7Geology and geochemistry of the Redrock Granite and anorthosite xenoliths … GEOLOGY AND GEOCHEMISTRY OF THE REDROCK GRANITE AND ANORTHOSITE XENOLITHS (PROTEROZOIC) IN THE NORTHERN BURRO

Mineral ages from the A-type granites and anorthosite xenoliths in the Red-rock area in the northwestern Burro Mountains in southwestern New Mexico cluster around ~1220–1225 Ma and provide yet another example of bimodal igneous activity during this time period in the southwestern United States. The metaluminous to peraluminous, marginally alkaline to subalkaline Redrock Gran-ite exhibits the textural, mineralogical, and geochemical features of A-type gran-ite that was emplaced at a relatively high crustal level. Field relationships, whole rock and mineral geochemical and isotopic trends suggest that the four phases of the Redrock Granite are genetically related, with the miarolitic biotite/alkali feldspar granite being the youngest phase. Spatial relationships and geochemi-cal data suggest that the anorthosite xenoliths were coeval with the Redrock Granite, which is consistent with the anorthosite being derived from the upper mantle, possibly due to deep mantle upwellings, and the Redrock Granite from the lower crust. The process involved melting in the upper mantle, emplace-ment of anorthosite in the crust resulting in partial crustal melting and thinning, and, finally, intrusion of shallow silicic plutons, the Redrock Granite. The Red-rock Granite and anorthosite were presumably derived from sources character-ized by subtle, long-term LREE depletion, with εNd (at 1220 Ma) values on the order of +1 to +2