Characterization of olivine fabrics and mylonite in the presence of fluid and implications for seismic anisotropy and shear localization

The Lindås Nappe, Bergen Arc, is located in western Norway and displays two high-grade metamorphic structures. A Precambrian granulite facies foliation is transected by Caledonian fluid-induced eclogite-facies shear zones and pseudotachylytes. To understand how a superimposed tectonic event may influence olivine fabric and change seismic anisotropy, two lenses of spinel lherzolite were studied by scanning electron microscope (SEM) and electron back-scattered diffraction (EBSD) techniques. The granulite foliation of the surrounding anorthosite complex is displayed in ultramafic lenses as a modal variation in olivine, pyroxenes, and spinel, and the Caledonian eclogite-facies structure in the surrounding anorthosite gabbro is represented by thin (<1 cm) garnet-bearing ultramylonite zones. The olivine fabrics in the spinel bearing assemblage were E-type and B-type and a combination of Aand B-type lattice preferred orientations (LPOs). There was a change in olivine fabric from a combination of Aand B-type LPOs in the spinel bearing assemblage to Band E-type LPOs in the garnet lherzolite mylonite zones. Fourier transform infrared (FTIR) spectroscopy analyses reveal that the water content of olivine in mylonite is much higher (approximately 600 ppm H/Si) than that in spinel lherzolite (approximately 350 ppm H/Si), indicating that water caused the difference in olivine fabric. Fabric strength of olivine gets weaker as the grain size reduced, and as a result, calculated seismic properties for the two deformation stages reveal that Pand S-velocity anisotropies are significantly weaker in the mylonite. Microtextures and LPO data indicate that the deformation mechanism changed from dominant dislocation creep in spinel lherzolite to dislocation creep accompanied by grain-boundary sliding in mylonite. Shear localization in the mylonite appears to be originated from the grain size reduction through (1) enhanced dynamic recrystallization of olivine in the presence of water and (2) Zener pinning of clinopyroxene or (3) by ultracomminution during the pseudotachylyte stage. Background Olivine is a dominant mineral in the upper mantle. Knowledge of the lattice preferred orientation (LPO) of olivine can be used to understand mantle flow and the seismic anisotropy of the upper mantle (Nicolas and Christensen 1987; Ben Ismail and Mainprice 1998; Long 2013). The water content of olivine is one of the most important factors affecting olivine LPO (Jung and Karato 2001a). In the early studies, olivine fabric in upper mantle conditions thought to be dominated by one type of olivine LPO in upper mantle conditions (Ben Ismail and Mainprice 1998; Nicolas and Christensen 1987), which is later named as * Correspondence: [email protected] School of Earth and Environment Sciences, Seoul National University, Seoul 151-747, South Korea Full list of author information is available at the end of the article © 2014 Jung et al.; licensee Springer. This is an Attribution License (http://creativecommons.or in any medium, provided the original work is p A-type (Jung and Karato 2001a). However, recent experimental studies (Jung and Karato 2001a; Jung et al. 2006; Karato et al. 2008; Katayama et al. 2004) show that A-, B-, C-, and E-types of olivine LPO can be observed under different deformation conditions. The relationship between olivine LPO and the water content of olivine has been established (Jung et al. 2006). Under dry conditions, A-type olivine LPO is observed and, characteristically, the [100] axes are aligned subparallel to lineation, and the [010] axes are aligned normal to foliation. Under water-rich conditions, olivine LPO can change to B-, C-, or E-types LPOs (Jung and Karato 2001a; Jung et al. 2006). For the B-type LPO, the [001] axes are aligned subparallel to lineation and the [010] axes are aligned normal to the foliation. For the C-type LPO, the [001] axes are Open Access article distributed under the terms of the Creative Commons g/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction roperly credited. Jung et al. Earth, Planets and Space 2014, 66:46 Page 2 of 21 http://www.earth-planets-space.com/content/66/1/46 subparallel to lineation and the [100] axes are normal to foliation. For the E-type LPO, the [100] axes are subparallel to lineation and the [001] axes are normal to foliation. Water-related olivine fabrics have been reported in some natural peridotites. For example, Frese et al. (2003) reported that prograde garnet peridotite from Cima di Gagnone in the Central Alps (Switzerland) has the C-type LPO of olivine. Mehl et al. (2003) found that peridotite from Talkeetna arc in the south central Alaska (USA) has the E-type LPO of olivine. Katayama et al. (2005) found that garnet peridotite from Otrøy in the Western Gneiss Region (Norway) has the C-type LPO of olivine. Michibayashi et al. (2007) reported that peridotites from the southern Mariana trench have B-type