GRANITE ACCUMULATION AND FRACTIONATION IN A DYNAMIC, OPEN-SYSTEM MAGMA CHAMBER, AZTEC WASH PLUTON, ELDORADO MTNS, NEVADA

The mid-Miocene Aztec Wash pluton can be subdivided into two distinct zones: "homogeneous" granite (HG) along the northern and northeastern margin and a larger heterogeneous zone (HZ) consisting of stratified and intermingled mafic and felsic rocks (e.g. Falkner et al., 1995). Detailed studies in the HG indicate that, rather than simply being a cap zone, the HG interfingers with depositional units of the HZ, and that the roof is on the ENE side rather than to the N. Evidence includes: (1) a weak but persistent, steeply dipping magmatic fabric in most of the granite that strikes NNW; this cumulate fabric is subparallel to paleohorizontal indicators in the HZ (Coiner et al., this volume); (2) localization of highly fractionated leucogranites with abundant miarolitic cavities along north-northeastern edges of the pluton, adjacent to N to NW striking pluton margins; (3) apparent debris from north-trending tongues of the HZ within granite cumulates along strike in the adjacent HG. Most of the HG is characterized texturally by relatively coarse, euhedral felspars that form a touching framework, with minor biotite and interstitial quartz; this cumulate texture varies in intensity but is generally strongest to the S and W (base and interior of magma chamber). Compositions of rare chilled margins, quenched felsic enclaves, and giant late dikes suggest that the input felsic magma was low-silica rhyolite (~73 wt% SiO2, 5% K2O, 1300 ppm Ba, 350 ppm Sr, 130 ppm Rb). Felsic cumulates trapped between mafic sheets in the HZ (~58-60 wt% SiO2; Ba to >2000, Sr >1000, Rb <100 ppm) are consistent with derivation from such liquids; in contrast, cumulate-textured HG rocks are variable but generally too evolved to have equilibrated with the input magmas (comparable to lower Ba and Sr, similar Rb).

We propose that both mafic and felsic magma replenishment occurred solely within the HZ, where granitic magma began crystallization, forming "primitive" Ba-Sr-rich cumulates and complementary fractionated melts. Compaction and melt extraction was enhanced by periodic emplacement of overlying mafic sheets. Fractionated melt accumulated roofward and laterally (to the north) and was the direct parent of the "evolved" HG cumulates. The most extreme fractionated melts, equivalent to high-silica rhyolites, accumulated at the roof and in wall overhangs.