MULTIPHASE PROTEROZOIC REWORKING OF THE SOUTHEASTERN MARGIN OF THE WYOMING ARCHEAN PROVINCE IN THE CENTRAL LARAMIE MOUNTAINS, WYOMING

Archean granitic gneiss and supracrustal rocks of the central Laramie Mountains, along the southeastern margin of the Wyoming Archean province, preserve a complex history of Paleoproterozoic and Mesoproterozoic reworking. Evidence for the timing of the deformation comes from deformation fabrics and features in ca. 2000 Ma mafic dikes of the Kennedy dike swarm that intruded the Archean rocks during rifting of the SE margin of the Wyoming province. Three major Proterozoic events are recorded in the dikes and their host Archean rocks, which are correlated here with the Trans-Hudson orogeny, the Medicine Bow orogeny (Cheyenne Belt suture), and the emplacement of the Laramie Anorthosite, respectively. Deformation associated with the events overprints Archean deformation, lumped as D1, and includes: 1) D2 - E-W shortening that produced upright, dominantly Z-symmetry folds and a southerly plunging L to LS fabric, 2) D3 - NW-SE shortening that reoriented and locally folded and crenulated the earlier fabrics and dikes, and 3) D4 - recumbent to open folding of earlier features along the northern and western margins of the Laramie Anorthosite complex (LAC).

All three of the Proterozoic phases of deformation are superposed in interlayered gneiss and supracrustal rocks in the area north of the Elmers Rock greenstone belt (ERGB), and to the northwest of the Red Mountain syenite of the LAC. In this area, mafic dikes with a strong L2 fabric are folded by F3 folds, and contain an S3 crenulation foliation. The dikes and associated F3 folds are refolded across a broad F4 synform that formed above the northwestern margin of the Red Mountain syenite.

Deformation fabric-porphyroblast relationships in pelitic schist in the ERGB indicate that each of the three events occurred at distinctly different metamorphic pressures. D2 deformation was accompanied by kyanite, D3 produced sillimanite and cordierite replacing staurolite, and D4 andalusite occurs in the contact zone of the LAC.