Mid‐Pleistocene basalt from the Seguam Volcanic Center, central Aleutian Arc, Alaska: Local lithospheric structures and source variability in the Aleutian Arc

Abstract

Recent geophysical and tectonic studies make it possible to examine the relation between local structural and tectonic evolution of the modern Aleutian Arc and the sources and processes that produce the broad spectrum of Aleutian basaltic lavas. Mid‐Pleistocene basalt from the Seguam Island volcanic center in the central Aleutian Arc erupted through strongly extended arc crust, contains 4–8% MgO, is low in K2O (<0.5%), high in Al2O3 (>18%), and is distinguished from Aleutian basalt erupted through less extended crust by a low abundance of incompatible and rare earth elements (REE) and low Sr and Pb content. The Ba/La (47–72) and Pb/La (0.7–1.5) ratios are high, incompatible element ratios are variable, and REE patterns cross one another. The 87Sr/86Sr ratio (0.70365 ± 0.00003) is the highest value obtained from the Aleutian islands, also 206Pb/204Pb (18.91 ±0.02), 207Pb/204Pb (15.60 ±0.02), and 208Pb/204Pb (38.51 ±0.04) ratios are among the most radiogenic values known, whereas the 143Nd/144Nd ratio (0.51297 ±0.00003) is among the lowest measured. Low abundances of incompatible elements, high Ba/La and Pb/La ratios, uniform Sr, Nd, and Pb isotope ratios, and high eruptive temperatures (∼1160° C) suggest that basalt erupted at Seguam ascends through extended subarc lithosphere without extensive chemical exchange with the upper mantle or lower crust. Partial melting of geochemically distinct portions of an isotopically uniform source region best explains observed geochemical characteristics. Trace element abundances rule out an eclogite source and suggest instead a peridotite‐dominated source with ∼5–10% of a subducted sediment component. This peridotite may reside in the mantle wedge or in the Amlia Fracture Zone (AFZ) of the Pacific plate that was subducted beneath Seguam ∼1 Ma. In addition, the AFZ possibly enhanced the subduction of terrigenous sediment into the source region beneath Seguam and this extra terrigenous component may explain the Ba/La, Pb/La and Sr, Nd, and Pb isotope ratios. The distinctive composition of Seguam basalt reflects a complex set of physical conditions, including fracture zone subduction and intraarc extension, that affected both the source region and thermo‐chemical history of magma ascent. Local structural and tectonic factors impact profoundly the nature of island arc magma sources and processes. Models that propose a common parental arc magma type or common processes on a global or arcwide scale are too generalized to explain the diversity actually observed in the Aleutian Arc.

Extension-related origin of magmas from a garnet-bearing source in the Los Tuxtlas volcanic field, Mexico

“…The island arcs represented in this database are: Aleutian (Kay et al 1982;Brophy 1986;Nye and Reid 1986;Romick et al 1990;Singer et al 1992;Myers et al 1985Myers et al , 2002; Burma (Stephenson and Marshall 1984); Izu-Bonin (Tatsumi et al 1992;Taylor and Nesbitt 1998); Japan (Sakuyama and Nesbitt 1986;Togashi et al 1992;Edwards et al 1994;Tamura 1994;Kita et al 2001); Kamchatka (Kepezhinskas et al 1997); Kuril (Zhuravlev et al 1987;Nakagawa et al 2002); Lesser Antilles (Arculus 1976;Brown et al 1977;Thirlwall and Graham 1984;Devine 1995;Smith et al 1996;Thirlwall et al 1997;Defant et al 2001); Luzon (Defant et al 1991a); Mariana (Hole et al 1984;Woodhead 1988;Bloomer et al 1989;Elliott et al 1997); New Britain (Woodhead and Johnson 1993); New Hebrides (Dupuy et al 1982); Papua-New Guinea (Hegner and Smith 1992); Philippines (Knittel et al 1997;Defant et al 1989;Bau and Knittel 1993); Sangihe (Tatsumi et al 1991); South Shetland (Smellie 1983); Sunda-Banda (Whitford et al 1979;…”

Subduction controls of Hf and Nd isotopes in lavas of the Aleutian island arc

“…3B). This westward increase results from the westward-thickening wedge of trench sediment that is banked against the east-facing escarpment formed by the Amlia Fracture Zone on the subducting Pacific Plate near Seguam (Scholl, et al, 1982, Singer, et al, 1992. West of Seguam, the sediment flux decreases systematically along the arc to approximately 20 m 3 /m/yr in the Komandorsky region (Fig.…”

Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc

“…In addition to the petrologic, isotopic, and thermal constraints on the plumbing system beneath Seguam [14,[26][27][28], InSAR imagery was used to reconstruct surface deformation of the island from 1992 to 2000 [30,31]. The complex syn-and post-eruptive inflation and deflation pattern associated with the 1993 basalt eruption is best explained as a result of transient magma and vapor fluxes into and through a shallow (b 7 km deep) plexus of dikes, driven from below by input of basaltic magma residing below 7 km [30].…”

“…1) [14]. Seguam was chosen for this study for the following reasons: 1) Compositions span a continuous range from 50 to 71 wt.% SiO 2 , with an unusually large proportion, about 30%, of the island comprising evolved dacitic and rhyolitic compositions [14], 2) the petrologic, geochemical, radiogenic (Sr-Nd-HfPb) isotope and O isotope characteristics of the erupted products are well-known [14,[26][27][28], 3) the entire eruptive history of the island has been mapped and is now exceptionally well-documented [29], 4) models of surface deformation based on pre-, syn-, and posteruptive magma and fluid migration through an upper crustal plumbing system have been developed from 10 years of interferometric synthetic aperture radar (InSAR) imagery [30,31], 5) geochemical data indicate that Seguam lavas reflect greater fluid and U-enrichments than other Aleutian volcanoes [32,33], and 6) the crustal structure is known from nearby seismic reflection and refraction profiles which indicate that the volcanic complex sits atop 25-30 km of arc crust [34]. An overall mafic composition for the crust, which comprises: porous or fractured extrusive and intrusive igneous rocks and volcaniclastic sediments in the upper 7 km, a MORB-like layer in the middle crust, and~19 km of gabbroic residua at the base, is inferred from the P-wave velocity structures [34].…”