PALEOMAGNETISM OF PROTEROZOIC MAFIC DIKES, SOUTHERN WIND RIVER MOUNTAINS, WYOMING

Northeast-striking, unmetamorphosed Proterozoic mafic dikes that cut Archean basement rocks are a common feature of the geology of the southern end of the Wind River Mountains. Although some dikes cut a north-northeast-striking quartz diorite dike that has yielded a U-Pb date of 2170±8 Ma, the age of the dikes is poorly known. K-Ar dates of whole rocks and mineral (pyroxene) separates range from 2010 to 1270 Ma, but the large spread of apparent ages and inconsistency of results precludes their use in understanding the emplacement history of the dikes. In order to better understand the age and tectonic setting of the dikes, we have obtained new paleomagnetic data from the southern Wind River dikes. Our results indicate that the remanence recorded by the dikes is complex: some dikes yield well-defined, in situ magnetizations that are of northeast declination and moderate to steep positive inclination; others yield magnetizations of south to southwest declination and moderate to steep positive inclination. A previous paleomagnetic study of the dikes has argued that the southern Wind River dikes may record evidence for dike emplacement at ca. 2.1 Ga and 1.4 Ga (Harlan et al., 2003). Emplacement of dikes at ca. 1.4 Ga is consistent with whole rock ⁴⁰Ar/³⁹Ar dates from two dikes in the area reported by Donohue (2002), with evidence for Mesoproterozoic magmatism along the northern margin of the Wyoming craton, and with dates from dikes elsewhere in the Wind River and Granite Mountains (Chamberlain and Frost, 1995); however, virtual geomagnetic poles (VGPs) from these dikes do not agree with 1.4 Ga poles from Laurentia, and correction for 10 to 20° northeast tilt of Paleozoic strata further accentuates this discordance. Emplacement of the Wind River dikes at 2.1 Ga is consistent with U-Pb dates from dikes elsewhere in Wyoming (Bowers and Chamberlain, 2006), but the VGPs from these dikes are inconsistent with time-equivalent poles from the Superior craton. It is possible that the dikes record multiple magnetization acquisition events over an extended period of the Mesoproterozoic, but additional high-precision geochronology of the southern Wind River dikes is required to better understand their age of emplacement and to facilitate their use in construction of apparent polar wander paths and testing of continental plate reconstructions.