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Volume 41 Issue 4
Aug.  2020
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Yi Cao, Xinzhuan Guo, Jinxue Du. Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau. Journal of Earth Science, 2020, 31(4): 683-692. doi: 10.1007/s12583-020-1290-1
Citation: Yi Cao, Xinzhuan Guo, Jinxue Du. Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau. Journal of Earth Science, 2020, 31(4): 683-692. doi: 10.1007/s12583-020-1290-1

Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau

doi: 10.1007/s12583-020-1290-1
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  • Understanding the electrical conductivity of high pressure metamorphic rocks is essential to constrain the compositions in the subduction zone and continental crust. In this study, we calculated the electrical conductivity for such rocks sampled from the central Qiangtang metamorphic belt in the northern Tibetan Plateau. The results reveal that, when aqueous fluids are absent, the conductivity of meta-mafic rocks (e.g., eclogite and amphibolite) is strikingly higher than that of meta-felsic rocks (e.g., garnet-quartz-mica schist). The conductivity of eclogite decreases due to the enrichment of amphibole, but this effect is diminished when a critical degree of amphibolization is reached. Our calculated conductivity of eclogite and amphibolite differs greatly from the experimentally derived results for the eclogites from other localities, partly owing to the strong effects of different mineral assemblages and chemical compositions on the conduction mechanisms and efficiencies. However, the disparity of conductivity between our calculated and the previously measured results for a similar amphibole-rich eclogite sampled from the same locality suggests that trails of highly conductive rutile-ilmenite aggregates may contribute to the higher bulk-rock conductivity in the laboratory measurements. Moreover, since the calculated conductivity of eclogite and amphibolite is not high enough at the temperatures relevant to their metamorphic thermal condition, partial melts or aqueous fluids originated from the upwelling asthenosphere are more likely to explain the anomalously high electrical conductivity zones in magnetotelluric images in the Qiangtang terrane in the northern Tibetan Plateau.
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Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau

doi: 10.1007/s12583-020-1290-1

Abstract: Understanding the electrical conductivity of high pressure metamorphic rocks is essential to constrain the compositions in the subduction zone and continental crust. In this study, we calculated the electrical conductivity for such rocks sampled from the central Qiangtang metamorphic belt in the northern Tibetan Plateau. The results reveal that, when aqueous fluids are absent, the conductivity of meta-mafic rocks (e.g., eclogite and amphibolite) is strikingly higher than that of meta-felsic rocks (e.g., garnet-quartz-mica schist). The conductivity of eclogite decreases due to the enrichment of amphibole, but this effect is diminished when a critical degree of amphibolization is reached. Our calculated conductivity of eclogite and amphibolite differs greatly from the experimentally derived results for the eclogites from other localities, partly owing to the strong effects of different mineral assemblages and chemical compositions on the conduction mechanisms and efficiencies. However, the disparity of conductivity between our calculated and the previously measured results for a similar amphibole-rich eclogite sampled from the same locality suggests that trails of highly conductive rutile-ilmenite aggregates may contribute to the higher bulk-rock conductivity in the laboratory measurements. Moreover, since the calculated conductivity of eclogite and amphibolite is not high enough at the temperatures relevant to their metamorphic thermal condition, partial melts or aqueous fluids originated from the upwelling asthenosphere are more likely to explain the anomalously high electrical conductivity zones in magnetotelluric images in the Qiangtang terrane in the northern Tibetan Plateau.

