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MIXING AND MELT SOURCES IN THE MIOCENE AZTEC WASH PLUTON (NEVADA, USA) AS REVEALED BY ZIRCON Hf AND O AND WHOLE ROCK Sr, Nd AND Hf ISOTOPES by Mark Ryan The 15.6 Ma Aztec Wash pluton, within the northern Colorado River extensional corridor (Nevada), displays abundant field evidence for open system processes. Previous geochemical studies documented physical and chemical mixing processes that produced a wide spectrum of compositional and textural variants. To better understand the mixing pathways and melt sources, whole rock and zircon isotopes were analyzed from samples that document the range of rock compositions and textural variety and that contain zircons that have trace element variations indicative of large thermal and chemical fluctuations. The new whole rock isotope data show that all rocks are mixtures of Precambrian crust and enriched lithospheric mantle components, with mixtures having a large mantle fraction (≥50%). New Hf (n=189) and O (n=241) isotope analyses of zircon from all samples show heterogeneous isotopic compositions (-5 to -18 εHf; 4.5-7.5‰ δO), which are interpreted to reflect recycling of crystals from many intrusive increments. Paucity of Precambrian zircons (n=1) indicates that initial melts were zircon-undersaturated and that zircon grew mainly from magmas in Aztec Wash magma chamber(s) or conduits. Silicic melt was derived from a deep crustal “hot zone” formed by injection of enriched mantle basalt into Proterozoic crust; mixing of this melt with enriched mantle-derived basalts produced the observed spread in isotopic data.

Mixing and Melt Sources in the Miocene Aztec Wash pluton (Nevada, USA) as Revealed by Zircon Hf and O and Whole Rock Sr, Nd and Hf Isotopes