Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

Abstract

The slab-mantle interface in subduction zones is a site of tectonic mixing of crustal and mantle rocks. It is the interface for fluid flow of slab-derived components into the mantle wedge. To assess the fluid-rock interaction along the slab-mantle interface, we studied the bleaching of pelitic schist in an ultramafic mélange. The Nishisonogi metamorphic rocks in Kyushu, Japan, comprise ultramafic mélanges intercalated with epidote-blueschist facies schists. The ultramafic mélange consists of tectonic blocks of various lithologies and a matrix of chlorite-actinolite schist and serpentinite. Along the contact with the mélange matrix, pelitic schist blocks are bleached mainly due to the modal increase of albite and the consumption of carbonaceous material. The bleaching is probably attributed to infiltration of Na-rich external fluid from the mélange matrix. Mass balance analysis indicates losses of C, Rb, K 2 O, Ba, Pb, and SiO 2 from the bleached pelitic schist, although Al 2 O 3 , TiO 2 , Sc, Y, Zr, Nb, La, Ce, and Nd remain immobile. This suggests fractionation of large-ion lithophile elements (LILE) and Pb from the high-field-strength elements and rare earth elements during the bleaching. If this ultramafic mélange is analogous to the slab-mantle interface, similar infiltration metasomatism will promote liberation of C, Si, LILE, and Pb from subducting metapelites and enhance metasomatism of the mantle wedge.

Hydrous oceanic crust hosts megathrust creep at low shear stresses

“…These observations together strongly suggest that albite is deformed by diffusion, despite having a grain size much larger than is usually reported for diffusion creep (25,26). One possibility to explain the large grain sizes in albite is grain growth, stimulated by Na-rich fluids inferred to be present at peak metamorphic conditions in the NMR (27). Given the slow strain rates that can be accommodated by diffusion creep in albite at Nishisonogi-like conditions and grain sizes (28) and the relatively weak strength of (001) planes in phyllosilicates (13,14), we expect that albite accommodated minor amounts of strain relative to the phyllosilicates.…”

Low‐Temperature Frictional Characteristics of Chlorite‐Epidote‐Amphibole Assemblages: Implications for Strength and Seismic Style of Retrograde Fault Zones

“…Replacement of load‐bearing strong phases with weak phyllosilicates as a consequence of cataclasis and alteration was also described in the Median Tectonic Line of Japan (Jefferies et al, 2006). While we focus here on retrograde structures, we also note that deformation localization within chloritic shear zones is also seen in subduction‐related thrusts, such as in the Nishisonogi metamorphic rocks of Japan, where chloritic shear zones developed on the prograde path contain fractured clasts of amphibole and feldspar (Figure 1e; Mori et al, 2014).…”

Secular Variation of Early Cretaceous Granitoids in Kyushu, SW Japan: The Role of Mélange Rocks as a Possible Magma Source

“…The Nishisonogi unit of the Nagasaki metamorphic rocks forms a Late Cretaceous subduction complex that consists mainly of epidote-blueschist facies pelitic, psammitic, and mafic schists with minor serpentinite (Nishiyama, 1990;Mori et al, 2014). The serpentinite, which is in tectonic contact with the schists, FIGURE 1 | Distribution of Cretaceous accretionary complex, metamorphic complex, plutonic rocks, volcanic rocks, and non-accretionary sedimentary rocks in Kyushu, SW Japan [modified from Miyazaki et al (2016) based on the seamless geological map of Japan (1:200,000), Geological Survey of Japan, https://gbank.gsj.jp/seamless/index_en.html] with solid circle indicating the locality of granitic rock samples of which the thermodynamic modeling was modeled in this study.…”

“…occurs as pod-like bodies of < 150 m in diameter and ultramafic melanges of < 350 m in thickness (Mori et al, 2014). The ultramafic melanges contain tectonic blocks of metagabbro, mafic schist, pelitic schist, and albitite in a matrix of chlorite-actinolite schist, talc schist, and schistose serpentinite (Nishiyama, 1989(Nishiyama, , 1990.…”