 2002 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; September 2002; v. 30; no. 9; p. 831–834; 5 figures; Data Repository item 2002097. 831 Basement complexes in the Wasatch fault, Utah, provide new limits on crustal accretion Stephen T. Nelson* Ronald A. Harris Michael J. Dorais    Department of Geology, S389 ESC, Brigham Young University, Provo, Utah 84602, USA Matthew Heizler New Mexico Bureau of Mines and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801, USA Kurt N. Constenius Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Daniel E. Barnett Parr Waddoups Brown Gee & Loveless, P.C., 185 South State Street, Suite 1300, Salt Lake City, Utah 84111, USA Figure 1. Distribution of exposed Precambrian crystalline rock as well as proposed crustal boundaries. Line A-A correlates to model of crust formation presented in Figure 5. ABSTRACT New and reinterpreted isotopic data for crystalline rocks ex- posed in the Wasatch Range require a reevaluation of Precambrian crustal boundaries in Utah. Crystalline rocks of the Santaquin Com- plex underwent metamorphism prior to ca. 1670 Ma, consistent with Sr and Nd isotope data. Mafic to intermediate rocks have major element, trace element, and isotope ratios indicative of derivation in an arc accreted to the Archean craton in Proterozoic time, requiring the crustal suture to be north of the Santaquin Complex. Farther north, the Farmington Canyon Complex has been considered Ar- chean based on published Nd model ages and discordant U/Pb zir- con ages. However, Nd model ages and zircons could be inherited from sedimentary protoliths. U/Pb and electron microprobe ages of monazite have a mode at 1650 to 1700 Ma, concordant with the Santaquin Complex, and lack inheritance. We propose that the Farmington Canyon Complex was first cratonized from Archean- derived sediments in the Proterozoic, requiring a crustal suture to be north of it as well. Accretion ages of arc terranes in southeastern Wyoming are 60–100 m.y. older than in Utah. Thus, a serious reevaluation of basement architecture in Utah is needed and a pre- viously unrecognized temporal complexity of accretion is indicated. Keywords: geochronology, igneous rocks, crustal origin, isotope geochemistry. INTRODUCTION The few Precambrian crystalline rock exposures in the Basin and Range and Colorado Plateau provide rare opportunities to document crust formation, tectonic setting, and deformational histories of the re- gion. Two basement exposures are found just east of Santaquin, Utah, and north of Salt Lake City, Utah, in the footwall of the Wasatch fault (Fig. 1). The geological, geochemical, and isotopic characteristics of the Santaquin Complex and Farmington Canyon Complex reveal the time and mode of crust formation, and help locate a concealed crustal bound- ary between Archean and Proterozoic crust. This study illustrates the complexity of crustal accretion in North America and the advantages of basing crustal boundary models upon the direct examination of basement blocks rather than relying solely on isotopic studies of young plutons. Geologic Setting The Santaquin Complex exposes 3 km 2 of igneous and meta- morphic rocks (Fig. 2A). Bounded westward by the Wasatch fault, clastic sediments of the Neoproterozoic Big Cottonwood Formation unconformably overlie the Santaquin Complex to the east (Fig. 2B). Of the mappable subunits (Fig. 2B), pegmatitic granites, mafic syenite, and fine-grained leucogranite are largely unstrained, whereas other rocks are foliated. Hintze (1993) and Witkind and Weiss (1991) in- ferred the Santaquin Complex to be Archean by correlation to the Far- mington Canyon Complex. Unlike the Santaquin Complex, the Far- *Corresponding author: stn@geology.byu.edu. mington Canyon Complex has received considerable attention (e.g., Hedge et al., 1983; Bryant, 1988; Barnett et al., 1993). RESULTS With the exception of electron microprobe monazite ages for the Farmington Canyon Complex, analytical results are reported only for the Santaquin Complex 1 . Data from previous studies of the Farmington Canyon Complex support the synthesis (Barnett et al., 1993; Bryant, 1988; Hedge et al., 1983). Isotopic Data A U/Pb age, 40 Ar/ 39 Ar ages, and microprobe ages of monazite determine the age of the Santaquin Complex. Zircons (method of Geh- rels, 2000) from mafic syenite define a chord with an upper intercept of 1673  23 Ma, a good estimate for the age of emplacement. This unstrained rock is younger than deformed rocks of the Santaquin Com- 1 GSA Data Repository item 2002097, Chemical, isotopic, electron micro- probe, U-Pb isotopic, and hornblende isotopic data, is available on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, editing@ geosociety.org, or at www.geosociety.org/pubs/ft2002.htm.