CALCIC SKARN IN THE TRES HERMANAS CONTACT-METAMORPHIC AUREOLE, LUNA COUNTY, NEW MEXICO

The Tres Hermanas Mountains of southern New Mexico were created when a quartz monzonite stock intruded Permian limestone and dolostone and Tertiary latite and andesite. This stock intruded its volcanic cover sequence and is thought to be a shallow pluton. Metamorphism of carbonate and volcanic rocks resulted in assemblages stable at low pressure and low to moderate temperatures. However, the high temperature mineral spurrite has been observed in some portions of this aureole. Carbonate protoliths are now found as calcic skarn, marble, and calc-silicate hornfels. In addition, metasomatism has greatly altered the bulk rock composition in some parts of the metamorphic aureole. Jasperoid was formed by replacement metasomatism acting on the carbonate protoliths. A preliminary study of one calc-silicate shows a complex zonation texture indicating a succession of metamorphic reactions. The rock is microcrystalline except for some fine-grained calcic pyroxene. The larger minerals are surrounded by reaction rims composed of plagioclase, barium-potassium feldspar, and epidote. The matrix is composed of the same assemblage plus quartz. Accessory minerals (e.g. sulfides, phosphates, sulfates, zircon, and oxides) are found in all parts of the rock even as inclusions in the pyroxenes. All mineral grains exhibit a reactive texture which may be evidence of fluid-rock interactions. The pyroxene grains exhibit heterogeneous zonation and range in composition from diopside to hedenbergite. This assemblage is not expected for the given protolith after a single contact-metamorphic event. Textures and phase relationships further suggest that this region may not be a typical contact aureole.