HR: 1330h
AN: V12D-0615    [PDF]
TI: Extreme Magma Differentiation in a Hawaiian Magma Chamber: An Analysis of Gabbro and Syenite Xenoliths From Hualalai Volcano
AU: * Shamberger, P J
EM: shamberg@hawaii.edu
AF: University of Hawaii - Manoa, 1680 East West Rd., Honolulu, HI 96822 United States
AU: Hammer, J E
EM: jhammer@soest.hawaii.edu
AF: University of Hawaii - Manoa, 1680 East West Rd., Honolulu, HI 96822 United States
AB: Large volumes of highly differentiated trachyte magma erupted from Hualalai Volcano 103$\pm$11 k.y.a. as a $>$275m thick flow on the north flank, a large pumice and obsidian cone, lavas at the NW tip of the main rift zone, and as blocks in a maar deposit. The principle flow is recognized as the largest single eruptive event on the island of Hawaii (Moore, et al., 1987). This episode contradicts previous notions of Hawaiian evolution, which generally associate highly evolved magmas with the conclusion of the post-shield alkalic stage (e.g., West Maui volcano). Hualalai differs from this pattern in that the evolved magmas appeared at the beginning of the post-shield alkalic stage and were erupted over a relatively narrow time interval. These distinctions have significant implications for our understanding of Hawaiian volcanoes' magmatic plumbing systems, including the depths, magma replenishment and extraction frequencies, and longevity of the reservoirs that feed eruptions, especially in the transitory interval between the tholeiitic shield and alkalic post-shield stages. Gabbro and syenite nodules erupted $<$10 k.y.a. from Hualalai's summit vents may represent cumulates, residual liquids, and/or crystallized magmas associated with trachyte differentiation, and thus provide insight into the processes leading to extreme fractionation. Because they contain a large number of phases and preserve reaction relationships in the form of mineral textures, the crystalline nodules may incorporate more information about magma differentiation conditions than can be extracted from the relatively homogeneous and micro-crystalline trachyte itself. The nodules were transported to the surface in alkali basalt at numerous vents and were found distributed in spatter and tephra deposits near the summit of the volcano over a region several km in diameter. They span broad compositional and textural spectra. Modes range from 58-93 vol% felsic minerals, dominantly plagioclase, with augite and Ca-poor pyroxene comprising most of the balance. Several nodules contain the hydrous minerals biotite and calcic amphibole. Accessory phases include ilmenite, magnetite, apatite, zircon, $\pm$ olivine or quartz. In many instances, the major minerals are clearly in reaction relationship with their surroundings. E.g., blebs of alkali feldspar dispersed throughout large plagioclase phenocrysts suggest the existence of precursor ternary feldspar; crystallographically-controlled oxide stringers along pyroxene planes suggest oxidation. We are considering deposit characteristics, mineral assemblages, and phase compositions of the basalt-hosted xenoliths to infer the depth and H$_{2}$O content of magma fractionation.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
DE: 8400 VOLCANOLOGY
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting