CENTRAL COLORADO TROUGH SEDIMENT SOURCE ISOLATION: PETROCHRONOLOGIC SOURCE DISCRIMINATION APPLIED TO AN ANCESTRAL ROCKY MOUNTAIN BASIN

Detrital zircon U-Pb data indicate that Middle-Late Pennsylvanian sedimentary rocks deposited in the Central Colorado Trough (CCT) of the Ancestral Rocky Mountains (ARM) were almost entirely sourced from Cambrian igneous rocks. The exclusivity of this source for ~ 10 My is striking because of the limited area and distribution of Cambrian igneous rocks throughout Laurentia. This unique provenance signal with limited potential sourcing provides a well-constrained natural laboratory to test the efficacy of multiple provenance tools. We analyze zircon U-Pb, Hf isotopes, and rare-earth elements (REE) from the sink and both distal (e.g., Amarillo-Wichita Uplift, and southern New Mexico) and local (e.g., Ancestral Front Range) potential source terrains. Results from potential sources suggest that igneous zircon Hf isotopic ratios decrease from east to west, and, coupled with REE data, should provide a discriminatory provenance tool to be applied in tandem with U-Pb data.

Our provenance interpretation integrates these data with facies relationships, sandstone petrography, and paleocurrent data within the basin. Results from this multi-disciplinary petrochronologic and basin approach adopted here are only consistent with local sourcing by Cambrian igneous rocks exhumed in the adjacent ARM uplift to west. Detrital zircons exhibit evolved Hf isotopic ratios consistent with local sourcing, yet REE data are inconsistent with REE abundances from the adjacent sampled nepheline syenite basement. We reconcile this discrepancy by integrating potential source rock mineralogy with the timing of isotopic versus elemental signatures acquired by zircons. The mineralogy of the sampled nepheline syenite precludes a significant negative Eu anomaly. However, the adjacent hornblende-biotite syenite of similar age contains abundant plagioclase, and is therefore expected to yield detritus which would match the mineralogy, Hf isotopic composition, and REE concentrations of the basin samples. These data are consistent with ARM-driven sediment source isolation of the CCT. Furthermore, these data document the petrochronologic character of North American Cambrian-age igneous zircons and present a case study for the application of REE concentrations and Hf isotopes as detrital zircon source-area fingerprinting tools.