CYCLIC STRATIGRAPHY IN THE AZTEC WASH PLUTON, NEVADA: VERTICAL CONSTRUCTION OF A COMPOSITE INTRUSION

Previous investigations have documented that the mid-Miocene Aztec Wash pluton (Eldorado Mtns., NV) is divisible into relatively homogeneous granite and a larger heterogeneous zone (HZ) where mafic-felsic magma interaction is documented clearly on outcrop and mineral scales; magmatic structures within the HZ provide paleohorizontal indicators (asymmetric depositional sheets, pipes, cumulate layering) and reflect post-magmatic tectonic rotation (theses and papers by C.M. Falkner, D.W. Patrick, D.M. Robinson, M.B. Cheversia). Ongoing detailed work is yielding a refined picture of the timing of emplacement and residence times of magmas (Cates et al., this vol.), the nature of input magmas, materials produced and physical processes within the chamber, and modification of the pluton by immediately post-solidification tilting (Harper et al., this vol., and below). The goal of this project is to achieve a clearer understanding of the magmatic processes active in this system and their relationships to regional volcanism and tectonism.

Field relations and geochemistry document repeated replenishment of the Aztec Wash magma chamber by (1) uniform granite (low silica rhyolite equivalent; ~73 wt% SiO2, 5 wt% K2O; expressed as chilled margins, quenched enclaves, and very large late dikes) and (2) basalt - basaltic trachyandesite (quenched sheets and pillows, ~48-53 wt% SiO2, ~1.5-3 wt% K2O, Mg# 50-65; range suggests variable mafic input). Detailed mapping reveals that lithologies that are extremely diverse and highly distinctive in composition (43-77 wt% SiO2, cumulate troctolite to fractionated high silica rhyolite), mineralogy, and texture recur in successively solidified zones formed in response to mafic recharge that constructed the HZ from bottom to top. Field, petrographic, and geochemical evidence indicate that most of the widespread intermediate rocks (~ 58-66 wt. % SiO2) are products of hybridization between mafic recharge magma and resident felsic magma in the HZ. Mafic + hybrid sheets from the HZ interfinger with contemporaneously deposited, cumulate, "homogeneous" granites. Very shortly after solidification of the pluton, the initially subhorizontal sheets were tilted ~45º to the ENE and intruded by very large composite dikes carrying very similar low silica rhyolite and basalt.