CAN ONE MELT BE RESPONSIBLE FOR THE PETROLOGIC ZONES OBSERVED IN THE MT. LOWE ZONED INTRUSION, SAN GABRIEL MOUNTAINS, CALIFORNIA? INSIGHT FROM MELTS THERMODYNAMIC MODELING

The Mt. Lowe intrusion (MLI) is a zoned igneous complex consisting of a southerly 'marginal' pluton, made up of megacrystic hornblende, hornblende+k-feldspar, and hornblende+k-feldspar+garnet monzodiorite to quartz monzodiorite, and a northerly 'central' pluton consisting largely of biotite quartz monzodiorite. Previous workers (Ehlig, 1981; Barth and Ehlig, 1988; Barth et al., 1990; Barth and Wooden, 2006) have estimated that the MLI formed at ~6.7 kbar at high fO2 from a mafic and/or eclogitic lower crustal source and concluded that variations in macroscopic texture and mineralogy are the product of fractionation of major and minor phases from a single parental magma, including plagioclase, hornblende, sphene (titanite), apatite, and magnetite. We used MELTS, a thermodynamically consistent model that tracks the evolution of a magma along paths such as isobaric cooling or isentropic decompression, to test whether the conclusions of previous workers regarding the primary melt of the MLI and its evolution are compatible with magmatic phase relations. For this study, 25 samples of diverse megacrystic assemblages and syn-intrusive dikes were collected in the marginal zone of the MLI, and 20 of these were measured for whole-rock major oxide and trace element composition. Normative mineralogy was calculated for all samples, and the majority are silica-saturated (n=17) and peraluminous (n=16). Six representative samples were chosen for MELTS modeling. For this project, we have added clinozoisite-epidote solid solutions to MELTS for the first time, in order to test whether the P-T-X-fO2 conditions for magmatic epidote formation add important constraints on the petrogenesis of the MLI. Preliminary models lacking the epidote-clinozoisite solid solution show that an assemblage of orthoclase-plagioclase-quartz-sphene-apatite-ilmenite, very similar to the MLI marginal zone, forms at P≥4 kbar, T≤~900°C, [H2O]>2-10 wt.%, and fO2 bounded by the QFM and NNO buffers. No modeling has yet been done on the central biotite zone, which may be a different pluton as suggested by previous workers.