Nd isotopic evidence for the antiquity of the Wyoming province

Sm-Nd isotopic data on Late Archean age metasedimentary rocks from around the Wyoming province yield ENd values at 2.6 Ga from -12.9 to +3.3, which correspond to depleted mantle model ages of 2.7 to 3.8 Ga. These results indicate that crust of a wide variety of ages was exposed and eroding in Late Archean time. The predominance of Middle Archean model ages suggests that continental crust was extensive in the Wyoming province by this time. No resolvable correlation exists between initial ENd value and geographic location or depositional environment of the metasedimentary rock samples; thus, there is no indication that any part ofthe province was established at a time different from the rest. The Nd isotopic characteristics of Archean rocks from the Wyoming province contrast with those of the Superior province, confirming that, although their Proterozoic evolution has been parallel, the two cratons did not share a common Archean history. INTRODUCTION Recent geochronological research has provided evidence of crust at least as old or older than 3.8 Ga in a number of Precambrian provinces around the world, including Australia, the Slave province, and Labrador. Ancient crust also appears to have been present in the Wyoming province, where zircon crystals as old as 3.8 Ga have been documented in a migmatite from the Wind River Range (Aleinikoff et aI., 1989), and detrital zircons from the Beartooth Mountains have been dated at 3.96 Ga (Mueller et aI., 1992). It is uncertain, however, how widespread Early Archean age crust was in the Wyoming province. Other evidence of material older than 3 Ga is limited and comes mainly from the northwestern part of the province (e.g., Nunes and Tilton, 1971; Stevenson and Patchett, 1990) and from the Granite Mountains (Fischer and Stacey, 1986). The volume of Early to Middle Archean age crust present in the Wyoming province is difficult to determine because Archean rocks are exposed only in the cores of late Mesozoic-early Tertiary Laramide uplifts; these exposures represent < 10% of the area of the Wyoming province. Moreover, the Archean exposures are dominated by Late Archean granitoids and granite gneisses that obliterated much of the earlier geologic record. I have documented the prevalence of Early and Middle Archean crust across the Wyoming province by examining the Sm-Nd isotopic characteristics of Late Archean metasedimentary rocks deposited prior to the emplacement of Late Archean granitoids. The rare-earth characteristics of fine-grained sediments are thought to reflect those of the exposed and eroding surface of the continental crust. As such, their Nd isotopic compositions provide a composite of the Nd isotopic character of their source areas. The Archean metasedimentary rocks studied here represent aggregates of detritus from the Wyoming craton, almost certainly including parts that are no longer exposed. Therefore, they represent a way to recover information about more of the Archean continental crust than can be investigated directly. The initial isotopic ratios of these sediments and their model crustal-residence ages indicate whether parts of the crust exposed in Late Archean time were largely juvenile or whether ancient materials dominated Late Archean detritus. Farmer and DePaolo (1984) first used Nd isotopic data to identify age provinces within Precambrian continental crust. They analyzed Phanerozoic granites, the Nd isotopic signatures of which were assumed to reflect the age ofthe crust through which they ascended. GEOLOGY, v. 21, p. 351-354, April 1993 The geographic pattern of ENd values and Nd model ages that emerged was used to delineate crust-formation age provinces. Many subsequent studies have adopted this approach, while emphasizing the importance of identifying granitoids that are pure crustal melts. Abstracts by Peterman and Futa (1987,1988) used Nd isotopic data from felsic crystalline rocks to compare the Wyoming and Superior provinces. My study differs from those above in that it examines metasedimentary rocks that represent composites of the subaerial crust instead of granitoids that represent a vertical sampling through the crust, which perhaps include an upper-mantle component. The Nd isotopic data from the Late Archean metasedimentary rocks presented here, together with ENd data from Late Archean igneous rocks, are used to assess the antiquity of the Wyoming province. SAMPLES The 23 samples of Archean metasedimentary rocks analyzed in this study were collected from the Beartooth, Madison, Gravelly, Wind River, Owl Creek, Granite, Laramie, and Hartville uplifts (Fig. 1). All samples have been variably metamorphosed from greenschist to granulite facies. They have major element compositions within the range typical of post-Archean shales and modern turbidite mudstones and sandstones. In general, the metasediment samples can be divided into three types. The first are those associated with greenstone-belt successions in the southern Wind River Range and the central Laramie Mountains. The second type appear to represent continental platform deposits such as those in the ranges of southwest Montana and the Hartville uplift where metacarbonates and shales predominate. The third type of metasedimentary samples includes those from the northern Wind River, Owl Creek, and Granite mountains. They are found as inclusions and disrupted belts within high-grade gneisses, and their depositional environment has not been determined. Intrusive contacts of Late Archean granitoids provide minimum depositional ages of2. 7 to 2.6 Ga for most samples; in addition, the depositional age of samples from the central Laramie Mountains is delimited by a 2.64 ±0.01 Ga age on a rhyolite from the same supracrustal succession (Snyder et aI., 1988). Sm-Nd ISOTOPIC RESULTS Sm and Nd concentration data, Nd isotopic ratio and calculated model ages are presented in Table 1. There is no obvious correlation