Primary magmas at thevolcanic front of the NE Japan arc: Coeval eruption of crustal low-K tholeiitic and mantle-derived medium-K calc-alkaline basalts at Azumavolcano

Abstract

It has long been thought that the low-K tholeiitic (TH) and medium-K calc-alkaline (CA) lavas in the NE Japan arc were produced by fractional crystallization from mantle-derived basalt magmas, and that the latter formed from mixing of mafic and felsic magmas, both derivedfrom a common primaryTH basalt through fractionation. An alternative view was recently proposed on the basis of Sr isotope microanalysis of plagioclase phenocrysts from the Zao volcano, suggesting that (1) the low-K TH basaltic andesites formed by melting of lower crustal amphibolite and that (2) the medium-K CA basalts to andesites formed by mixing of mantle-derived basalt and crustal TH melts. To investigate further the origin of the 'primary' low-K TH and medium-K CA basalts, we investigated basalts and andesites from Azuma volcano. Azuma is a Quaternary eruption center at the volcanic front in the NE Japan arc that has erupted two types of basalt: (1) radiogenic-Sr (87Sr/ 86Sr=0.7058-0.7062) low-K TH basalt lavas without evidence of magma mixing and assimilation; (2) unradiogenic-Sr (87Sr/86Sr=0.7039-0.7041) medium-K CA basalt lavas with subtle evidence for magma mixing. Associated intermediate lavas are voluminous and are all (3) mildly radiogenic-Sr (87Sr/86Sr=0.7044-0.7055) medium-K andesites, all ofwhich have CA affinities with evidence for rigorous magma mixing but no crustal assimilation. The low-K TH basalt has an isotopic composition similar to that of crustal granitoids beneathAzuma and has a composition indicating that it potentially formed from a high-degree lower crustal amphibolite melt. The medium-KCA basalt has a basaltic groundmass with Mg-rich olivine (Fo89) and calcic plagioclase phenocrysts (An90) and the most unradiogenic Sr (87Sr/ 86Sr=0.7037-0.7038), suggesting that it originated from a primary mantle melt. Major and trace elementmicroanalysis of the basaltic groundmass indicates that the primary magma composition iscloseto high-K. We conclude that the mantle-derived basalt at Azuma is the result of a high- to medium-K magma that was later mixed with a low-K TH basalt melt from the amphibolitic lower crust to form medium-K CA basalts and andesites. This supports the view of a lower crustal origin of the low-KTH basalts and simultaneously requires a reappraisal of the origin of theacross-arc variation in K contents of the mantle-derived primary arc basalts, as the high- to medium-K CA basalt is geochemically fairly similar to the high-K rear-arc basalt in the NE Japan arc. © The Author 2012. Published by Oxford University Press. All rights reserved.