Rocks of the Miocene and Pliocene Timber Mountain--Oasis Valley (TM-OV) caldera complex in the southwest Nevada volcanic field are dominantly rhyolites and quartz latites (trachytes). Minor basaltic to dacitic rocks were erupted peripherally to the caldera complex and in the moat of the youngest caldera. We divided the petrologic evolution of the caldera complex into a series of petrochemical cycles based on systematic changes in rock chemistry, modal petrography, and mineral chemistry as a function of time. Each petrochemical cycle is characterized by periods of systematic differentitiation toward more siliceous or rhyolitic compositions. Breaks between cycles are generally abrupt and follow either times of major ash flow tuff eruption or episodes of relatively mafic (basaltic to dacitic) volcanism.

New cycles begin with magma compositions that are less silicic than rhyolites erupted at the end of the preceding cycle. The systematic chemical changes toward more silicic compositions indicate that each cycle represents periods of progressive magmatic differentiation and suggest that members of a cycle successively evolved from a common parental magma. Some of the chemical trend within cycles are consistent with crystal fractionation of the observed phenocrysts and accessory minerals. However, other mechanisms must have operated in conjunction with crystal fractionation to produce the observed chemical variations in these rocks. Progressive buildup of volatiles and the depression of liquidus temperatures is suggested in some cycles by the decline in phenocryst abundances and the resorption of quartz. The occurrence of multiple petrochemical cycles is consistent with the interpretation that a series of magma bodies were successively emplaced and differentiated in the upper crust beneath TM-OV. Members of petrochemical cycles were periodically erupted during the evolution of these magma bodies, providing a record of their differentiation. An alternative interpretation is that a large upper crustal silicic magma body was present beneath TM-OV for most of its history and that new cycles of differentiation began after major ash flow eruptions, episodes of magma replenishment, and breakdown in compositional zonation. ¿ American Geophysical Union 1989