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Volume 30 Issue 6
Dec.  2019
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Deliang Liu, Rendeng Shi, Lin Ding, Shao-Yong Jiang. Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau. Journal of Earth Science, 2019, 30(6): 1253-1265. doi: 10.1007/s12583-019-1250-9
Citation: Deliang Liu, Rendeng Shi, Lin Ding, Shao-Yong Jiang. Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau. Journal of Earth Science, 2019, 30(6): 1253-1265. doi: 10.1007/s12583-019-1250-9

Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau

doi: 10.1007/s12583-019-1250-9
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  • The origin of the central Qiangtang metamorphic belt (CQMB) has long been in debate, which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block, or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks. Here we report field observations, zircon U-Pb ages and Lu-Hf isotopes, as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB. The ca. 229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount. Their geochemical and isotopic compositions are similar to OIB-type lavas. Unlike other metabasalts (eclogites and blueschists) in the CQMB, the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism; this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab. This work provides new solid evidences for an in situ origin of the CQMB.
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Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau

doi: 10.1007/s12583-019-1250-9
    Corresponding author: Deliang Liu;  Shao-Yong Jiang

Abstract: The origin of the central Qiangtang metamorphic belt (CQMB) has long been in debate, which is not clear whether this belt is the exhumed Jinsha oceanic plate that had been subducted and underthrusted beneath the Qiangtang Block, or the in situ Longmu Co-Shuanghu suture that separated the south and north Qiangtang blocks. Here we report field observations, zircon U-Pb ages and Lu-Hf isotopes, as well as whole rock geochemistry and Sr-Nd isotopes of the Late Triassic volcanic rocks near the Chabo Co within the southern margin of the CQMB. The ca. 229 Ma Chabo Co volcanic rocks and limestones possess characteristic lithologies of a seamount. Their geochemical and isotopic compositions are similar to OIB-type lavas. Unlike other metabasalts (eclogites and blueschists) in the CQMB, the Chabo Co volcanic rocks are OIB-type lavas that did not experience high-grade metamorphism; this is likely because that the Chabo Co seamount was detached from the subducting Longmu Co-Shuanghu oceanic slab. This work provides new solid evidences for an in situ origin of the CQMB.

Deliang Liu, Rendeng Shi, Lin Ding, Shao-Yong Jiang. Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau. Journal of Earth Science, 2019, 30(6): 1253-1265. doi: 10.1007/s12583-019-1250-9
Citation: Deliang Liu, Rendeng Shi, Lin Ding, Shao-Yong Jiang. Survived Seamount Reveals an in situ Origin for the Central Qiangtang Metamorphic Belt in the Tibetan Plateau. Journal of Earth Science, 2019, 30(6): 1253-1265. doi: 10.1007/s12583-019-1250-9
  • The Earth's continental crust in general comprises a mafic lower crust, a dioritic middle crust and a felsic crystalline plus sedimentary upper crust, according to the classic model (Rudnick and Gao, 2014). However, there are places in which the continental crust has different vertical lithological components with the classic model. For example, in the Pamir-Tibetan Plateau, lower crustal metaclastics may underlie upper portions of the southern Pamir Block in central Asia (Hacker et al., 2005; Ducea et al., 2003) and the north Qiangtang Block in Tibet (Hacker et al., 2000), which were sampled as xenoliths by Late Cenozoic volcanics. Nevertheless, the spatial distribution, as well as the timing and mechanism for the incorporation of these metasedimentary rocks in the lower crustal level are still unclear.

    One of the most prominent tectonic patterns in the central Tibetan Plateau is the > 500-km-long and up to 100-km-wide eclogite- and blueschist-bearing metamorphic belt (central Qiangtang metamorphic belt, CQMB) in the Qiangtang Block (Fig. 1a). The origin of this belt is debatable. It was initially argued that the CQMB was the remains of the Jinsha oceanic plate, which has been subducted to great depths beneath the Qiangtang Block, and underwent metamorphism up to blueschist- and eclogite-facies. The subducted Jinsha oceanic plate with the overlying massive clastic sedimentary rocks continued to underthrust beneath the Qiangtang Block as a result of the convergence between the Jinsha Ocean and the Qiangtang Block. Then the exhumation of this underthrusted Jinsha oceanic plate formed the CQMB (Kapp et al., 2003, 2000). This hypothesis indicates that the CQMB was connected with the Jinsha suture to the north through the lower crust (which was the underthrusted Jinsha oceanic plate) of the north Qiangtang Block, and the protoliths of the eclogites and blueschists in the CQMB were the Jinsha oceanic crust. Zhang et al. (2006b) compared the lithology and geochemistry of the (meta)mafic rocks from the CQMB with these from the Jinsha ophiolite; they found a distinct geochemical difference in the basaltic composition, in that the Jinsha suture hosts tholeiitic mid-oceanic-ridge basalts (MORB) whereas the CQMB contains metabasalts of a within-plate oceanic island (OIB) character with alkalic affinities. Therefore, they proposed an in situ suture model, which suggested that the CQMB was formed by the subduction and exhumation of an oceanic plate between the north and south Qiangtang blocks. However, recent investigations on the CQMB (Dan et al., 2018; Zhai et al., 2011a) and the Jinsha suture (Liu et al., 2016; Yang et al., 2012) have revealed both MORB and OIB occurring in the CQMB and the Jinsha suture, these findings again resulted in an ambiguous origin for the CQMB.

