CO-CRYSTALLIZATION OF ZIRCON AND FE-TI OXIDES AND THEIR RELATION TO THE CHEMICAL AND THERMAL EVOLUTION OF THE POCO CANYON CALDERA SYSTEM, NEVADA

The Oligocene Poco Canyon Caldera (PCC) system in the southern Stillwater Range of central Nevada, consists of a steeply tilted crustal block that exposes an intact ~6 km cross-section containing coeval and likely cogenetic intrusive and extrusive units (John, 1995). This exposure presents an ideal setting to investigate magmatic evolution through integrating field and geochemical datasets. Previous CA-ID-TIMS U-Pb zircon geochronology from all PCC units (Freeman Creek and Kspar granites, granite porphyry dike, Poco Canyon Tuff) give ca. 25.3 Ma dates, suggesting rapid construction in ≤100 ka. Field observations indicate that following the caldera-forming eruption of the Poco Canyon Tuff (~360 km³), the composite granite porphyry dike intruded through the eruption conduit and into the base of the tuff.

We present mineral chemical data (feldspar, amphibole, oxides), Ti contents in quartz, and crystal orientation maps for zircon and Fe-Ti oxides from the caldera margin dike and Kspar granite. Alkali feldspars in the dike and granite do not display zoning in backscatter electron images but show resorption textures. Feldspar compositions deep in the dike and granite include near end-member orthoclase and albite and a mixed Kspar (An_1-2Ab_36-51Or_44-63). At shallow depths, the dike contains only orthoclase and albite. Amphiboles from all samples are hastingsite–ferro-hornblende. Zircon grains (30-80 um) display oscillatory zoning and are ubiquitously associated with magnetite containing exsolved ilmenite lamellae, suggesting co-crystallization of these phases. In relation to magnetite, zircons occur as both single euhedral grains or clusters of grains attached to magnetite, and as inclusions within it. Magnetite thermometry yields sub-solidus temperatures with notable intra-sample variability (~200–700 °C), suggesting equilibration during hydrothermal overprinting. Preliminary Ti-in-quartz results from the Kspar granite are consistent (15-70 ppm), suggesting crystallization at a constant magmatic temperature likely at the minimum eutectic. Further testing of zircon/Fe-Ti oxide nucleation via EBSD, trace element thermometry (qtz and zrn), and zircon trace element analyses will further constrain the magmatic evolution of the intrusive and extrusive units of the PCC system.