The cheyenne belt: analysis of a proterozoic suture in Southern Wyoming

Abstract

The Cheyenne belt of southeastern Wyoming is a major shear zone which separates Archean rocks of the Wyoming province to the north from 1800-1600 Ma old eugeoclinal gneisses to the south. Miogeoclinal rocks (2500-2000 Ma old) unconformably overlie Archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift-formed continental margin. Intrusive tholeiitic sills and dikes are interpreted as rift-related intrusions and a date of 2000 Ma on a felsic differentiate of these intrusions gives the approximate age of rifting. There are no known post-2000 Ma felsic intrusions north of the Cheyenne belt. Volcanogenic gneisses and abundant syntectonic calc-alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks. The volcanics are a bimodal basalt-rhyolite assemblage. Plutons include large gabbroic complexes and quartz diorite (1780 Ma), syntectonic granitoids (1730-1630 Ma) and post-tectonic anorthosite and granite (1400 Ma). There is no evidence for Archean crust south of the Cheyenne belt. Structural data (thrusts in the miogeoclinal rocks, vertical stretching lineations, and the same fold geometries north and south of the shear zone) suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane (island arc) over the northern terrane (craton and miogeocline), probably as a continuation of the south-dipping subduction which generated calc-alkaline plutons of the southern terrane. A metamorphic discontinuity across the shear zone, with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south, also suggests thrusting of the southern terrane (deeper crustal levels) over the northern terrane (shallower levels). The Cheyenne belt may be a deeply-eroded master decollement, perhaps analogous to a ramp in the master decollement in the southern Appalachians. This interpretation of the Cheyenne belt as a Proterozoic suture zone provides an explanation for the geologic, geochronologic, geophysical, metallogenic, and metamorphic discontinuities across the shear zone.