Metamorphic Phase Equilibria in a Contact Aureole: Tres Hermanas Mountains, Luna County, New Mexico.

To characterize the metamorphism in the Tres Hermanas Mountains contact–metamorphic aureole in Luna County, New Mexico, I mapped and collected samples of the Paleozoic strata near the quartz monzonite pluton along the northeast flank of North Peak. Stratigraphy in this area includes the Pennsylvanian undivided unit that is composed of thin-medium interbedded limestone, shale, and sandstone and the Hueco Formation that could be broken out into four members: a pebble–cobble conglomerate marks the base, a medium to massive bedded siliceous limestone is next, followed by a pure limestone member and topped by a breccia bed in places marking the fault contact with extrusive andesite. These country rocks were intruded first by andesite sills and stocks and a second time by the large quartz monzonite pluton. The metamorphic assemblages are similar for all the sedimentary rock units and include quartz, calcite, diopside, plagioclase, K-feldspar, clinozoisite, garnet, and wollastonite. Mineral composition often vary within individual samples (e.g. various compositions along the diopsidehedenbergite, grossular-andradite, and albite-anorthite solid solution series). I was able to determine that the Paleozoic stratigraphy endured a lithostatic pressure between 22.5 and 37.5 MPa, at temperatures between 245 and 505 °C, and in the presence of a very water-rich metamorphic fluid having an X(CO2) less than 0.01. High-temperature minerals spurrite, merwinite, and monticellite were not observed in the contact aureole and may have formed as a result of the low pressure, water-rich fluids, and higher temperatures within the surrounding magma.