Yi Cao, Xinzhuan Guo, Jinxue Du. Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau. Journal of Earth Science, 2020, 31(4): 683-692. doi: 10.1007/s12583-020-1290-1
Citation: Yi Cao, Xinzhuan Guo, Jinxue Du. Electrical Conductivity of Eclogite, Amphibolite and Garnet-Quartz-Mica Schist with Implications for the Conductivity in the Qiangtang Terrane of Northern Tibetan Plateau. Journal of Earth Science, 2020, 31(4): 683-692. doi: 10.1007/s12583-020-1290-1
  • Eclogite is a meta-mafic rock consisting mainly of garnet (Grt) and omphacite (Omp) that are formed by high-pressure to ultrahigh-pressure (HP-UHP) metamorphism. Tectonically, eclogite commonly represents the mafic components in the subducted oceanic/continental crust or in the thickened orogenic crust during continental collision (Austrheim, 2013; Godard, 2001; Peacock, 1993; Carswell, 1990). However, except Grt and Omp, other mineral assemblages are also common and their volume proportions can be highly variable. For example, hydrous minerals such as amphibole (e.g., glaucophane and hornblende), epidote, lawsonite, mica (e.g., phengite and biotite) and chlorite, and nominally anhydrous minerals such as quartz, plagioclase, rutile, titanite, calcite and ilmenite can also be observed in the eclogite; and their presences indicate different equilibrated pressure-temperature (P-T) conditions and hydrous states of the HP-UHP metamorphism (Wei et al., 2009; Hacker et al., 2003; Liou et al., 1998). In the outcrops, eclogite often occurs as hard blocks within soft matrix that consists of meta-sediments, serpentinite and other weak meta-mafic rocks (i.e., oceanic-type mélange) or gneiss, marble, and quartzite (i.e., continental-type mélange), forming so-called "tectonic mélange" (e.g., Ernst, 2016; Angiboust et al., 2011; Spandler et al., 2008; Davis and Whitney, 2006; Baldwin et al., 2004; Yang et al., 2002; Liou et al., 2000).

    By comparison with the electrical conductivity (EC) results (especially anomalously high EC zones) deduced from field magnetotelluric (MT) surveys, the laboratory-derived EC data for various minerals, rocks, fluids/melts and their combinations can be adopted to constrain the composition and its heterogeneities in the Earth's crust and mantle (e.g., Guo and Keppler, 2019; Yoshino et al., 2018; Sakuma and Ichiki, 2016; Guo et al., 2015; Pommier et al., 2015; Karato and Wang, 2013; Dai et al., 2012). Since eclogite is an important rock type occurring at the subduction interface and the roots of orogenic belts, its EC has been experimentally measured at varied P-T conditions and employed to explain the regional MT observations (Dai et al., 2016; Guo et al., 2014; Bagdassarov et al., 2011; Čermák and Laštovičková, 1987; Laštovičková and Parchomenko, 1976). Moreover, one can calculate the EC of a multiphase rock using the available experimentally measured EC data for single phases (Garber et al., 2018; Wang et al., 2014). The calculation can estimate the approximate EC for the rocks whose chemical compositions and mineral assemblages are different from the samples directly analyzed by experiments, and thus can be better used to interpret the MT data based on the collections of natural rocks.

    In this study, we calculated the EC for the eclogites with varying contents of amphibole (Amp) and their garnet-quartz- mica (GQM) schist country rocks sampled from the Pianshishan region at the central Qiangtang metamorphic belt in the northern Tibetan Plateau. The results give clues about (1) how amphibolization process affects the EC of eclogite and (2) EC distinctiveness between meta-mafic and meta-felsic rocks. Besides, it is noteworthy that Guo et al. (2014) actually measured the EC for one eclogite that was collected from the same locality as our samples, which provides a good opportunity to make a comparison between our calculated and their measured results.

