CONTRASTING LEVELS OF EXPOSURE OF MESOPROTEROZOIC PLUTONS IN THE WET MOUNTAINS, COLORADO: IMPLICATIONS FOR EMPLACEMENT UNDER DYNAMIC VS. SYNTECTONIC CONDITIONS

The Wet Mountains of Colorado expose an oblique section across an 8-10 km thickness of Precambrian crust, indicated by hornblende geobarometry on plutonic rocks ranging in age from 1.44 to 1.36 Ga. In support of that finding by Cullers et al. 1993, a comparison of two study areas in the Wet Mountains of Colorado reveals a north-to-south variation in metamorphic grade, crustal position and deformational style of gneisses hosting Mesoproterozoic intrusions. In the north, Paleoproterozoic gneisses of the Arkansas River Canyon reached middle amphibolite grade and saw only minor foliation development at 1.4 Ga. Deformation was localized along discrete, crosscutting shear zones. Granite bodies were mostly undeformed, but some developed weak foliation within their discordant margins.

In contrast, the central Wet Mountains experienced widespread migmatization at upper amphibolite grade, with fabric development in host gneisses and intrusions alike. Assimilation of migmatite blocks within sills suggests that temperatures in the host rocks were comparable to granite emplacement temperatures. Strong concordant foliation exists within widespread sills whose contacts parallel the metamorphic layering in migmatitic gneisses, a product of dominant flattening deformation. The heterogeneous layered sequence is interpreted to represent a zone of lateral spreading and flow of material in the middle crust during Mesoproterozoic plutonism. A minimum age for deformation is provided by overprinting mineral textures that record an episode of mineral growth coincident with the youngest plutonism. Large, unoriented hornblende porphyroblasts give 40Ar-39Ar ages of 1369±4 Ma to 1342±6 Ma.

Relationships in the Wet Mountains illustrate very complex structural-plutonic relationships at ~1.4 Ga. They reveal little evidence for deformation in middle grade gneisses of higher crustal levels, while abundant evidence for syn-plutonic deformation exists in coeval, higher grade migmatites from a deeper level. Thus, examples of both anorogenic and syntectonic emplacement can be found. These findings suggest that interpretations of the tectonic setting for magmatism at 1.4 Ga (e.g. Nyman et al. 1994) may depend on the level of crustal exposure.