Journal of Petrology, Volume 40, Issue 12: December 1999.

Polybaric Petrogenesis of Mafic Layers in the Horoman Peridotite Complex, Japan

E. TAKAZAWA¹, F. A. FREY¹, N. SHIMIZU², A. SAAL^1,2 AND M. OBATA³

¹MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
²WOODS HOLE OCEANOGRAPHIC INSTITUTION, WOODS HOLE, MA 02543, USA
³KYOTO UNIVERSITY, KYOTO, 606-8502, JAPAN

Two major types of mafic granulite layers occur within the Horoman peridotite, an 8 km * 10 km * 3 km orogenic lherzolite exposed in the high-T and low-P Hidaka metamorphic belt of Hokkaido, Japan. The mineral assemblages and textures of these layers reflect subsolidus reactions occurring during uplift from the upper mantle to the crust. Nevertheless, their whole-rock compositions can be used to infer the primary mineralogy of these layers, and a genetic relationship to melts geochemically similar to mid-ocean ridge basalts (MORB). The intralayer compositional variation of Type I layers (Al-Ti augite type mafic granulites) shows that the centers formed as garnet clinopyroxenites in equilibrium with an incompatible element depleted melt that crystallized to form the margins. In contrast, the Type II layers (Cr-diopside type mafic granulites) formed at relatively shallow depths and are much older, ~830 Ma, than the Type I garnet pyroxenites, which formed at ~80 Ma. The temporal sequence supports the hypothesis that the Horoman peridotite represents shallow MORB-related oceanic mantle that had subsided to deeper mantle depths before crustal emplacement.

Keywords: orogenic lherzolite;mafic layer;pyroxenite; trace element; isotope

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