SHRIMP U-Pb zircon ages for Big Creek gneiss, Wyoming and Boulder Creek batholith, Colorado: Implications for timing of Paleoproterozoic accretion of the northern Colorado province

Abstract

Sensitive, high-resolution, ion microprobe (SHRIMP) U-Pb zircon ages from a sample of the high-grade, hornblende-feldspathic Big Creek gneiss of the southeastern Sierra Madre, along with samples of a quartz monzonitic phase of the Boulder Creek batholith, help define timing of three major Paleoproterozoic thermo-tectonic events within the northern Colorado province at approximately 1810, 1710, and 1610 Ma. Previous ages determined for these key rock units were problematic; they hindered regional tectonic interpretations of the Paleoproterozoic crustal accretion history of the Colorado province that extends from the Cheyenne belt of southern Wyoming to north-central New Mexico. The Colorado province has been popularly modelled as a series of accreted oceanic volcano-plutonic arc systems and associated sediments, although alternative interpretations suggest that the series represents continental-margin arc rocks. The Big Creek gneiss has been interpreted as a high-grade basement equivalent of the oldest arc volcanic rocks exposed within the Green Mountain arc terrane, but it also has been suspected of being either an older block of pre-arc basement or perhaps an allochthonous piece of crust from slightly older orogens to the east and north. Previous ID-TIMS work on mg-size zircon fractions yielded U-Pb concordia upper-intercept ages of 1618 ± 22 and 1684 ± 5 Ma as well as negative lower-intercept ages, indicating complex U-Pb isotopic systematics involving at least two ages of zircon growth overprinted by at least one episode of Pb-loss. Zircons from this gneiss were analyzed using the SHRIMP, and a total of 32 spot analyses on both centers and rims produced a range of different 207 Pb/ 206 Pb ages between ∼1840 and ∼1560 Ma. The weighted mean of the oldest 207 Pb/ 206 Pb ages is 1812 ± 12 Ma and is interpreted to estimate the age of the protolith that appears to be slightly older than lower-grade metabasalts and associated plutons at ∼1790–1775 Ma. This protolith age of 1812 Ma further implies that significantly older crust (> 1820 Ma; e.g., Penokean orogeny) is not found in the Green Mountain magmatic arc. The youngest 207 Pb/ 206 Pb ages of ∼1610 Ma are interpreted to represent a time of new zircon growth during highly localized high-grade metamorphism—an event that also produced local granitic magmatism at ∼1625 Ma. The Boulder Creek batholith had been dated previously using the ID-TIMS, U-Pb zircon technique that yielded ages at ∼1670 and ∼1714 Ma, a 45-m.y. discrepancy that left the true age of the batholith in doubt. Zircons from two samples, previously dated using the ID-TIMS method, were analyzed using SHRIMP, and yielded concordia upper-intercept ages of 1713 ± 10 and 1721 ± 15 Ma. These results, combined with two earlier U-Pb zircon determinations, help to establish the age of the Boulder Creek batholith at 1714.4 ± 4.6 Ma (weighted mean), an age more compatible with those for the other large, tonalitic to quartz monzonitic, syntectonic plutons within the northern Colorado province. The new Boulder Creek age helps to establish a discrete period of plutonism (∼1735–1705 Ma) that is syn- to post-tectonic with respect to major regional structures of deformation and metamorphism in the northern Colorado province. Assuming the multiple oceanic arc accretion model, the new age for the mid-crustal emplacement of this batholith into a deforming composite back-arc basin may date the closure of that basin during crustal shortening.