CENOZOIC METAMORPHISM AND ALONG-STRIKE PEAK TEMPERATURES IN THE PENNSYLVANIAN MINTURN FORMATION IN THE SANGRE DE CRISTO RANGE: UNRAVELING THE COMPLEX THERMAL HISTORY OF THE RIO GRANDE RIFT IN SOUTHERN COLORADO

The Sangre de Cristo Range in Colorado records some of the highest-grade Phanerozoic metamorphism in the Rocky Mountain region, but the conditions and origins of this metamorphism are not well understood. This study uses field mapping, petrographic analysis, and Raman spectroscopy of carbonaceous material (RSCM) of the Pennsylvanian Minturn Fm. along an ~80 km, range-parallel transect to characterize the variation in metamorphic grade. In the Garner Creek (GC) and Crestone Creek (CC) areas, porphyroblastic assemblages locally include andalusite, staurolite, and biotite, indicating a higher-grade metamorphism than previously suggested. We have also identified a porphyroblastic zone in the Minturn Fm. in the Mosca Creek (MC) area, where retrograde chloritoid overprints higher grade fabrics coeval with penetrative top-SW shear. RSCM records peak metamorphic temperatures of 460–590°C in the GC zone, 360–380°C in the CC zone, ~650°C in the MC zone, and fluctuation from 350–590°C along the remainder of the transect. Previously published and new ZFT dates presented in this study from Pennsylvanian strata along this western flank span from Permian to middle Cenozoic, consistent with variable degrees of reheating. The GC, CC and MC metamorphic zones are characterized by cross-cutting leucogranite dikes and quartz veins. At GC we interpret metamorphism to be contact in origin and late Eocene to early Oligocene in age based on a 33.1 ± 1.3 Ma zircon U-Pb date of a leucogranite intrusion and a 35.5 ± 1 Ma monazite U-Pb date. We interpret the variation between metamorphism and deformation to record temporal differences: during Laramide shortening cleavage developed coevally with growth of muscovite and locally andalusite at CC; late Eocene to early Oligocene magmatism resulted in localized, heterogeneous deformation or static metamorphism; and late Oligocene magmatism during early stages of the Rio Grande rift resulted in penetrative top-SW-directed extensional shear associated with quartz dislocation creep and post-kinematic chloritoid growth at MC. This study identifies a heterogenous Cenozoic thermal structure in the Sangre de Cristo Range that reflects spatial and temporal variations in magmatism, and RSCM data indicate that the entire range was heated to much higher temperatures than previously recognized.