THE ANGEL LAKE GNEISS COMPLEX OF NORTHEASTERN NEVADA AND THE SOUTHWESTERN LIMITS OF ARCHEAN TO PALEOPROTEROZOIC BASEMENT IN NORTH AMERICA

The gneiss complex of Angel Lake in the East Humboldt Range of northeastern Nevada marks the southwestern extent of Archean or near-Archean basement rocks in North America. Although recent work questioned the protolith age of some of these rocks, new U-Pb SHRIMP analyses of zircons from orthogneiss and paragneiss now establishes the existence of allochthonous Neoarchean to Paleoproterozoic rocks in the core of a large, southward-closing recumbent fold, the Winchell Lake nappe. The core of the Winchell-Lake fold consists of strongly migmatitic biotite schist and impure metapsammite yielding a suite of Archean detrital zircons with secondary zircon overgrowths at ~1.8 Ga, constraining the protolith age to Neoarchean or Paleoproterozoic. Folded around the core paragneiss is a striped biotite monzogranitic orthogneiss that was strongly migmatized during Late Cretaceous peak metamorphism. A sample of this orthogneiss with minimal migmatization yields a 207Pb/206Pb age of 2449 +/- 3 Ma. This unit probably intrudes the above-mentioned paragneiss, but original contact relationships remain unclear and could have been unconformable or even faulted. In any case, the zircon systematics of some of the orthogneiss so closely resemble the core paragneiss that it seems likely either that the orthogneiss was a close and dominant source for the paragneiss, or that the orthogneiss originated from partial melting of the paragneiss. Taken together, the Angel Lake results resemble previously reported data sets from nearby Neoarchean to Paleoproterozoic terrains in the Albion, Raft River and Grouse Creek Ranges (Strickland et al., 2007) and Farmington Canyon complex, Utah (Stroud et al., 2007, 2009), thus suggesting a genetic link between these otherwise isolated occurrences. In addition, the existence of near-Archean crust in northeastern Nevada may bear important implications for models of mid-Tertiary gold mineralization at shallower crustal levels in the area. Folded around both the core paragneiss and the orthogneiss is an outer quartzite and schist unit with a strong Grenvillian spike in detrital zircon ages, supporting correlation with the Neoproterozoic McCoy Creek Group. All three units also host widespread small amphibolite bodies interpreted as metamorphosed mafic dikes and sills.