The geology and geochemistry of Sierra de Cristo Rey and its inclusions, Dona Ana County, New Mexico
The plug-like Sierra de Cristo Rey intrusion represents some of the oldest magmatism (48 my) in the Tertiary Trans-Pecos magmatic province. It is located in the western portion of the Trans-Pecos province. The intrusion consists of phenocryst-rich biotite hornblende trachyandesite and contains a suite of enclaves that includes biotite hornblende monzodiorite (81%), medium to coarse-grained hornblende diorite (3%), fine-grained hornblende diorite (IX), crustal xenoliths that range from hornblende anorthosite to anorthositic gabbro (14%), and sedimentary xenoliths incorporated from the surrounding Cretaceous sediments (1%). The crustal xenoliths display distinctive texture and are unlike any rock type exposed in the El Paso area. They probably represent an underlying Precambrian anorthositic body of the middle crust. An aphanitic rhyolitic sill crops out on the southern and southwestern flanks of the main trachyandesite intrusion. Field relationships suggest that the rhyolitic magma intruded prior to the trachyandesite magma. The Cristo Rey host and most enclaves are characterized by steep REE patterns and abundant Sr (1200ppm) and Ba (1300-1900ppm). Zr abundances range from 130-160ppm. Crustal xenoliths have similar Sr abundances but lower Ba (500ppm) and Zr (60ppm). The presence of the rhyolitic sill and the enclave population suggest that the Cristo Rey magma was affected by a combination of fractional crystallization, magma mixing and/or mingling, and possibly partial melting continental crust.