THE CRATONIC CROSS-ROADS: NEW U-PB AGE CONTROL FROM THE WILLISTON BASIN BASEMENT PROVIDES INSIGHT INTO THE ASSEMBLY OF LAURENTIA

The Great Falls Tectonic Zone (GFTZ) is an area of high angle structures striking northeast to southwest through Montana. It is part of a series of collisional belts, including the north-striking Trans-Hudson Orogen (THO) to the east, formed during the ~1.7-1.9 Ga amalgamation of Laurentia. Evaluating models for these collisions depends on high quality geochronology; however, sampling is hampered by the extensive Mesoproterozoic and younger cover rocks. One critical area is the basement to the Williston Basin, near the presumed juncture of the GFTZ and THO. Deep test wells (DTW) penetrating the Williston Basin basement provide valuable samples for high quality U-Pb geochronology. The current data set for this region is limited to poorly constrained Rb-Sr whole-rock model ages, Rb-Sr whole-rock isochrons, and K-feldspar thermochronolgy, yielding a range of ages from 0.65-2.9 Ga. Extant U-Pb zircon geochronology is restricted to 7 discordant sample analyses, yielding upper intercept ages ranging from 1.7-2.9 Ga.

New elemental and isotopic data vastly improve the controls on protolith ages, thermal history, and petrogenesis of the basement at this critical juncture. Preliminary basement geochronology from cores Madison-1 (northeast Wyoming) and Madison-3 (west-central Montana) yields ages of ~2.9 and ~3.0 Ga, while a third sample from the Sundance diatreme in northeast Wyoming yielded an age of ~2.6 Ga. These ages overlap with previously observed data from the Wyoming province, helping refine the location of the northern margin in the THO and GFTZ, respectively. DTW AMJ-6 is located at the presumed intersection of the GFTZ and THO in north-western Montana, and a basement sample yielded an age of ~1.8 Ga. This age overlaps with observed data from both the GFTZ and the THO, confirming the affinity of the basement. DTW MT-3 located in northeast Montana yielded a U-Pb age of ~1.8 Ga revealing further influence from the Paleoproterozoic orogenies. Further work on these samples will provide a valuable opportunity to characterize the age and nature of the basement from the marginal region of both the Wyoming and Superior cratons, and the concealed portions of the GFTZ and THO.