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Volume 31 Issue 1
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Magnetic Fabric and Petrofabric of Amphibolites from the Namcha Barwa Complex, Eastern Himalaya

  • The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility (AMS) and the crystallographic preferred orientation (CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.3×10-4 to 3.314×10-2 SI, with the Jelínek's anisotropy values (Pj) ranges from 1.094 to 1.487. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction (EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the[001] axes along the lineation and the[100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.
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  • Almqvist, B. S. G., Mainprice, D., 2017. Seismic Properties and Anisotropy of the Continental Crust: Predictions Based on Mineral Texture and Rock Microstructure. Reviews of Geophysics, 55(2): 367-433. https://doi.org/10.1002/2016rg000552
    Biedermann, A., 2018. Magnetic Anisotropy in Single Crystals: A Review. Geosciences, 8(8): 302. https://doi.org/10.3390/geosciences8080302
    Biedermann, A. R., Koch, C. B., Pettke, T., et al., 2015. Magnetic Anisotropy in Natural Amphibole Crystals. American Mineralogist, 100(8/9): 1940-1951. https://doi.org/10.2138/am-2015-5173
    Biedermann, A. R., Kunze, K., Hirt, A. M., 2018. Interpreting Magnetic Fabrics in Amphibole-Bearing Rocks. Tectonophysics, 722: 566-576. https://doi.org/10.1016/j.tecto.2017.11.033
    Biedermann, A. R., Pettke, T., Angel, R. J., et al., 2016. Anisotropy of Magnetic Susceptibility in Alkali Feldspar and Plagioclase. Geophysical Journal International, 205(1): 479-489. https://doi.org/10.1093/gji/ggw042
    Biedermann, A. R., Jackson, M., Bilardello, D., et al., 2017. Effect of Magnetic Anisotropy on the Natural Remanent Magnetization in the MCU IVe' Layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway. Journal of Geophysical Research: Solid Earth, 122(2): 790-807. https://doi.org/10.1002/2016jb013506
    Booth, A. L., Zeitler, P. K., Kidd, W. S. F., et al., 2004. U-Pb Zircon Constraints on the Tectonic Evolution of Southeastern Tibet, Namche Barwa Area. American Journal of Science, 304(10): 889-929. https://doi.org/10.2475/ajs.304.10.889
    Booth, A. L., Chamberlain, C. P., Kidd, W. S. F., et al., 2009. Constraints on the Metamorphic Evolution of the Eastern Himalayan Syntaxis from Geochronologic and Petrologic Studies of Namche Barwa. Geological Society of America Bulletin, 121(3/4): 385-407. https://doi.org/10.1130/b26041.1
    Borradaile, G. J., Henry, B., 1997. Tectonic Applications of Magnetic Susceptibility and Its Anisotropy. Earth-Science Reviews, 42(1/2): 49-93. https://doi.org/10.1016/s0012-8252(96)00044-x
    Borradaile, G. J., 2001. Magnetic Fabrics and Petrofabrics: Their Orientation Distributions and Anisotropies. Journal of Structural Geology, 23(10): 1581-1596. https://doi.org/10.1016/s0191-8141(01)00019-0
    Borradaile, G. J., Jackson, M., 2010. Structural Geology, Petrofabrics and Magnetic Fabrics (AMS, AARM, AIRM). Journal of Structural Geology, 32(10): 1519-1551. https://doi.org/10.1016/j.jsg.2009.09.006
    Cao, S. Y., Liu, J. L., Leiss, B., 2010. Orientation-Related Deformation Mechanisms of Naturally Deformed Amphibole in Amphibolite Mylonites from the Diancang Shan, SW Yunnan, China. Journal of Structural Geology, 32(5): 606-622. https://doi.org/10.1016/j.jsg.2010.03.012
    Chadima, M., Hansen, A. K., Hirt, A. M., et al., 2004. Phyllosilicate Preferred Orientation as a Control of Magnetic Fabric: Evidence from Neutron Texture Goniometry and Low and High-Field Magnetic Anisotropy (SE Rhenohercynian Zone of Bohemian Massif). Geological Society, London, Special Publications, 238(1): 361-380. https://doi.org/10.1144/gsl.sp.2004.238.01.19
    Chung, S. L., Chu, M. F., Zhang, Y. Q., et al., 2005. Tibetan Tectonic Evolution Inferred from Spatial and Temporal Variations in Post-Collisional Magmatism. Earth-Science Reviews, 68(3/4): 173-196. https://doi.org/10.1016/j.earscirev.2004.05.001
    Ding, L., Zhong, D. L., 1999. Metamorphic Characteristics and Geotectonic Implications of the High-Pressure Granulites from Namjagbarwa, Eastern Tibet. Science in China Series D: Earth Sciences, 42(5): 491-505. https://doi.org/10.1007/bf02875243
    Dunlop, D. J., Özdemir, Ö., 1997. Rock magnetism: Fundamentals and Frontiers (Vol. 3). Cambridge University Press, Cambridge. https://doi.org/10.1017/cbo9780511612794
    Geng, Q. R., Pan, G. T., Zheng, L. L., et al., 2006. The Eastern Himalayan Syntaxis: Major Tectonic Domains, Ophiolitic Mélanges and Geologic Evolution. Journal of Asian Earth Sciences, 27(3): 265-285. https://doi.org/10.1016/j.jseaes.2005.03.009
    Grégoire, V., de Saint Blanquat, M., Nédélec, A., et al., 1995. Shape Anisotropy versus Magnetic Interactions of Magnetite Grains: Experiments and Application to AMS in Granitic Rocks. Geophysical Research Letters, 22(20): 2765-2768. https://doi.org/10.1029/95gl02797
    Hirt, A. M., Evans, K. F., Engelder, T., 1995. Correlation between Magnetic Anisotropy and Fabric for Devonian Shales on the Appalachian Plateau. Tectonophysics, 247(1/2/3/4): 121-132. https://doi.org/10.1016/0040-1951(94)00176-a
    Holland, T., Blundy, J., 1994. Non-Ideal Interactions in Calcic Amphiboles and Their Bearing on Amphibole-Plagioclase Thermometry. Contributions to Mineralogy and Petrology, 116(4): 433-447. https://doi.org/10.1007/bf00310910
    Hrouda, F., Kahan, Š., 1991. The Magnetic Fabric Relationship between Sedimentary and Basement Nappes in the High Tatra Mountains, N. Slovakia. Journal of Structural Geology, 13(4): 431-442. https://doi.org/10.1016/0191-8141(91)90016-c
    Hrouda, F., Schulmann, K., Suppes, M., et al., 1997. Quantitive Relationship between Low-Field AMS and Phyllosilicate Fabric: A Review. Physics and Chemistry of the Earth, 22(1/2): 153-156. https://doi.org/10.1016/s0079-1946(97)00094-3
    Jelínek, V., 1981. Characterization of the Magnetic Fabric of Rocks. Tectonophysics, 79(3/4): T63-T67. https://doi.org/10.1016/0040-1951(81)90110-4
    Ji, S. C., Shao, T. B., Michibayashi, K., et al., 2015. Magnitude and Symmetry of Seismic Anisotropy in Mica- and Amphibole-Bearing Metamorphic Rocks and Implications for Tectonic Interpretation of Seismic Data from the Southeast Tibetan Plateau. Journal of Geophysical Research: Solid Earth, 120(9): 6404-6430. https://doi.org/10.1002/2015jb012209