    Figure 1.  (a) Simplified geologic map of the Tibetan Plateau modified after Liu et al. (2018). Basemap is from GeoMapApp (www.geomapapp.org); inset shows the location of the Tibetan Plateau; the distribution of the Late Triassic volcanic rocks and granites in the Qiangtang Block is modified after Wu et al. (2015). ATF. Altyn Tagh fault; KF. Karakoramfault; MFT. main frontal thrust; IYS. Indus-Yarlung suture; BNS. Bangong-Nujiang suture; JS. Jinsha suture; AKMS. Ayimaqin-Kunlun Mutztagh suture. Black rectangle shows the location of Fig. 1b. (b) Geologic map of the Chabo Co area based on the field observations of this study, showing the distribution of lavas and limestone. Straight lines with numbers indicate cross sections for observations and sampling. Line A-B show the location of cross section in Fig. 1c. (c) Cross section based on observations in the sampling sections in Fig. 1b.

    In this study, we report the relic of a Triassic seamount from the southern margin of the CQMB, where the volcanic rocks and limestones of the seamount have not been metamorphosed. We therefore suggest that this seamount did not come from the underthrusted and exhumed Jinsha oceanic plate; instead, it is a remnant of a seamount scraped off from the subducting Longmu Co (Co=lake, in Tibetan language)-Shuanghu Paleo-Tethyan Ocean (LSO).

  • The Tibetan Plateau holds a 2.5-million km2 area with 60–80 km thick crust and > 4 km surface elevation, and is the largest plateau on Earth (Zhang et al., 2011; Fielding et al., 1994). It was formed by sequential amalgamation of continental and oceanic arc blocks over several orogenic cycles since the Paleozoic (Kapp and DeCelles, 2019). The Qiangtang Block in the central Tibetan Plateau is separated from the Songpan-Ganzi Block to the north by the Jinsha suture, which closed in the Triassic, and from the Lhasa Block to the south by the Bangong-Nujiang suture, which formed during the Lhasa-Qiangtang collision during the Cretaceous.

    The first order tectonic framework of the Qiangtang Block is characterized by a 600-km-long and up to 270 km-wide east plunging anticlinorium in its central and west portion (Yin and Harrison, 2000). This anticlinorium consists of Carboniferous– Permian shallow marine strata and metamorphic rocks in its core and Triassic–Jurassic limestones and sandstones interbedded with volcanic rocks along its north and south limbs (Kapp et al., 2003). The eclogite- and blueschist-bearing CQMB crops out in the core of the Qiangtang anticlinorium between the Gangma Co in the west and the Shuanghu County in the east (Dan et al., 2018; Liang et al., 2017; Pullen et al., 2011; Zhai et al., 2011b; Zhang et al., 2006a, b; Kapp et al., 2000). The scattered outcrops of mantle peridotite, mafic dyke, massive and pillow basalt and radiolarian chert between the Longmu Co to the west and Shuanghu County to the east define the surface trace of the Longmu Co-Shuanghu suture zone (LSS), which separates the Qiangtang Block into the north and south Qiangtang blocks. The surface trace of the LSS rocks sometime spatially overlaps with that of the northern part of the CQMB and is roughly parallel to the CQMB (Li et al., 2007; Zhai et al., 2007).

