LATE EOCENE AND OLIGOCENE INTRUSIONS IN THE RUBY-EAST HUMBOLDT CORE COMPLEX, NEVADA: MAGMATIC PROCESSES IN THE MIDDLE CRUST IN RELATION TO TECTONIC EXTENSION

Tertiary plutonic rocks in the Ruby-East Humboldt (R-EH) core complex, northeastern Nevada, range in composition from gabbro to leucogranite. New and published geochronometric data indicate that magmatism can be subdivided into two age groups: ~40–36 (late Eocene) and ~29 Ma (late Oligocene). The largest of the late Eocene intrusions is the ~36-Ma Harrison Pass pluton, which forms an approximately 140-km², composite pluton in the southern Ruby Mountains. Elsewhere in the core complex, late Eocene intrusive rocks are smaller in areal extent, but are widespread. As in the Harrison Pass pluton, they encompass a wide range of bulk compositions. Mafic-intermediate rocks include hornblende-pyroxene gabbro, biotite-hornblende quartz diorite, and hornblende-biotite tonalite/ granodiorite. Silicic rocks range from hornblende-biotite monzogranite to garnet two-mica leucomonzogranite. Coeval volcanic rocks are part of the allochthonous, hanging-wall Tertiary volcanic-sedimentary sequences that occur along the margins of the R-EH core complex. We have studied several late Eocene intrusions in the Lamoille Canyon area (Ruby Mountains) where they form subconcordant intrusive sheets, one of which is ~125-m-thick. These intrusive bodies, chiefly monzogranitic, were emplaced into older granitic and metasedimentary host rocks that record a complex Mesozoic history developed during crustal shortening. Al-in-hornblende barometry on samples from the late Eocene intrusive bodies yield pressure estimates ~5.5 kb, indicating emplacement depths of ~20–22 km prior to core-complex development. The late Oligocene intrusive suite is characterized by steeply dipping biotite monzogranite dikes and on the basis of U-Pb (zircon) ages is interpreted to have intruded the core complex in a narrow time interval at ~29 Ma. No late Oligocene volcanic rocks are known from the Tertiary volcanic sequence, either locally or regionally. In summary, the mid-Tertiary intrusive rocks in the R-EH core complex indicate that abundant mantle-derived magmas invaded and interacted with continental crust of the Sevier hinterland prior to and perhaps during core-complex development. This phenomenon yielded a heterogeneous intrusive suite that was eventually unroofed through large-magnitude crustal extension during late Oligocene–Miocene time.