  • For our samples, we divided them into 5 categories in terms of their mineral assemblages and proportions: amphibole- poor eclogite (1 sample), amphibole-rich eclogite (8 samples), garnet-bearing amphibolite (3 samples), plagioclase-rich GQM schist (2 samples) and plagioclase-poor GQM schist (3 samples). The mineral species and their volume proportions of studied samples were measured using electron backscattered diffraction (EBSD) and were further corrected based on optical images. As shown in the EBSD phase maps, Amp-poor eclogite consists dominantly of Omp and Grt, in which Grt grains scatter as large porphyroblasts and Amp as a minor phase occurs sporadically in the Omp-rich matrix (Figs. 1a and 2a). In contrast, Amp-rich eclogite has comparable proportions of Amp and Omp, and dispays layering of Grt and trails of rutile and ilmenite (Rut-Ilm) aggregate (Figs. 1b and 2b). It is noted that this aggregate is dominated by Rut grains whose rim is often altered by dark-colored thin Ilm film (Fig. 2b). Minor phases such as Quartz (Qtz), plagioclase (Pl) and phengite (Phn) are also common. Different from the eclogite, Grt-bearing amphibolite is made up of Grt porphyroblasts and surrounding Amp dominated matrix, while Omp is a minor phase (Figs. 1c and 2c, 2d). Besides, one amphibolite sample displays abundant needle- shaped epidote (Ep) grains (Fig. 2d). Pl-rich GQM schist shows high volume proportion of Pl that often occurs together with Grt as large porphyroblasts in the fine-grained matrix consisting mainly of Phn and Qtz (Figs. 1d and 2e). In contrast, Pl-poor GQM schist has much smaller Pl content and exhibits alternated Grt-bearing layers of Qtz and Phn (Figs. 1e and 2f).

    Figure 1.  EBSD phase maps showing the mineral assemblages and microstructures in the representative rock types. (a) Amp-poor eclogite (P1502-2); (b) Amp-rich eclogite (P1511-1); (c) Grt-bearing amphibolite (P1507-15); (d) Pl-rich GQM schist (P1502-DB1); (e) Pl-poor GQM schist (P1507-8). Red dashed lines in the upper left corners denote the foliation in each sample. Red arrows in (b) indicate the rutile-ilmenite trails.

    Figure 2.  Optical photomicrographs showing the mineral assemblages and microstructures in the representative rock types. (a) Amp-poor eclogite (P1502-2); (b) Amp-rich eclogite (P1506-7); (c) Grt-bearing amphibolite containing minor amount of Ep (P1507-15); (d) Grt-bearing amphibolite containing abundant needle-shaped Ep (P1507-12); (e) Pl-rich GQM schist (P1503-6); (f) Pl-poor GQM schist (P1507-8).

  • To estimate the bulk-rock EC of eclogite, amphibolite and GQM schist, we used Hashin-Shtrikman (HS) averaging scheme to constrain the conductivity bounds (Berryman, 1995; Hashin and Shtrikman, 1962). This method treats the bulk rock which is a multiphase material as an isotropic polycrystal aggregate and can provide tighter constraints than the series and parallel solutions (Berryman, 1995)

    where σHS- and σHS+ are the lower and upper bounds, respectively; xi and σi are volume fraction and EC of mineral i and N is the number of minerals; σmin and σmax are the minimum and maximum EC among all individual phases, respectively. The σHS- represents isolated conductive inclusions in a resistive matrix, whereas σHS+ can be regarded as non-interconnected resistive inclusions in a conductive matrix.

    The temperature dependence of the EC of individual minerals was calculated using the Arrhenius equation.

    where σ0 is the reference pre-exponential factor (S/m), ΔH is the activation enthalpy in kJ/mol, R is gas constant, and T is absolute temperature in Kelvin. The Eq. (3) is inserted into Eqs. (1) and (2) to obtain the HS bounds.

    For our eclogite, amphibolite and GQM schist samples, more than 10 individual minerals are present (Fig. 1). This large number of phases increases the complexities of calculation and interpretation, we thus used a simplified mineral assemblage by neglecting those minor phases whose volume proportions are less than 5% (Table 1). This treatment ignores the contributions of minor phases to the conductivity, but is not considered to compromise the true conductivity remarkably, because all minor phases are small in contents and not inter- connected, and their contributions to the bulk-rock conductivity should not be substantial. The experimentally measured parameters of all rock-constituting minerals and natural eclogites— including the minerals not employed for the bulk-rock conductivity computation—used in our calculation are given in Table 2.