    Kitamura, K., 2006. Constraint of Lattice-Preferred Orientation (LPO) on Vp Anisotropy of Amphibole-Rich Rocks. Geophysical Journal International, 165(3): 1058-1065. https://doi.org/10.1111/j.1365-246x.2006.02961.x
    Ko, B., Jung, H., 2015. Crystal Preferred Orientation of an Amphibole Experimentally Deformed by Simple Shear. Nature Communications, 6: 6586. https://doi.org/10.1038/ncomms7586
    Leake, B. E., Woolley, A. R., Arps, C. E. S., et al., 1997. Nomenclature of Amphiboles; Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names. Mineralogical Magazine, 61(405): 295-310. https://doi.org/10.1180/minmag.1997.061.405.13
    Li, Z. Y., Zheng, J. P., Liu, Q. S., et al., 2015. Magnetically Stratified Continental Lower Crust Preserved in the North China Craton. Tectonophysics, 643: 73-79. https://doi.org/10.1016/j.tecto.2014.12.012
    Li, Z. Y., Zheng, J. P., Moskowitz, B. M., et al., 2017. Magnetic Properties of Serpentinized Peridotites from the Dongbo Ophiolite, SW Tibet: Implications for Suture-Zone Magnetic Anomalies. Journal of Geophysical Research: Solid Earth, 122(7): 4814-4830. https://doi.org/10.1002/2017jb014241
    Liu, Q. S., Wang, H. C., Zheng, J. P., et al., 2013. Petromagnetic Properties of Granulite-Facies Rocks from the Northern North China Craton: Implications for Magnetic and Evolution of the Continental Lower Crust. Journal of Earth Science, 24(1): 12-28. https://doi.org/10.1007/s12583-013-0314-5
    Liu, Y., Zhong, D., 1997. Petrology of High-Pressure Granulites from the Eastern Himalayan Syntaxis. Journal of Metamorphic Geology, 15(4): 451-466. https://doi.org/10.1111/j.1525-1314.1997.00033.x
    Liu, Y., Zhong, D. L., 1998. Tectonic Framework of the Eastern Himalayan Syntaxis. Progress in Natural Science, 8(3): 366-370
    Mainprice, D., Hielscher, R., Schaeben, H., 2011. Calculating Anisotropic Physical Properties from Texture Data Using the MTEX Open-Source Package. Geological Society, London, Special Publications, 360(1): 175-192. https://doi.org/10.1144/sp360.10
    Mainprice, D., Humbert, M., 1994. Methods of Calculating Petrophysical Properties from Lattice Preferred Orientation Data. Surveys in Geophysics, 15(5): 575-592. https://doi.org/10.1007/bf00690175
    Punturo, R., Mamtani, M. A., Fazio, E., et al., 2017. Seismic and Magnetic Susceptibility Anisotropy of Middle-Lower Continental Crust: Insights for Their Potential Relationship from a Study of Intrusive Rocks from the Serre Massif (Calabria, Southern Italy). Tectonophysics, 712/713: 542-556. https://doi.org/10.1016/j.tecto.2017.06.020
    Robinson, P., Heidelbach, F., Hirt, A. M., et al., 2006. Crystallographic-Magnetic Correlations in Single-Crystal Haemo-Ilmenite: New Evidence for Lamellar Magnetism. Geophysical Journal International, 165(1): 17-31. https://doi.org/10.1111/j.1365-246x.2006.02849.x
    Schmidt, V., Hirt, A. M., Leiss, B., et al., 2009. Quantitative Correlation of Texture and Magnetic Anisotropy of Compacted Calcite-Muscovite Aggregates. Journal of Structural Geology, 31(10): 1062-1073. https://doi.org/10.1016/j.jsg.2008.11.012
    Tatham, D. J., Lloyd, G. E., Butler, R. W. H., et al., 2008. Amphibole and Lower Crustal Seismic Properties. Earth and Planetary Science Letters, 267(1/2): 118-128. https://doi.org/10.1016/j.epsl.2007.11.042
    Tarling, D. H., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. Chapman & Hall, London. 217
    Uyeda, S., Fuller, M. D., Belshé, J. C., et al., 1963. Anisotropy of Magnetic Susceptibility of Rocks and Minerals. Journal of Geophysical Research, 68(1): 279-291. https://doi.org/10.1029/jz068i001p00279
    Wang, H. C., Liu, Q. S., Zhao, W. H., et al., 2015. Magnetic Properties of Archean Gneisses from the Northeastern North China Craton: The Relationship between Magnetism and Metamorphic Grade in the Deep Continental Crust. Geophysical Journal International, 201(1): 486-495. https://doi.org/10.1093/gji/ggv036
    Xu, H. J., Jin, Z. M., Mason, R., et al., 2009. Magnetic Susceptibility of Ultrahigh Pressure Eclogite: The Role of Retrogression. Tectonophysics, 475(2): 279-290. https://doi.org/10.1016/j.tecto.2009.03.020
    Xu, H. J., Jin, Z. M., Ou, X. G., 2006. Anisotropy of Magnetic Susceptibility of the Cores from the Mainhole (100-2 000 m) of the Chinese Continental Scientific Drilling: Implications for the Ultrahigh-Pressure (UHP) Metamorphic Rocks. Acta Petrologica Sinica, 22(7): 2081-2088 (in Chinese with English Abstract)
    Xu, Z. Q., Ji, S. C., Cai, Z. H., et al., 2012. Kinematics and Dynamics of the Namche Barwa Syntaxis, Eastern Himalaya: Constraints from Deformation, Fabrics and Geochronology. Gondwana Research, 21(1): 19-36. https://doi.org/10.1016/j.gr.2011.06.010
    Xue, Z. H., Martelet, G., Lin, W., et al., 2017. Mesozoic Crustal Thickening of the Longmenshan Belt (NE Tibet, China) by Imbrication of Basement Slices: Insights from Structural Analysis, Petrofabric and Magnetic Fabric Studies, and Gravity Modeling. Tectonics, 36(12): 3110-3134. https://doi.org/10.1002/2017tc004754
    Yin, A., Harrison, T. M., 2000. Geologic Evolution of the Himalayan- Tibetan Orogen. Annual Review of Earth and Planetary Sciences, 28(1): 211-280. https://doi.org/10.1146/annurev.earth.28.1.211
    Zhang, J. F., Green, H. W. Ⅱ, Bozhilov, K. N., 2006. Rheology of Omphacite at High Temperature and Pressure and Significance of Its Lattice Preferred Orientations. Earth and Planetary Science Letters, 246(3/4): 432-443. https://doi.org/10.1016/j.epsl.2006.04.006
    Zhang, Z. M., Zhao, G. C., Santosh, M., et al., 2010. Two Stages of Granulite Facies Metamorphism in the Eastern Himalayan Syntaxis, South Tibet: Petrology, Zircon Geochronology and Implications for the Subduction of Neo-Tethys and the Indian Continent beneath Asia. Journal of Metamorphic Geology, 28(7):719-733. https://doi.org/10.1111/j.1525-1314.2010.00885.x
    Zhang, Z. M., Dong, X., Santosh, M., et al., 2012. Petrology and Geochronology of the Namche Barwa Complex in the Eastern Himalayan Syntaxis, Tibet: Constraints on the Origin and Evolution of the North-Eastern Margin of the Indian Craton. Gondwana Research, 21(1): 123-137. https://doi.org/10.1016/j.gr.2011.02.002
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Magnetic Fabric and Petrofabric of Amphibolites from the Namcha Barwa Complex, Eastern Himalaya

