A REFINEMENT OF THE TIMING FOR PROTEROZOIC CRUSTAL GROWTH IN WESTERNMOST COLORADO FROM SHRIMP-RG U-PB ZIRCON GEOCHRONOLOGY

Nineteen new SHRIMP-RG U-Pb zircon ages from mainly Proterozoic metaplutonic rocks exposed in westernmost Colorado (near Glenwood Springs, Colorado National Monument, Glade Park, and Unaweep Canyon) define the timing of two distinct magmatic episodes at 1722 - 1703 Ma and 1458 - 1434 Ma, with the exception of two samples at ~1760 Ma. Composed of minor gabbro and diorite, and abundant granodiorite through granite, these two magmatic episodes are extensions of a broader Proterozoic magmatic history previously documented by over 150 SHRIMP-RG U-Pb zircon ages from basement exposures throughout western Colorado.

In that broader history, over 50 of these samples have ages between ca. 1785 and 1725 Ma, and likely define at least two separate magmatic events that predate the batholith-size, granitic intrusions between 1725 and 1675 Ma that extend throughout southwestern North America. The oldest event defines the formation of the Green Mountain arc at the Colorado-Wyoming border between 1782 and 1771 Ma. A slightly younger suite of gabbro and Na-granite is found within and to the south of the Green Mountain arc and range from ca. 1775 to 1763 Ma. A similar coeval suite of Na-granitoids is present west of Denver, over 100 km to the south. The two blocks are now thought to have been part of the southern edge of the Green Mountain arc that was rifted at ca. 1770 - 1730 Ma to form the metasedimentary terrane between them, the Poudre basin. Two samples from small exposures at Glenwood Springs (granitic gneiss) and Colorado National Monument (melanosome of migmatitic gneiss) are coeval with these older suites and are interpreted as small remnants of the rift-related, extensional granites. Metapelitic rocks and minor quartzites of the western region are intruded by the ca. 1.7-Ga plutonic rocks and are considered to be analogous to the Poudre Basin rocks of the Colorado Rocky Mountains.

A comparison of SHRIMP-RG U-Pb zircon ages for Mesoproterozoic magmatism in Colorado indicates the possibility of multiple pulses: a more mafic magmatic event at ~1450 to 1430 Ma, a slightly younger granitic pulse between 1430 and 1410 Ma, a granitic pulse between 1405 and 1390 Ma, and the San Isabel batholith (granite) at ca. 1360 Ma. Thus far, Mesoproterozoic zircon ages for samples from westernmost Colorado are coeval only with the older mafic magmatic event.