GEOCHEMISTRY AND GEOCHRONOLOGY OF PROTEROZOIC COMINGLED JACK CREEK RAPAKIVI GRANITE AND MINETTE IN THE NORTHERN BURRO MOUNTAINS, GRANT COUNTY, NEW MEXICO

A complex Proterozoic terrain in the northern Burro Mountains, southwestern New Mexico consists of metamorphic rocks that are intruded by granitic and mafic rocks. One of the granitic suites, the Jack Creek Rapakivi Granite is transitional between A-type and I-type, is characterized by large K-feldspar phenocrysts, some of which are mantled by plagioclase forming the rapakivi texture, and contains synplutonic dikes and enclaves of minette. The crenulate to cuspate margins of the synplutonic dikes and many of the enclaves show evidence for intensive mingling with the rapakivi granite host; quartz and K-feldspar megacrysts have been mechanically incorporated into the minette and the quartz xenocrysts are locally mantled by biotite or hornblende. The minette dikes are up to 30 m wide and several tens of meters long and mingled with the granite forming swarms of enclaves. A minette stock has intruded the metamorphic rocks south of the contact between the metamorphic rocks and the rapakivi granite. The minette is mafic and geochemically primitive (e.g., Mg number ~70). Zircons from the rapakivi granite and minette are pervasively altered and discordant, but yield upper intercept ages of 1461 ± 24 Ma and 1465 ± 16 Ma (2s) that are interpreted as fair approximations of the crystallization ages of the granite and minette, respectively. The ~1460 Ma Jack Creek Rapakivi Granite is similar in age and chemistry to many Proterozoic "anorogenic" granites of the western United States. The association that the Jack Creek Rapakivi Granite shows with minette, however, is rare. The Jack Creek pluton shows relatively radiogenic initial Nd isotopes with epsilon Nd (at 1460 Ma) values of +2.3 to +2.8 (rapakivi granite) and +2.2 to +4.0 (minette). The Jack Creek Rapakivi Granite and minette were presumably emplaced inboard of an extensional arc in response to mafic underplating associated with backarc extension. Nd isotopic data imply an overall juvenile craton margin and presence of a light-rare-earth-element-depleted mantle.