    Corresponding author: Haijun Xu, hj_xu@sina.com
    Corresponding author: Junfeng Zhang, jfzhang@cug.edu.cn
  • State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China; School of Earth Sciences, China University of Geosciences, Wuhan 430074, China

Abstract: The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility (AMS) and the crystallographic preferred orientation (CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.3×10-4 to 3.314×10-2 SI, with the Jelínek's anisotropy values (Pj) ranges from 1.094 to 1.487. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction (EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the[001] axes along the lineation and the[100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.

0.   INTRODUCTION
  • Magnetic fabric and petrofabric, commonly described by second-rank tensors represented by ellipsoids, interest several fields of Earth sciences such as structural geology and geodynamics. Geologists and geophysicists have paid long and deep attention on these two studies, and often use them as tectonic indicators which can provide important information about the geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism (e.g., Punturo et al., 2017; Xue et al., 2017; Borradaile, 2001; Borradaile and Henry, 1997).

    Magnetic fabric is the magnetic anisotropy of rocks and minerals. The main parameters related to magnetic fabric include anisotropy of low-field magnetic susceptibility (AMS), anisotropy of anhysteretic remnant magnetization (AARM), anisotropy of isothermal remanence (AIRM) and anisotropy of gyroremanence (AGRM). The AMS describes the variation in magnetic susceptibility caused by the distribution, shape and crystalline anisotropy of magnetic material. Compared with other magnetic fabric parameters, the AMS is easy to acquire and is often used as a proxy for mineral texture in geological applications such as grain formation on deformation processes in metamorphic rocks or flow processes in igneous rocks (e.g., Biedermann et al., 2015; Borradaile and Jackson, 2010). Though this magnetic method is easy to use, researchers have found it difficult to explain exactly the origin as well as the implication of magnetic fabrics of natural rocks. It is now well known that, in rock magnetism, magnetic fabric originates from shape anisotropy and crystalline anisotropy which often operate individually (Uyeda et al., 1963). The shape anisotropy plays a leading role in cubic magnetite crystal and rocks containing magnetite (Grégoire et al., 1995), whereas crystalline anisotropy dominates in many rock-forming minerals such as phyllosilicates, amphiboles and ilmenites (Schmidt et al., 2009; Chadima et al., 2004; Hrouda et al., 1997; Hirt et al., 1995).

    Petrofabric describes the deformation features in natural rocks or experimental samples focusing on the grain scale. The most studied features of petrofabric including distribution, morphology and orientation of mineral grains, referred as crystallographic preferred orientation (CPO) and shape preferred orientation (SPO). Traditional methods for unraveling petrofabric include U-stage optical microscopy, X-ray goniometry, neutron diffraction and the upstart electron backscatter diffraction (EBSD). Recent years, scientists have paid more attention to establish relation between physical properties and intrinsic anisotropy of minerals as well as their fabrics in a rock. For instance, preferred orientations of minerals with crystallographic anisotropy are considered the main cause of some geophysical anomalies, such as seismic anisotropy and magnetic anomaly (Almqvist and Mainprice, 2017; Biedermann et al., 2017; Punturo et al., 2017; Ji et al., 2015; Li et al., 2015; Wang et al., 2015). Quantifying petrophysical properties and relations to lithology and rock fabric are important to understand the behavior of middle-lower continental crust (Biedermann, 2018; Almqvist and Mainprice, 2017; Punturo et al., 2017; Xu et al., 2009, 2006). More gratifying is that the AMS of many rock types have been successfully modeled based on the intrinsic magnetic property of rock-forming minerals and their model contents as well as their CPOs measured by EBSD, which casts light for better understanding the origin of magnetic fabrics in rocks (e.g., Biedermann, 2018; Biedermann et al., 2018, 2015; Robinson et al., 2006).

