Geochemistry of the quaternary volcanic rocks of the northeast Japan arc

Abstract

Major and trace-element data are presented for a series of lavas from 17 volcanic centres in the NE Japan arc. These represent a transect of the Quaternary arc from its volcanic front (type A volcanoes) in the east, across its central zone (type B) to its western margin (type C). Rocks range from basalt to dacite in composition and the variation is attributed to fractionation of plagioclase, mafic silicates and titanomagnetite. For comparative purposes trace-element data for each volcano are presented on a 55 wt.% SiO 2 normalised basis. The volcanoes display the characteristic features of arc volcanism viz enrichment in large ion lithophile elements (Rb, Sr, Ba, K) and depletion in high field strength elements (Zr, Nb, Hf, Ta). Using these data and derived primary magma compositions (Tatsumi et al., 1983), mixing calculations give the percentages of fractionating mineral phases necessary to derive the 55 wt.% SiO 2 liquids. These data together with published distribution coefficients allow the calculation of trace-element abundances in the primary magmas. Reciprocal trace-element plots indicate that irrespective of position with respect to the volcanic front, the elements, La, Ba, K, Sr, Nb, Y and Zr of the primary magmas all lie on a common line strongly suggesting that the melts are the products of varying degrees of melting of a common homogeneous source. However, those melts close to the volcanic front (type A) show anomalous Rb, Th and Pb values suggesting that their mantle source was enriched in these elements. Further calculations suggest that the HFS depletion is not due to retention by residual mineral phases but is a feature of the source. Thus the data suggest: (a) that the source of the arc volcanism was homogeneous; and (b) that the characteristic chemical features were an inherent feature of the source. Since Zr/Nb ratios in the source melts are close to those found in N-type MORB it is suggested that a major component of arc-volcanic source rocks was a depleted mantle of the type which has given melts of N-type MORB composition. LIL elements added to such a depleted source are derived by the loss of silica-rich aqueous fluids from the descending (subducting) slab. Such losses occur within the first 100 km and are fixed in the overlying mantle wedge. This contaminated material is transported down by drag-induced convection, producing a homogeneous, but metasomatised mantle within the melting zone. Diapiric uprise produced by small-scale melting, produces further melting, with the major control of melt chemistry being the degree of partial melting. Superimposed on this chemistry is a localised addition of Pb, Rb and Th in the source of those volcanoes nearest the volcanic front.