THE MAZATZAL PROVINCE IN SOUTHERN NEW MEXICO: U-PB DATING OF PALEOPROTEROZOIC ROCKS TO ASSESS MAGMATISM, DEPOSITION, METAMORPHISM, AND DEFORMATION

The origin and evolution of the Mazatzal province in southern New Mexico is being evaluated through structural studies, U-Pb dating of zircon (LA-ICPMS and SHRIMP), and argon dating. The earliest magmatism is represented by strongly deformed granitic orthogneiss and amphibolite at 1684–1674 Ma. Some amphibolites of the region are dikes with entirely Pl + Hbl and have igneous protoliths. Other amphibolites are sedimentary based on abundant quartz and exposure as layers alternating with quartzite in an association that may represent turbidites. The main phase of magmatism at 1665–1640 Ma consists of felsic gneisses with minor amphibolite. Deformation is variable, with no correlation between age and degree of deformation, suggesting deformation was localized. The compositions of the plutons are not consistent with a juvenile island arc, but a continental arc setting is possible. The next phase occurred at 1633–1624 Ma. These rocks include felsic gneiss, boudinaged amphibolite dikes, undeformed granite, metavolcanic rocks, and gabbro. The bimodal distribution of rock types suggests a rift setting. Detrital zircon ages from metasedimentary rocks are 1660–1630 Ma and older.

Both regional and contact metamorphism occurred. An undeformed 1631 Ma granite in the San Andres Mtns. is flanked by Qtz-Bt schist with contact metamorphism resulting in Grt growth adjacent to the pluton. These relationships provide evidence for foliation development prior to pluton emplacement and synplutonic static metamorphism. In the Burro Mtns., a 1633 Ma gabbro lies within metamorphic rocks with a field gradient that goes from Grt-Sil schist to andalusite-bearing hornfels.

Most of the ⁴⁰Ar/³⁹Ar dates from hornblende and biotite in the region are ~1460 Ma and overlap with a U-Pb metamorphic zircon date from amphibolite, monazite dates from schist, and a U-Pb titanite date from amphibolite. These dates are coeval with voluminous granitic magmatism at ~1460–1430 Ma in the region. High-T shear zones are present, but their orientations are not consistent, nor are the orientations of the regional foliations in metamorphic rocks. Proterozoic rocks in the San Andres Mtns. have the least amount of ~1.4 Ga and Paleogene heating and thus are the ongoing focus of our efforts to decipher the Paleoproterozoic history of the southern Mazatzal province.