SHRIMP U-PB ZIRCON AGES FOR PALEOPROTEROZOIC BASEMENT ROCKS FROM THE NORTHERN AND CENTRAL COLORADO FRONT RANGE: A REFINEMENT OF THE TIMING OF CRUSTAL GROWTH IN THE COLORADO PROVINCE

One hundred SHRIMP U-Pb zircon ages have been determined on Paleoproterozoic basement rocks of the Colorado Front Range and adjacent ranges to the west. Twelve ages from metavolcanic and metaplutonic rocks north of the Skin Gulch shear zone trend (SGSZ) refine the timing of formation of this basement to between 1782 and 1767 Ma. Ten ages from tonalitic to granitic plutons within this northern zone possibly define two later magmatic pulses at 1763 ± 7 Ma and 1741 ± 8 Ma.

South of the SGSZ, but north of the Idaho Springs–Ralston Creek shear zone (ISRCSZ), ages from over 15 metaplutonic rocks ranging from gabbro to monzogranite that intrude mainly high-grade biotite gneiss range from 1722 to 1696 Ma. This age range may represent three closely spaced magmatic pulses at 1722 to 1717 Ma (Rawah batholith age), 1715 to 1705 Ma (Boulder Creek batholith age), and 1705 to 1696 Ma. Several plutons of this age were identified in the northern zone.

South of the ISRCSZ, 8 ages from gneissic rocks of suspected volcano-plutonic origin suggest a basement that formed from 1780 to 1766 Ma. This basement is overlain by variably metamorphosed sedimentary rocks of the Clear Creek-Idaho Springs area in which 4 samples define a predominant 1755-Ma provenance that also contains zircons ~1780 Ma, 1800-1880 Ma (Trans-Hudsonian), and 2500-2620 Ma in age. Depositional timing for these sediments is further constrained by intrusion of the Boulder Creek batholith (~1710 Ma). 1740-Ma plutons of the northern zone are conspicuously missing in both the central and southern zones.

Also south of the ISRCSZ, the formation of migmatites and granulite that crosscut the older rock assemblages is constrained to 1693 ± 5 Ma. Minor, post-tectonic felsic plutons occur at ~1680 Ma and ~1645 Ma. Plutons of this age are not found north of the ISRCSZ, although a localized 1627-Ma monzogranite intruded gneisses just south of the Cheyenne Belt.

These new data indicate that: (1) there is no simple younging of crustal ages from north to south; (2) the protoliths of the central Front Range gneisses are among the oldest known rocks within the Paleoproterozoic southwest of North America; (3) rocks of the same age as the Boulder Creek batholith are the only suite found in all three zones; and (4) no evidence for a high-grade metamorphic event at ~1693 Ma or post-metamorphic plutonism are found north of the ISRCSZ, except the 1627-Ma monzogranite.