Contrib Mineral Petrol (1989) 101:198-206 Contributions to Mineralogy and Petrology 9 Springer-Veriag1989 Evidence for an Early Archean component in the Middle to Late Archean gneisses of the Wind River Range, west-central Wyoming: conventional and ion microprobe U-Pb data John N. Aleinikoff 1, Ian S. Williams z, William Compston 2, John S. Stuckless 1, and Ronald G. Worl 1 1 US Geological Survey, Box 25046, Denver, CO, 80225, USA z Research School of Earth Sciences, Australian National University, GPO Box 4, Canberra, ACT, Australia Abstract. Gneissic rocks that are basement to the Late Ar- chean granites comprising much of the Wind River Range, west-central Wyoming, have been dated by the zircon U- Pb method using both conventional and ion microprobe techniques. A foliated hornblende granite gneiss member from the southern border of the Bridger batholith is 2670___ 13 Ma. Zircons from a granulite just north of the Bridger batholith are equant and faceted, a typical mor- phology for zircon grown under high grade metamorphic conditions. This granulite, which may be related to a second phase of migmatization in the area, is 2698 + 8 Ma. South of the Bridger batholith, zircons from a granulite (char- nockite), which is related to an earlier phase of migmatiza- tion in the Range, yield a discordia with intercept ages of about 2.3 and 3.3 Ga. However, ion microprobe analyses of single zircon grains indicate that this rock contains sever- al populations of zircon, ranging in age from 2.67 to about 3.8 Ga. Based on zircon morphology and regional geologic relationships, we interpret the data as indicating an age of ---3.2 Ga for the first granulite metamorphism and mig- matization. Older, possibly xenocrystic zircons give ages of -~3.35, 3.65 and -~3.8 Ga. Younger zircons grew at 2.7 and 2.85 Ga in response to events, including the second granulite metamorphism at 2.7 Ga, that culminated in the intrusion of the Bridger batholith and migmatization at 2.67 Ga. These data support the field and petrographic evi- dence for two granulite events and provide some temporal constraints for the formation of continental crust in the Early and Middle Archean in the Wyoming Province. Introduction The Wind River Range is a northwest-trending Laramide uplift located in west-central Wyoming. It is composed mainly of Late Archean granites, with subsidiary thin belts and xenoliths of Middle to Late Archean high-grade gneis- ses and migmatites. The crystalline rocks are in thrust fault contact with Phanerozoic sedimentary rocks of the Green River basin to the west and south. To the east, Phanerozoic sedimentary rocks of the Wind River basin unconformably overlie the Precambrian basement; these sediments are folded and faulted against the northern and northwestern margins of the Range (Fig. 1). Geologic mapping in the Offprint requests to . J.N. Aleinikoff Range is incomplete and map coverage is widely scattered. After early studies established the geologic framework of the Range (Baker 1946), subsequent work in the Precam- brian rocks has been mostly related to mineral resource assessment of Wilderness and Primitive areas (Bayley 1973; Granger et al. 1971 ; Pearson et al. 1971 ; Worl et al. 1986). The basement gneisses throughout the Range are com- posed of orthogneiss and a migmatitic complex that con- tains lenses of metaigneous and metasedimentary rocks of diverse composition. A migmatitic pyroxene gneiss, that occurs only in the central part of the Range, contains xeno- liths of supracrustal rocks, thought by Worl et al. (1986), Link et al. (1985), and Frost et al. (1986) to be the oldest rocks in the area. The southern part of the Range is domi- 43' 1090 Fig. l. Generalized geologic map of the Wind River Range, Wyo- ming. Sample localities marked by circles