SHRIMP U-PB DATING OF ZIRCON REVEALS OLIGOCENE, LATE CRETACEOUS, AND LATE JURASSIC AGES IN TROY GRANITE, EAST-CENTRAL NEVADA

The Grant Range in east-central Nevada exposes a thick section of Lower Cambrian to Permian continental shelf strata overlain unconformably by Oligocene to Miocene volcanic and sedimentary rocks. The Paleozoic rocks underwent Mesozoic compressional deformation and were later exhumed by polyphase Tertiary extension after 35 Ma. In the west-central part of the range, extensional faulting exhumed an overturned anticline that involves metamorphosed Lower Cambrian strata intruded by muscovite-biotite granite (Fryxell, 1985). Called the Troy granite and dated at 86.4 ± 4.6 Ma (TIMS U-Pb, Taylor and others, 2000), it was mapped as a single body composed of several textural phases. It is relatively undeformed in the core of the range but exhibits top-to-north mylonitic fabrics along its western and upper margins. New SHRIMP U-Pb ages from the Troy granite reveal a more complex history of intrusion and deformation. Medium-grained, garnet-muscovite-biotite granite is dated at 163.3 ± 1.2 Ma. It exhibits folds and mylonitic fabrics at outcrop scale and biotite foliation with superposed crush zones in thin-section. Crosscutting relations and deformation suggest that this granite was anticlinally folded with the country rocks. Relatively undeformed biotite granite, also in the core of the range, is dated at 83.7 ± 0.8 Ma. It contains muscovite flakes aligned along a spaced cleavage that appears as brittle crush zones at thin-section scale. A set of west-dipping hornblende-biotite granite dikes along the highly extended western range flank are dated at 31.7 ± 0.8 Ma. They contain distinct dipyramidal quartz grains and very fine-grained groundmass. There is no evidence of crushing in the dike samples, but the quartz grains exhibit undulatory extinction and aligned fluid inclusions. These textures, together with aligned biotite in chilled margins of the dikes, suggest minor syn-emplacement ductile deformation. All three granite samples contain inherited zircon (ages of about 1100, 1400, and 1800 Ma) presumably derived from detrital grains in sedimentary rocks partially melted into the granites. Isotopic dating of the granites allows discrimination of fabrics among the early contractional orogenic and later extensional orogenic events in the Grant Range.