Nature and timing of metamorphism within the San Isabel granite aureole, Wet Mountains, south Colorado , The

Abstract

The Wet Mountains, southern Colorado, expose Mesoproterozoic metamorphic and magmatic rocks that record the early deformational history of the east-central continental USA. An increase in metamorphic grade from northwest to southeast indicates that these lithologies represent an exposed cross-section through a tilted crustal block. Of particular interest to the geological history of the region are Cu- and Zn-bearing metamorphic rocks adjacent to the San Isabel A-type granite, situated in the southern Wet Mountains. Previous studies of these units have focused on the timing of base metal mineralization, although the pressure–temperature (P–T) conditions of equilibration and age (t) of metamorphism are unknown. Conventional thermobarometry and phase diagram-based thermodynamic modeling of metapelitic and metacarbonate rocks indicates P–T conditions of metamorphism within the upper amphibolite facies (~700 °C) at middle-crustal pressures (~6 kbar). These data implies a geothermal gradient of ~30–35 °C/km, which is typical of metamorphism in the core of a deforming orogen, although they mostly lack the foliated microstructures that often characterize regionally metamorphosed lithologies. In-situ U–Pb geochronology of monazite within sapphirine-bearing metasediments and garnet-biotite schist indicate that peak P–T conditions were reached at c. 1.36–1.34 Ga. These units are slightly younger than nearby lithologies situated outside of the thermal aureole (c. 1.43–1.39 Ga), but approximately the same age as the intrusion itself (c. 1.37–1.36 Ga). These rocks are thus interpreted to be of contact metamorphic origin, in contrast to those found elsewhere in the Wet Mountains, which do display prominent foliations and ductile deformation features indicative of regional metamorphism. However, as the temperature of emplacement and crystallization of the San Isabel granite is unlikely to have exceeded ~800–850 °C, these rocks document an unusual occurrence of normally granulite-facies and (ultra)high-temperature metamorphic minerals (e.g. sapphirine, corundum, orthopyroxene, and spinel) stabilizing at lower grade, potentially due to substitution of minor elements. Thus, caution must be used when using such parageneses as field indicators of geodynamic settings associated with extreme heat flow, such as continental rifting and/or voluminous emplacement of dry, mafic magma.