Windows into Basement Structure Exposed in the Footwall of the Wasatch Fault: The Santaquin and Farmington Canyon Complexes

The Santaquin complex [SC] and Farmington Canyon complex [FCC] are two of very few basement rock exposures in the footwall of Wasatch Fault, Utah. The SC is dominated by igneous and metamorphic lithologies. Mineral ages show that SC rocks underwent amphibolite-facies metamorphism prior to ~1650 Ma. Unstrained mafic syenite has as a 1673+/-±23 Ma U-Pb age [zircon] and a 1623+/-±5 Ma ⁴⁰Ar/³⁹Ar plateau age [hornblende]. Sr- [errorchron] and Nd-isotope data [T_dms] are also consistent with crust formation at ~ 1650 Ma. Whole-rock Pb-isotope data are discordant, however, and give a ²⁰⁷Pb/²⁰⁶Pb age of ~1400 Ma. Mafic to intermediate rocks of the SC have high LILE/HFSE ratios, whereas granitic rocks have low Nb and moderate Y, indicative of derivation in an arc. ⁸⁷Sr/⁸⁶Sr_i and ¹⁴³Nd/¹⁴⁴Nd_i are also consistent with arc magmatism. Thus, the SC represents an arc terrane accreted to the Archean craton, requiring a suture [Cheyenne belt?] to lie northward. North of the SC, the FCC is considered to be Archean basement based on published T_dms and discordant U-Pb zircon ages, albeit strongly overprinted at 1800 Ma (Hedge et al., 1983). However, Nd T_dms and zircons can be inherited from sediment. Published U-Pb ages on monazite (Barnett et al., 1993) are 1650 to 1700 Ma. Although monazite, once formed, is as retentive as zircon, it shows no sign of inheritance. Instead, monazite is believed to form during prograde metamorphism of sedimentary phosphate. Thus, we propose that the FCC was cratonized from Archean-derived sediments at 1800 Ma. Thus, new data from SC and reinterpretation of the FCC suggest that a serious re-evaluation of basement architecture in Utah is warranted. Indeed, the Cheyenne belt may lie to the north and east of the FCC.