EXPLORING THE CONNECTIONS BETWEEN VERY LARGE VOLUME IGNIMBRITES AND INTRACALDERA PLUTONS: INTRUSIONS RELATED TO THE OLIGOCENE WAH WAH SPRINGS TUFF, GREAT BASIN, USA

The Indian Peak caldera complex was a major focus of explosive silicic activity in the subduction-related Great Basin ignimbrite province during a middle Cenozoic ignimbrite flareup. The Indian Peak caldera is the largest in the complex and formed 30 Ma when an estimated 5,900 km³ of crystal-rich dacitic magma erupted as the Wah Wah Springs tuff. Basin and range faulting has exposed the internal structure of the caldera clarigying relations between the intracaldera tuff and a series of intrusions. The inner ring fault extends ~40 km north-south and the area within it is about 1,000 km². In the Indian Peak Range, about 20 km² of the porphyry is exposed and it probably underlies an area of at least 75 km². After the caldera collapsed, magma continued erupting explosively to create a thick accumulation of lithic-rich intracaldera tuff. As vesiculation and fragmentation ceased, dacitic magma rose along the inner ring fault of the caldera and initiated resurgent uplift and reversal of displacements along the inner ring faults. This magma solidified as texturally varied granodiorite in contact with intracaldera tuff. The mineral assemblages of the tuff and the granodiorite are the same: plagioclase, hornblende, and biotite with smaller amounts of quartz, pyroxene, and Fe-Ti oxides, with K-feldspar in the groundmass. The granodiorite is also chemically indistinguishable from the tuff, showing, for example, a high Cr content that is a distinctive feature of the Wah Wah Springs tuff. The tuff is not metamorphosed by the intrusion, although Paleozoic limestones intruded by the granodiorite have been metamorphosed to marble. Furthermore, the intrusion along contacts with the tuff does not appear quenched. Propyllitic alteration is prevalent in the intrusion. All of these features imply that the tuff was still hot when the granodiorite intruded. Contacts between the tuff and intrusion that had been previously mapped as faults are instead almost horizontal intrusive contacts. Thus, field and geochemical evidence suggest that the very large volume tuff and the intrusion are cogenetic and the granodiorite intruded simultaneously or shortly after the tuff was emplaced. The magma may have accumulated incrementally but a large body of magma in an equally large chamber of magma existed before eruption of the tuff.