    The volcanic rocks in this study crop out 10 km south of the Chabo Co, and they can be traced by an approximately east-west elongated 6×1.5 km2 sliver of dark volcanic rocks bounded by two thrust faults in the south and north with the light-colored limestone (Figs. 1b, 2a, 2b and 2f). The limestones consist mainly of light-gray-colored micritic limestone and light-colored bioclastic limestone, and minor limestone breccia. The limestone is intercalated with the volcanic rocks, and the contact between the limestone and volcanic rock was reactivated as thrust fault (Figs. 2a and 2b). Both the two thrust faults dip moderately to the NNE, and are likely part of the Lugu thrust system mapped by Kapp et al. (2003) at the NW shore of the Chabo Co. The volcanic sliver consists of mainly basalt in the central, minor andesite and dacite in the north and south (Figs. 1c and 2). The lithological assemblage is comparable to that of a modern seamount, which consists of OIB type volcanic rocks and limestone.

    Figure 2.  (a) Field photograph showing the Chabo Co lavas and limestone. The lavas are dominanted by basalt, with minor andesite and dacite. (b) Field photograph showing the contact between the limestone and the volcanic rock. (c), (d), (e), (f) Field photographs showing the outcrops of basalt, andesite, dacite and limestone, respectively. (g), (j) Photomicrographs showing the basalt. (h), (k) Photomicrographs showing the andesite. (i), (l) Photomicrographs showing the dacite, see text for details. Pl. Plagioclase; Qtz. quartz; Cal. calcite; Cpx. clinopyroxene; Hbl. hornblende; Pmp. pumpellyite.

    The basalt is dark-green in color and shows massive structure. It has porphyritic texture; the phenocrysts consist of euhedral to subhedral Pl±Cpx, with 0.5–2 mm in size, the groundmass shows intergranular texture, with interstices between angular plagioclase occupied by pyroxene and iron titanium oxides (Figs. 2g and 2j). The andesite and dacite have porphyritic texture and contain amygdales filled or partly filled with calcite, quartz or pumpellyite (Figs. 2h, 2i, 2k and 2l), the phenocrysts are mainly euhedral Pl of 1–2.5 mm long. The volcanic rocks have experienced slight alteration, but unlike other metabasalts (eclogites and blueschists) in the CQMB, they show no evidence of any high-grade metamorphism.

  • In this study, we dated one andesite sample (13GZ07, GPS: 33°13'15.67"N, 84°13'13.34"E, height 4 848 m a.s.l.), which was approximately 5 kg in weight. This sample was crushed and sieved for separating zircon grains using standard magnetic and heavy liquid procedures, at the Hebei Provincial Institute of Geological and Mineral Survey. Zircon grains were mounted in epoxy and polished to expose the cores of the crystals. Cathodoluminescence (CL) images were prepared at the Beijing GeoAnalysis Co. Ltd. Zircon U-Pb isotopic analyses were conducted on a new wave UP193FX excimer laser coupled with an Agilent 7500a inductively coupled plasma-mass spectrometer (ICP-MS) at the Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing. The ablation system was operated at a wavelength of 193 nm with a spot diameter of 30 μm. Every five analyses of sample were followed by analysis of Plesovice zircon and NIST 610 standard glass. The weighted mean 206Pb/238U age of the Plesovice zircon is 337.6±1.8 Ma (n=5, MSWD=0.1), which is identical to the recommended age (337±0.37 Ma, Sláma et al., 2008). Common Pb corrections were made following methods of Andersen (2002). U-Pb ages were calculated using the GLITTER 4.0 software (Jackson et al., 2004). Age plots were made using the Excel macro program "Isoplot 3.0" (Ludwig, 2003).

  • In situ zircon Lu-Hf isotopes were measured on a Nu Plasma Ⅱ MC-ICP-MS coupled with a Resonetics-S155 excimer ArF laser ablation system, at the State Key Laboratory of Geological Processes and Mineral Resource, China University of Geosciences, Wuhan, China. A 50-μm-diameter laser beam with repetition rate of 10 Hz and energy density of ~8 J/cm2 were used for the analysis, and the ablation process was set to last for 40 seconds. The 179Hf/177Hf value (0.732 5) was applied to calibrate the mass discrimination (Chu et al., 2002), while mass discrimination of Yb isotope was corrected, using the equation (βYb=0.872 5×βHf) proposed by Xu et al. (2004). The interference of 176Lu and 176Yb on measuring 176Hf was calibrated using the ratios of 176Yb/172Yb (0.588 6) and 176Lu/175Lu (0.026 55) which was proposed by Chu et al. (2002). Penglai zircon was selected as the standard during the analysis, whose analytical result (176Hf/177Hf=0.282 910±0.000 055, n=73, MSWD=0.37) is in good agreement with the published data (0.282 906±0.000 010, Li et al., 2013).