    Amphibolite is one of the dominant rock types in the middle-lower crust, containing valuable information of the composition, structure and evolution of the continental crust, and has been extensively documented in literatures. The mineral fabrics of amphibole, plagioclase and quartz in amphibolite are well established, and there exists considerable knowledge about the relation between mineral fabric and deformation, flow pattern and seismic anisotropy (e.g., Ko and Jung, 2015; Kitamura, 2006). In contrast, only a few studies attempted to establish relation between magnetic fabric and petrofabric of amphibolite (e.g., Biedermann et al., 2018).

    In this study, fresh amphibolites were collected from the Namcha Barwa Complex in the eastern Tibet. We have studied in detail of petrography and fabrics of amphibolites, simulated the magnetic fabrics on the base of amphibole CPOs. The results indicate a strong correlation between magnetic fabric and petrofabric of natural amphibolites.

1.   GEOLOGICAL SETTINGS
  • The amphibolite samples were collected near the Danniang Village (29°26'45″N, 94°41'38″E), about 40 km southeast of the Linzi County, Tibet (Fig. 1). The study area belongs to the Namcha Barwa Complex (NBC) on the eastern terminus of Himalayan orogenic belt. The Himalayan-Tibetan orogeny is a composite orogeny that formed under sequential northward indention of microcontinents, accretionary belts and island arcs onto the southern margin of Eurasia since the Early Paleozoic (Xu et al., 2012; Zhang et al., 2012). The NBC, also referred as the Greater Himalayan Sequence (GHS), comprises Late Proterozoic to Early Cambrian metasedimentary rocks (Booth et al., 2009, 2004; Yin and Harrison, 2000). The main rock comprises granulite, amphibolite, high-grade gneiss, schist, migmatite, quartzite and marble. Detailed petrological and geochronological studies indicate three evolutionary stages: initial subduction under Lhasa terrane before ~40 Ma, subsequent amphibolite and granulite-facies metamorphism at 37–32 Ma, and fast exhumation with migmatization at 25–18 Ma (Xu et al., 2012; Zhang et al., 2012). The mafic granulite have yielded peak metamorphic conditions at 750–900 ℃ and 1.4–1.8 GPa (Zhang et al., 2010; Ding and Zhong, 1999; Liu and Zhong, 1998, 1997).

    Figure 1.  Simplified geological map of the Namcha Barwa Complex (modified after Geng et al., 2006), showing the location of the studied samples (a); and (b) principal terrane boundaries in East Asia and the location of Namcha Barwa (modified after Chung et al., 2005). ATF. Altyn Tagh fault; KF. Kunlun fault; KJFZ. Karakorum-Jiali fault zone; RRF. Red River fault; WCF. Wang Chao fault.

2.   METHODS
  • Six samples from the Namcha Barwa Complex were analyzed in detail. Thin sections were cut perpendicular to the foliation and parallel to the lineation (XZ-plane). For optical microscopic observations and EBSD analyses, all the thin sections were polished into standard thickness about 30 μm using a series of diamond powders of decreasing grain size for surface grinding. Remaining surface damages were then removed by vibration polishing with 0.05 μm colloidal silica for at least 2 h.

  • EBSD was applied for unravel mineral fabric. The EBSD data were obtained using a Quanta 450 Field Emission Gun (FEG) scanning electron microscope (SEM), equipped with an HKL Nordlys EBSD detector, housed at the State Key Laboratory of Geological Processes and Mineral Resources (GPMR) of China University of Geosciences, Wuhan. Working conditions were set as following: 15–20 kV accelerating voltage, 20–25 mm working distance, 6 spot size, 70° sample tilt in low vacuum mode (20–60 Pa of gaseous water) to avoid excessive electron charging of sample. Only those measurements with mean angular deviation values below 1.0° (between detected and simulated EBSD patterns) were accepted for analyses in manual interactive mode.

  • Quantitative chemical analyses of minerals were conducted with a JEOL electron probe microanalyzer (EPMA) JXA-8100, equipped with four wavelength-dispersive spectrometers, housed by the Key Laboratory of Submarine Geosciences, State Oceanic Administration at Hangzhou, China. The accelerating voltage, beam current and counting time were set to 15 kV, 20 nA and 30 s, respectively. Pure oxides and simple silicates of known compositions were selected as standards for calibration.

  • Low-field AMS were measured on an AGICO MFK1-FA susceptibility bridge at the State Key Laboratory of Biogeology and Environmental Geology of China University of Geosciences, Wuhan. Measurements were performed at a frequency of 976 Hz, and in a field of 200 mA-1, allowing anisotropy discrimination below 0.2% over a wide range of susceptibility. Samples TOO-38 and 16LZ-21 were drilled into oriented cylinders for AMS measurement. The cylinders were drilled with the long axes parallel to foliation and perpendicular to lineation. Each cylinder is approximately 22 mm in length and 25 mm in diameter, the standard size for magnetic measurement.

    In the low-field AMS measurement, the mean susceptibility was determined from the volume-normalized directional measurements and is referred to as volume susceptibility. The magnetic susceptibility is described mathematically by a second-rank symmetric tensor k, expressed by its principal eigen vectors and eigen values k1k2k3, representing the maximum, intermediate, minimum principal susceptibility respectively, and the corresponding eigenvectors define the direction of each principal susceptibility axis. Logarithmic parameters of the corrected anisotropy degree (Pj) and the shape parameter (U) are defined according to Jelínek (1981) and Biedermann et al. (2018), respectively.

