Structural and petrological constraints on the tectonic evolution of the garnet‐lherzolite facies Higashi‐akaishi peridotite body, Sanbagawa belt, SW Japan

The Higashi‐akaishi peridotite body in the Sanbagawa metamorphic belt is a unique example of a kilometer‐scale body including garnet‐lherzolite facies rocks from an oceanic subduction‐type orogen. In this body, four distinct deformational phases D 1 , D 2 , D 3 , and D 4 can be defined. The tectonic significance of D 1 is unclear. Microstructural observations and application of garnet‐orthopyroxene geothermobarometry suggest that D 2 took place during high‐temperature subduction up to depths of ∼100 km (from 2.3 GPa to above 2.8 GPa at temperature of 700–800°C) or more. D 3 represents a major phase of exhumation. After exhumation the Higashi‐akaishi body became juxtaposed with the adjacent Besshi and Eclogite units at a depth of around 35 km (1 GPa). The common history of the two units after D 3 is shown by the intertectonic growth of tremolite between D 3 and D 4 that corresponds to a stage of intertectonic growth of plagioclase in the Besshi and Eclogite units. Both the spatial distribution and the estimated pressure‐temperature (P‐T) conditions of this intertectonic phase of mineral growth in the different units are in excellent agreement. The development of an antigorite schistosity during the late stage of D 2 indicates an anticlockwise P‐T path with cooling at depths of around 100 km. The association of the antigorite‐bearing fabric and the onset of exhumation after the D 2 stage suggest that the density decrease due to antigorite formation may have been sufficient to trigger the exhumation of dense ultramafic body from UHP conditions in subduction zones. Syn‐D 2 water introduction and the anticlockwise P‐T path are expected when convection in the mantle wedge brings peridotite toward a subduction boundary through mantle wedge or when peridotite is dragged down along a subduction boundary that is progressively cooling immediately after initiation of subduction.