700–850 °C and final pressures of at least 2 GPa. In order to constrain the initial pressures and temperatures, we estimate the degree of melting (F) experienced by the peridotites using a model for depletion of whole rock Yb concentration via fractional melting. Next, we correlate F with an upper bound on pressure to which the peridotite must have decompressed to undergo this extent of melting. This method indicates that some xenoliths were melt depleted at pressures as low as ~1 GPa. Based on these estimates, our thermobarometric analysis indicates that, after experiencing low-pressure melt depletion, these peridotites have followed a path of significant subsolidus compression associated with cooling. Therefore, we hypothesize that mature continental arcs, such as the Sierra Nevada, may spend their terminal stages in a state of tectonic compression. The exact details and timing of this compression is not constrained by our study.