    Sample Rock type Omp Grt Phn Ep Amp Qtz Pl
    P1502-2 Amp-poor eclogite 1.000 0 - - - - - -
    P1502-1 Amp-rich eclogite 0.238 7 0.263 0 - - 0.498 3 - -
    P1502-5 Amp-rich eclogite 0.445 7 0.243 7 - - 0.310 6 - -
    P1503-3 Amp-rich eclogite 0.264 2 0.306 3 - - 0.351 1 0.078 3 -
    P1503-4 Amp-rich eclogite 0.169 7 0.344 9 - - 0.485 5 - -
    P1506-2 Amp-rich eclogite 0.201 0 0.214 7 - - 0.502 0 0.082 3 -
    P1506-7 Amp-rich eclogite 0.319 1 0.272 3 - - 0.408 6 - -
    P1507-2 Amp-rich eclogite 0.181 8 0.236 1 0.073 4 - 0.417 5 0.091 2 -
    P1511-1 Amp-rich eclogite 0.233 7 0.233 1 - - 0.533 3 - -
    P1504-6 Grt-bearing amphibolite - 0.073 5 - - 0.926 5 - -
    P1507-12 Grt-bearing amphibolite - 0.055 0 - 0.144 7 0.733 4 0.066 9 -
    P1507-15 Grt-bearing amphibolite - 0.080 9 - - 0.826 6 - 0.092 5
    P1502-DB1 Pl-rich GQM schist - 0.082 8 0.397 9 - - 0.281 7 0.237 5
    P1503-6 Pl-rich GQM schist - 0.068 4 0.351 8 - - 0.416 1 0.163 8
    P1507-4 Pl-poor GQM schist - 0.089 7 0.319 0 - - 0.591 3 -
    P1507-7 Pl-poor GQM schist - 0.079 4 0.452 2 - - 0.468 4 -
    P1507-8 Pl-poor GQM schist - 0.140 4 0.497 5 - - 0.362 2 -
    Omp. Omphacite; Grt. garnet; Phn. phengite; Ep. epidote; Amp. amphibole; Qtz. quartz; Pl. plagioclase.

    Table 1.  Simplified and normalized phase volume proportions

    Mineral/rock Pressure (GPa) Temperature (K) σ0 (S/m) ΔH (kJ/mol) Note References
    Garnet 1 873–1 273 54.33 73.32 Grt composition: pyrope 73, almandine 14, grossular 13 and H2O content of 465 ppm Dai et al. (2012)
    Clinopyroxene 1.2 523–1 273 3 630.78 71.00 Cpx composition: aegrine and H2O content of 375 ppm Yang et al. (2011)
    Amphibole 1 523–973 53.91 70.42 Amp composition: pargasite Zhou et al. (2011)
    Phengite 2.3 500–900 1.720 62.71 Chen et al. (2017)
    Epidote 1 523–1 073 158.05 65.60 Hu et al. (2017)
    Plagioclase 1 700–1 100 14.13 82.00 Pl composition: albite Guo et al. (2015)
    Quartz 1 710–1 073 0.417 85.86 Alpha-quartz, measurement parallel to c-axis Yoshino and Noritake (2011)
    Calcite 1 773–1 148 1.20E+08 159.18 Bagdassarov and Slutskii (2003)
    Rutile 1.01E-04 873–1 373 7.59E+05 146.64 Greener et al. (1965)
    Chlorite 2 673–923 0.101 54.02 Manthilake et al. (2016)
    Ilmenite 1.01E-04 400–1 050 1.28E+06 12.54 Andreozzi et al. (1996)
    Eclogite 1 873–1 173 67.61 81.04 Bimineralic eclogite, Dabie-Sulu Orogen Dai et al. (2016)
    Eclogite 1.5–3.5 500–650 1.995 43.40 Amp-bearing eclogite, Pianshishan, central Qiangtang, Tibet. ΔH calculated for 1 GPa using the given ΔV Guo et al. (2014)
    665-800 25 119 95.80
    Eclogite 2.5 646–1 142 29.42 80.07 Gln-bearing eclogite, At-Bashi, Tianshan Orogen Bagdassarov et al. (2011)
    Eclogite 1.01E-04 623–973 7.943 46.31 Eclogites from Bohemian Massif Laštovičková and Parchomenko (1976)
    633–1 173 2.818 62.71
    653–1 173 4.898 68.50
    473–1 173 11.22 71.39
    823–1 173 0.447 62.71
    633–873 0.631 59.81
    923–1 173 10 69.46
    583–1 173 7.943 77.18
    623–1 173 2.818 85.86

