Petrology of Hayachine ultramafic complex in contact aureole, NE Japan.(II) metamorphism and origin of the complex.

Abstract

The thermal history imprinted on the Hayachine ultramafic complex in the Tono granodiorite aureole has been reconstructed. The contact aureole in the complex can be divided into four zones, from north to south, using critical mineral assemblages as follows: Zone 1 olivine+antigorite+clinopyroxene Zone 2 olivine+talc±tremolite Zone 3 olivine+anthophyllite±tremolite Zone 4 olivine+orthopyroxene±tremolite The mineral paragenesis indicates that the temperature reached at approximately 400°C in the north of the complex, and 750°C at the southern margin. Abundant carbonate minerals were formed in the northern marginal area of the zone 1. The mineral assemblages change (from the northern rim to the middle of the zone) from magnesite+talc, magnesite+talc+antigorite to magnesite+antigorite. The change should have been caused by decreasing in CO2 pressure. The original rock of the complex is probably a residual mantle material (harzburgite with dunite).