Tectonic setting of high-Mg andesite magmatism in the SW Japan arc: K-Ar chronology of the Setouchi volcanic belt

Abstract

SUMMAR YThe Setouchi volcanic belt in SW Japan is characterized by the occurrence of andesites with an unusually high Mg concentration (high-Mg andesites, HMAs). Such HMAs may have formed under unusual tectonic settings, as basaltic magmas are produced dominantly in the mantle wedge of modern subduction zones. A total of 50 new K±Ar ages for the Setouchi rocks con®rmed that intermediate to felsic magmatism including the eruption of HMAs took place in this volcanic belt within a short period of 13.2t0.4 Ma. This is synchronous with the clockwise rotation of the SW Japan arc sliver that was associated with the opening of the Japan Sea back-arc basin. Such rotation of an arc sliver forced the initiation of subduction and partial melting of a young and hot lithosphere of the Shikoku Basin beneath the arc. Interaction between slab melts and mantle wedge peridotites may be a possible generation process of unusual HMA magmas. When the SW Japan arc sliver ceased to drift, the Setouchi magmatism was switched off by the cessation of subduction. The eruption of HMA magmas may also be attributed to an extensional stress ®eld in the Setouchi region, in contrast to the compressional stress regime documented for other regions of the SW Japan arc.

Thermal structure due to solid-state flow in the mantle wedge beneath arcs

“…(1) Partial melting of subducted metabasalt (or metagabbro) at eclogite facies conditions is inferred based on the similarity of trace element patterns in rare, primitive andesites with partial melts of eclogite [e.g., Defant and Kepezhinskas, 2001;Grove et al, 2001;Tatsumi et al, 2001;Yogodzinski et al, 2001;Rapp et al, 1999;Yogodzinski and Kelemen, 1998;Yogodzinski et al, 1995;Yogodzinski et al, 1994;;Defant and Drummond, 1990;Drummond and Defant, 1990;Kay, 1978]. Such lavas are primarily observed where subduction rates are slow, and/or the subducting oceanic crust is young (< 20 million years old), and/or the subducting plate terminates along strike allowing heating from the side as well as from the top [Yogodzinski et al, 2001;Defant and Drummond, 1990].…”

Rare earth elements compositions of Setouchi high Mg andesites (HMAs) and basalt; an implication for a mantle compositional shift beneath the SW Japan arc during the Japan Sea opening.

“…This rotational movement caused the young Philippine Sea plate subduction (15 ~ 26 Ma; Okino et al, 1994) and induced the Setouchi magmatism around 13 Ma (Tatsumi et al, 2001). It has been further suggested that the magmatism in the Osaka area preceded that in the Shodo-Shima area (Tatsumi et al, 2001).…”

“…The K-Ar ages and paleomagnetic data have revealed that the SW Japan arc sliver has rotated clockwise about 60 o (Otofuji et al, 1985(Otofuji et al, , 1991 in the late stage of the Japan Sea opening Tamaki, 1995). This rotational movement caused the young Philippine Sea plate subduction (15 ~ 26 Ma; Okino et al, 1994) and induced the Setouchi magmatism around 13 Ma (Tatsumi et al, 2001). It has been further suggested that the magmatism in the Osaka area preceded that in the Shodo-Shima area (Tatsumi et al, 2001).…”

“…This backarc basin was formed during the middle Miocene following the rapid rotational movement of the SW Japan arc sliver (Otofuji et al, 1985(Otofuji et al, , 1991Kaneoka et al, 1990Kaneoka et al, , 1992. This megatectonic event caused the young Philippine Sea plate subduction beneath the SW Japan arc to induce magmatism in the Setouchi area and Outer zone area in SW Japan (Shinjoe, 1997;Tatsumi et al, 2001).…”

Discovery of Ferropicrites and High-Magnesian Andesites From the Erdenetsogt Formation, Central Mongolia

“…It is widely accepted that most arc magmas are derived from hydrous melting of peridotites in the mantle wedge induced by fluids released from subducted oceanic crust or overlying sediments. However, the HMA may not be generated by steady subduction but only rarely by subduction of a young and hot oceanic slab or at the initiation of subduction [18,25,40,41]. 2) The first detailed geochemical and petrological study combined with field observations of the greenstones from the Hangay region revealed that the Hangay greenstones were generated in two distinct geotectonic settings and from two different magma sources: (1) deep-seated, mantleplume derived, high Fe-and Mg-tholeiitic greenstones that formed an oceanic plateau and (2) arc-derived calc-alkalic HMAs related to subduction of young oceanic plate.…”