    Table 2.  Experimentally derived parameter values for the electrical conductivity of minerals and natural eclogite

  • The calculated EC varies greatly among individual minerals. As shown in Fig. 3, the most conductive mineral is ilmenite, showing very high EC of 105‒106 S/m in the temperature range of 500‒1 100 K at ambient pressure condition. In contrast, all the other minerals are much more resistive and their EC is lower than 0.1 S/m in the same temperature range except for calcite. The EC of calcite and rutile increases more markedly with temperature than the other minerals as characterized by their contrasting slopes. For the minerals that we used to calculate the bulk-rock EC, they have very similar temperature dependence of conductivity and exhibit EC in order of epidote > clinopyroxene (~375 ppm H2O) > amphibole > garnet (~463 ppm H2O) > phengite > quartz. Chlorite and albite are more resistive than most of other minerals and have disparate temperature dependences from each other (crossover at ~700 K). Notably, quartz is the least conductive mineral within the whole temperature range.

    Figure 3.  Calculated electrical conductivity of individual minerals (polycrystals or single crystal) as a function of temperature. Lines with markers are the minerals used for calculating the bulk-rock EC.

    As displayed in Fig. 4, the calculated bulk-rock EC at ~1‒2 GPa (i.e., depth of ~30‒60 km) condition is the highest for the Amp-poor eclogite. Since only single phase of Omp was used, the HS upper and lower bounds are coincident. Compared to the Amp-poor eclogite, the Amp-rich eclogite and Grt-bearing amphibolite have lower EC (both HS upper and lower bounds), but their HS upper bounds are close to the EC of Amp-poor eclogite (10-6–10-2 S/m at 500‒1 100 K) and their HS lower bounds are about 1.5‒2 orders of magnitude lower than their upper bounds—the discrepancy between upper and lower bounds decreases with increasing temperatures. The Pl-rich and Pl-poor GQM schists have similar magnitudes of EC, but the HS upper bounds of both schists (10-7‒10-3 S/m) fall by about one order of magnitude below those of eclogite and amphibolite. Similarly, the HS lower bounds of GQM schist are also about one order of magnitude lower than those of eclogite and amphibolite.

    Figure 4.  Calculated electrical conductivity of studied samples as a function of temperature: Amp-poor eclogite (red), Amp-rich eclogite (blue), Grt-bearing amphibolite (green), Pl-rich GQM schist (cyan) and Pl-poor GQM schist (magenta). The upper and lower HS bounds are denoted by solid and dashed lines, respectively. Experimentally determined and modeled conductivity of natural eclogites (black and gray lines) are also shown for comparison. The light and dark gray shade regions indicate the MT-derived electrical conductivity for lines 500 and 600 which section the Qiangtang terrane (modified after Guo et al. (2014)).

  • Based on the calculated EC, the most compelling finding is that the meta-felsic GQM schist has much lower conductivity than the meta-mafic eclogite and amphibolite, regardless of temperature (Fig. 4). This striking difference can be explained by the markedly lower EC of Qtz and Phn than that of Grt, Omp and Amp (Fig. 3). Therefore, this result suggests that, in the absence of aqueous fluids, the meta-mafic rocks are much more conductive than the meta-felsic rocks at depth of ~30‒60 km in the subduction interface or the continental lower crust. However, it must be cautious that the assumption of fluid absence may actually not be applicable to the wet tectonic settings, especially subduction zones. In fact, previous studies have proposed that, when aqueous fluids with significant amounts of dissolved ionic species (e.g., Na+, Cl and Al3+) are in the presence with quartz or albite, even a small amount of fluids (~1 vol.%) with salinity of ~3 wt.%‒10 wt.% can increase the conductivity of meta-felsic rocks tremendously up to 0.1 S/m at 800 K and 1 GPa (Guo et al., 2015; Shimojuku et al., 2014, 2012). This value is about 100 times greater than the HS upper bound conductivity of eclogite and amphibolite.

