ISOTOPIC CONSTRAINTS ON THE EVOLUTION OF FELSIC MAGMA IN THE AZTEC WASH PLUTON, ELDORADO MOUNTAINS, NEVADA

The mid-Miocene Aztec Wash pluton is a solidified shallow magma chamber that preserves a record of the progressive evolution of felsic magma. The pluton is broadly divisible into relatively homogeneous granite (HG) and a highly heterogeneous zone (HZ) of mingled and mixed magmas ranging from gabbro to granite. Previous work has shown that both mafic and felsic recharge occurred and that, despite their apparent uniformity, granites in the HG were derived from repeated, multi-stage fractionation of inputs of felsic magma in the HZ.

New Sr, Nd, and Pb isotope data from the Aztec Wash pluton help to clarify the open and closed system evolution of the magma chamber, particularly with regard to the formation of HG. HG does not appear to represent a homogeneous, entirely closed system, as isotopic values show appreciable isotopic heterogeneity (Sr_i from 0.7089 to 0.7116, eNd_t from -12.9 to -8.4, ^206/204Pb from 17.83 to 18.21, ^208/204Pb from 38.57 to 39.15).

Quenched margin material and microgranite enclaves have been interpreted as potential felsic input compositions. Isotopic data support the notion of different input magmas for a border porphyry (Sr_i 0.7105), fine-grained phaneritic border granite (0.7110), and enclaves (0.7089-0.7116). Upon input, the magmas may have experienced wall rock interaction or interaction with mafic magmas of the HZ (0.7070-0.7090). Field and geochemical data suggest that initial input magmas entered the HZ and deposited felsic cumulates during fractionation. Evolved magma ascended from the HZ to fractionate HG cumulates, granites, and highly fractionated miarolitic granite (0.7102-0.7113). Isotopic values for some multi-stage fractionates are distinct enough to suggest minor contamination of magma by wall rock in the HG (particularly for samples near the margins), and possibly by earlier contamination of mafic magma in the HZ. The HZ granite cumulates (0.7088-0.7093) are notably similar to the mafic sheets that surround them, suggesting that cumulates in the HZ may have experienced diffusive isotopic exchange with surrounding mafic magma before complete solidification.