Transformation-induced strain localization in a Iherzolite mylonite from the Hidaka metamorphic belt of central Hokkaido, Japan

The Uenzaru peridotite complex in the northern part of the Hidaka metamorphic belt of central Hokkaido, Japan, contains a mylonitized, plagioclase Iherzolite. The plagioclase-Iherzolite mylonite consists of porphyroclasts of olivine, orthopyroxene, clinopyroxene and spinel, and fine-grained matrix of olivine, orthopyroxene, clinopyroxene, plagioclase and spinel. Symplectite composed of plagioclase, olivine and chromian spinel occurs around porphyroclasts of orthopyroxene, clinopyroxene and spinel. A tine-grained aggregate of plagioclase, olivine and chromian spinel also occupies the pressure shadows around porphyroclasts of orthopyroxene, clinopyroxene and spinel. The fine-grained aggregates in the pressure shadows laterally connect with each other to form layers, which characterize the mylonitic foliation. Symplectite with the same mineral assemblage as the fine-grained aggregate, a bulk rock chemistry of the mylonite close to that of pyrolite, and a reverse zoning of plagioclase such that the anorthite component increases toward its rim, indicate that the fine-grained aggregate is derived from the subsolidus phase-transformation reaction from spinel Iherzolite to plagioclase Iherzolite, but not from melt. The mylonitic foliation defined by layers of fine-grained aggregate implies that strain is localized into the reaction products. Therefore the phase-transformation reaction may have enhanced mylonitization of the Iherzolite. Compositional zoning of pyroxenes and plagioclase in the mylonite suggests an adiabatic decompression at temperatures above 960 degrees C within the spinel-lherzolite stability field, followed by a rapid isobaric cooling down below 760 degrees C at about 700-800 MPa. The Iherzolite which initially ascended as a mantle diapir was mylonitized through the phase-transformation reaction and thrusting onto crustal rocks, the contact with which resulted in a rapid cooling of the Iherzolite. (C) 1999 Elsevier Science B.V. All rights reserved.