NEW CONSTRAINTS FROM FIELD MAPPING AND U-PB TIMS GEOCHRONOLOGY ON THE MAGMATIC HISTORY OF THE NEEDLE MOUNTAINS PROTEROZOIC COMPLEX, SOUTHWESTERN COLORADO

The Proterozoic basement exposed in the Needle Mountains of southwestern Colorado records an intriguing 400 Myr magmatic, metamorphic, and depositional history ranging from a 1.8 Ga high Na/K trondhemdite emplaced in a supracrustal sequence of greenstone, greywacke, and BIF to a 1.4 Ga low Na/K red granite with A-type geochemical affinities. This shift from sodic to potassic magmatism parallels the well-known decrease in average magmatic Na/K at the end of the Archean caused by declining "TTG" (trondhjemite, tonalite, and granodiorite) magmatism. As such, the Needle Mountains provide a potentially novel case study. However, geochemical and geochronological constraints in the region are relatively sparse, and complicated by open-system behavior given multiple metamorphic and hydrothermal overprinting events.

In order to better resolve the petrology and magmatic history of the Needle Mountains plutonic complex we have applied techniques ranging from field mapping to zircon U/Pb CA-ID-TIMS-TEA geochronology. Even with modern single-crystal chemical abrasion techniques, open system behavior remains a formidable obstacle in the western portion of the Needle Mountains – resulting in Pb-loss discordia arrays even after chemical abrasion (HF, 12h, 185C). However, discordia upper intercept ages may still provide useful constraints when coupled with field observations. For instance, while previous geochronology suggested approximately 100 Ma of age difference between the two units (Bickford, 1969) the central Trimble Granite displays comagmatic textures at the contact with the Eolus Granite – an interpretation supported by new upper intercept ages of 1429.6 ± 1.0 Ma for the Trimble Granite and 1431.7 ± 0.5 Ma for the adjoining Eolus. Other new constraints from the western Needle Mountains include upper intercept dates of 1707.4 ± 2.4 Ma and 1707.9 ± 2.3 Ma for two adjacent samples of Whitehead Granite.

Notably, the large and little studied Pine River pluton in the eastern Needle Mountains is relatively unaffected by tertiary hydrothermal alteration and yields nonmetamict zircons with precise, concordant U-Pb ages ranging from 1433.7 ± 1.0 Ma to 1425.9 ± 0.9 Ma in a single sample of hornblende-biotite granite – potentially suggesting protracted zircon crystallization or remobilization at the hand-sample scale.