Geochemistry of Quaternary basaltic lavas in the Norikura area, central Japan: Influence of the subcontinental upper mantle on the trace elements and Sr isotope compositions

The trend of across-arc major- and trace-element compositions and 87Sr86Sr ratios for Quaternary lavas from the western part of central Japan are different from those from NE Japan. Basalts from the Norikura area, located at the backarc-side of the western part of central Japan are characterized by lower KZr and higher 87Sr86Sr (~0.705–0.706) than those of the backarc-side of NE Japan. Beneath the Norikura area, the Pacific plate is subducting at the depth where dehydration of phlogopite in the down-dragged peridotite occurs. Through dehydration, fluid is added to the mantle wedge which consists of a N-MORB-like source, triggering partial melting, and producing uprising mantle diapirs. These diapirs have high KZr and low 87Sr86Sr as indicated by basalts in the backarc-side of NE Japan. Subcontinental mantle materials which have low LILHFS and high 87Sr86Sr exist in the uppermost portions of the mantle wedge beneath the Norikura area, and interact geochemically with the uprising diapirs. The geochemical characteristics of the Norikura area may be attributed to the involvement of the subcontinental upper mantle.