THE PALEOPROTEROZOIC TECTONIC HISTORY OF THE NORTHERN WYOMING PROVINCE CONSTRAINS ITS INTEGRATION INTO THE CRATONIC BASEMENT OF LAURENTIA

Crystalline basement rocks in the Tobacco Root and Highland Mtns, and the Ruby, Greenhorn, Gravelly, and Madison Ranges of SW Montana constitute the northern Wyoming Province (WP) and preserve a record of Paleoproterozoic tectonics that constrains the role of the WP in the amalgamation and breakup of Precambrian supercontinents and in the final assembly of the Laurentian craton as it exists today. The southern WP can be linked to the southern Superior craton in the earliest Paleoproterozoic, putting the WP in an inverted position (relative to present coordinates) within a larger supercraton and leaving the northern WP exposed along the flank of that supercraton. In the northern WP, metamorphism, leucocratic melt intrusion, and mylonitization indicate crustal thickening consistent with collisional orogenesis at ≈2.55 Ga and ≈2.45 Ga. It seems likely that this resulted in containment of the WP within a larger supercontinent as collision was followed by an extended period of stability and tectonic quiescence in the northern WP. Tectonism resumed in the northern WP at 2.06-1.96 Ga with the intrusion of mafic dikes and sills that we associate with a period of widespread rifting within the supercontinent. Following rifting, a marine platformal setting (Ruby Range), a fringing volcanic arc (Little Belt Mtns), and an intervening back-arc basin (Highland and Tobacco Root Mtns) were established across the northern WP and indicate the presence of a convergent plate boundary. Clockwise rotation of the WP during subduction would bring it to its present orientation and into the realm of the Manikewan Ocean. This subduction system culminated in the collisional Big Sky orogeny in the period ≈1.78-1.71 Ga. Relative to collisional tectonism in the Trans-Hudson orogen, it appears that the WP was the last craton to amalgamate into the stable core of Laurentian basement.