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Singer, B.S., 1990

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Singer, B.S., 1990, Petrology and geochemistry of mid-Pleistocene lavas from Seguam Island, central Aleutian Islands, Alaska: Implications for the chemical and physical evolution of oceanic island arc magmatic centers: University of Wyoming, Laramie, Ph.D. dissertation, 188 p.


Theoretical, field, and laboratory investigations examine the origin and evolution of mid-Pleistocene magmas erupted from the central Aleutian volcanic center of Seguam Island. Thermal modeling demonstrates that repeated magma ascent and heating of the lithosphere is a dynamic process, which over the lifetime of a volcanic center may alter the crystallization and hybridization history of successive eruptive products. Superimposed on this thermal and chemical evolution, intra-arc extension has modified the thermal and mechanical properties of the central Aleutian lithosphere. K-Ar dates verify that voluminous tholeiitic basalt and basaltic andesite and subordinate andesite and dacite lavas erupted 1.07 to 0.07 Ma. Mineralogic evidence for closed-system equilibrium crystallization, low abundances of K2O, Na2O, Rb, Ba, Sr, and LREE, and uniform but anomalously high 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios and remarkably low 143Nd/144Nd ratios indicate that Seguam basalt is unlike other Aleutian basalts. Seguam's geochemical uniqueness corresponds to its unusual position atop strongly extended arc crust. Extended and presumably hotter lithosphere allowed basaltic magmas to crystallize under closed-system equilibrium conditions at ca. 1,200o C and <8 kb and to erupt readily at Seguam. Extensive plag + cpx + mt +/- ol +/- opx fractionation at 1,200-1,000o C and 1-4 kb produced the evolved lavas. In contrast, basalts from adjacent volcanic centers on unextended and presumably cooler lithosphere crystallized under disequilibrium conditions, at higher pressures, assimilated lower crustal and upper mantle rocks, and rarely erupted. The source of central Aleutian basalts may reside in the mantle wedge or subducted crust. Seguam basalt compositions cannot be derived by eclogite melting, but are consistent with partial melting of depleted peridotite plus a small fraction of subducted sediment. Along arc variations in the composition of the mantle wedge (proportion of fertile/depleted peridotite) or the subducted crust (fracture zone peridotite beneath Seguam, eclogite beneath Adak and Atka) may explain geochemical differences between these centers. Superimposed on this potential source variability are contrasting styles of magma ascent, such as open vs. closed system evolution, which may be facilitated by differing degrees of intra arc extension.

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