PALEOPROTEROZOIC AND MESOPROTEROZOIC GRANITIC MAGMATISM IN THE CENTRAL SANGRE DE CRISTO MOUNTAINS, SOUTHERN COLORADO

New U-Pb geochronology from the central Sangre de Cristo Mountains, southern Colorado, reveals at least two discrete episodes of granitic magmatism during the Proterozoic and provides additional constraints on the timing and style of intracontinental deformation during the Mesoproterozoic in southern Colorado. The Marshall Gulch pluton and Crestone stock, granitic intrusions exposed on the western flank of the range near Crestone, Colorado, were both emplaced during the Paleoproterozoic at ca. 1690 Ma. The Marshall Gulch pluton consists of coarse-grained syenite and monzogranite that typically display a magmatic foliation, and it is a tabular body (2 x 6 km) that was emplaced parallel to the pre-existing regional NW-SE striking, sub-vertical foliation in surrounding gneisses. The southern margin of the pluton contains a diffuse, NE-SW striking zone of solid-state deformation and is cut by a swarm of thick (~10 meter), NW-SE striking, sub-vertical pegmatite dikes of Mesoproterozoic age. The Crestone stock is a smaller intrusion of medium-grained, typically undeformed quartz monzonite that was emplaced into greenschist-facies amphibolites. Its southern margin exhibits a moderately developed, NE-SW striking biotite foliation that is subvertical. The deformation fabrics contained in these two plutons were likely developed during the Paleoproterozoic Mazatzal Orogeny and may have been reactivated during younger (i.e., Mesoproterozoic) tectonism. To the south, along the eastern side of the range, the Music Pass pluton was emplaced during the Mesoproterozoic, ca. 1435 Ma, into gneisses and a thin (<30 meter) band of quartzite. This intrusion comprises coarse-grained quartz monzonite and locally contains a well-developed magmatic foliation oriented parallel to the pluton margins. Like the Marshall Gulch pluton, the Music Pass pluton was emplaced parallel to the pre-existing regional WNW-ESE striking, sub-vertical wall rock foliation, and it is locally cut by discrete zones of solid-state, and locally mylonitic, deformation that strike NE-SW. Orientation of these deformational features and the emplacement geometry of the Music Pass pluton and Mesoproterozoic pegmatite dikes are all consistent with regional stress fields that have been proposed for Mesoproterozoic intracontinental tectonism.