TIMESCALE FOR THE CONSTRUCTION OF AZTEC WASH PLUTON, SOUTHERN NEVADA

Tilted Miocene plutons of the Eldorado Mtns. (Colorado River Extensional Corridor, Nevada) are exceptional targets for evaluating the growth of magma bodies in the mid-upper crust. Previous work on one of these, Aztec Wash pluton, has documented E-NE tilting, resulting in 5 km of continuous exposure below the pluton roof. Spectacular sequences of gabbro/diorite sheets display evidence for deposition and build-up on the local floor of a granite magma-mush body. Mixing between intruding mafic sheets and resident felsic magma is widespread and corroborated by whole rock and isotope data showing open system evolution. Previous U-Pb zircon dating by SIMS suggested an approximately 200 kyr intrusion timescale. New CA-ID-TIMS U-Pb zircon geochronology and zircon geochemistry from 8 mafic sheets, from the deepest to shallowest parts of the pluton, has been conducted to better constrain this timescale. Samples were taken from isotropic gabbro/diorite well away from mixing zones. Zircon Hf isotope data were also obtained for a subset of the sheets. Mafic sheets were chosen because: (1) they are the most easily discernable “increments” in the pluton; (2) their compositions indicate that they were zircon-undersaturated when intruded and should therefore lack inherited zircon and precisely date the time of solidification.

CL imaging of zircon shows simple internal zoning. Zircon geochemistry and saturation modeling are consistent with late saturation and show down-T crystallization trends. Zircon Hf isotopes show minimal intrasample variability (<2 ε_Hf), which is consistent with closed system zircon crystallization. Thus, the mafic sheets are excellent candidates for dating the intrusion timescale. Preliminary weighted mean ²⁰⁶Pb/²³⁸U dates from individual mafic sheets range from 15.729±0.024 Ma to 15.654±0.018 and indicate Aztec Wash pluton was emplaced over a maximum duration of 75,000 yrs. An obvious age trend the between structurally highest and lowest sampled sheets is not evident in the data obtained thus far, although the three youngest samples are from the central part of the pluton. The overall short timescale of sheet intrusion over the 5 km exposure depth is consistent with a persistent, fractionating (but possibly waxing and waning), granitic magma-mush body that was thermally maintained by repeated mafic injections.