ABSTRACT The Montana metasedimentary terrane (MMT) forms the NW margin of the Wyoming Province in present coordinates. The MMT preserves a multistage Paleoproterozoic tectonic history that clarifies the position of the Wyoming craton during assembly and breakup of the Precambrian Kenorland supercontinent and the subsequent assembly of Laurentia’s Precambrian basement. In SW Montana, burial, metamorphism, deformation, and partial melting attributed to orogeny were superimposed on Archean quartzofeldspathic orthogneisses and paragneisses at ca. 2.55 and ca. 2.45 Ga during the Tendoy and Beaverhead orogenies, respectively. Subsequent stability was disrupted at 2.06 Ga, when probable rift-related mafic dikes and sills intruded the older gneisses. The MMT was profoundly reworked by tectonism again as a consequence of the ca. 1.8–1.7 Ga Big Sky orogeny, during which juvenile metasupracrustal suites characteristic of an arc (the Little Belt arc) and back-arc basin collapsed against the Wyoming craton continental margin. The northern margin of the Wyoming craton occupied an upper-plate position south of a south-dipping subduction zone at that time. Lithostratigraphic correlations link the southeastern Wyoming and southern Superior cratons at ca. 2.45 Ga with the Wyoming craton joined to the Kenorland supercontinent in an inverted position relative to present coordinates. This places the MMT along an open supercontinental margin, in a position permissive of collision or accretion and orogeny during a time when other parts of Kenorland were experiencing mafic volcanism and incipient rifting. The ca. 2.45 Ga Beaverhead orogeny in the MMT was most likely the consequence of collision with one of the Rae family of cratons, which share a history of tectonism at this time. The Beaverhead collision enveloped the Wyoming craton in a larger continental landmass and led to the 2.45–2.06 Ga period of tectonic quiescence in the MMT. Breakup of Kenorland occurred ca. 2.2–2.0 Ga. In the MMT, this is expressed by the 2.06 Ga mafic dikes and sills that crosscut older gneisses. The Wyoming craton would have been an island continent within the Manikewan Ocean after rifting from Kenorland on one side and from the Rae family craton on the MMT side. Subduction beneath the MMT in the Wyoming craton started no later than 1.87 Ga and was active until 1.79 Ga. This opened a back-arc basin and created the Little Belt arc to the north of the craton, contributed to the demise of the Manikewan Ocean, and culminated in collision along the Big Sky orogen starting ca. 1.78 Ga. Collision across the Trans-Hudson orogen in Canada occurred during a slightly earlier period. Thus, docking of the Wyoming craton reflects the final stage in the closure of the Manikewan Ocean and the amalgamation of the Archean cratons of Laurentia.

Abstract This field guide covers the Precambrian geology of the western portion of the Clearwater complex and surrounding area in north-central Idaho in the vicinity of Marble Creek within the St. Joe National Forest. The regional geology of the Marble Creek area includes Precambrian basement orthogneisses, possible basement metasupracrustal rocks, and overlying metamorphosed Belt Supergroup strata. These rocks are exhumed within the western portion of the Cretaceous-Eocene Clearwater metamorphic core complex. This guide focuses on the western part of the Clearwater complex in the vicinity of Marble Creek. Outcrops of Paleoproterozoic basement and overlying Mesoproterozoic metasedimentary units provide a better understanding of the Precambrian magmatic and metamorphic history of this region. The road log in this guide describes the regional geology in a south to north transect from Clarkia, Idaho, to the confluence of Marble Creek with the St. Joe River.