    Two cylinders of TOO-38 were chosen for thermomagnetic analysis. Temperature dependence of the magnetic susceptibility was measured in an argon atmosphere using an AGICO MFK1-FA susceptibility bridge equipped with a CS-3 high-temperature furnace (AGICO Ltd., Brno) at the State Key Laboratory of Biogeology and Environmental Geology of China University of Geosciences, Wuhan. Measurements of susceptibility were made at a rate of about 9.5 ℃ per minute from room temperature up to maximum temperature of 700 ℃, and then back to room temperature.

  • The bulk anisotropy of magnetic susceptibility in a rock is determined by the single crystal magnetic anisotropy, modal contents and orientation of the magnetic carriers. In the last three decades, the whole-rock AMS for different rock types such as peridotite, amphibolite and gneiss have been successfully modeled based on these three factors (e.g., Biedermann et al., 2018, 2015; Mainprice et al., 2011; Mainprice and Humbert, 1994).

    In this study, the amphibolite samples are mainly composed of amphibole, plagioclase, quartz and minor garnet, biotite, magnetite, ilmenite. According to the single crystal AMS study, plagioclase and quartz have very low bulk susceptibility and weak anisotropy compared to amphibole (Biedermann et al., 2016), which should have little contribution to the principal susceptibility directions. Meanwhile, the minor magnetite without shape anisotropy would increase the mean susceptibility and lower the anisotropy degree. As second rank of magnetic carrier in amphibolite, the ilmenite may have certain contribution for the low magnetic susceptibility. In order to simplify the calculation, we only input the single crystal AMS and the measured CPO of amphibole during numerical modeling of the magnetic anisotropy of amphibolite.

    The AMS of the amphibolites were characterized by Hill average tensors using software of Matlab (version R2015a) with toolbox of MTEX (version 4.5.2). The difference of iron content in hornblende can slightly affect the magnetic susceptibility (Biedermann et al., 2018, 2015). Therefore, the following averaged single crystal AMS tensor of hornblende is used with a*=(100)||X, b=[010]||Y, c=[001]||Z (all entries in 10-7 m3 kg-1)

3.   RESULTS
  • The studied amphibolites are generally fresh and slightly retrogressed. The gray rock samples contain major minerals of amphibole (60 vol%–77 vol%) and plagioclase (15 vol%–27 vol%), and minor minerals of quartz, garnet, biotite, ilmenite and magnetite, showing medium- to coarse-grained inequigranular texture or porphyroblastic texture (Fig. 2a). All the samples are characterized by clear macroscopic foliation and lineation defined by banded composition domains and aligned minerals of amphibole, plagioclase and ilmenite (Fig. 2). The amphibole grains are subhedral to anhedral without clear evidence of intracrystalline deformation (Figs. 3a, 3b). In contrast, the plagioclase and quartz grains show mechanical twinning, serrated grains boundaries and intracrystalline deformation lamella (Fig. 3c). In sample TOO-38, euhedral magnetite grains are occasionally found along the grain boundaries of amphibole (Fig. 3d). Ilmenite commonly exists together with amphibole with three distinct characteristics: elongated aggregates along grain boundaries (Fig. 2b); as inclusions in amphibolite (Fig. 3e); netlike morphology along grain boundaries or factures (Fig. 3f).

    Figure 2.  Microphotographs showing mineral assemblage and texture of amphibolites from the Namcha Barwa Complex. Well oriented amphibole (Amp), plagioclase (Pl) and ilmenite (Ilm) showing clear lineation in XZ section. (a) Sample 16LZ-14, (b) 16LZ-20, (c) TOO-38 and (d) 16LZ-21. Grt. Garnet. Bi. biotite.

    Figure 3.  Transmitted-light microphotographs (a)–(c) and BSE images (d)–(f) of amphibolite. (a) Millimeter-sized amphibole grains with tabular shapes. (b) Amphibole showing serrated grain boundaries. (c) Recrystallized fine-grained quartz grains. (d) Euhedral cubic magnetite along amphibole grain boundaries. (e) Anhedral ilmenite grains included in an amphibole grain. (f) Ilmenite along amphibole grain boundaries showing a netlike morphology. Qtz. quartz; Mag. magnesite.

    The chemical compositions of amphibole and plagioclase are summarized in Table 1. The magnesium number (Mg#=Mg/ (Mg+Fe2+)) and iron contents (FeO in wt.%) of amphibole varies between 0.48 and 0.71 and between 13.07% and 19.49%, respectively. According to the general classification of the amphiboles by Leake et al. (1997), the amphibole can be classified into pargasite, magnesiohornblende or tschermakite. The composition of plagioclase varies between oligoclase and andesine. Based on the geothermometer of Holland and Blundy (1994), the formation temperatures of these amphibolites are estimated to be 607–647 ℃.

    Sample 16LZ-14 16LZ-21 16LZ-23 16LZ-25 16LZ-26 TOO-38
    Amphibole
    SiO2 41.87 46.98 46.37 42.62 40.55 43.37
    Al2O3 13.09 8.55 10.13 12.86 14.65 14.63
    TiO2 1.68 0.88 0.46 2.01 1.09 0.56
    Cr2O3 0.08 0.05 - 0.05 0.06 0.04
    FeO 19.49 15.00 13.07 16.21 17.12 15.81
    MnO 0.14 0.31 0.17 0.25 0.26 0.17
    MgO 7.99 12.58 13.37 9.86 10.13 10.17
    CaO 11.14 12.03 11.88 11.52 10.62 9.87
    Na2O 1.52 1.04 1.52 1.56 2.21 2.56
    K2O 1.55 0.88 1.06 1.56 0.84 0.26
    Cl 0.00 0.14 0.28 0.02 1.05 0.10
    Total 98.55 98.44 98.31 98.52 98.58 97.54
    Mg# 0.48 0.67 0.71 0.56 0.70 0.69
    parg Mg-hbl Mg-hbl parg tsch tsch
    Plagioclase
    SiO2 60.73 57.83 61.91 58.68 62.47 63.30
    Al2O3 23.67 25.98 23.12 25.35 22.79 22.49
    TiO2 0.02 0.02 0.00 0.00 0.02 0.00
    FeO 0.11 0.11 0.08 0.13 0.10 0.11
    MnO 0.01 0.00 0.04 0.03 0.03 0.00
    MgO 0.01 0.00 0.00 0.01 0.00 0.00
    CaO 6.08 8.80 5.36 7.84 4.82 4.36
    Na2O 7.78 6.40 8.16 6.68 8.41 9.04
    K2O 0.43 0.30 0.38 0.47 0.13 0.07
    Total 98.84 99.44 99.05 99.19 98.77 99.37
    Ab 0.68 0.56 0.72 0.59 0.75 0.79
    An 0.29 0.42 0.26 0.38 0.24 0.21
    Or 0.03 0.02 0.02 0.03 0.01 0.00
    Ab=Na/(Na+K+Ca); An=Ca/(Na+K+Ca); Or=K/(Na+K+Ca); parg. pargasite; Mg-hbl. magnesiohornblende; tsch. tschermakite.

