PALEOPROTEROZOIC SUTURING OF THE WYOMING CRATON AND MEDICINE HAT BLOCK AND IT’S INFLUENCE ON PHANEROZOIC CRUSTAL EVOLUTION

The accretion of the Archean Wyoming Craton to the rest of North America has long been an outstanding issue in the Precambrian history of the continent. The boundary between the NW margin of the Wyoming Craton (WC) and the Medicine Hat Block (MHB) is generally taken to be the Great Falls Tectonic Zone (GFTZ), which in Montana, is delineated by a zone of NE-trending Proterozoic-Tertiary faults and intrusions. These features coincide with a zone of basement-related potential field anomalies, leading to the suggestion that their position is controlled by Precambrian basement structures. Our recent work in the Little Belt Mtns. suggests the presence of a Paleoproterozoic suture along the GFTZ and allows construction of a model for the accretion of the WP.

Field mapping and U-Pb ion-probe dates indicate that the GFTZ was affected by voluminous calc-alkaline magmatism over a protracted interval spanning ~1870 to 1790 Ma. This magmatism is interpreted as the result of subduction between the WC and the MHB to the north, prior to suturing. The Paleoproterozoic magmas have intruded ca. 2600-2800 Ma meta-igneous rocks as well as pelitic gneiss, and metavolcanics. The metasedimentary unit contains detrital zircons with concordant ages of ~2600 Ma, ~2370 Ma, and ~1950-1850 Ma., indicating a Paleoproterozoic depositional age. The lack of detrital zircons older than ~2600 Ma argues against a derivation from the WC. Together, the U-Pb data suggest that the Little Belt magmatic arc was not built on the WC, but more likely the MHB or an intervening crustal fragment. This hypothesis is being tested with geochemical data.

The NE trend of the GFTZ would suggest a simple southwest projection of the Little Belt arc and the WP-MHB suture zone into SW Montana. Two new dates of ~1863 Ma and 1892 Ma from the assumed SW extension of the GFTZ are compatible with this suggestion, however, other data, argue against a simple southwestward extension of the magmatic arc and suture.

There appears to be a strong spatial correlation between Paleoproterozoic crust/structures along the GFTZ (and SW extension) and Phanerozoic tectonic and magmatic features in SW Montana and Idaho. Thus, in the context of Earthscope, the GFTZ suture zone is important not only to Precambrian crustal amalgamation processes, but also to the subsequent crustal evolution of western North America.