FIELD INVESTIGATION OF SMALL MESOPROTEROZOIC INTRUSIONS IN THE SOUTHERN COLORADO: LACK OF DEFORMATION FABRICS GIVES EVIDENCE FOR COMPARATIVELY SHALLOW EMPLACEMENT

A new field study in the Arkansas River Canyon of Colorado focused on structural relationships in rocks hosting Mesoproterozoic intrusions. Gneisses and schists north of the river near Texas Creek host a systematic set of granitic sills and irregular masses of coarse pegmatite. The country rocks exhibit a strong NW- to W-striking foliation, identified as a second-phase fabric, S2, with S1 recorded as a fabric within large mineral poikiloblasts. Well-developed concordant foliation in Paleoproterozoic granodiorite in the sequence suggests that S2 developed during regional plutonism at 1.66-1.7 Ga.

Particular focus was given to fabric development in small Mesoproterozoic granitic bodies in this area, as a gauge of dynamic conditions for plutonism around 1.4 Ga. Elsewhere in this region, small intrusions developed penetrative foliation and concordant margins whereas large plutons and stocks did not. Examination of small intrusive bodies in the Texas Creek area, however, showed that only a weak discordant foliation was rarely developed. While the contacts between granite phases were often gradational and indistinct, the intrusive contacts cleanly cut across S2 in host rocks. Findings from this study area support the hypothesis that intrusions of the Arkansas River Canyon were "anorogenic," and emplaced within the upper portion of the crust. They lay outside of the zone of deformation and more voluminous magmatism in the "dynamic" middle to lower crust, represented in the Wet Mountains to the south.