Origin of eclogitic metagabbro mass in the Sambagawa belt: Geological and geochemical constraints

Eclogite-bearing metagabbro masses in the high-P/T Sambagawa metamorphic belt in SW Japan have long been regarded as tectonic blocks derived from the mantle wedge or hanging-wall side of the former Sambagawa subduction zone. The origin of one of the metagabbro masses (the Seba metagabbro) is reexamined by: (i) geological (chemico-structural) study focusing on the relationship between this body and the surrounding metasedimentary schists; and (ii) comparative major- and trace-element bulk-chemistry with other gabbros/metagabbros from the world. The geological studies reveal the presence of a marginal shear zone that: (i) is located between the enveloping pelitic schist and the central metagabbro; (ii) has a bulk chemical composition intermediate between the two; and (iii) shows up to 10 cm-scale prominent mafic–felsic banding. Based on the difficulty of forming such marginal shear zones by a tectonic block hypotheses, and key field observations in neighboring regions indicating formation of the marginal shear zone as sediments, the Seba metagabbro is concluded to have originated on the ocean floor as an olistolith. Comparative bulk-chemical studies show that Sr content of the Seba metagabbro is too high for gabbros derived from fracture zones associated with a spreading ridge. A comparison with other eclogitic metagabbro masses in the Sanbagawa belt reveals numerous similarities with the Seba metagabbro. This suggests all the metagabbro masses can be regarded as members of an ‘olistostrome complex’ that subducted and underwent the eclogite-facies metamorphism. The eclogitic complex includes the Western Iratsu mass that is considered to be an ancient seamount. This suggests that the metagabbro masses represent fragments of the ancient Western Iratsu seamount.