RAPID CONSTRUCTION OF THE MIOCENE AZTEC WASH PLUTON (NEVADA) FROM HIGH-PRECISION U-PB ZIRCON GEOCHRONOLOGY

Aztec Wash pluton is one of several 15-17 Ma plutons emplaced during regional extension in the the Eldorado Mtns of southern Nevada (northern Colorado River Extensional Corridor). Previous work indicates that Aztec Wash pluton was constructed by numerous injections of mafic sheets into a granitic magma body (or bodies). Mapping and paleomagnetism of Aztec Wash pluton shows that it was tilted moderately to steeply E-NE, resulting in continuous exposure from the pluton roof to a depth of 5 km. Field relationships, whole rock geochemistry, and isotopes attest to extensive hybridization of mafic sheets and granite magma throughout the intrusion, from the deepest to shallowest exposures. The current model for construction of Aztec Wash pluton posits three major tongues or pulses of mafic magma invaded a granitic magma body with an overall roofward (E-NE) “younging”. Early U-Pb SIMS geochronology gave and approximately 150-200 ka construction timescale.

New high-precision CA-ID-TIMS U-Pb geochronology has been undertaken to better constrain the duration of construction of Aztec Wash pluton. Sampling was conducted on 8 mafic sheets (from the 3 major mafic pulses) from the deepest to shallowest parts of the pluton; mafic sheets were chosen to constrain the timing because the magmas that formed these sheets were initially undersaturated in zircon when intruded, and thus reached saturation late in residual melts and contain little inherited zircon. This is corroborated by CL imaging that shows relatively simple internal zoning and similar intersample trace element geochemistry. Preliminary weighted mean U-Pb dates suggest that the entire Aztec Wash pluton was emplaced over a maximum duration of 60,000 yrs but an age difference the between structurally highest and lowest mafic lobes cannot currently be resolved; more data are needed to better refine the dates of several samples.

Although the construction of the intrusion may have waxed and waned between major mafic pulses, the short emplacement timescale over the entire 5 km exposure depth implies a persistent and dynamic granitic magma body that was thermally maintained by repeated mafic injections. This would also have permitted repeated extensive hybridization between mafic input and granitic magma and mush as observed throughout Aztec Wash pluton.