24 Igneous Petrology/Volcanology (Posters)

Saturday, 30 April 2005
9:00 AM - 5:00 PM

PRODUCTION AND DISTRIBUTION OF HYBRIDIZED MAGMA IN A REPLENISHED, OPEN-SYSTEM MAGMA CHAMBER; AZTEC WASH PLUTON, ELDORADO MOUNTAINS, NEVADA

Paper number: 24-2
Program: Technical Sessions
Time: 9:00 AM – 5:00 PM

Authors

Abstract

The Heterogeneous Zone (HZ) of Aztec Wash pluton comprises a stratified sequence that records repeated injections of basaltic (~50 wt% SiO2) and granitic (~73% SiO2) magmas into an active chamber. A relatively homogeneous Granite Zone (GZ) both interfingers with the HZ and caps it. Sequences (m's - 100s of m) of intermediate rocks localized at contacts between the upper part of major HZ lobes and overlying GZ include (1) fine-grained, homogeneous, equigranular "Grey" rock (60-63% SiO2); (2) variants with coarse, partly resorbed and reacted feldspars similar in size and shape to those in the adjacent GZ, enclosed in Grey matrix; (3) coarse feldspar-rich granite and quartz monzonite with abundant cm-scale more mafic enclaves, rapakivi textures, and dm to m-scale, rounded clasts of Grey rock; (4) assemblages with rounded Grey rock clasts enclosed in a texturally contrasting Grey matrix. Sr(i) and εNd of the sequences fall between the extremes that appear to represent mafic (~0.708, -8) and felsic (~0.712, -12) end members, but they do not form well-defined arrays. Elemental compositions also lie broadly between the mafic and felsic input compositions, but the intermediate rocks are richer in Al, Ti, Sr, Ba, and light REE than either constituent.

The intermediate sequences reflect dynamic transport processes that led to mechanical and at least local chemical mixing. Slope processes facilitated by topography on the piles of accumulating mafic and intermediate material probably were important. Compositions that differ from simple mixtures of end members suggest that mixtures included fractionate from mafic magma and accumulating crystals (fspars, accessory minerals) from overlying granite; uniform Grey rocks may reflect solution of captured crystals in fractionated mafic melt. Chemical mixing was probably enhanced by repeated replenishments and fractionation of the mafic magmas to produce hot, wet, low-density melts. Both spreading of dense replenishments at the base of the chamber and ascent of light fractionates through overlying magma would initiate fluid shear mingling and reduce the necessary transport scale for diffusive mixing. Mechanisms and relative extents of mechanical and diffusive mixing are being evaluated by elemental and isotopic zoning studies of crystals and physical analog experiments.