New models for Paleoproterozoic orogenesis in the Cheyenne belt region: Evidence from the geology and U-Pb geochronology of the Big Creek Gneiss, southeastern Wyoming

Abstract

The disputed age of the deep crust of the Colorado Province is central to hypotheses for Paleoproterozoic crustal growth in the region. We studied the high-grade Big Creek Gneiss, southeastern Wyoming, as a potential exposure of pre-1780 Ma basement rocks. New geologic mapping and U-Pb geochronological data indicate that the Big Creek Gneiss exposes a deeper, but coeval, level of the Green Mountain arc relative to the predominantly supracrustal section to the west. The Big Creek Gneiss is composed of: supracrustal rocks; a ca. 1780 Ma Green Mountain arc-correlative, bimodal intrusive suite; a ca. 1763 Ma extensional(?) bimodal intrusive suite; and widespread ca. 1630 Ma pegmatitic leucogranite. The mafi c member of the younger bimodal suite is documented here for the fi rst time. U-Pb zircon ages from migmatite leucosomes indicate penetrative deformation of the Big Creek Gneiss at ca. 1750 Ma.We fi nd that the postarc intrusive suite is mantle-involved, implying a second period of crustal growth. Shortening postdates arc magmatism by ~20 m.y., implying that termination of arc magmatism and accretion were separate events. Finally, criteria previously used to constrain the polarity of subduction for the Green Mountain arc are not reliable. We propose two competing models: (1) southward-dipping Green Mountain arc subduction (present coordinates), with slab breakoff-related magmatism following arc accretion; or (2) northward-dipping subduction, with extensional postarc magmatism. In both models, high-temperature deformation coincides with accretion along the Cheyenne belt, and extensional magmatism is an impor-tant component of crustal growth. We prefer the northward-dipping subduction model because it can be better integrated with regional tectonic events and published isotopic compositions of the igneous rocks.

A new view of an old suture zone: Evidence for sinistral transpression in the Cheyenne belt

“…A wide variety of igneous intrusive, metavolcanic, and metasedimentary rocks are exposed south of the Cheyenne belt in the Sierra Madre and Medicine Bow Mountains Houston et al, 1989;Houston and Graff, 1995;Jones et al, 2010). These rocks are generally considered part of the Green Mountain arc (Condie and Shadel, 1984;Reed et al, 1987;Tyson et al, 2002).…”

“…Arc-related igneous on August 13, 2013 gsabulletin.gsapubs.org Downloaded from rocks in the Green Mountain terrane include ca. 1774-1782 Ma layered mafi c complexes, gabbro , diorite, tonalite, granite, and rhyolite porphyry (Premo and Van Schmus, 1989;Premo and Loucks, 2000;Jones et al, 2010Jones et al, , 2011. In the southeastern Sierra Madre, a voluminous ca.…”

“…These rocks are generally considered part of the Green Mountain arc (Condie and Shadel, 1984;Reed et al, 1987;Tyson et al, 2002). Because igneous rocks in this terrane record both arc magmatism and postarc extension (Jones et al, 2010, we refer to it as the Green Mountain terrane to avoid the genetic connotation of the term "arc." Arc-related igneous on August 13, 2013 gsabulletin.gsapubs.org Downloaded from rocks in the Green Mountain terrane include ca.…”

“…Historically, these shear zones have been interpreted as part of a strike-slip fault system (Warner, 1978); a high-angle reverse fault system representing a paleo-subduction zone (Hills and Houston, 1979); a gently dipping, crustal-scale thrust fault system rotated into its present subvertical orientation during late-stage, orogen-scale folding (Karlstrom and Houston, 1984;Houston, 1986, 1987;Duebendorfer, 1988); and a crustal-scale stretching fault that accommodated differential crustal thickening during the Medicine Bow orogeny (Sullivan et al, 2011). The thrust fault model is the most widely accepted, and it has led to plate-tectonic models that invoke nearorthogonal collision during the Medicine Bow orogeny (Hills and Houston, 1979;Duebendorfer and Houston, 1987;Chamberlain, 1998;Jones et al, 2010). The thrust fault model is based on fi ve lines of evidence: (1) Lineations in the Cheyenne belt shear zones are typically steeply plunging; (2) kinematic analyses of L-S tectonites from the frontal shear zone indicate southeast-side-up motion; (3) foliations and bedding are generally subparallel with the tectonic contacts; (4) metamorphic grade increases abruptly across the shear zones toward the southeast; and (5) large horizontal displacements are needed to juxtapose the widely different rock units exposed across the Cheyenne belt (Hills and Houston, 1979;Karlstrom and Houston, 1984;Houston, 1986, 1987;Duebendorfer, 1988).…”

“…1) (Whitmeyer and Karlstrom, 2007). Rocks in this area record: (1) rifting and formation of a passive margin along the southern margin of the Archean Wyoming Province beginning around 2010 Ma, followed by (2) reactivation of the passive margin and accretion of the Green Mountain terrane of the Colorado Province during the 1780-1740 Ma Medicine Bow orogeny (e.g., Hills and Houston, 1979;Karlstrom and Houston, 1984;Duebendorfer and Houston, 1987;Chamberlain, 1998;Jones et al, 2010). The tectonic reactivation of the southern margin of the Wyoming Province was the start of a rapid period of continental amalgamation as an ~1000-km-wide belt of relatively juvenile crust collectively known as the Colorado Province was added to the Archean cratons of North America over a 140-170 m.y.…”