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

Abstract

[1] 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 pressuretemperature (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. Citation: Mizukami, T., and S. R. Wallis (2005), Structural and petrological constraints on the tectonic evolution of the garnet-lherzolite facies Higashi-akaishi peridotite body, Sanbagawa belt, SW Japan, Tectonics, 24, TC6012,

What CO 2 well gases tell us about the origin of noble gases in the mantle and their relationship to the atmosphere

“…These have been exhumed from the depths of approximately 120 km and originate through dewatering of the subducting slab and subsequent hydration reactions with the host mantle-wedge olivine (Mizukami & Wallis 2005). These inclusions contain atmosphere-derived noble gases in seawater proportions and halogen ratios indistinguishable from marine pore fluids.…”

Recognition of the ‘early’ Sambagawa metamorphism and a schematic cross-section of the Late-Cretaceous Sambagawa subduction zone

“…Speculated subduction-boundary geotherm at about 89 Ma Figs. 2b, 2d Ds Wallis, 1990Hara et al, 1992Wallis et al, 1992;Aoya, 2002;Mizukami and Wallis, 2005;Mori and Wallis, 2010 -3 Wallis and Ar-Ar Itaya and Takasugi, 1988;Takasu and Dallmeyer, 1990;Dallmeyer and Takasu, 1991;Itaya and Fukui, 1994 -91- Kabir and Takasu (2011) (for Seba pelite in SB), Endo and Tsuboi (2013) (for EI) and Weller et al (2015) (for SB and Kotsu). For Ota et al (2004), only P-T estimates obtained from mineral assemblages involving garnet, clinopyroxene and phengite are plotted.…”

“…Abbreviations of rock-body names other than KT (Kotsu) follow Fig.3. Petrologically derived P-T paths (Mizukami & Wallis (2005) for HA, Endo (2010) and Endo et al (2012) for WI, Aoya et al (2003) and Weller et al (2015) for KT, and Enami (1998) for the Shirataki unit are shown by bold arrows, and those derived using thermal model of Endo et al (2012) by dashed bold arrows. (a) 117-109 Ma.…”

Antigorite crystallographic preferred orientations in serpentinites from Japan

“…These results imply that in nature, static antigorite serpentinization precedes shear deformation. However, Nozaka (2005) and Mizukami and Wallis (2005) reported the direct formation of foliated antigorite serpentinite from the Happo-one ultramafic complex and the Higashi-Akaishi peridotite body, Japan. There is uncertainty regarding the formation mechanism and the processes that result in the serpentinization of peridotite.…”