Sedimentary Provenance of the Upper Cambrian Worm Creek Quartzite, Idaho, Using U-Pb and Lu-Hf Isotopic Analysis of Zircon Grains
Mary K. Todt and Paul K. Link
Idaho State University, Pocatello, ID
The Worm Creek Member of the Upper Cambrian St. Charles Formation is an anomalous feldspathic sandstone within the Cordilleran carbonate platform in southeast Idaho. It contains two or three decameter-scale siliciclastic to carbonate cycles, ranging from 5 to over 150 meters thick. Detrital zircons within the Worm Creek display a strong unimodal 500 Ma age-peak. U-Pb zircon results from Beaverhead-Big Creek belt plutons yield ages of 505-490 Ma and are identical to the unimodal age peaks of detrital zircons in the Worm Creek. This suggests exhumation and erosion of crust overlying the hypabyssal plutons immediately after intrusion. Four samples from the Beaverhead and Deep Creek plutons yielded εHf values between -6.3 and 2.7, suggesting an isotopically evolved source. εHf values for the 500 Ma-age peaks of six sandstones from the Worm Creek have similar evolved εHf signatures with a range of -8.0 to 5.4. The simplest mantle derivation model for the plutons suggests segregation from the mantle at 1370 to 1470 Ma, which overlaps the age of Mesoproterozoic granitic rocks west of Salmon, Idaho. The overlap of εHf values, along with the same depleted mantle age for the detrital and igneous zircons, suggests that Beaverhead-belt plutons are the source of the 500 Ma detrital zircon population in the Worm Creek. Evidence of active Cambrian magmatism and exhumation is important because syndepositional plutonism is not normally considered part of the “passive” margin tectonic setting and the areal extent of siliciclastic influx from the unroofed plutons gives insight into the size of lowstand (Sauk II-III) depositional systems within the overall carbonate platform.
AAPG Search and Discovery Article #90181©2013 AAPG/SEG Rocky Mountain Rendezvous, University of Wyoming, Laramie, Wyoming, September 27-30, 2013