    Besides, we noted that Amp-poor eclogite is the most conductive meta-mafic rock (calculated using 100 vol.% Omp), while Amp-rich eclogite and most Grt-bearing amphibolite are obviously more resistive (Figs. 4 and 5). This result indicates that amphibolization actually reduces the conductivity of eclogite. However, the absence of correlation between Amp content and conductivity (especially HS upper bound) between Amp-rich eclogite and Grt-bearing amphibolite (Fig. 5) implies that the conductivity is reduced to a relatively constant level once the Amp content reaches a value—between the Amp contents of Amp-poor and Amp-rich eclogites—so that it can form an inter- connected network. This observation is consistent with the relatively higher resistivity of Fe-bearing amphibole prior to its dehydration or dehydrogenation (< 850 K) (Hu et al., 2018; Wang et al., 2012) than the hydrated clinopyroxene (Yang et al., 2011). One amphibolite sample presents similar intensity of HS upper bound EC to the Amp-poor eclogite (Figs. 4 and 5), owing to the enrichment of epidote (Table 1) which is the much more conductive than amphibole and omphacite (Fig. 3) (Hu et al., 2017).

    Figure 5.  Calculated electrical conductivity at ~1 GPa and 800 K for eclogite and amphibolite. Black arrow indicates the amphibolite sample (P1507-12) that has similar HS upper bound EC to the Amp-poor eclogite. The electrical conductivity of Cpx, Grt, Amp and Ep is shown for comparison.

    Compared with the existing data, our calculated HS upper bound EC for eclogite and amphibolite is overall about 1‒2 orders of magnitude higher than most laboratory-measured EC for the eclogites from Bohemian Massif, Dabie-Sulu and Tianshan Orogen (Dai et al., 2016; Bagdassarov et al., 2011; Laštovičková and Parchomenko, 1976) and modeled HS upper bound EC for the eclogites with varied water contents (Liu et al., 2019) (Fig. 4). The reason for this discrepancy is complicated and can be attributed to multiple sources such as the variations of mineral assemblage and composition affecting the conduction mechanisms and efficiencies. For example, the EC data we adopted for Omp, Amp and Grt are actually measured from hydrated clinopyroxene (~375 ppm H2O) of mainly aegrine composition (Yang et al., 2011), amphibole of mainly pargasite composition (Zhou et al., 2011), and hydrous garnet (465 ppm H2O) with pyrope dominated composition (Dai et al., 2012). The compositions of these minerals are different from the studied eclogites which typically have omphacitic clinopyroxene, glaucophane-dominated amphibole and almandine- dominated garnet with varied amounts of incorporated hydrogen. Besides, large variations in the measured EC derived at the similar experimental conditions (e.g., eclogites from Bohemina Massif, see Fig. 4) may also imply the strong influence of chemical compositions on the EC. Because the chemical compositions are not always specified in the literature, it is noted that a comparison between the calculated EC from our samples and the measured EC from other samples is not straightforward.

