PRESERVATION OF FOUNDERED LOWER CRUSTAL CUMULATES IN THE HIGASHI-AKAISHI ULTRAMAFIC BODY, JAPAN

Recycling of ultramafic lower crustal cumulates via delamination or foundering is linked to continental crust formation, as a mechanism to return mafic material to the mantle. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths and the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan. Dunite is the dominant lithology with garnet pyroxenite found as lenses. Although the petrogenetic history of the HA body is still debated, our new bulk major and trace element compositions, radiogenic isotope data, as well as petrologic and field observations, are consistent with a lower crustal cumulate origin for the HA dunite and pyroxenite, with a later slab-derived fluid overprint. Clinopyroxene and olivine in the foliated HA dunite have compositions consistent with ultramafic cumulates with high Mg#s (Mg# cpx = 0.94, Mg# oliv = 0.88), high NiO in olivine (~0.26 wt%) and low-Al clinopyroxene. In addition, the bulk major element chemistry of the HA dunite and garnet pyroxenite follow systematic behavior in Mg# vs. SiO₂ wt%, similar to those observed in other lower crustal cumulate lithologies. Our new thermobarometry (peak P-T at 2.6 GPa, 713ºC) is consistent with a hot slab surface subduction path rather than lower crustal temperatures (Kohistan & Talkeetna: 1 GPa, 800ºC). Slab-fluid influence is also indicated in the HA lithologies by LREE & Ce enrichments, and strong Nb and Zr depletions. Both the trace elements and the P-T estimates necessitate removal of the HA body from the lower crust and incorporation in the mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, facilitating lower crustal foundering into a mantle wedge where it was likely subducted along a hot P-T path to their peak conditions. Hydration of the HA body likely provided a change in density that could facilitate its fairly rapidly exhumed to the surface. The HA cumulate thus likely represents a piece of the subduction system that is rarely preserved, as well as key component in the compositional evolution of the continental crust.