Mid-Pleistocene lavas from the Seguam volcanic center, central Aleutian arc: closed-system fractional crystallization of a basalt to rhyodacite eruptive suite

Petrogenesis of the early Eocene I-type granites in west Yingjiang (SW Yunnan) and its implication for the eastern extension of the Gangdese batholiths

“…Several samples contain amphibole and can be classified as amphibole-rich calc-alkaline granites. These rocks can be produced by the fractional crystallization of a mantle-derived low-K basaltic magma (e.g., Singer et al, 1992;Barth et al, 1995). Our samples show a decrease of FeO t , MgO, Al 2 O 3 , CaO, Sr, Eu/Eu* and Dy/Yb with an increase in SiO 2 , suggesting significant fractional crystallization of plagioclase, K-feldspar, hornblende and biotite during magma evolution (Figs.…”

Long‐term volumetric eruption rates and magma budgets

“…Closed magmatic systems which exchange heat but little material with their surroundings (i.e., neither assimilation nor recharge is important) may be rather uncommon. What is more likely is that specific systems may behave as closed systems for restricted portions of their history [e.g., Singer et al, 1992;Zielinski and Frey, 1970]. It is important to note, however, for the olivine basalt-trachyte series at Gough Island where fractional crystallization appears dominant, Pb and Sr isotopic data indicates that assimilation of hydrothermally altered country rock and/or recharge of isotopically distinct magma has taken place [Oversby and Gast, 1970].…”

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

“…18 O plag data from Seguam lavas combined with a comparison of the modal mineralogy to published low pressure cotectics suggest that basaltic parental magmas crystallized at 3-5 kb, or about 10-15 km depth, followed by closed-system differentiation to dacite and rhyolite between 1-2 kb (i.e., crustal depths of~3-6 km) [14,28].…”

“…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].…”

“…However, a suite of young lavas and ash may fail to capture the full extent of erupted compositions, magmatic processes and growth rates that characterize large arc volcanoes. Where geochronologic control is good, it is clear that monotonic chemical evolution is rare, and that many abrupt or gradual shifts in composition, processes (e.g., crystallization vs. mixing vs. assimilation), and growth rate are expected to occur over periods of 10 4 to 10 5 yrs at arc volcanoes (e.g., Mt Adams, [12]; Tatara San Pedro, [13]; and Seguam, [14]). Variations of Th isotope composition over these relatively long time periods offer a potentially powerful means of distinguishing shifts between open-and closed-system magmatic processes and constraining changes in magma ascent and crystallization rates [6,[15][16][17].…”