T54 Phase Relations, High P-T Terrains, P-T-ometry and Plate Pushing II: A Tribute to W.G. Ernst

Sunday, 2 November 2003
1:00 PM - 3:45 PM

GRANITES, DYNAMIC MAGMA CHAMBER PROCESSES, AND PLUTON CONSTRUCTION: AZTEC WASH PLUTON, ELDORADO MOUNTAINS, NEVADA

Paper number: T54-10
Program: Technical Sessions
Time: 3:15 PM – 3:30 PM

Authors

Abstract

Granites comprising the northern third of the Aztec Wash pluton, though far less variable than the mafic/felsic mingled southern two-thirds, record deposition of the solidifying products of the felsic portion of a recharging, open system magma chamber. Abundant paleohorizontal indicators in the mingling zone document ~50ยบ ENE tilting (Coiner et al., 2003). Distribution of lithologies and structures in the northern granite zone substantiate this orientation. Cumulate granites with abundant coarse, euhedral, touching feldspars and interstitial quartz are concentrated in the SW part of the granite zone. They are weakly foliated and grade to the N and E into finer-grained granite, unfoliated and with more prominent quartz. Near the ENE margin, the granite is fine-grained and more felsic, becoming extremely felsic and rich in miarolitic cavities at the contact. Less felsic granite porphyry is sparsely preserved at the eastern contact. We interpret km-scale NNW-trending lobes of mingled zone material to be prograding deposits formed during periods of increased mafic input - thus, lateral equivalents of the cumulate granites with which they are in contact. Slurries of intermediate hybrid blocks in a cumulate granite matrix immediately east of the lobes represent debris flows shed from sloping upper surfaces of the lobes. The upper margin porphyry is likely a chilled margin or solidification front that marks the first stage of intrusion. Subsequent recharging by felsic and mafic magma rejuvenated and maintained the chamber.

Elemental modelling suggests that the cumulate granites represent a secondary accumulation from an already evolved melt, and that more mafic, feldspar-bio-hbl-accessory mineral rocks trapped between mafic sheets in the mingling zone are the initial cumulates. Fractionated melt accumulated roofward (E) and laterally (N) and was the direct parent of the "evolved" cumulates. Fluids accumulated at the roof, marked now by miarolitic cavities in fractionated granite. SHRIMP analysis indicates that a majority of zircon growth (~final accumulation and solidification) occurred over a period of <~200,000 years (15.75-15.60 Ma). A persistent 16.2 Ma population of zircons in multiple samples suggests an earlier episode of solidification, likely represented by the marginal porphyry (dating in progress).