CIRCA 1.4 GA PENETRATIVE DEFORMATION IN THE SOUTHERN WET MOUNTAINS, COLORADO: IMPLICATIONS FOR MESOPROTEROZOIC INTRACONTINENTAL TECTONISM ACROSS THE SOUTHERN ROCKY MOUNTAINS

Recent field studies and U-Pb geochronology in the southern Wet Mountains, Colorado, reveal a prolonged history of ca. 1.4 Ga penetrative deformation accompanied by metamorphism and pulses of A-type (sensu stricto) granitic magmatism. The depth of exposure and overwhelming effect of 1.4 Ga events in the southern Wet Mountains make these rocks the most sensitive recorders of tectonic events and the regional state of stress during Mesoproterozoic granitic magmatism. Extensive coarse-grained granitic sills were emplaced at ca. 1434 Ma into amphibolite and locally migmatitic gneisses. These sills are texturally, temporally and geochemically correlated with a regional suite of A-type granitoids occurring elsewhere in the Wet Mountains and across the Rocky Mountain region. Emplacement of these granites was accompanied by growth of metamorphic zircon in an amphibolite wall rock at ca. 1436 Ma, and the granites contain a gneissic foliation that dips moderately NNW associated with a NNW-plunging biotite mineral lineation. These gneissic granites exhibit asymmetric mineral fabrics and folds that indicate top-up-to-the SSE kinematics across the southern Wet Mountains, broadly synchronous with granite emplacement. These fabrics are in turn cut by a suite of ca. 1386 Ma fine-grained granitic sills that contain a concordant biotite foliation, mineral lineation, and asymmetric mineral fabrics and folds also recording top-up-to-the SSE. These consistent fabrics and crosscutting relationships suggest long-lived penetrative deformation broadly synchronous with pulses of concordant, synkinematic granitic magmatism that was eventually punctuated by the emplacement of the San Isabel granite ca. 1361 Ma (Bickford et al., 1989). These fabrics indicate protracted NNW-SSE shortening, consistent with models for emplacement of ca. 1.4 Ga granites during a long-lived episode of intracontinental tectonism likely related to northward convergence along the southern margin of Laurentia.