Formation and deformation mechanisms of pyroxene-spinel symplectite in an ascending mantle, the Horoman peridotite complex, Japan: An EBSD (electron backscatter diffraction) study

Abstract

Symplectites, fine vermicular intergrowth (< 10 μm) of orthopyroxene, clinopyroxene, and spinel, and the surrounding lenticular coarser grained (100 200 μm) aggregate (seam) with the same mineral assemblage of the symplectites define remarkable foliation and lineation in spinel lherzolites of the Horoman complex, northern Japan. They are inferred to be products of reaction between garnet and olivine during decompression of the host peridotite accompanying deformation. Microstructure of a symplectite was investigated with automated electron backscattered diffraction (EBSD) analysis using a field emission gun SEM (FE SEM) in order to clarify reaction and deformation mechanisms and thereby better constraining mechanical interaction between the Earth's upper mantle and lower crust. The symplectite is composed of two segments with a large misorientation angle of ~ 60° only for spinel, and the two spinel crystals are in mirror symmetry with the segment boundary approximately parallel to the mirror plane. The segment boundary is interpreted as spinel law twin formed during phase transition from garnet. Each segment is further subdivided into several sectors with gradual lattice distortion smaller than a few degrees/mm and intra sector misorientation mostly smaller than 25° for all constituent minerals and with misorientation axes nearly perpendicular to the lineation and parallel to the foliation. The sector boundaries are inferred to be subgrain boundaries formed by dislocation creep of pyroxenes and spinel in the spinel stability field. The spinel twin suggests that a garnet was decomposed directly into entangled aggregate of pyroxenes and spinel, which grew from an embryo nucleated on the surface of the reactant garnet. The symplectite minerals in each sector show systematic crystallographic orientations (topotaxy) with each other. The topotaxial relationship in the fine intergrowth with subgrain structure demonstrates that the systematic crystallographic orientations were acquired when the crystals grew by decomposing the garnet and were later modified by deformation during the consecutive ascent of the complex.

Orogenic, Ophiolitic, and Abyssal Peridotites

“…3.4.2.1.1.2 'Symplectite-bearing' peridotites Although equilibrated in the spinel and plagioclase peridotite facies, the orogenic peridotites from Hokkaido, Japan (Horoman and Nikanbetsu, Figure 2), are characterized by the widespread occurrence of symplectitic aggregates that bear evidence for early equilibration of the whole peridotite bodies in the garnet stability field (Morishita and Arai, 2003;Odashima et al, 2008;Ozawa and Takahashi, 1995;Takahashi, 2001;Takahashi and Arai, 1989;Takazawa et al, 1996;Tazahi et al, 1972). The massifs were uplifted from conditions comparable to those inferred for the 'HP-HT' garnet peridotites (>1000 C and >70 km deep).…”

Myrmekite and strain weakening in granitoid mylonites

“…The processes of grainsize evolution are probably related to the minimization of interfacial energy in the vermicular microstructure of pristine myrmekite (e.g. Odashima et al, 2007;Dégi et al, 2010). Qtz grain coarsening reflects annealing of the pristine vermicular microstructure after the reaction front moved further into the Kfs (Fig.…”

Myrmekite and strain weakening in granitoid mylonites

“…These processes are probably related to the minimization of interfacial energy in the vermicular 15 microstructure of the pristine myrmekite (e.g. Odashima et al, 2007;Dégi et al, 2010). Quartz grain coarsening occurred as a consequence of annealing of the pristine vermicular microstructure after the reaction front moved further into the K-feldspar ( Hopson and Ramseyer, 1990;Dell'Angelo and Tullis, 1996;Xiao et al, 2002).…”