LPO of olivine. Webber et al. (2008) noted that mantle peridotites from Red Hills (New Zealand) also have the B-type LPO of olivine. Jung (2009) reported that peridotites from Val Malenco (Italy) have both Band E-type olivine LPO. Park et al. (2014) recently reported that spinel peridotites from Adam's Diggings, Rio Grande Rift (USA) contain C-type LPO of olivine. Jung et al. (2013) reported that peridotites from North Qaidam UHP collision belt (China) have C-type olivine LPO with olivine containing a large amount of water (1,130 ± 50 ppm H/Si). Park and Jung (2014) recently reported the Band E-type LPO of olivine in the mantle xenoliths from Shanwang, eastern China. However, the effects of water and dynamic recrystallization on olivine fabric strength and two-stage deformations under water-rich conditions on seismic anisotropy are not yet well understood. To use mantle fabrics to decipher plate movements, it is important to understand the timing of fabric development. This issue is closely linked to understanding mantle anisotropy, where studies of this topic generally assume that seismic anisotropy developed during a single tectonic event. Lindås Nappe, located in western Norway, contains rocks exposed to eclogite-facies conditions, and the anorthosites in this area possess two fabrics. Grenvillian granulite facies rocks are transected by eclogite-facies shear zones that developed during the Caledonian collision between Baltica and Lurentia. Lindås Nappe contains numerous small peridotite lenses that allow us to investigate fabric development in peridotites exposed to two high-grade metamorphic events with different amounts of fluid. Fluids may also promote metamorphic reactions, like transformation from spinel peridotite to garnet peridotite. Metamorphic reactions and changes in mineral assemblages can be associated with a change in fabric. We show here that the peridotites, like the anorthosites, display two fabrics of different strengths and that these differences can be related to the water content of olivine. Microstructures are used to infer the deformation mechanism of olivine. Dislocation microstructures of olivine for the two structures are also reported. Consequences of our findings are discussed in relation to shear localization and seismic anisotropy in the upper mantle. Shear localization may have been initiated by intermediate deep earthquakes represented by pseudotachylytes. The earthquakes induced inhomogeneity in the rock through ultracomminution and allowed fluid influx. Geological setting and sample description Bergen Arc is composed of several Nappe units that are distributed in an arc shape and centered on the town of Bergen, Western Norway (Figure 1A). Lindås Nappe is the largest of these units. It is believed that the eclogite facies overprinting occurred when Lindås Nappe formed part of the root zone to the Caledonian mountain belt (Austrheim 1987; Austrheim et al. 1997; Boundy et al. 1997). The main component of Lindås Nappe is anorthosite and includes other members of the anorthosite-mangeritegranite-charnockite (AMGC) suite of rocks (Austrheim 1990). The Lindås Nappe anorthosite complex has undergone two main phases of metamorphism and deformation the Grenvillian orogeny and the Caledonian orogeny. During the Grenvillian orogeny, the complex developed a dry granulite-facies mineralogy (800°C to 900°C, <1 GPa) (Austrheim and Griffin 1985) and a granulite-facies foliation. Typically, this foliation is characterized by disc-shaped coronas that are oriented in the same direction and by alternating plagioclase-rich and mafic layers that produce a strong banding. The granulitefacies anorthositic rocks were transformed, locally, to dense eclogite-facies rocks during the Caledonian orogeny (650°C to 700°C, 1.5 to 2.1 GPa) (Jamtveit et al. 1990; Glodny et al. 2008). Eclogitization during the Caledonian orogeny is associated with deformation and fluid infiltration (Austrheim 1987). Eclogites are formed along fractures and shear zones, and in breccias, where angular blocks of granulites are surrounded by eclogite. The Lindås Nappe anorthosite complex can be classified based on the percentage of eclogite under 40%, 40% to 80%, and over 80% (Austrheim 1990). In the anorthositic rock body, there are a large number of pseudotachylyte veins that contain evidence of deep crustal seismic activity (Austrheim and Boundy 1994; Austrheim et al. 1996). The exact time of this seismic activity is unclear, but it must have taken place before or syn-eclogite facies metamorphism since the veins display an eclogite facies mineralogy. The northwest part of the island of Holsnøy is a part of Lindås Nappe. Within this region, there are numerous strongly banded ultramafic lenses, typically 5 to 50 m across. They consist of lherzolites interlayered with websterites and garnet pyroxenites. Calculated equilibrium P-T conditions and age data of these lenses (Kühn et al. 2000) give 1 GPa, 743°C to 977°C, and 842 ± 12 Ma, which are consistent with those of the Grenvillian granulite-facies