NEW SHRIMP-RG U-PB ZIRCON GEOCHRONOLOGY OF PALEOPROTEROZOIC BASEMENT ROCKS FROM WESTERN COLORADO: A REFINEMENT OF THE TIMING FOR PROTEROZOIC CRUSTAL GROWTH IN THE COLORADO PROVINCE

About 40 new SHRIMP-RG U-Pb zircon ages have been obtained from plutonic, metaplutonic, and metasedimentary (MS) samples collected from three western Colorado exposures of Paleoproterozoic basement [the greater Colorado National Monument (CNM) area near Grand Junction, the White River uplift (WRU) area near Glenwood Springs, and the Gunnison Igneous Complex (GIC) south of Gunnison] in an attempt to refine the major ages of crustal formation.

A 1762-Ma age for WRU gneissic granite is very similar to ages obtained from diorite to granite in the Dubois Greenstone of the GIC (1770-1751 Ma). This older suite is coeval with a suite of granitoids west of Denver that range in age from 1786-1754 Ma (avg = 1767 Ma). This age is also recognized in the Big Creek gneissic terrane of southern Wyoming (1770-1760 Ma) and in the Needle Mountains (1769 to 1763 Ma). Rocks of Green Mountain arc affinity (ca. 1780-1770Ma) or older were not found in any of these western areas.

Twelve samples from the CNM area define dioritic to granodioritic magmatism at 1720-1710 Ma and dioritic to granitic magmatism at 1450-1435 Ma. Psammite to biotite gneisses in the area have not yielded zircons yet. Six samples from the WRU area define granodioritic to granitic magmatism at 1745 and between 1725-1710 Ma; none are of 1.4-Ga affinity. A MS sample exhibits an average provenance age of 1786 Ma, but minimum zircon population age ~1770 Ma (N > 4). So, it is possible that this MS rock was deposited prior to intrusion of the oldest plutonic suite at ca. 1762 Ma, although a younger depositional age is allowable.

A new SHRIMP zircon age for the Gold Basin granitoid of the GIC is 1722 ± 7 Ma which is similar to a new SHRIMP age of 1724 ± 4 Ma for the Pitts Meadow granodiorite from the western end of the Black Canyon area. These plutons reportedly intrude a series of MS rocks that are distinguished by two distinct provenance signatures; a volumetrically-dominant suite that is characterized by a very young provenance age of ~1740 Ma, and an older suite with average provenance ~1785 Ma, similar to that found in the WRU. The differing provenances of these two MS rock suites are very similar to other MS provenances in the Front Range. Several of the older MS provenances also contain significant amounts of earliest Proterozoic to late Archean grains, that are not indicative of MS with younger provenances.