AN ASSESSMENT OF THE EMPLACEMENT HISTORY AND TECTONIC SETTING OF THE ~1.7 GA TENMILE GRANITE, WESTERN NEEDLE MOUNTAINS, COLORADO

The Tenmile Granite pluton was emplaced in an orogenic event that culminated at ~1.71 Ga. This intrusion represents an important element of the Paleoproterozoic record in the Needle Mountains because it marks the transition from a syn- to post-orogenic period, but it has not been studied in any great detail.

Emplacement of this pluton involved numerous granitic to dioritic sills and dikes that intruded pre-1.75 Ga basement. Nearly all parts of the Tenmile Granite contain pendants and xenoliths of older metamorphic rock, and field evidence indicates that layering in the metamorphic rocks was a dominant control on magma emplacement. Our field research documents that the pluton is a composite of numerous deformed and undeformed felsic to intermediate pulses of magma that were emplaced during and after ~N-S compressive strain with ~E-W extension.

New and existing geochemical data, Sm/Nd isotope, and petrographic data establish that rocks in the Tenmile Granite are calc-alkaline to slightly alkaline with peraluminous to metaluminous signatures. All the data are consistent with emplacement of the Tenmile Granite in a volcanic-arc system in which mantle magmas invaded and melted older juvenile crust. Intermediate to felsic magmas in the pluton are a result of partial melting and fractionation of mantle and crustal sources.

A regional S-pattern of foliation in pre-1.75 Ga basement rocks in the western Needle Mountains involves some of the ~1.7 Ga intrusive rocks. We suggest that this regional deflection of foliation could be the response of transpressive strain that caused local extension and dilation in pre-1.75 Ga crust creating pathways for magmas that infiltrated older rocks to create the sill-dike complex in the Tenmile Granite.