Record of magma chamber processes preserved in accessory mineral assemblages, Aztec Wash pluton, Nevada

Field relations and geochemistry indicate that Aztec Wash pluton had a comp lex, open-system history. The tilted pluton represents a 2.5 km thick chamb er that was recharged with both felsic and mafic magma. The lower portion i s highly heterogeneous, with mafic sheets: cumulates; hybrid rocks; mafic, felsic, and composite dikes; and sheets and pods of granite (heterogeneous [H] zone). The upper part is granite that is generally homogeneous in textu re and geochemistry (granite [G] zone). At the base of the G zone, a discon tinuous zone (buffer [B] zone) records interaction between the Cr and H zon es. Complexity of the H zone makes detailed reconstruction of magma chamber history difficult, and the relatively homogeneous G zone appears to offer few clues about the evolution of the pluton or the interaction between the felsic and underlying more mafic magmas. Accessory mineral textures, zoning , and assemblages in the G zone, however, are far from homogeneous and prov ide clear evidence for fluctuating conditions that elucidates magma chamber history. Mafic rocks of the H zone contain the accessory mineral assemblage ilmenite + magnetite + quench apatite +/- late sphene and zircon. G zone rocks have magnetite + apatite + sphene + zircon +/- allanite, ilmenite, and chevkini te. The magnetite + allanite + early sphene, apatite, and zircon associatio n that characterizes much of the G zone indicates a lower temperature and p ossibly higher f(O2) than the H zone assemblage. Mineral textures and zonin g, however, document fluctuations in the stable Cr zone assemblage: (1) as many as five rounded surfaces truncate internal zones in zircon, each indic ating a dissolution event; (2) in addition to euhedral concentric zoning, s phene contains regions of highly irregular zoning that are rich in inclusio ns, especially anhedral ilmenite; (3) ilmenite and allanite are mutually ex clusive, but allanite is present in the matrix of rocks that contain sphene with ilmenite inclusions, and sphene grains in some samples have alternati ng regions with allanite and ilmenite inclusions. We attribute fluctuations in the stable G zone accessory assemblage to fluc tuations in temperature and possibly f(O2) with appearance of the high-T, r educed assemblage indicating interaction with hot, mafic, H zone magma. The se interactions certainly involve heat transfer and may involve limited che mical contamination. We infer that they must have taken place near the II z one-G zone boundary. The most frequent and intense fluctuations (marked by zircon with the highest number of truncation surfaces, and by sphene with i rregular zoning and abundant ilmenite inclusions) affected rocks that are n ear the boundary, but ilmenite inclusions in sphene and truncation surfaces in zircon are present to the top of the pluton. We conclude that granitic ma,oma was subjected to multiple cycles of thermally induced vertical trans fer-convection-that, at least initially, affected the entire upper part of the chamber.