    Table 1.  Representative mineral compositions of amphibolites from Namcha Barwa

  • The CPOs of amphibole are shown in Fig. 4. The amphiboles of all five samples show similar patterns, with [001] axes subparallel to the lineation and [100] axes forming high concentrations or girdles nearly normal to the foliation. However, a major difference between the measured samples is the spread in distribution of crystallographic axes of [100] axis and (010) pole. In the three samples of 16LZ-14, 16LZ-21 and 16LZ-23, the [100] axes form a point distribution normal to the foliation, while the (010) poles form a large girdle with a weak point distribution in the foliation normal to the lineation. These patterns are similar to the first type CPO of amphibole experimentally deformed by simple shear (Ko and Jung, 2015), which is common in natural amphibolites (e.g., Biedermann et al., 2018). In contrast, the other two samples of 16LZ-26 and TOO-38 are characterized by strong point distribution of [001] axes around the lineation, whereas the [100] axes and the (010) poles form typical girdle patterns normal to the lineation.

    Figure 4.  Crystallographic preferred orientation of amphibole in amphibolites from the Namcha Barwa Complex. Pole figures are plotted in lower hemisphere equal area projections with a half width of 20°. The color coding refers to the density of data points, and the contours correspond to the multiples of uniform distribution. N is the number of discrete grains measured. Horizontal line represents plane of foliation (XY) with lineation direction at X.

  • Thermomagnetic curve, representing the temperature dependence of low field magnetic susceptibility, is sensitive to subtle changes of magnetic minerals during thermal treatment. The thermomagnetic curve thus provides useful information on magnetic susceptibility behavior versus temperature and for identification of magnetic mineralogy (e.g., Dunlop and Özdemir, 1997). The irreversible thermomagnetic curves during cooling indicate the formation of new magnetic phases during heating (Fig. 5). The heating curves show a distinct Hopkinson- type peak in the vicinity of 580 ℃ (the Curie point of magnetite), indicating the presence of magnetite in one amphibolite sample (TOO-38-2). In contrast, the other sample (TOO-38-1) shows relatively higher susceptibility during cooling, which may indicate unknown high susceptibly magnetic minerals. The appearance of a distinct λ-shaped peak around 250 ℃ during cooling in TOO-38-2 may probably be interpreted as the formation of titanium-bearing magnetic phases during cooling. Therefore, these thermomagnetic susceptibility behaviors indicate that the dominant magnetic carriers are magnetite, ilmenite and paramagnetic amphibole.

    Figure 5.  Normalized thermomagnetic curves for amphibolite rocks from the Namcha Barwa Complex

  • The magnetic properties of amphibolite are listed in Table 2. The bulk magnetic susceptibility ranges from 7.06×10-3 to 3.314×10-2 SI, with an average of 1.971×10-2 SI for sample TOO-38. The bulk magnetic susceptibility of sample 16LZ-21 is one order of magnitude lower, ranging from 7.3×10-4 to 7.6×10-4 SI. The corresponding mean mass susceptibility for TOO-38 and 16LZ-21 are 6.24×10-6 and 2.48×10-7 m3/kg, respectively. The corrected Jelínek anisotropy values (Pj) of low field magnetic susceptibility varies between 1.094 and 1.487, whereas the κ' ranges from 0.9×10-8 to 1.11×10-6 m3/kg. The shape parameter of magnetic susceptibility U varies from -0.38 to 0.87 at room temperature. These parameters indicate that the measured samples have AMS ellipsoids with both prolate and oblate shapes.

    Sample Bulk susceptibility Mass susceptibility Susceptibility axes Anisotropy shape Anisotropy degree
    κ (10-3 SI) χ (10-6 m3/kg) κ1 κ2 κ3 U κ' (10-6 m3/kg) Pj
    TOO-38
    1 22.77 7.21 1.191 7 0.993 7 0.814 6 -0.05 1.110 1.463
    2 24.30 7.69 1.090 9 1.014 7 0.894 4 0.22 0.622 1.223
    3 22.64 7.16 1.128 4 1.022 6 0.848 9 0.24 0.826 1.335
    4 7.06 2.23 1.223 3 0.949 6 0.827 0 -0.38 0.370 1.487
    5 20.64 6.53 1.149 6 1.017 8 0.832 6 0.17 0.849 1.385
    6 22.04 6.97 1.094 7 1.044 1 0.861 2 0.57 0.700 1.289
    7 14.49 4.59 1.126 7 0.990 7 0.882 6 -0.11 0.458 1.277
    8 9.02 2.85 1.187 2 0.971 8 0.841 0 -0.24 0.407 1.414
    9 13.93 4.41 1.160 8 0.975 6 0.863 6 -0.25 0.540 1.346
    10 18.42 5.83 1.105 2 1.009 9 0.884 9 0.13 0.526 1.251
    11 25.76 8.15 1.159 4 1.013 2 0.827 3 0.12 1.108 1.405
    12 22.06 6.98 1.130 7 1.008 3 0.861 0 0.09 0.770 1.315
    13 33.14 10.49 1.083 0 1.024 7 0.892 3 0.39 0.837 1.221
    16LZ-21
    1 0.73 0.24 1.034 4 1.028 1 0.937 5 0.87 0.011 1.117
    2 0.76 0.25 1.034 0 1.016 4 0.949 6 0.58 0.009 1.094

