NEW U-PB ZIRCON GEOCHRONOLOGY OF PRECAMBRIAN PARAGNEISSES AND LATE PHANEROZOIC ORTHOGNEISSES OF THE ANGEL LAKE – LIZZIES BASIN REGION OF THE EAST HUMBOLDT RANGE, NORTHEASTERN NEVADA: A COMPARISON WITH THE THERMAL CHRONOLOGY AT LAMOILLE CANYON IN THE ADJACENT RUBY MOUNTAINS

Previous zircon age results indicate that the allochthonous gneiss complex of Angel Lake in the northern East Humboldt Range (EHR) represents the southwesternmost exposures of Archean to near-Archean basement rocks in North America. Zircon from monzogranitic orthogneiss yielded an age of 2449 +/- 3 Ma. New U-Pb SHRIMP-RG zircon results from migmatitic biotite schist, quartzite, and metapsammite define several distinctive sequences of paragneiss. The oldest rocks are Late Archean to Paleoproterozoic and occur within the core of a large recumbent fold, the Winchell Lake nappe. Detrital zircon results from the oldest sequence indicate a near-singular provenance age of ~2550 Ma (range = 2460 to 2660 Ma). These rocks could be Archean in age, but they could also be a different facies of the next oldest sequence that is characterized by a detrital zircon, minimum provenance age of ~2450 Ma, but also contain zircon that are near concordant between 2500 and 2770 Ma, including two analyses at 3340 and 3740 Ma. Three samples also have evidence for metamorphic zircon growth at ~1770 Ma and three show the same evidence for growth between 70 and 85 Ma.

The next oldest sequence of mainly quartzite and schist is folded around the core gneisses, and their detrital zircons show a strong minimum provenance of Grenville-age (800 to 1250 Ma), supporting a correlation with the McCoy Creek Group. Other detrital zircon ages in the sequence include 1360-1450, 1600-1880, 2000-2050, and 2600-2940 Ma. All samples from this sequence (N=8) also record metamorphic zircon growth between 80 and 100 Ma.

Zircon from a quartz dioritic orthogneiss yielded an age of 162 Ma, similar to ages in Lamoille Canyon (LC). Leucocratic orthogneiss within the Winchell Lake nappe yielded a zircon age of 84 Ma, similar to metamorphic zircon ages from nearby amphibolite and growth zones on older zircon from many of the EHR gneisses. Lastly, zircon from a dioritic orthogneiss from the cirque east of Humboldt Peak yielded an age of 40.9 ± 0.9 Ma, similar to ages from gabbro and diorite from LC and Secret Pass. In contrast to LC, we have yet to document younger magmatic zircon ages (32-37 Ma) from EHR rocks. Likewise, the near-ubiquitous metamorphic event at ~85 ± 10 Ma found in EHR rocks is very minor in LC rocks, suggesting that these two regions record significantly different thermal histories.