VOLCANISM IN THE FISH CREEK MOUNTAINS, BATTLE MOUNTAIN AREA, CENTRAL NEVADA: A MICROCOSM OF CENTRAL GREAT BASIN IGNEOUS ACTIVITY DURING THE CENOZOIC

The Fish Creek Mountains (FCM) and neighboring regions of north-central Nevada have undergone diverse igneous events at ca. 34 (Eocene lavas and domes), 25 (Fish Creek Mountains Tuff), 16 (northern Nevada rift), 5.5 (HAOT), and 4-1 (Buffalo Valley) Ma, spanning most of the Cenozoic igneous history of the Great Basin. All but the 24.7 Ma FCM Tuff event include mafic lavas that record changing mantle source compositions with time. Rare Eocene basalts and basaltic andesites, interpreted to be related to slab rollback and subduction of the Farallon plate, have normalized trace element patterns typical of subduction-related magmas and eNdT from -2 to -10. Eocene mafic rocks have high MREE/HREE indicating a garnet-bearing source. Northern Nevada rift basalts also have subduction-type trace element patterns and eNdT between -1 and -8, but patterns are flatter and LILE-enrichment is less, and MREE/HREE are lower, consistent with a spinel peridotite source. Rare HAOT north of the Fish Creek Mountains have near-flat trace element patterns with minor LILE enrichment, and eNdT of +0.5. Alkaline basalts of the 4-1 Ma Buffalo Valley field have intraplate-type trace element patterns and eNdT between +2 and +5, and MREE/HREE indicate melting at the spinel-garnet peridotite transition. Mafic magmas record thinning of the lithospheric mantle and a transition from lithospheric to asthenospheric mantle sources with time. Crustal rocks with variable end member isotopic compositions contaminated evolved magmas erupted at each time period. Eocene FCM basaltic andesites have very negative eNdT values (-7 to -10), but Eocene FCM andesites through rhyolites have eNdT of -4 to -5, as do tuffs and pre-caldera volcanics from the nearby 34.0 Ma Caetano caldera. The 24.7 Ma FCM Tuff event, 9 Ma younger than nearby ignimbrite flare-up caldera complexes, has higher eNdT (-0.5 to -1.5), whereas evolved northern Nevada rift lavas (trachydacite and rhyolite) have eNdT from -6.5 to -8. Eocene basaltic andesites interacted with garnet-bearing lower crust, whereas other evolved magmas assimilated upper crust of variable age/isotopic composition. The observed changes in tectonic setting (subduction, to distal hotspot, to intraplate) and in mantle sources coincides with a progressive decrease in the overall volume of erupted material through time.