Geochemistry and origins of Ueno and On-take basaltic to andesitic rocks (< 3 Ma) produced by distinct contributions of subduction components, central Japan

Ueno basaltic rocks (2.7-1.6 Ma) and On-take volcanic rocks (less than or e qual to 0.75 Ma) crop out in a restricted area in central Japan and were an alyzed for major and trace elements and Sr and Nd isotopes. The Ueno rocks were erupted as numerous monogenetic volcanoes largely of basaltic composit ions, whereas the On-take lavas form a composite stratovolcano dominated by andesites. The Ueno basalts are primarily geochemical mixtures of two comp onents; (1) sub-alkalic common basalts generated from the lithospheric subc ontinental mantle with Sr-87/Sr-86 similar to 0.706, epsilon(Nd) similar to 1000, and rare within-plate basalts (WPB) of alkali basaltic composition w ith Sr-87/Sr-86 less than or equal to 0.705, epsilon(Nd) greater than or eq ual to + 5, K/Nb less than or equal to 180. Subordinate Ueno andesites were formed by assimilation-fractional crystallization processes within the low er crust having Sr-87/Sr-86 similar to 0.709, epsilon(Nd) similar to -8, bu t without appreciable assimilation in the upper crust, On the other hand, t he On-take basalts have Sr-87/Sr-86 similar to 0.705 +/- 0.0005, epsilon(Nd ) similar to + 2 +/- 1, K/Nb greater than or equal to 1200 and geochemical signatures characteristic of continental are basalts. The On-take andesites appear to have been contaminated frequently by upper crustal materials. It is proposed that the injection of a WPB-type diapir yielded partial meltin g of the subcontinental mantle, forming the common Ueno basalt magmas. The are-type basaltic magmatism, which was probably the result of subduction of the Pacific plate, occurred at similar to 1 Ma after the Ueno volcanism. A ddition of slab-derived components with Sr-87/Sr-86 < 0.7045 and K/Nb > 230 0 modified the subcontinental mantle, forming the source of On-take basalts . (C) 2000 Elsevier Science B.V. All rights reserved.