Pleistocene intraplate magmatism in the Goto Islands, SW Japan: Implications for mantle source evolution and regional geodynamics

Abstract

We present geochemical, Sr, Nd and Pb isotopic data for the youngest back-arc tholeiitic and alkali basalts in the southern tip of Goto Islands along the Taiwan–Shinji Folded Belt in northwestern Kyushu, SW Japan. The data are compared with those more-or-less contemporaneous back-arc basalts elsewhere in the region and interpreted accordingly. Our sampling loci included Ukujima (ca. 1Ma), Ojikajima and Kamigoto (from 0.6Ma to 0.14) and six locations in the Fukue island area (ages between 0.7 and 0.02Ma). The Ukujima tholeiites show the highest SiO2 (ca. 52.5wt%), FeO* (13wt%), TiO2 (>2.5wt%) and lowest MgO (4wt%) and CaO (7.5wt%) contents whereas the alkali Kamigoto and most Ojikajima magmas show lower SiO2 (48wt%) and higher MgO (8.5wt%) and CaO (11wt%). The Ukujima tholeiites also show the least radiogenic lead with closely similar 87Sr/86Sr and 143Nd/144Nd ratios ranging between 0.7038 and 0.7042, and between 0.51280 and 0.51285, respectively. Mantle-normalized incompatible element distributions are of alkali basalt-type exhibiting, along with the major element, significant spatio-temporal variations. Regardless of such differences the younger basalts, as compared with older 6–15Ma eruptive products in Hirado (and those in Kita-Matsuura), northeastward along the belt, are the least radiogenic. The 15Ma volcanics were erupted during episodes of lithospheric extension and show effects of crustal contamination. The 6–9Ma tholeiites and alkali basalts appeared as regional extension gave way to compression and show hybrids of depleted mid-ocean ridge basalt-like (N-MORB) mantle and heterogeneous EM2 (enriched mantle type 2) (Uto et al., 2004). As extension diminished, deeper asthenospheric sources were tapped producing a range of enriched mantle type 1 (EM1)-contaminated N-MORB-like magmas in the younger (<1Ma) Ukujima, Ojikajima-Kamigoto and Fukue volcanoes. This composition is believed to present throughout East and Southeast Asian asthenosphere.