    Table 2.  Magnetic parameters of two amphibolites from the Namcha Barwa Complex

    Principal directions of the low field anisotropy magnetic susceptibility at room temperature are shown in Fig. 6. The samples yield a mean tensor with recognizable orthorhombic symmetry which has close relationships with the macroscopic rock foliation and lineation defined by mineral orientation and compositional banding. That is, the maximum susceptibility axes (Kmax) are subparallel to the mineral lineation (X), and the intermediate susceptibility axes (Kint) are normal to the lineation (X) in the foliation plane (XY), while the minimum susceptibility axes (Kmin) are subnormal to the foliation plane. There is no clear correlation between the magnetic anisotropy degree (Pj) and magnetic susceptibility in all measurements (Fig. 7). However, there's a positive correlation in their mean values, which probably indicate that high susceptibility of magnetite and ilmenite control the AMS of the amphibolite.

    Figure 6.  Measured AMS of amphibolites. (a) Sample TOO-38; (b) 16LZ-21. Mean directions of Kmax and Kmin are displayed as cross marks in circle and diamond respectively. Note that the maximum susceptibility axes (Kmax) of TOO-38 and 16LZ-21 are subparallel to the mineral lineation (X), and the intermediate susceptibility axes (Kint) are normal to the lineation (X) in the foliation plane (XY), while the minimum susceptibility axes (Kmin) are subnormal to the foliation plane.

    Figure 7.  Plot of corrected magnetic anisotropy Pj versus mean magnetic susceptibility Km.

  • The AMS ellipsoid of amphibolite was modeled from the amphibole texture based on the EBSD data. In order to simplify the calculation, we assume that the low field magnetic susceptibility of amphibolite attribute solely to the main mineral of paramagnetic amphibole with high model content of 60 vol%–70 vol%. However, it should be pointed out that the minor mineral of magnetite (without shape anisotropy) will increase the bulk magnetic susceptibility and decrease the anisotropy degree to some extent depending on their model contents if taken into account during simulation calculation (Biedermann, 2018).

    The modeled AMS ellipsoids (Fig. 8) has principal susceptibility axes that agree well with the orientation of the measured principal axes (Fig. 6). The modeled maximum susceptibility of each sample is subparallel to the rock lineation, whereas the modeled minimum susceptibility is normal to the foliation. These results are similar to previous results (Biedermann et al., 2018).

    Figure 8.  Lower hemisphere stereoplots showing the modeled AMS (color coded, in 10-7 m3·kg-1) based on the EBSD-derived CPO data of amphibole in amphibolites at room temperature. White symbols indicate eigenvector directions: square maximum, triangle intermediate and circle minimum susceptibility.

4.   DISCUSSION
  • The magnetic fabrics of a rock depends on several factors including the intrinsic magnetic properties, model contents, SPOs and CPOs of rock-forming minerals (e.g., Biedermann, 2018; Robinson et al., 2006). The calculated magnetic susceptibility of amphibolite is 5.9×10-4 to 1.2×10-3 SI if presume that the magnetic susceptibility of amphibolite is solely controlled by 60%– 80% amphibole, consistent with the measured magnetic susceptibility of sample 16LZ-21 (7.3×10-4 to 7.6×10-4 SI) (Fig. 9). In contrast, the bulk magnetic susceptibility of sample TOO-38 varies between 7.06×10-3 and 3.314×10-2 SI, with an average of 1.971×10-2 SI, which is one magnitude higher than the theoretical value of amphibolite. Comparing with previous studies, these values are much higher than those reported previously for amphibolites (Biedermann et al., 2018) and other rock types such as granulite-facies rocks in the North China Craton and serpentinized peridotites from Southwest Tibet (Li et al., 2017, 2015; Wang et al., 2015; Liu et al., 2013). The high magnetic susceptibility of sample TOO-38 can be attributed to minor minerals with high susceptibility such as magnetite (Fig. 9), ilmenite and some unknown titanium-bearing minerals (Fig. 5). Only 0.2%–1% magnetite is needed to account for bulk susceptibility of sample TOO-38. Euhedral magnetite grains are occasionally found along the grain boundaries of amphiboles (Fig. 3d), indicating the possible secondary origin during retrogression. Ilmenite (model content up to 3 vol%–4 vol%) is common in all the studied amphibolites. The ilmenite grains have different origins including the primary generation of ilmenite with amphibole and secondary netlike ilmenite during retrogression of amphibolite. When compared with magnetite, the effect of ilmenite on bulk magnetic susceptibility is neglectable due to its low content (Tarling and Hrouda, 1993). Therefore, we propose that the magnetic susceptibility of sample 16LZ-21 is mainly contributed by paramagnetic amphibole, while the magnetic susceptibility of sample TOO-38 is mainly contributed by magnetite. Because of the heterogenous distribution of garnet and magnetite in drilled cores, the relatively high and larger variations of magnetic susceptibility of sample TOO-38 are conceivable (Table 2).

    Figure 9.  Mineral contributions to bulk susceptibility of a rock. The measured bulk susceptibility are denoted by grey-shaded area for sample TOO-38 and the red line for sample 16LZ-21, respectively. The yellow-shaded area is the calculated susceptibility of amphibolite by assuming 60%–80% amphibole. Mineral susceptibilities are from Hrouda and Kahan (1991).

    In amphibolites, the presence of plagioclase and quartz lower the rock's mean susceptibility because of their weak magnetic susceptibility comparing to amphibole. However, their effects on the AMS are limited and neglectable in modeled calculations (Biedermann et al., 2018, 2016). The magnetic anisotropy of magnetite is controlled by its shape anisotropy and distribution (Grégoire et al., 1995). It is known that the high susceptibility mineral of magnetite is nearly isotropic, therefore the presence of magnetite increase the bulk magnetic susceptibility while lowering the bulk magnetic anisotropy of amphibolite rocks. Besides it should be noted that for those high magnetic rocks in which the mean susceptibility is higher than 5×10-3 SI, the iron-titanium oxides such as titanomagnetite and ilmenohaematite solid solutions are particularly important owing to their high magnetic susceptibility and anisotropy (Tarling and Hrouda, 1993).

