Volcanism in nascent back-arc basins behind the Shichito Ridge and adjacent areas in the Izu-Ogasawara arc, northwest Pacific: evidence for mixing between E-type MORB and island arc magmas at the initiation of back-arc rifting

Abstract

Mineral chemistry, major and trace elements, and 87Sr/86Sr ratios are presented for 29 igneous rocks dredged from the northern portion of the Izu-Ogasawara arc. These rocks are compositionally bimodal. Basement gabbro and trondhjemite from the arc are extremely poor in K2O (0.05–0.19%) and Rb (0.48–0.62 ppm), and their REE patterns and Sr isotope ratios indicate that there are island arc tholeiites. Quaternary volcanic rocks from the present volcanic front (Shichito Ridge; active arc), back-arc seamounts (east side; inactive arc) and Torishima knoll between the two back-arc depressions (incipient back-arc basins) behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in K and Rb. Rhyolite pumice from the backarc depression is also the depleted island arc tholeiite, whereas basalts from the depression have compositions that are transitional between MORB and island arc tholeiites in trace element (Ti, Ni, Cr, V, Y and Zr) and mineral chemistries. The back-arc depression basalts have relatively high BaN/CeN(0.66–1.24), Cen/YbN(1.1–1.9) and K/Ba(45–105) and low 87Sr/86Sr (0.70302–0.70332) and Ba/Sr (0.1–0.2), which are similar to other back-arc basin basalts and E-type MORB, but are quite unlike the depleted island arc tholeiites. The diverse trace element and Sr isotope compositions of basalt-andesite from the back-arc depressions imply the interplay between E-type MORB and island arc tholeiite. These chemical characteristics and the relationships of (Ce/Yb)N vs (Ba/Ce)N and (Ce/Yb)N vs 87Sr/86Sr suggest that the back-arc depression magmas are generated by mixing of E-type MORB and depleted island arc tholeiite magmas. Geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust.