The mantle wedge is an important locus for material recycling, magma generation, and fluid transportation from the slab, and should therefore be thoroughly examined to better understand these processes. Peridotite xenoliths transported to the surface by arc magmas, or by other magmas, may be representative of the upper part (lithosphere) of the mantle wedge. Fore-arc peridotites exposed on the seafloor also represent the uppermost part of the mantle wedge. We summarize their modal composition, mineral chemistry, equilibrium temperature, and redox state, and discuss the implications for mantle-wedge processes. The arc peridotites are thought to derive mainly from the spinel to plagioclase-peridotite stability fields. They are varied in character, depending on their history as well as the tectonic setting (e.g., fore-arc, volcanic front, and back-arc) of their source regions. Some arc peridotites, especially those from the fore-arc to the volcanic front, are harzburgites and contain high-Mg olivine and high-Cr spinel, with high degrees of partial melting. They also show metasomatism, silica enrichment (i.e., formation of secondary orthopyroxene at the expense of olivine), and hydration (i.e., precipitation of Ca-amphiboles and/or phlogopites). The presence of tremolite, which is indicative of low temperatures and/or depleted (Al-poor) chemistry, is characteristic of sub-arc mantle peridotites. The equilibrium temperature is relatively low (＜1100℃) with the exception of the Noyamadake peridotites, SW Japan arc, which are characterized by high temperatures (～1200℃). Some peridotites from the Western Pacific show high oxygen fugacities relative to abyssal peridotite (although a few peridotites show relatively low oxygen fugacities or contain secondary veins composed of highly reduced minerals such as metals and alloys). This indicates the importance of local reducing agents in the mantle wedge.
This paper critically reviews pressure estimation methods for spinel peridotite xenoliths that are frequently included in Phanerozoic alkali basalts. Geobarometries used for pressure ranges in the stability field of spinel peridotites are critically examined and problems clarified. A guide to the solutions to these problems and methods for improving the reliability and accuracy of geobarometers for spinel peridotites are presented. Methods for the validation of estimated pressures are proposed, based on estimates of the time taken for the transport of xenoliths from depth to the surface.
The last quarter-century has been a period of worldwide study of ophiolites by Japanese geologists, as described in this paper. The Oman Mountains expose the world largest and best preserved ophiolite that provides insights into crustal and mantle processes below a fast-spreading system and transformation of oceanic lithosphere to subarc crust and mantle. The Mirdita Ophiolite exposes mantle peridotites covered by mid-ocean ridge basalt (MORB) and arc tholeiitic to boninitic lavas, recording the transition from a spreading to a subduction environment. The Luobusa Ophiolite is well-known for its ultrahigh-pressure minerals such as coesite and micro-diamond inclusions in chromite, considered to represent recycling of subducted slab deep into the mantle at >380 km depth. The 5-3 Ma ophiolites in the Timor and Tanimbar Islands are the world's youngest, and are considered to have formed the forearc crust and mantle that collided with and obducted onto the northern edge of the Australian continent. Volcanic rocks have geochemical characteristics that are intermediate between arc tholeiite and MORB, although the mantle peridotites are not cognate with the overlying volcanic rocks. Another young ophiolite (6-5 Ma) of Taitao, southern Chile, was part of the eastern limb of the Chile Ridge, which was subducted and accreted in an accretionary complex along the western coast of Chile. In spite of their mid-ocean-ridge origin, the lavas and sheeted dikes of ophiolite have geochemical characteristics typical of arc magmas. Isua and Pilbara contain the world's oldest accretionary complexes consisting of superposed slices of oceanic crust that form a duplex structure, indicating the beginning of plate tectonics in the early Archean.
All the granitoids in the Japanese Islands are Phanerozoic and of arc-type. They are part of the Late Mesozoic Circum-Pacific granite superprovince. Most of the Japanese granitoids were formed when the Japanese Islands belonged to the Eurasian continent, as the growing front of the continent. They are mostly of I-type, and S-type granitoids are very small in amount. The origin of these granitoids is mostly partial melt of mantle-derived mafic lower crust of arc without an involvement of ancient cratons or their derivatives.
The granitic magmatism was quite episodic. 80% of their surface exposure area is occupied with 50-130Ma, Paleogene to Cretaceous granitoids. In Southwest Japan, they constitute three arc-parallel granitic provinces called Ryoke, San-yo and San-in belts. A transect from the Ryoke to San-yo belt represents the hypothetical crustal cross section of the Cretaceous Eurasian continental margin. The Hidaka belt in Hokkaido is another example of a crustal cross section, exposing the deep Kuril arc at Miocene. On the fore-arc side of the Southwest Japan, 13-15Ma, Middle Miocene granitic rocks are exposed sporadically but widely. The magmatism was very short-lived, supposed to be generated in an unusual tectonic setting related to back-arc opening and incipient subduction of the Philippine Sea plate. Middle Miocene and still younger granitoids are exposed in the Izu Collision Zone. The Quaternary granitoids of ~1Ma are exposed at the Central Highlands in central Japan. Jurassic and Triassic granitoids occur in the Hida belt, which is the most back-arc side unit of the Japanese Islands. Paleozoic granitoids are rare. They are exposed as geologically isolated small bodies or tectonic blocks.