Petrogenesis of the Higashi-Akaishi Peridotite Body, Japan

The Higashi-Akaishi (HA) Peridotite is an ultramafic body in the Besshi region of the Sanbagawa metamorphic belt in Japan. Dunite is the dominant lithology with minor occurrences of garnet pyroxenite and harzburgite found as lenses within foliated dunite. New pressure-temperature estimates using garnet-orthopyroxene geobarometry and two-pyroxene thermometry suggests the garnet pyroxenite records peak pressure-temperature conditions of 1.8-2.6 GPa and 609-713 ºC, which are on average lower than those reported by Enami et al. (2004). Previous studies proposed the HA body protolith to be (1) a lower crustal cumulate, (2) part of the mantle wedge, or (3) remnants of a subducted oceanic plateau. Our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a mantle wedge origin for the dunite and pyroxenite in the HA. New ¹⁴³Nd/¹⁴⁴Nd vs. ⁸⁷Sr/⁸⁶Sr data support a mantle wedge origin of garnet pyroxenite and dunite but suggest that these lithologies do not represent residues of melting. Field and petrographic observations of monomineralic and bimineralic layering in garnet pyroxenite and chromite layers within dunite are characteristic of cumulates. The strong Nb and Zr depletions and low Ni and Cr concentrations in the bulk rocks also support a sub-arc cumulate origin for the dunite and pyroxenite. We propose the garnet pyroxenite and dunite studied here represent sub-arc cumulates, which have been subsequently metamorphosed along a hot subduction path, reaching their peak pressure-temperature conditions 2.6 GPa and 713 ºC, and rapidly exhumed without significant retrograde metamorphism. The presence of high pressure hydrous minerals such as serpentine and chlorite and bulk trace element compositions indicate that fluids and/or fluid-rich melts played a role in both the initial formation of the sub-arc cumulate protolith as well as during the subduction and metamorphic phase of the HA's history. Thus, the HA represents another natural record of sub-arc geochemical exchange and melting processes, like Kohistan, and Talkeetna, among others, which are essential to understanding processes occurring beneath arc volcanoes and within the mantle wedge.

Enami et al., J. Metamorphic Geol., 22, 1-15 (2004).