  • Whole-rock major and trace element concentrations were measured at the Hebei Provincial Institute of Geological and Mineral Survey. Major elements were measured by an Axiosmax X-ray fluorescence spectrometer, and trace elements were determined by an X Series Ⅱ ICP-MS (Thermo Fisher Corp., USA). The loss on ignition (LOI) was determined by gravimetric method. Precision for major and trace elements are better than 2% and 5%, respectively.

  • Whole-rock Sr-Nd isotopic compositions were analyzed at the Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences in Beijing. Approximately 100 mg of powdered sample was dissolved using standard HF-HNO3 methods. Sr and Nd fractions were separated by chromatographic techniques. Isotope ratios were measured using a Nu Plasma Ⅱ MC-ICP-MS. Measured values for NBS 987 Sr standard and JNDi Nd standard were 0.710 252±0.000 014 (n=6) for 87Sr/86Sr and 0.512 127±0.000 028 (n=5) for 143Nd/144Nd.

  • Zircon grains in sample 13GZ07 are euhedral, and 60 to 100 μm long and 30 to 70 μm wide. They show oscillatory zoning patterns (Fig. 3), and have low Th (34 ppm–241 ppm) and U (24 ppm–212 ppm) concentrations with high Th/U ratios (0.78–1.22). The U-Pb geochronologic data are listed in Table S1. Two of the total nine analyses yield a young (139 Ma) and an old age (861 Ma), the other seven analyses give ages of 228–232 Ma. The weighted mean 206Pb/238U age of these seven analyses is 229.3±2.9 Ma (n=7, MSWD=0.18, Fig. 3), this age is interpreted to be the timing of volcanism.

    Figure 3.  LA-ICP-MS zircon U-Pb concordia and representative CL image of zircons (a) and weighted average of the 206Pb-238U age (b) for the andesite sample in the Chabo Co area.

  • Lu-Hf isotope data are given in Table S2. Zircons from the Chabo Co andesite have 176Hf/177Hf ratios ranging from 0.282 508 to 0.282 507, the initial 176Hf/177Hf ratios calculated using the corresponding zircon ages are 0.282 506–0.282 554, and the εHf(t) values range from -2.7 to -4.4. TDM ages range from 0.98 to 1.04 Ga.

  • We analyzed 27 volcanic samples for major and trace elements, the results are shown in Table S3. The LOI of the volcanic samples are not high, ranging from 1.27 wt.% to 3.73 wt.%, the average value is 2.61 wt.%. The major element data reported as oxides in weight percentage below in the text and figures are recalculated to 100% on an anhydrous basis. The volcanic rocks have low to moderate SiO2 (48.66 wt.%–66.19 wt.%), high TiO2 (1.81 wt.%–4.37 wt.%) and Na2O (2.24 wt.%–4.98 wt.%), varying MgO (0.72 wt.%–6.77 wt.%) contents but high Mg# (37–70), and extremely low K2O (0.09 wt.%–0.50 wt.%) contents.

    The volcanic rock data plot in fields of basalt, basalt andesite/andesite and dacite in the total alkali-silica (TAS) diagram (Fig. 4a), and they plot in the fields of sub-alkaline basalt, andesite and rhyodacite/dacite in a SiO2 versus Zr/TiO2×0.000 1 diagram (Fig. 4b). Therefore, these samples are classified as basalt, andesite and dacite, respectively, based both on geochemical discrimination diagrams and on petrographical characteristics. In addition, the samples belong to low-K tholeiite series (Fig. 4c), sodium volcanic rocks (Fig. 4d).

    Figure 4.  Total alkali-silica (TAS, La Bas et al., 1986) diagram (a), Zr/TiO2×0.000 1-SiO2 diagram (b) (Winchester and Floyd, 1977); SiO2-K2O (c) and Na2O-K2O (d) plots for the classification of the Chabo Co lavas.

    Trace element patterns of the Chabo Co basalt, andesite and dacite are essentially identical (Figs. 5a and 5b). They are enriched in light rare earth element (LREE) relative to heavy rare earth element (HREE) (LREE/HREE=3.9–6.2), and display weakly negative to slightly positive Eu anomalies (Eu/Eu*=0.88–1.13). Though ranges of the rare earth element (REE) concentrations of the three types of rocks largely overlap with each other, the andesite and dacite have slightly higher LREE than most of the basalt. Trace element concentrations of the Chabo Co basalt, andesite and dacite are higher than both E- and N-MORB, but similar to OIB (Figs. 5a and 5b). Furthermore, all the samples have high Nb (19 ppm–36 ppm, with an average of 26 ppm) concentrations and thus belong to high-Nb volcanic rocks (Reagan and Gill, 1989).

