HR: 16:45h
AN: V44C-04    [Abstracts]
TI: Evolution of Volcano-Plutonic Centers in the Northern Colorado River Extensional Corridor, Nevada-Arizona: Protracted Cycles of Replenishment, Mush Accumulation, Fractionation, and Melt Extraction
AU: * Miller, C F
EM: calvin.miller@vanderbilt.edu
AF: Dept. of Earth and Environmental Sci., Vanderbilt U., Nashville, TN 37235, United States
AU: Miller, J S
AF: Dept. of Geology, San Jose State U., San Jose, CA 95192, United States
AU: Claiborne, L L
AF: Dept. of Earth and Environmental Sci., Vanderbilt U., Nashville, TN 37235, United States
AU: Walker, B A
AF: Dept. of Earth and Environmental Sci., Vanderbilt U., Nashville, TN 37235, United States
AU: Walker, B A
AF: Dept. of Geosciences, Oregon State Univ., Corvallis, OR 97331-5506, United States
AU: Faulds, J E
AF: Bureau of Mines & Geology, U. of Nevada, Reno, NV 89557, United States
AU: Wooden, J L
AF: USGS-SUMAC, Stanford U., Stanford, CA 94305-2220, United States
AB: Three major magmatic systems were simultaneously active during early development of the northern Colorado River extensional corridor. These systems, centered on Spirit Mountain batholith, Searchlight pluton, and Aztec Wash and Nelson plutons, include hypabyssal intrusions as well as coarser, deeper-seated rocks emplaced at depths of 5-13 km. Erupted products of Searchlight are clearly exposed; connections between the other systems and extensive coeval volcanic sequences, while very likely, remain unverified. Intrusion at each center began at 17-18 Ma and terminated with a dike swarm at ca. 15.5 Ma. Dikes, sills, and evidence for mingling document the frequent replenishment suggested by longevity of the centers. Quenched magmas in pillows, dikes, and chilled margins indicate that input included trachybasalt (49- 52 wt pct SiO2), trachydacite (quartz monzonite; 62-65 wt pct SiO2), and low-Si rhyolite (granite; ca. 73 wt pct SiO2); similar magmas formed a large part of the regional volcanic sequence. Some of the basalt may represent juvenile magma from enriched mantle, but Sr and Nd isotopic data indicate that all other input magmas are hybrids with both juvenile and ancient crustal components. Although local mixing is evident from field and geochemical evidence, the system-wide hybridization occurred at deeper crustal levels prior to emplacement into the upper crust. Whole-rock elemental compositions, field relations, crystal-size distributions and textures within the volcanic rocks and co-genetic intrusions indicate repeated cycles of magma emplacement and extraction of fractionated melt from cumulate mush. Cumulates are enriched in Sr and Ba and have positive Eu anomalies relative to input magmas. The melt-rich extracts have high-silica rhyolite compositions. They are exposed in plutons as small dikes and large subhorizontal sheets and roof zones comprising fine-grained, commonly vesicle-rich aplitic granite, and they erupted from the Searchlight center and probably from the others. These rocks are extremely depleted in Sr, Ba, and Eu, and middle REE, low in light REE, P, Ti, and Zr, and enriched in Rb, reflecting fractionation of feldspars and accessory minerals. Their high SiO2 (77-79 wt pct) attests to fractionation at shallow levels, consistent with emplacement depths of the upper parts of the plutons. SHRIMP U-Pb ages and compositional zoning in zircon also indicate repeated cycles of growth from fractionating melts and recycling into less evolved melts. Most samples have two or more distinguishable age populations, and many individual grains show evidence for resorption. Compositions of zones indicate that grains experienced dramatic temperature fluctuations and were transferred from highly fractionated to unfractionated melts. Taken together, these data indicate that the intrusions formed and were modified by repeated (a) felsic replenishment that eventually formed thick crystal mush; (b) mafic replenishments that helped to maintain a thermal balance; (c) extraction of fractionated melt into local conduits or ponding zones, to the roof, or to erupt at the surface; (d) rejuvenation of the mush by the preceding processes, entraining and transporting crystals and blurring previous intrusive contacts.
DE: 1065 Major and trace element geochemistry
DE: 3618 Magma chamber processes (1036)
DE: 3640 Igneous petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting