MIXING PROCESSES AND MELT SOURCES IN THE AZTEC WASH PLUTON AS REVEALED BY O AND HF ISOTOPIC ANALYSIS OF ZIRCON

The 15.6 Ma Aztec Wash Pluton (AWP) is one of several Miocene intrusions located within the northern Colorado River extensional corridor. Extensive E-W tilting of fault blocks in the region has allowed excellent exposure of the pluton from the roof to 5 km structural depth. Earlier field and petrologic studies subdivided the AWP into two distinct zones: (1) a Granite Zone (GZ) comprised of relatively homogeneous granite with subtle differences in textures and mineralogy; (2) a Heterogeneous Zone (HZ), which interfingers the GZ, contains evidence for mafic and felsic magma input with a wide compositional range (42-78 wt% SiO₂) and abundant field evidence for hybridization. Previous whole rock geochemistry and zircon trace element analyses also indicate compositional variation is produced by multi-component mixing between mafic and felsic melts within the HZ, and recycling and transfer of zircon into contrasting chemical and thermal environments.

Recent Hf (n=189) and O (n=241) isotope analysis of zircon from samples in the HZ provide new information on the melt source and a broader perspective on hybridization processes within the AWP. Overall, zircon grains from all samples show variable Hf isotopic composition (-5 to -18 εHf). Intragrain variability is unresolvable at the error limits (ca. 1.5 εHf), and Precambrian cores were absent; only one zircon was clearly from older Precambrian rocks (εHf = -25 and unzoned). However, zircons from the same hand sample are distinctly heterogeneous (6-10 εHf for granites, 5-7 εHf for ”hybrids”, 5-6 εHf for mafic sheets). Oxygen isotope values for the same zircons range from 5-7‰ δ¹⁸O, and also show intrasample heterogeneity.

The Hf and O isotopic data indicate mixing of enriched mantle magmas with melts of local basement rocks but the paucity of inherited Precambrian cores or grains (n=1) indicates that crustal melts were initially undersaturated in zircon, and/or that mixing resulted in strong undersaturation and eradication of old zircon. However, the observed intrasample variability requires repeated and efficient recycling of zircon antecrysts in the growing pluton once zircon saturation was reached within any given pulse of magma. This likely occured by both magma mixing and mixing of new melts with crystal-rich, zircon-bearing cumulates.