Journal of the Faculty of Science, Hokkaido University. Series 4, Geology and mineralogy;Vol.XXI, No.2


Petrology of the Horoman Ultramafic Rocks in the Hidaka Metamorphic Belt, Hokkaido, Japan

Niida, Kiyoaki

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The Horoman ultramafic massif, covering more than 8×10 km in the Hidaka Metamorphic Belt, is an "alpine-type" peridotite which is a gently warped sheet about 3,000 m in thickness. The massif exhibits a conspicuous layered structure which consists of layers of dunite, lherzolite, plagioclase lherzolite, and a small amount of gabbro and pyroxenite. Coexisting minerals from all the rock types of the layered ultramafic to mafic sequence were analysed by EPMA. The ferromagnesian minerals show large compositional variations in accordance with lithological change in the series of dunite → lherzolite → plagioclase lherzolite → gabbro. Forsterite content of olivine varies successively from Fo92.5 to Fo64.5. Enstatite content of orthopyroxene also varies from En93.0 to En85.5. Large and continuous compositional variation was also obtained for clinopyroxenes, e.g. Ca46Mg51Fe3 from dunite, Ca48Mg48Fe4 from lherzolite, Ca49Mg46Fe5 from plagioclase lherzolite, Ca50Mg44Fe6 from the margin of gabbro, and Ca38Mg39Fe23 from the center of gabbro. Al and Ti contents of clinopyroxcncs and pargasitic amphiboles increase with decrease of the Mg/ Mg + Fe ratio. The mineralogical characteristics indicate that the Horoman layered sequence represents a magmatic series formed by fractional crystallization. The gabbroic seams in the plagioclase lherzolite and the layers of gabbro were probably formed by crystallization of residual liquid which were slightly alkaline in chemical nature. The Horoman ultramafic rocks are strongly modified by deep-seated deformation and recrystallization, and additionally by mylonitization during the up-thrusting intrusion into the Earth's crust. The primary composition of minerals, obtained by step-scanning EPMA analyses are recognized as a distinct and uniform compositional plateaus at the cores of large, porphyroclastic, primary grains. The equilibration temperatures were calculated for the orthopyroxene-clinopyroxene pairs of primary porphyroclasts, using the Opx-Cpx geothermometer. The temperatures range from 900℃ to 1,100℃. The estimates for the neoblastic pyroxene pairs are slightly lower, ranging between 850℃ and 1,000℃. The rocks might have re-equilibrated under the subsolidus conditions in the upper mantle.