METAMORPHISM AND DEFORMATION OF PROTEROZOIC ROCKS IN THE 7.5 MINUTE ROSILLA PEAK QUADRANGLE, SOUTHERN SANGRE DE CRISTO MOUNTAINS, NORTHERN NEW MEXICO

In the southern Sangre de Cristo Mountains of northern New Mexico, the ca. 1.72 Ga Pecos Complex preserves a partial record of ductile deformation and metamorphism from 1.65 Ga to 1.4 Ga. This history has been documented through mapping, petrography, structural analysis, and ⁴⁰Ar/³⁹Ar thermochronology. The Pecos Complex includes the ca. 1720 Ma Jones Rhyolite Complex, a metavolcanic and metasedimentary sequence, which was intruded by the broadly contemporaneous Windy Bridge tonalite. An isoclinally folded foliation (S₁), preserved only in the Windy Bridge tonalite, suggests that D₁ occurred prior to intrusion of the Indian Creek granite. Thus deformation associated with the regionally recognized Mazatzal orogeny locally ceased by ca. 1650 Ma. At ca. 1480 Ma the Macho Creek granite intruded the Pecos Compex. The Macho Creek granite has no magmatic foliation, but locally possesses a mylonitic foliation (S₂). S₁ is partially transposed into S₂ in the Windy Bridge tonalite. S₂ is the only foliation visible in both the Jones Rhyolite Complex and the Indian Creek granite. Microstructures found throughout the area and Windy Bridge tonalite mineral compositions suggest amphibolite grade metamorphic conditions persisted through D₂. Kinematic indicators consistently record dextral strike-slip with a component of normal south-side-down shearing on ENE-striking S₂. D₂ is constrained to be between ca. 1480 Ma, the age of crystallization of the locally mylonitized Macho Creek granite, and ca. 1372 Ma, the mean of ⁴⁰Ar/³⁹Ar cooling ages of largely post-mylonitic hornblende in the Windy Bridge tonalite. The cooling age, metamorphic grade, and kinematic indicators collectively indicate that ca. 1.4 Ga D₂, was transtensional rather than extensional or contractional, as has been previously suggested.