[S-MP21] Oceanic and Continental Subduction Processes: petrologic and geochemical perspective

[SMP21-04] Microstructural attributes of (ultra-) high-pressure eclogite and retrograded amphibolite from the Tso Morari Complex

*DRIPTA DUTTA¹、Takeshi Imayama¹、Dyuti Prakash Sarkar²、Jun-ichi Ando²、Kaushik Das² (1.Research Institute of Frontier Science and Technology, Okayama University of Science, Okayama 7000005, Japan、2.Department of Earth and Planetary Systems Science, Hiroshima University, Higashi Hiroshima 7398526, Japan)

キーワード：CPO Inheritance, Symplectite, Phase transformation, Interphase misorientation, Deformation mechanism

Breakdown of peak pressure omphacite to amphibole and plagioclase during exhumation is typical in (ultra-) high-pressure eclogites. We studied one such eclogite sample from the Tso Morari Crystalline Complex. The eclogite preserves symplectites characterized by either partial replacement, i.e., at the periphery of the omphacite grains and along transgranular fractures (Site 1), or complete replacement of the omphacite grains (Site 2). A garnet amphibolite sample is studied to examine the dislocation creep-mediated deformation of amphibole. The samples are non-oriented, but the shape-preferred orientations of omphacite and amphibole grains in the eclogite and amphibolite, respectively, are conspicuous. The jadeite contents of the omphacite grains range between 43-47%. The plagioclase grains are albite or oligoclase in composition. The amphibole grains of the eclogite are higher in Mg# values and Na content than those of the garnet amphibolite. The omphacite, amphibole, and plagioclase grains in the eclogite exhibit strong crystallographic preferred orientations (CPOs). We identified the following crystallographic relationships from the pole figures: <001>_Omp||<001>_Amp, (100)_Omp||(100)_Amp, (010)_Omp||(010)_Amp, and (110)_Omp 30° to (110)_Amp. The intragrain misorientation axes (LAXs) of omphacite are mostly parallel to their <001>, and those of the amphibole grains mainly lie on (100) with two maxima sub-parallel to <001> and <010>. Plagioclase LAXs from Site 1 are sub-parallel to the poles of (010) and (0-10), whereas those from Site 2 do not show any preferred orientation. The higher frequencies of the neighbour-pair misorientation angle distributions of amphibole and plagioclase than their respective random-pair and theoretical distributions at lower angles (10–30°) and point distributions of their CPOs suggest that their CPOs originated by inheritance. Preliminary interphase misorientation analysis also demonstrates statistically significant differences between the neighbour- and random-pair distributions for both omphacite-amphibole and omphacite-plagioclase pairs testifying to the influence of omphacite CPOs on those of amphibole and plagioclase. Though characterized by point and girdle distributions, the amphibole CPOs from the garnet amphibolite are much weaker than those from the eclogite. The quartz grains are few and do not exhibit any preferred orientation. The amphibole LAXs are mostly sub-parallel to the poles of (100), suggesting the dominance of (010)<001> slip-system activity (assuming tilt boundaries). The distribution of the quartz LAXs implies activity of the high-T (>600°C) {m}<c + a> slip-system. However, since only a few amphibole grains preserve subgrains, detailed assessments of their slip-system activities and intragranular plasticity will be performed along with thermobarometric studies to obtain better constraints on the deformation temperature.