NEW SHRIMP U-PB ZIRCON AGES AND ND ISOTOPIC SIGNATURES FOR PLUTONISM IN THE NORTHERN RUBY - EAST HUMBOLDT RANGES OF NE NEVADA: IMPLICATIONS FOR THE TIMING OF TERTIARY CORE COMPLEX FORMATION

Polyphase zircons from the orthogneiss of Angel Lake and three two-mica leucogranites from Lamoille Canyon (LC) were analyzed using the SHRIMP-RG to better elucidate their multiple histories of formation that work to define the timing of Tertiary core complexing. In addition, Sm-Nd whole-rock isotopic data from each sample were obtained to constrain the source(s) for these granitic rocks. Preliminary SHRIMP analyses on zircon from the Angel Lake orthogneiss confirm the presence of mostly dark, well-rounded, Archean zircon cores (~2540 Ma), but further define a Late Cretaceous (~84 Ma) growth of clear, low Th/U (<0.015), magmatically-zoned, euhedral zircon. Several analyses from rounded cores yielded discordant ²⁰⁷Pb/²⁰⁶Pb (7/6) ages between 1880 and 2440 Ma. Mylonitized, two-mica leucogranitic orthogneiss of Thorpe Creek in LC yielded complex results, but a concentration of concordant analyses at 90.6 ± 1.9 Ma from magmatic zones of zircon grains with normal Th/U, and an εNdi of -8.8. Ages of inherited zircon cores from the Thorpe Creek orthogneiss are ~1150 and ~2590 Ma. Medium-grain, two-mica leucogranite gneiss from deeper within the canyon yielded complex U-Pb results with a concentration of concordant, low Th/U analyses at 67.9 ± 0.9 Ma. An εNdi of -15.3 is indicative of its predominant derivation from Proterozoic crust. Fine-grained, foliated, two-mica leucogranite from LC also yielded complex zircon results with a small concentration of concordant analyses from magmatically zoned rims at 32.1 ± 0.9 Ma (εNdi = -11.8), and inherited cores with 7/6 ages of ~1760, ~1875, and ~2690 Ma. These ages are slightly younger than two pre- to syn-mylonitic, mafic intrusive rock units; a quartz diorite from Secret Creek gorge (38.8 ± 0.5 Ma; εNdi = -17.0) and a qtz-bearing gabbro from LC (36.6 ± 0.5 Ma; εNdi = -11.7).

These zircon ages suggest four Late Cretaceous to Oligocene plutonic pulses at ~84-92 Ma, ~68 Ma, ~37 to 39 Ma, and ~32 Ma, and help to constrain at least the last major mylonitic event in the northwestern Ruby Mtns to between ~32 and 37 Ma. Furthermore, very low Th/U signatures for the last three pulses indicate that these granitic rocks were produced from anatectic melts, influenced by U-rich metamorphic fluids. Initial Nd signatures (-9 to -15) indicate that the source of the melts was mostly Proterozoic crust.