THE ANGEL LAKE GNEISS COMPLEX REVISITED: PRECAMBRIAN GEOLOGY OF THE NORTHERN EAST HUMBOLDT RANGE, ELKO COUNTY, NEVADA

Recently reported U-Pb SHRIMP age results for the orthogneiss of Angel Lake in the northern East Humboldt Range of northeastern Nevada challenge the Archean age of this unit and renew interest in the regional tectonic significance of this orthogneiss and associated paragneiss (Lush et al., 1988; Premo et al., 2008). As mapped by McGrew (1992), the Precambrian gneiss of Angel Lake occurs in the core of the Late Cretaceous Winchell Lake fold-nappe, with a sequence of paragneiss folded around the orthogneiss unit, and both units enfolded by inferred Paleozoic metasediments. Aside from lithologic contrast, the Precambrian rocks differ from the surrounding Paleozoic sequence and Neoproterozoic rocks farther south in the presence of widespread amphibolite bodies interpreted as metamorphosed Proterozoic mafic intrusions. Unfortunately, the amphibolite failed to yield magmatic zircon, so the protolith age remains unknown, but one sample yielded metamorphic zircons indicating Late Cretaceous peak metamorphism consistent with previous results for the age of migmatization. New mapping largely confirms the above field relationships, but reveals that enclaves of paragneiss within the orthogneiss sequence form a coherent mappable unit in the core of the Winchell Lake fold. We suggest that this newly recognized internal paragneiss unit could represent either an infold or a pre-metamorphic fault repetition of the external paragneiss unit, or some components could even represent Archean country rock to the orthogneiss intrusion. Both the internal and the external paragneiss sequences include diverse metasedimentary packages ranging from quartzite to migmatitic biotite schist that may represent a range of protolith ages. However, newly obtained U-Pb SHRIMP data indicate that at least some of the paragneiss is probably Neoproterozoic as it contains detrital zircons with concordant 207Pb/206Pb ages ranging from >2400 Ma to 900 Ma, with a prominent peak near 1000 Ma (Grenville). These zircon results strongly suggest a correlation at least in part with the McCoy Creek Group of the eastern Great Basin, although the lithologic character and the relative thinness of the paragneiss units at Angel Lake contrasts with the thick, marble-bearing McCoy Creek paragneiss at Lizzies Basin farther south.