    The studied amphibolites show obvious macroscopic foliation and lineation defined by aligned amphibole, plagioclase and ilmenite (Fig. 2). Amphibole (composition between pargasite, magnesiohornblende and tschermakite) and plagioclase (composition between oligoclase and andesine) are the two major minerals in amphibolites, which have model contents of 60 vol%–77 vol% and 15 vol%–27 vol%, respectively. The EBSD results indicate two kinds of amphibole CPOs in six rock samples (Fig. 4). The first is similar to the type-Ⅰ amphibole fabric reported in simple shear experimental studies (Ko and Jung, 2015) and many natural amphibolites from different tectonic settings (e.g., Ji et al., 2015; Cao et al., 2010; Tatham et al., 2008). That is the [100] axes forming a point distribution normal to the foliation, while the (010) poles forming a large girdle with a weak point distribution in the foliation normal to the lineation (Figs. 4a4c). The second was found in samples 16LZ-26 and TOO-38, characterized by strong point distribution of [001] axes around the lineation, whereas the [100] axes and the (010) poles form typical girdle patterns normal lineation (Figs. 4d4e). The second type is similar to the "L-type" CPO of clinopyroxene (Zhang et al., 2006), which may result from a simple shear or extruding strain.

    Despite the magnetic susceptibility of samples TOO-38 and 16LZ-21 are dominated by magnetite and amphibole, respectively, the low field AMS of two amphibolite samples have a recognizable orthorhombic symmetry and an identical relationship with macroscopic rock fabrics (Fig. 6): the maximum susceptibility axis subparallel to the mineral lineation; the intermediate susceptibility axis normal to the lineation in the foliation plane, the minimum susceptibility axis subnormal to the foliation plane. Meanwhile, the modeled maximum susceptibility of each sample is subparallel to the rock lineation, whereas the modeled minimum susceptibility is subnormal to the foliation (Fig. 8). In the Cartesian coordinates defined by macroscopic lineation and foliation plane, the modeled AMS ellipsoids has principal susceptibility axes that agree well with the orientation of the measured principal axes. In contrasts with three model rocks correspond to lineation-dominated (L-type), strong lineation and strong foliation (LS type) and foliation- dominated (S-type) textures (Biedermann et al., 2018), the natural amphibolites from NBC in Tibet are similar to the lineation-dominated patterns. These results indicate that magnetic fabric data from AMS contain useful information about petrofabric especially amphibole CPO in natural amphibolite. The consistency of magnetic lineation and mineral lineation of natural amphibolites provide a reliable tool to predict mineral slip direction in the field. Meanwhile, numerical simulations with variety of natural samples help us predict the real magnetic fabrics in nature, and give reference when dealing with interpretations of AMS in middle-lower crustal rocks.

    In addition, it should be noted, the bulk magnetic susceptibility of the natural amphibolites show strong variations between 7.3×10-4 and 3.314×10-2 SI, with the Jelínek anisotropy values (Pj) ranging from 1.094 to 1.487 (Table 2). Theoretically, the high magnetic susceptibility and the anisotropy of amphibolite sample TOO-38 are primarily carried by the ferromagnetic fraction (Tarling and Hrouda, 1993). Indeed, the thermomagnetic curves reveal that the magnetic carriers are magnetite and titanium oxides (Fig. 5). In thin sections, magnetite and ilmenite are commonly found along amphibole grain boundaries (Figs. 3d, 3e). Interestingly, many ilmenite grains show well shape orientations in consistent with amphibole which determine the rock lineation and foliation (Fig. 2). Though great difference between bulk magnetic susceptibility, the amphibolites TOO-38 and 16LZ-21 show similar patterns of measured and modeled AMS (Figs. 6, 8). Therefore, we propose that the well oriented titanium oxides such as ilmenite and iron-titanium solid solutions along rock foliation and lineation will greatly increase both the bulk susceptibility and the anisotropy of a rock.

5.   CONCLUSIONS
  • We have investigated the low field AMS and magnetic fabrics simulated based on EBSD-derived CPO data and isolated paramagnetic anisotropy of amphiboles collected from the Namcha Barwa Complex in the eastern Tibet. The bulk magnetic susceptibility show strong variations between 7.3×10-4 and 3.314×10-2 SI, with the Jelínek anisotropy values (Pj) ranging from 1.094 to 1.487. EBSD analyses show clear CPOs of amphibole in all samples, with a preferred alignment of [001] axes along the lineation, whereas [100] spread along a girdle normal to the lineation along with relatively weak point distributions normal to the foliation plane. Numerical simulations of magnetic anisotropy show well consistency with the measured magnetic anisotropy, i.e., the maximum susceptibility is approximately parallel to lineation and minimum susceptibility is subnormal to foliation plane, suggesting that the magnetic fabrics of amphibolites are largely controlled by the CPOs of amphibole. The well oriented iron-titanium oxides along rock foliation and lineation could greatly increase the susceptibility and the anisotropy of a rock. These results confirmed that the magnetic anisotropy can be used to obtain information about mineral fabrics amphibolite.

ACKNOWLEDGMENTS
  • We thank Prof. Zongmin Zhu for facilitating access to the measurement of magnetic properties, Drs. Feng Shi and Wenlong Liu for assistance with EBSD analyses and magnetic modeling, Dr. Jing Chen and Prof. Yiming Ma for help with the sample preparation. Dr. Zhenhua Xue and an anonymous reviewer are gratefully acknowledged for their constructive comments and advices on the manuscript. This study was supported by the National Natural Science Foundation of China (Nos. 41425012, 41872230, 41772222), the National Key Basic Research Program of China (No. 2015CB856101), and the MOST Special Fund from the State Key Laboratory of GPMR. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1021-7.

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