IS THE CHEYENNE BELT THE MAZATZAL DEFORMATION FRONT?: EVIDENCE FOR REACTIVATION OF THE CHEYENNE BELT AT ~1.65–1.63 GA

Several lines of evidence suggest that the Cheyenne belt in the Sierra Madre, Wyoming, was reactivated by thrusting at ~1.65–1.63 Ga. New geologic mapping shows that the ~1.65–1.63-Ga pegmatitic white quartz monzonite (WQM), originally mapped as a small intrusive body of uncertain tectonic affinity actually pervasively intrudes the 1.78–1.75- Ga, high-grade Big Creek Gneiss. Foliation in the Big Creek Gneiss was folded during ~N–S shortening under upper amphibolite-facies conditions prior to intrusion of the WQM. SHRIMP analyses document widespread metamorphic zircon growth in the Big Creek Gneiss at ~1.63–1.60 Ga. WQM samples have ε_Nd(1630 Ma) (+2.4 to -1.0) intermediate between the older rocks of the Big Creek Gneiss (+3.7 to -0.1,) and Barber Lake block (-2.2 to -6.2; Ball and Farmer, 2001). Electron-microprobe analyses of medium- to high-Y monazite growths, attributed to garnet breakdown, yield Th-U-total Pb dates of 1636–1614 Ma. These observations are consistent with contractional tectonism at or before ~1.63 Ga. In this model, thrusting of the Big Creek Gneiss over the Barber Lake block resulted in melting to produce the WQM, by tectonic thickening and/or strain heating. According to this new interpretation, the Big Creek Gneiss constituted the hanging wall in this contractional system and experienced synchronous folding and thrusting before the widespread emplacement of numerous bodies of pegmatitic WQM. These pegmatites and their associated silica-rich fluids resulted in widespread metamorphic zircon growth in the Big Creek Gneiss. Garnet became unstable at post-exhumation pressures and partially decomposed in the presence of added fluid, producing monazite growth in paragneiss. If correct, this contractional deformation constitutes a newly recognized period of Paleoproterozoic orogenesis in the region, following the ~1.75-Ga Medicine Bow orogeny (Chamberlain, 1998) and preceding ~1.60-Ga greenschist-facies cataclastic faulting (Duebendorfer et al., 2006). The simplest interpretation is that it represents foreland deformation associated with the Mazatzal orogeny. This interpretation, together with that of workers in the Lake Superior region (Holm et al., 1998), suggests reactivation along the length of the southern Archean-Proterozoic boundary in the late Paleoproterozoic.