Testing the early Paleoproterozoic connection of the Superior and Wyoming cratons with geochronology and geochemistry

Abstract

Paleogeography is critical as a backdrop for environmental changes, yet the Paleoproterozoic configuration of Archean cratons remains controversial. The Snowy Pass Supergroup (SPS), a Paleoproterozoic succession on the SE Wyoming craton (Medicine Bow Mountains, MBM), has been correlated with the Huronian Supergroup (HS) of the Superior craton, suggesting deposition in the same basin, and indicating that these cratons were adjoined in the Paleoproterozoic Superia supercraton. We present U-Pb volcanic and detrital zircon data for the underlying Phantom Lake Metamorphic Suite (PLMS), SPS, and Snowy Pass Group (SPG; Sierra Madre, SM) and U-Pb zircon ages for two mafic sills and Baggot Rocks granite. The PLMS contains a statistically significant mode of ca. 2.45 Ga zircon dates in the SM; zircons of this age also occur in one to two grain quantities in the PLMS in the MBM. The Baggot Rocks granite and Colberg Metavolcanics of the PLMS also yielded the ca. 2.45 Ga U-Pb zircon ages, indicating the early Paleoproterozoic age for this suite and the ca. 2.45 Ga extensive, rift-related mafic magmatism on the southern margin of the Wyoming craton, allowing correlation with the basal HS units of the Superior craton. Zircon age spectra for lithostratigraphically correlative units on the Wyoming and Superior cratons show strikingly similar, primarily Archean patterns, which, along with paleocurrent directions, establish provenance of both cratons for the shared basin; ca. 2.55 and 2.3 Ga modes are common for the uppermost formations. Fine-grained siliciclastic rocks throughout the SPS yield similar TDM and εNd ages as well as μ and κ values to the HS, further supporting similar provenance for the two supergroups. Mafic sills in the MBM yielded U-Pb zircon dates of 2220 ± 8 Ma and 2094.3 ± 1.6 Ma that closely correspond to the age of the Nipissing mafic intrusions in the HS and the Cauchon Lake dikes in the NW Superior craton. Correlation between the HS and SPS breaks down in the upper SPS at the Nash Fork Formation, which bears detrital zircon age modes at ca. 2.2 and 2.1 Ga, whereas deposition of the HS terminated before ca. 2.22 Ga when the Nipissing dikes intruded and shortly after a ca. 2.31 Ga volcanic event recorded by the Gordon Lake and Bar River formations. The matching detrital zircon dates to a globally unique, ca. 2.3 Ga, mode first appeared in the Lookout Schist of the SPS and steadily decreased up-section through the upper SPS, pointing to a similar volcanic source. Maximum depositional ages for the uppermost formations of the SPS and SPG, the purportedly correlative Nash Fork and Slaughterhouse formations, are at 2081 ± 10 Ma and 2334 ± 20 Ma, respectively. The former detrital zircon mode corresponds to that recorded by the basal Animikie basin units (Denham Formation of the Mille Lacs Group in MN and East Branch Arkose of the Dickinson Group, MI) and are linked to rifting, breakup, and ultimate separation of the Wyoming and Superior cratons at ca. 2075 Ma.