"NEW" NEOPROTEROZOIC COMES TO LIGHT IN THE COLORADO FRONT RANGE: DETRITAL ZIRCON PROVENANCE ANALYSIS RESOLVES THE AGE OF BASEMENT-HOSTED INJECTITES AND REMOBILIZED MATURE SANDSTONES

Detrital zircon (DZ) provenance analysis is used to resolve the time of emplacement of remobilized sandstones hosted by Mesoproterozoic plutono-metamorphic rocks of the Colorado Front Range (CFR). Informally named Tava sandstone, the formation represents a foremost example of sedimentary injectites within a non-sedimentary host, due to the scale of the system. Basement-hosted sandstones occur over a distance ≥75 km, with single dikes up to 6 meters in width, dike sets up to 7 km long, and putative parent bodies with volumes ≥6x10⁶ m³. Introduced to the geological community 130 years ago by Cross (1894), the sandstone dikes have been attributed to nearly every geological time period, including the Holocene, however paleomagnetic attributes constrain the dikes’ emplacement to Pennsylvanian or older. DZ attributes may be used to refine the age brackets yet further, under the assumption that proximal, same-aged siliciclastic units contain similar diagnostic age distributions. To this end, U-Pb DZ ages were obtained for three Tava sandstone dikes and one parent body, to be compared with new U-Pb DZ ages for mature sandstone beds from four Paleozoic units of differing age. All samples are mature quartz arenite.

All four Tava samples exhibit a dominant broad peak of 0.94 - 1.31 Ga in normalized relative age probability curves, indicative of detritus of the Grenville orogen that is prominent in mid-Cryogenian strata of the western USA. Two CFR samples exhibit pronounced peaks at ca. 1.46 and ca. 1.65 Ga, attributable to proximal sources in the Berthoud and Routt plutonic suites, that are absent in the other two samples. The DZ data from four CFR Paleozoic quartz arenite beds yield contrasting results. Basal Cambrian sandstone lacks DZs <1.30 Ga. Upper Paleozoic samples contain ca. 430 Ma zircons that are absent from remobilized sandstones, and only a small group of 0.97 - 1.30 Ga recycled zircons. The lack of correlation in DZ characteristics allows a proposed Paleozoic age for sandstone remobilization to be ruled out. A statistical similarity to published DZ distributions for Cryogenian strata in the western USA suggests instead that the remobilized sandstones’ ancestry can be traced to the interval 770–740 Ma in Rodinia, opening avenues to deeper understanding of continental paleoenvironments and paleogeography of the time.