GEOTECTONIC IMPORTANCE OF VERY FERTILE LHERZOLITE AMONG THE CIRCUM-PACIFIC OPHIOLITES WITH SPECIAL REFERENCE TO THE L-TYPE UST'-BELAYA OPHIOLITE IN NE RUSSIA

Recent geological-geochemical works of Dilek and Furnes (2009; Lithos, 113, 1-20) and Pearce and Robinson (2010; Gondwana Res., 18, 60-) convincingly demonstrate that the harzburgite(H)-type Tethyan ophiolites (Troodos, Oman, etc.) formed through subduction initiation and slab roll-back stages in the supra-subduction zone spreading settings analogous to those of the current Western Pacific marginal basins. Nearly forty years after the Troodos debate provoked by Miyashiro (1973; EPSL, 19, 218-), the marginal-basin origin of typical Tethyan ophiolites is likely to become a new consensus.

However, another distinct type of ophiolite is present in the West Mediterranean area; the lherzolite(L)-type Alpine-Ligurian ophiolite (Ishiwatari, 1985; EPSL, 76, 93-), which represents fragments of narrow oceanic lithosphere formed between the rifted continents. Its mantle section bears some very fertile lherzolite of subcontinental origin.

The Western Pacific marginal basins have long been thought as an analogue of the Mesozoic terrane accretion (including ophiolites) of North American Cordillera (e.g. Silver & Smith, 1983; Geology, 11, 198-), and analogous view may be applicable to other circum-Pacific areas such as Northeast Russia and Japan. Indeed, most circum-Pacific ophiolites are H-type of supra-subduction zone origin, but there are a few exceptions such as the Southwest Oregon peridotite, USA (Medaris, 1972; GSA Bull., 83, 41-) and Isabela ophiolite, Philippines (Andal et al. 2005; Isl. Arc, 14, 272-), that bear extremely fertile lherzolite containing Cpx with >1.5 wt.% Na₂O and highly aluminous spinel, resembling those of subcontinental lherzolite.

Our Japanese-Russian joint research has revealed that the Ust’-Belaya ophiolite (Devonian or older) in NE Russia with a huge lherzolitic mantle section (Sokolov et al. 2003; GSL Spec. Publ., 218, 619-) with troctolitic cumulate is associated with extremely fertile lherzolite. Although the ophiolite resides on an accretionary complex of oceanic origin, the presence of the Pekulney garnet metagabbro-ultramafic complex 50 km to the east (Ishiwatari et al. 2007; Isl. Arc, 16, 1-) proves the presence of thick crust. Pervasive occurrence of low-grade metamorphic minerals in the lherzolite and occasional occurrence of glaucophane schist in the ophiolite suggest later hydration and metamorphism above a subduction zone, but the occurrence of extremely fertile lherzolite suggests continental rift-zone affinity of this ophiolite.

We stress the importance of the L-type ophiolites with extremely fertile lherzolite as they suggest that the circum-Pacific ophiolites formed not only through subduction initiation of the pre-existing oceanic lithosphere but also through continental (margin) rifting as in the case of the West Mediterranean ophiolites. Early Miocene opening of the Japan Sea basin provides an example of continental margin rifting in the Western Pacific area. The continental fragments rifted in the Paleozoic and Mesozoic time may have mostly been lost by transcurrent movement or subduction.