IN SITU MECHANICAL AND DIFFUSIVE MIXING IN AZTEC WASH PLUTON, NV: EVIDENCE FROM ZIRCON TI THERMOMETRY AND ELEMENTAL ZONING

Aztec Wash pluton consists of a texturally and compositionally diverse Heterogeneous Zone (HZ) interfingered with a less extensive, more uniform Granite Zone (GZ).  Within the lobes of the HZ and at their upper contacts with GZ, mixing of mafic and felsic magmas is indicated by mafic enclaves in granites and cumulates; enclaves, pods, and initially subhorizontal sheets of “grey,” intermediate composition rock; and coarse resorbed alkali feldspar in intermediate rocks.  Mixing is further suggested by ⁸⁷Sr/⁸⁶Sr and e_Nd of grey rocks that are intermediate between mafic and felsic end members.  Non-linear arrays in bulk rock chemistry, including elevated Al, Sr, Ba, Ti, and LREE for intermediate ("grey") compositions, suggest that mixing occurred between materials previously modified by fractional crystallization.  Enclaves within felsic cumulate material and the presence of resorbed feldspar phenocrysts in grey rock, together with elemental and isotopic data, suggest that mafic magma from prograding lobes was both mechanically contaminated by and hybridized with felsic cumulate.  Fine-grained, equigranular grey rock that appears to be thoroughly mixed hybrid liquid has liquidus T >1000C, suggesting effective heating by invading mafic magma with near-complete dissolution of pre-existing crystals.

Zircons from HZ rocks preserving evidence of mafic-felsic interaction record Ti-in-zircon temperatures (T_Ti) in excess of 900C (Watson et al. 2006; a_TiO2 ~ 0.7 [ubiquitous sphene ± ilmenite]), and prominent resorption surfaces suggest periods of undersaturation at higher temperatures.   T_Ti's within grains also reflect heating and cooling, consistent with more complicated histories as products of both fractionation and mixing, while GZ zircons record relatively simple, low T (<850C) growth with a dominant trend of progressive cooling.  Though they record a similar thermal history, trace element zoning patterns differ between zircons from mechanically and diffusively mixed HZ samples.  Shallower Eu anomalies in zircons from grey rocks suggest that cumulate feldspars were largely dissolved back into the melt from which zircon grew, whereas Eu anomalies from zircons from contaminated cumulates are more pronounced, documenting mechanical mixing with limited feldspar dissolution.