Mid-Miocene rhyolite sequence, Highland Range, NV: record of magma evolution and eruption from the Searchlight pluton magma chamber
Colombini, Lindy Lee
EARTH AND ENVIRONMENTAL SCIENCES MID-MIOCENE RHYOLITE SEQUENCE, HIGHLAND RANGE, NV: RECORD OF MAGMA EVOLUTION AND ERUPTION FROM THE SEARCHLIGHT PLUTON MAGMA CHAMBER LINDY L. COLOMBINI Thesis under the direction of Professor Calvin Miller The Highland Range in southern Nevada contains a ~3 km-thick sequence of pre- to synextensional volcanic rock that records both large-magnitude Miocene extension and the evolution of large magma system. A km-thick sequence of rhyolite in the southeast part of the range was emplaced above a thick section of trachyandesite and trachydacite at 16.2-16.0 Ma. The lower half of the sequence comprises trachydacite lavas, which abruptly give way to tuffs and inter-fingered rhyolite lavas that mark a transition to a more explosive period of eruption. The uppermost unit of the sequence is a rhyolite lava that is heavily contaminated by mafic enclaves, lithics, and xenocrysts. Basaltic trachyandesite overlies this upper rhyolite. SHRIMP analysis of Ti and other trace elements in zircon from two samples near the top of the sequence documents strongly fluctuating T (720-920°C) and evolution of melt compositions; generally, rims grew at lower T from more evolved melt. Zr-in-sphene thermometry indicates that these phenocrysts grew at the lower temperatures recorded by the zircon rims. The rhyolite sequence appears to have erupted from the middle granite zone of the nearby Searchlight pluton, which is the same age and similarly evolved toward highly silicic compositions, and overlies slightly older quartz monzonite (cf. Bachl et al. 2001; J Miller et al. 2007). MREE depletion in both the rhyolite lavas and in the high-silica granites of Searchlight pluton indicates sphene fractionation.