U-Pb age constraints on the timing and duration of Proterozoic and Archean metamorphism along the southern margin of the Archean Wyoming craton.

摘要

U-Pb ages of zircon, sphene, monazite, and apatite from the Richeau Hills, Medicine Bow Mountains (MBM), and Sierra Madre of southeastern Wyoming constrain the timing and duration of metamorphism and deformation on the Archean Wyoming craton associated with ca. 1.78 Ga collision between the craton and a Proterozoic island-arc terrane to the south (i.e., Medicine Bow orogeny). In addition, U-Pb ages of sphene form the basis for a model invoking significant modification of the distribution of metasedimentary rocks in the MBM at ca. 1.5 Ga.; Isotopic and structural analysis suggest that the Richeau Hills are located within the Cheyenne belt arc-continent suture zone. The Richeau Hills are cut by several belts of mylonitic shear zones. A pink granitic gneiss intruding banded gneiss is found only north of the central mylonite belt and contains zircons interpreted to indicate Archean crystallization with Proterozoic disturbance. In contrast, orthogneiss south of the central mylonite belt is ca. 1.75 Ga in age. A concordant U-Pb age of 1778.0 {dollar}pm{dollar} 1.1 Ma on metamorphic zircon from a migmatitic semi-pelitic paragneiss indicates high-grade metamorphism was associated with the Medicine Bow orogeny. Deformation ceased before intrusion of a post-kinematic granite at 1737.4 {dollar}pm{dollar} 5.1 Ma.; In the MBM, U-Pb ages of sphene growth during the Medicine Bow orogeny range from 1760 to 1725 Ma. This indicates that metamorphism continued for up to 50 m.y. after the initial collision, with the thermal metamorphic peak following deformation by as much as 15 m.y. Growth of synkinematic sphene at ca. 1.5 Ga in a greenschist-grade shear zone is interpreted to suggest that normal faulting occurred on the western flank of the MBM at this time. Normal faulting during extension may have caused structural emplacement of metasedimentary rocks metamorphosed at moderate pressures between high-pressure gneisses of the western MBM and the central Laramie Mountains. The model presented herein for modification of the post-Medicine Bow orogeny tectonic configuration accounts for the inconsistency between the present distribution of metamorphic rocks in southern Wyoming and the arc-continent collisional model.