An overview of world ophiolite studies by Japanese researchers

Abstract

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.