SERPENTINIZATION AND METASOMATISM IN THE MANTLE WEDGE; A CASE STUDY IN THE UST'-BELAYA OPHIOLITE

The Ust’-Belaya ophiolite is exposed in the 80 km x 40 km area on the south of Ust’-Belaya (N65 30', E173 17’), Far East Russia (Sokolov et al., 2003, Geol. Soc. London, Spec. Publ., 218, 619-). The associated limestone suggests Devonian or older age of this ophiolite. It is an important character of this ophiolite that glaucophane-bearing rocks occur. Here we report the petrographical features and mineral chemistry of the peridotite from the Ust’-Belaya ophiolite and discuss about their metamorphism and metasomatism.

Mantle section of the Ust’-Belaya ophiolite is composed of fertile lherzolite to moderately depleted harzburgite. Those peridotites are characterized by significant hydration and low grade metamorphism, which causes formation of secondary olivine, secondary cpx, amphibole, chlorite, antigorite, and opaque minerals. They are divided into three major types on the basis of the mineral assemblage; (1) olivine + amphibole + chlorite +/- talc, with or without relict minerals, (2) olivine + antigorite + amphibole + chlorite and (3) olivine + antigorite + chlorite +/- secondary clinopyroxene. In some of antigorite-bearing peridotites, olivine shows an apparent “cleavage”. Basically secondary olivine occurs along with antigorite replacing primary olivine or amphibole replacing primary pyroxene. Such petrographical features resemble those of the antigorite-bearing serpentinite from Mariana forearc (Ohara & Ishii, 1998, Island Arc, 7, 541-; Murata et al., 2009, Geosphere, 5, 90-).

Amphiboles show different compositional trend corresponding to the mineral assemblage. Amphiboles in mineral assemblage (1) are calcic amphiboles, showing a pargasite/edenite-tremolite trend. On the other hand, amphiboles in mineral assemblage (2) show a richterite-tremolite trend with some pargasites.

Pargasite/edenite in the mineral assemblage (1) and richterite in the mineral assemblage (2) may be formed at relatively high temperature and low temperature, respectively. Several amphiboles in the mineral assemblage (2) shows zoning composed of pargasitic core, tremolitic mantle and rihiteritic rim. This zoning indicates a multiple-stage addition of Na₂O by fluid.

Relict minerals of the Ust’-Belaya peridotite do not resemble those of common forearc peridotite (Parkinson & Pearce, 1998, J. Petrol., 1577-) in terms of their degree of partial melting, suggested from Cr# (0.1-0.6) in spinel. On the other hand, they are similar to forearc peridotite in terms of their low equilibrium temperature suggested from Mg# in spinel and pervasive low-grade metamorphism as described above.

Glaucophane-bearing rocks, low equilibrium temperature of peridotites and the evidence of fluid-peridotite interaction are suggesting the Ust’-Belaya peridotite may represent a fragment of the Early Paleozoic forearc mantle wedge, which has been effectively cooled and metasomatized by H₂O-rich fluids released from the subducting slab. Absence of highly depleted peridotite suggests that expected subduction zone had been “cold subduction zone”