Petrogenesis of ultramafic rocks from the Miocene Setogawa Group, Shizuoka, central Japan: insights into the Philippine Sea Plate mantle

Ultramafic rocks occur as tectonic blocks of varying sizes in the northern part of the Izu Massif, situated roughly parallel to the Philippine Sea Plate boundary with the Eurasian and North American Plates in Japan. These ultramafic rocks likely originated from the uppermost mantle, influenced by magmatic processes associated with the Izu-Bonin-Mariana (IBM) intraoceanic arc and back-arc basin development within the Philippine Sea Plate. This study focuses on representative ultramafic rocks from the Setogawa Group, a Palaeogene accretionary complex. Petrographic observations and whole-rock chemistry classify the rocks as harzburgite-lherzolite, dunite, and clinopyroxenite. Harzburgite-Lherzolite samples contain minor, but generally saussuritized plagioclase, interpreted as additions to the residual mantle after variable degrees of partial melting. These additions likely resulted from the infiltration of melts with compositions similar to the melt forming gabbros recovered from the IBM arc. The Setogawa harzburgite-lherzolite exhibit similarities to those observed in back-arc basins of the Philippine Sea Plate. Dunite with high-Cr# spinel (Cr/(Cr+Al) atomic ratio >0.7) occurs with chromitite layers containing medium Cr# spinel (~0.5). Sharp contacts are evident between the dunite and surrounding harzburgite. The high-Cr# spinel-bearing dunite may have formed through the reaction of pyroxene-bearing peridotite (harzburgite) with arc-derived magmas. The primitive mantle-normalized trace element patterns of estimated melt compositions from clinopyroxene in clinopyroxenite resemble geochemical features associated with arc-related magmatism, akin to the frontal volcanic rocks of the IBM arc. In conclusion, the Setogawa ultramafic rocks represent a portion of the upper mantle significantly influenced by a wide range of arc-related magmatism. These rocks are interpreted as a northern extension of the IBM arc-back arc system. Arc-related magmatic processes can generate pyroxene-rich ultramafic rocks, which contribute, at least in part, to the low Vp observed in the uppermost mantle beneath the thick arc crust of the IBM intraoceanic arc.