Petrogenesis of the Higashi-Akaishi Ultramafic Body: Implications for Lower Crustal Foundering and Mantle Wedge Processes

Abstract

Recycling of ultramafic lower crustal cumulates via delamination or foundering is often invoked as a mechanism to return mafic material to the mantle during continental crust formation. These recycled pieces of the lower crust are rarely sampled but are preserved in several locations including the Kohistan and Talkeetna arc sections, Sierra Nevada and Colorado Plateau pyroxenite xenoliths, and as discussed here for the first time, the exhumed Higashi-Akaishi (HA) ultramafic body in Japan. The HA is located in the Besshi region of the Sanbagawa metamorphic belt in southwestern Japan and is dominantly composed of dunite with lesser garnet pyroxenite and harzburgite 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# clinopyroxene = 0.94, Mg# olivine = 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 and corresponding intrusive lithologies, pointing to different liquid lines of descent for the corresponding melts. Our new thermobarometric estimates (peak pressure-temperature at 2.6 GPa, 713ºC) are consistent with a hot slab surface subduction path, rather than the lower crustal temperatures recorded in arc sections (Kohistan & Talkeetna: 1 GPa, 800ºC). A pervasive slab-fluid influence is also indicated in the HA lithologies by LREE & Ce enrichments and strong Nb & Zr depletions. The trace elements and the pressure-temperature estimates, as well as the thermodynamic modeling results necessitate removal of the HA body from the lower crust and incorporation into cooler portions of a mantle wedge. At lower crustal conditions, the bulk density of the HA lithologies is greater than the background mantle, indicating the feasibility of lower crustal foundering into a mantle wedge where the HA was incorporated in the subduction channel to reach its peak conditions. Hydration of the HA body while in the subduction channel likely provided the change in density necessary to facilitate its rapid exhumation to the surface. Thus, the HA cumulate likely represents a piece of the subduction system that is rarely preserved, as well as a key component in the compositional evolution of the continental crust.

Mantle wedge olivine modifies slab-derived fluids: Implications for fluid transport from slab to arc magma source

“…Shallow mantle wedge serpentinites have high δ 11 B (> +10‰) (Savov et al, 2004), whereas lower δ 11 B values (< +5‰) were observed in serpentinites from the deep mantle wedge, as slab-derived fluids become increasingly lower in δ 11 B with increasing slab depth (Martin et al, 2016). The B isotope values for olivine of the HA body correspond to slab fluids from 90-120 km depth (Scambelluri and Tonarini, 2012;Yamada et al, 2019), consistent with its estimated P-T conditions (Enami et al, 2004;Guild et al, 2020).…”

“…1). The body contains mantle wedge peridotites including cumulates and residual mantle (Hattori et al, 2010) that were fluxed by slabderived fluids at high pressure in a subduction channel (Aoya et al, 2013;Guild et al, 2020;Hattori et al, 2010;Mizukami and Wallis, 2005;Sumino et al, 2007), and may ultimately have been derived by delamination from the lithospheric mantle section of the overriding plate (Guild et al, 2020).…”

“…The rocks record four stages of deformation (D1-D4; Mizukami and Wallis, 2005;. The first stage (D1) resulted in preferred orientation of coarse-grained olivine under low H2O activity, and high T (715-800°C) and P (2.1-2.6 GPa) (Enami et al, 2004;Guild et al, 2020).…”

Preferential Formation of Chlorite Over Talc During Si‐Metasomatism of Ultramafic Rocks in Subduction Zones

“…Other examples of exhumed mantle wedge peridotite metasomatized by slab‐derived fluids include the Higashi‐Akaishi peridotite (Japan) and chlorite harzburgites in Cima di Gagnone (Switzerland). These exhumed metaperidotites record peak P‐T conditions of ∼700°C–800°C and <3 GPa (Guild et al., 2020; Scambelluri et al., 2014). Metasomatism of these metaperidotites by slab‐derived fluids may have occurred at or before the rocks had reached peak‐metamorphic temperatures.…”