Abstract
Ueno basaltic rocks (2.7–1.6 Ma) and On-take volcanic rocks (≤0.75 Ma) crop out in a restricted area in central Japan and were analyzed for major and trace elements and Sr and Nd isotopes. The Ueno rocks were erupted as numerous monogenetic volcanoes largely of basaltic compositions, whereas the On-take lavas form a composite stratovolcano dominated by andesites. The Ueno basalts are primarily geochemical mixtures of two components; (1) sub-alkalic common basalts generated from the lithospheric subcontinental mantle with 87Sr/ 86Sr∼0.706, ε Nd∼+2 and K/Nb∼1000, and (2) rare within-plate basalts (WPB) of alkali basaltic composition with 87Sr/ 86Sr≤0.705, ε Nd≥+5, K/Nb≤180. Subordinate Ueno andesites were formed by assimilation-fractional crystallization processes within the lower crust having 87Sr/ 86Sr∼0.709, ε Nd∼−8, but without appreciable assimilation in the upper crust. On the other hand, the On-take basalts have 87Sr/ 86Sr∼0.705±0.0005, ε Nd∼+2±1, K/Nb≥1200 and geochemical signatures characteristic of continental arc 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 melting of the subcontinental mantle, forming the common Ueno basalt magmas. The arc-type basaltic magmatism, which was probably the result of subduction of the Pacific plate, occurred at ∼1 Ma after the Ueno volcanism. Addition of slab-derived components with 87Sr/ 86Sr<0.7045 and K/Nb>2300 modified the subcontinental mantle, forming the source of On-take basalts.
Published Version
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