CONSTRAINING THE METAMORPHIC AND DEFORMATIONAL HISTORY OF CRYSTALLINE BASEMENT ROCKS, NEEDLE MOUNTAINS, COLORADO

The interval from 1800 to 1700 million years ago in southwestern Colorado was marked by the accretion of new continental crust to the edge of North America. Although detailed studies in the past fifteen years have provided a snapshot of principal geologic events in this period of crust formation and growth, definitive constraint on the conditions of metamorphism preserved in basement rocks in the Needle Mountains has remained elusive. Understanding the pressure and temperature conditions under which these rocks were metamorphosed is important to gain insight into the tectonic processes involved in their formation. This information will help us understand the broader geologic history and crustal dynamics for this part of Laurentia during the Paleoproterozoic.

The prior consensus from mineral assemblages alone is that basement rocks in the Needle Mountains were metamorphosed at lower greenschist- to lower amphibolites-facies conditions. In the western Needle Mountains, near Coal Bank Pass, an isolated zone of highly deformed garnet-amphibole-plagioclase gneiss is exposed within the ~1.78 Ga metamorphosed arc-plutonic complex of the Twilight Gneiss. Field observations and detailed petrographic work delineate the extent of this zone, and establish several generations of syn- to post-deformational prograde mineral growth. Growth of all dominant prograde-mineral phases was initiated during deformation, but static thermal growth outlasted deformation. The associations of minerals and fabrics in these rocks are consistent with protracted middle-amphibolite to granulite-facies conditions from a strain- to thermal-dominated state. New microprobe data will be presented to further test this hypothesis and constrain potential P-T-t paths during deformation and metamorphism.