GEOCHEMICAL COMPARISON OF IGNEOUS ROCK IN THE TUSAS MOUNTAINS, NEW MEXICO

Paleoproterozoic basement rocks of southwestern North America formed during a series of orogenic events that developed and accreted primarily juvenile arc rocks to Laurentia from the Mojave ~2300 through the Picuris Orogeny to ~1300 Ma. The northern Tusas Mountains of New Mexico provides a rare window through the Picuris, back to the 1710-1680 Ma Yavapai and possibly earlier orogenies in that area. A-type Tres Piedras and Tusas Mountain metagranites intruded ~1700 Ma, were likely derived by shallow rift-related melting of pre-existing continental crust, which doesn’t align with the Yavapai terrane being produced by island arc tectonic processes. To better understand the local early tectonic processing, conglomerate clasts in the oldest lithologic unit, Moppin complex were sampled. The Moppin is greenschist to amphibolite grade immature metasediments intruded by the Maquinita Granodiorite ~1,755 Ma. The conglomerate north of Hopewell Lake includes rounded to subangular intermediate metaigneous conglomerate clasts deposited before the intrusion of the Maquinita and therefore can provide insights to the earliest regional tectonic history. ICP-MS/XRF bulk compositions were collected on ~10 clasts. The clasts are subalkaline and tholeiitic rocks that do not compare favorably to any exposed metaigneous rocks currently in the region, including nearby adakites. Trace element data showed a relatively flat REE pattern with a slight negative Eu anomaly, extremely low Ni (1-7 ppm), low Cr (9-34 ppm), moderate Zr (80-94 ppm), extremely low Hf (1.5-2.0 ppm), Nb (4.4-6.3 ppm), Ta, (0-1 ppm), extremely high Sr (436-550 ppm), and high Ba (136-629 ppm). These data suggest that the clasts are the first product of the partial melting of a mafic lower crust, probably by back-arc extensional setting is a more appropriate model for this earlier crustal development, rather than an island arc subduction related that is the current model.