EBSD-measured crystal preferred orientation of eclogites from the Sanbagawa metamorphic belt, central Shikoku, SW Japan

Abstract

Electron back-scattered diffraction (EBSD) maps and crystal-preferred orientation (CPO) of eclogite-facies (omphacite and garnet) and amphibolite-facies (hornblende and actinolite) phases are reported for understanding the rheological behaviour of crust during subduction. Two types of eclogites from the subduction-related high-pressure/low-temperature type Sanbagawa metamorphic belt, Japan, have been investigated. Type-I eclogite (sample Sb-1) is composed of garnet, omphacite, secondary actinolite and hornblende. Type-II eclogite (samples Sb-2 and Sb-2a) are mainly composed of omphacite, garnet, and retrograde hornblende with no actinolite. Omphacite, the peak eclogite-facies phase, exhibits L-type CPO (maximum density of [001] axes parallel to and high density of {110} poles normal to the lineation) in Type-I eclogite, suggesting intra-crystalline plasticity with [001] {110} and 〈110i{110} active slip systems, indicating a constrictive strain regime at mantle depths. Omphacite in Type-II eclogite exhibits a similar fabric but with much weaker CPO. Using the LS-index symmetry analysis (one for the end-member L-type, zero for the end-member S-type, and intermediate values for LS-types), a progressive change in LS index of 0.80 for Type-I and 0.61 to 0.44 for Type-II eclogites is observed. These values suggest a transition from axial extension parallel to the lineation for Sb-1 and weaker CPO associated with pure or simple shear for Sb-2 and Sb-2a. Garnet, the second dominant phase in the eclogite-facies stage, exhibits weak and complex fabric patterns in all eclogite types, behaved like rigid bodies and does not show plastic deformation. Amphibolite-facies phases (e.g., hornblende and actinolite) exhibit more than two types of CPO. Hornblende and actinolite in Type-I eclogite have a strong CPO along [001] axes aligned parallel to the lineation, indicating homotactic crystal growth probably by the replacement of omphacite during the early stages of retrogression. Type-II eclogites have weak CPO in hornblende but with characteristic alignment of [001] parallel to the lineation and other poles to planes (100), (010), and {110} normal to the lineation. This fabric might have resulted from a cataclastic deformation and could be related to the late-"D1" deformation stage in the Sanbagawa metamorphic belt.

Electron Backscatter Diffraction Study of Ultrahigh-Pressure Tso Morari Eclogites (Trans-Himalayan Collisional Zone): Implications for Strain Regime Transition from Constrictional to Plane Strain during Exhumation

“…In brief, the omphacite LPOs indicate transition from constrictional strain regime at peak (HP) metamorphic conditions to a plane strain regime dur-ing the postpeak (HP) decompression stage. A similar kind of transition has been observed by various workers in omphacite from eclogite in different tectonic settings around the world [29,35,36,80]. Detailed LPO analyses of omphacite from different eclogite-bearing nappes of Alps [80] show that an L-type fabric in omphacite is formed in the constrictional field during the high-pressure metamorphism and subsequent retrogression stage.…”

“…where R T ðθÞ is the theoretical distribution of misorientation angle for a random fabric and R 0 ðθÞ is the distribution for observed misorientation angles. Low-angle boundary (2 °-10 °) misorientation analyses for both inner (subgrain boundaries) and outer grain boundaries were carried out for all the eight samples and are plotted following the principles and methodologies described by [35], and the slip systems were inferred from [24][25][26]. For misorientation analyses of low-angle grain boundaries of quartz, the plotting convention of [67] was followed.…”

“…Figure 8 shows the stereoplots (lower hemisphere) of LPOs for omphacite, and Figure 9 shows the inverse pole figures for misorientation analyses of low-angle (2 °-10 °) inner and outer "neighbour-to-neighbour" grain boundaries in crystallographic reference frame. This "neighbour-to-neighbour" grain boundary misorientation analyses helps identify the slip systems that are active during deformation (see [25,35] and references therein for detailed descriptions of identifying slip systems from grain boundary misorientation analyses), and in some cases, especially for quartz, the identified slip systems can also provide an assessment of the temperature of deformation [67]. LPOs of sample 3XA show girdle distribution of (010) poles perpendicular to the foliation (Z) with a maximum MUD (multiples of uniform distribution) of 6.…”

“…One of the most important devices used for the textural and microfabric analyses of HP/UHP rocks is electron backscatter diffraction (EBSD) [20,21]. Lattice preferred orientation (LPO) patterns of omphacite in eclogite have been studied in the past by various workers using either the universal stage ( [22,23]; also see [24,25] for reviews) or EBSD [26][27][28][29][30][31][32][33][34][35][36][37]. These studies have demonstrated that in omphacite LPOs, lineationparallel [001] maxima and foliation-parallel girdle distribution represent constriction and flattening strains, respectively (Figure 1) ( [25] and references therein).…”

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

“…However, the EBSD analyses of garnet grains in the quartz eclogites studied here, although limited, suggest that these grains are single crystals and formed by simple concentric growth (Figure ). Rehman, Mainprice, Barou, Yamamoto, and Okamoto () measured crystal preferred orientations of eclogite facies phases of quartz eclogites from the Gongen area, and did not find a possibility of polycrystalline growth of garnet. Therefore, we might not need to consider the case that the difference in zoning patterns in garnets of this study is due to polycrystalline growth.…”