    Furthermore, our calculated HS upper bound EC for eclogite and amphibolite is about 1‒2 orders of magnitude lower than the experimentally determined EC for one eclogite sample from central Qiangtang in the northern Tibet (Fig. 4). This result is noteworthy, because this eclogite sample was collected from the same locality as in our study (Guo et al., 2014). A change in the temperature dependence of EC was observed experimentally at ~650 K, which can result from the transition of conduction mechanism from hopping of small polarons or free protons at lower temperatures to ionic diffusion at higher temperatures (Dai and Karato, 2014; Guo et al., 2014; Karato and Wang, 2013). Since we used a single Arrhenius equation for the entire temperature range, this EC transition was not seen in our calculation. The similar mineral assemblage and chemical composition between our Amp-rich eclogite samples and the eclogite (Grt ~40 vol.%, Omp ~30 vol.%, Gln ~25 vol.%, Rut+Qtz ~5 vol.%) studied by Guo et al. (2014) suggest that mineral assemblage and chemical composition, though cannot be excluded, may not be the key factors affecting the discrepancy between calculated and measured EC. Although Rut is less conductive than Cpx, Grt and Amp at temperature less than 900 K, the frequent occurrence of interconnected highly conductive Ilm film along the boundaries of Rut grains (i.e., Rut→Ilm alteration, Fig. 2b) suggests that despite minor in proportion Ilm can contribute high EC to the Rut-Ilm aggregate at low temperature (< 900 K) (Fig. 3). The possible presence of such highly conductive trails formed by rutile-ilmenite aggregate—millimeters in length parallel to the foliation (Fig. 1b) which is equivalent to the thickness of rock slice (~1 mm) used for EC measurement—may produce markedly elevated conductivity in the experiments but is not modeled in the calculations. Therefore, the layering microstructure may also significantly affect the intensity of EC, as well as its anisotropy— conductivity difference between foliation-parallel and foliation- perpendicular directions, which is expected to be tested by future experimental measurements.

    Lastly, anomalously high EC zones of 10-3‒100.5 S/m were imaged at depths of 45‒100 km in the Qiangtang terrane (Wei et al., 2007, Wei et al., 2001). This EC range is higher than our calculated EC for eclogite and amphibolite at temperature lower than ~800 K, but overlaps with the calculated values at higher temperatures above 800 K (Fig. 4). Because the maximum metamorphic temperature was estimated to be ~830 K at ~2.5 GPa (Du et al., preliminary data), eclogite and amphibolite (absence of dehydration or partial melting due to sub-solidus thermal condition) are not supposed to interpret the abnormally high EC conductivity in the Qiangtang terrane (a similar conclusion was drawn for the Dabie-Sulu high pressure terrane by Dai et al. (2016)). Instead, highly conductive partial melts or aqueous fluids that are probably originated from the upwelling asthenosphere beneath Qiangtang terrane (Zhang et al., 2015; Tilmann and Ni, 2003) are thus more favored (Guo et al., 2018; Wei et al., 2001).

  • Based on the calculated electrical conductivity for the high-pressure metamorphic rocks from central Qiangtang in the northern Tibetan Plateau, we draw the following conclusions. In the absence of aqueous fluids, the conductivity of meta- mafic rocks (e.g., eclogite and amphibolite) is remarkably larger than the meta-felsic rocks (e.g., garnet-quartz-mica schist). Enrichment of amphibole can decrease the conductivity of eclogite, but this effect is diminished when a critical level of amphibolization degree is reached. Our calculated conductivity of eclogite and amphibolite differs greatly from the experimentally measured results for the eclogites from other localities, attributing partly to the strong effects of different mineral assemblages and chemical compositions on the conduction mechanisms and efficiencies. Besides, the calculated conductivity of eclogite and amphibolite is markedly lower than the measured data for the eclogite sample from the same locality, possible owing to the contribution of trails of highly conductive rutile-ilmenite aggregates to elevate the bulk-rock conductivity. The calculated conductivity of eclogite and amphibolite is not high enough, partial melts or aqueous fluids originated from the upwelling asthenosphere can better to explain the anomalously high electrical conductivity zones in the Qiangtang terrane in the northern Tibetan Plateau.

  • This study was supported by the National Natural Science Foundation of China (Nos. 41902222, 41502059) and 111 Project of China (No. BP0719022). We thank three anonymous reviewers for their thoughtful comments, which improved greatly the quality of our manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-020- 1291-0.

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