SOME OF THE OLDEST ROCKS IN COLORADO ARE RIFT-RELATED? RESULTS FROM SHRIMP U-PB ZIRCON GEOCHRONOLOGY AND MAJOR AND TRACE-ELEMENT GEOCHEMISTRY

More than a 150 SHRIMP U-Pb zircon ages and 300 geochemical analyses have been determined on samples from Proterozoic rocks of Colorado as part of a 5-year project to assist new mapping efforts. Fifty of these samples yielded ages between ca. 1790 and 1725 Ma, and define what we believe to be 3 separate magmatic events that predate the better-known intrusions of batholith-size, granitic magmas between 1725 and 1695 Ma. Whole-rock Ndi values from this older suite are positive (+3 to 6), indicating their derivation from mantle sources or mantle-derived crust. Zircons from this suite are generally low in U (avg. ~350 ppm; Th/U ~0.25 - 0.35), well-preserved, devoid of inherited cores, and produce concordant to near-concordant ages (>65% of the data are less than 5% discordant).

The oldest event defines the formation of the Green Mountain arc at the Colorado-Wyoming border. Seven ages on metavolcanic and plutonic units limit the timing of this event between 1782 and 1771 Ma (avg = 1779 Ma). These rocks are calc-alkalic basalt to alkali-calcic, quartz latite and rhyolite, which formed in a continental arc - not a primitive oceanic setting - and were deposited upon a thin, passive-margin wedge.

A slightly younger suite of bimodal gabbro and granite is found within and to the south of the Green Mountain arc. Nine ages from this suite range from ca. 1775 to 1763 Ma (mean age of 1772 Ma). Most granitic rocks are average to sodic (Na)-enriched; none are potassic (K)-enriched. These strongly bi-modal, Fe-rich, calc-alkalic rocks formed as the arc split apart.

A similar suite of Na-granitoids is present west of Denver over 100 km to the south. Thirteen samples from this central suite range in age from 1786 to 1754 Ma (avg = 1767 Ma). Rocks of this central belt cut psammitic rocks that probably were basement to the Green Mountain arc before being rifted to their present location west of Denver. This age of crust is only otherwise recognized in the Gunnison area (1770 to 1751 Ma) and the Needle Mountains (1769 to 1763 Ma).

In the northern part of the state, other slightly younger rocks (1745 to 1733 Ma) are similar in age to those in the Gunnison-Salida area and Sangre de Cristo Mountains - almost 200 km to the south that include the Cochetopa Canyon arc rocks (new ages from 1751 to 1724 Ma (avg age = 1733 Ma).