METAMORPHISM AND STRAIN LOCALIZATION ASSOCIATED WITH YAVAPAI-MAZATZAL-AGED SYNTECTONIC PLUTON EMPLACEMENT IN THE TUSAS MOUNTAINS, NEW MEXICO

Yavapai-Mazatzal aged metamorphic rocks in the northern Tusas Mountains of New Mexico record the conditions of deformation and tectonic processes during assembly and stabilization of the southern Laurentian margin. Metasedimentary and metavolcanic supracrustal rocks of the Vadito and Hondo groups comprise the majority of exposures in the Tusas Mountains. Detrital and igneous zircon from within these units constrain the age of deposition to be ca. 1.70 Ga. P-T estimates from metamorphic assemblages in supracrustal units indicate mid-crustal conditions of ~425-600¢ªC and 4-6 kbars.

Two orthogneiss bodies, the Tres Piedras (TP) and Tusas Mountain (TM) granites, were emplaced into supracrustal host rocks at ca. 1.69 Ga. Detailed field based mapping and anisotropy of magnetic susceptibility analyses suggest that magmatic fabrics within these plutons are consistent with principal regional fabrics (D1) recorded in the supracrustal host rocks. These data, and patterns of hornblende Ar-Ar cooling ages, suggest syntectonic emplacement of the TP and TM granite during 1.70 Ga deformation.

Microstructures within and proximal to the TP and TM bodies show contact areole metamorphism and strain localization during the cooling of the plutons. Quartz and feldspar microstructures suggest that the body of both plutons record non-coaxial deformation over a range of temperatures from near solidus to high-T conditions (~600¢ªC). Electron backscattered diffraction analyses from quartz document changes in the active deformation mechanisms with respect to position in the plutonic body, ranging from high-T (>600¢ªC) prism-c slip to medium-T (~500¢ªC) prism-a slip. Together with field mapping, these deformation patterns suggest progressive partitioning of strain from within the plutons into an ultra-mylonite zone at the contact of the pluton during cooling. This pattern of strain partitioning is further indicated by the distribution and orientation of thin (1-10cm) apalite dikes, which increase in frequency near the pluton margin.

In this study region, ca. 1.7 Ga tectonothermal activity is preserved by a contact areole metamorphism in the host rock and strain localization during pluton emplacement. Furthermore our study shows that regional ca. 1.4 Ga metamorphism is absent in portions of northern New Mexico.