    Figure 5.  Chondrite-normalized (Boynton, 1984) REE patterns and primitive mantle-normalized (Sun and McDonough, 1989) trace element patterns of the Chabo Co lavas (a) and (b), mafic rocks (Liu et al., 2016; Yang et al., 2012; Zhang et al., 2006b) from the Jinsha suture suture ophiolite (c) and (d), mafic rocks (Zhai et al., 2013c) from the Longmu Co-Shuanghu suture ophiolite (e) and (f), blueschists (g) and (h) and eclogites (i) and (j) from the central Qiangtang metamorphic belt (Dan et al., 2018; Tang and Zhang, 2013; Zhai et al., 2011a; Zhang et al., 2006b). Data for OIB, E-MORB and N-MORB are from Sun and McDonough (1989).

  • The Chabo Co volcanic rocks have variable Sr-Nd isotopic compositions (see Table S4), the basalt has initial 87Sr/86Sr ratios of 0.703 9–0.705 8, initial 143Nd/144Nd ratios of 0.512 663– 0.512 763, and εNd(t) of 6.24 to 8.19. The andesite has initial 87Sr/86Sr ratios of 0.704 9–0.707 4, initial 143Nd/144Nd ratios of 0.512 324–0.512 683, and εNd(t) of -0.37 to 6.63. The dacite has initial 87Sr/86Sr ratios of 0.707 0–0.707 6, initial 143Nd/144Nd ratios 0.512 323–0.512 327 and εNd(t) of -0.32 to -0.39. The Sr-Nd isotopes of the Chabo Co volcanic are comparable to that of the Society volcanic rocks (see next section).

  • Rocks cropping out near Chabo Co include lavas of mainly basaltic and minor intermediate compositions as well as limestones, however no other terrigenous clastic sediments were found. This lithology assemblage is similar to that of a seamount. The low SiO2 and high TiO2 contents (Figs. 4a and 4b), as well as the OIB-like REE and trace element patterns indicate an OIB origin for the Chabo Co lavas (Fig. 5). This inference is also supported by tectonic setting discrimination diagrams as shown in Fig. 6, the Chabo Co basalt samples have high Zr and Ta concentrations and are plotted in within-plate volcanic zone in different diagrams. Hereafter, the combination of lavas and limestones near Chabo Co is named the Chabo Co seamount.

    Figure 6.  Tectonic setting discrimination plots. (a) Zr-Zr/Y plot (Pearce and Norry, 1979), WPB. within plate basalts; MORB. mid-ocean ridge basalts; VAB. volcanic arc basalts. (b) Ta/Hf-Th/Hf diagram (Wang et al., 2001); Ⅰ. divergent plate margin, N-MORB; Ⅱ. convergent plate margin basalts (Ⅱ1. oceanic island arc basalts; Ⅱ2. continental margin volcanic arc basalts); Ⅲ. oceanic within-plate basalts; Ⅳ. continental within-plate basalts (Ⅳ1. intracontinental rift and continental margin rift tholeiites; Ⅳ2. intracontinental rift alkali basalts; Ⅳ3. Continental extensional zone or initial rift-related basalts); Ⅴ. mantle plume basalts. (c) Ta/Yb-Th/Yb and (d) Yb-Th/Ta plots (Gorton and Schandl, 2000); OA. oceanic arc; ACM. active continental margins; WPVZ. within-plate volcanic zones; WPB. within-plate basalts; MORB. mid-ocean ridge basalts. Data sources are the same as in Fig. 5.

    The basalt has low MgO contents (4.53 wt.%–6.77 wt.%) and low concentrations of compatible elements (e.g., Cr=23 ppm–179 ppm, Ni=40 ppm–93 ppm), suggesting fractionation for the parental magma (Fig. 7). The variation of La at a relatively constant La/Sm ratio defines a fractionation trend for the Chabo Co lavas (Fig. 7a). The MgO decreases with increasing SiO2 contents and the Ni decreases along a concave-down curve with Cr implying the fractionation of olivine (Figs. 7b and 7c). The relatively constant TiO2 and P2O5 contents with decreasing MgO suggest that the fractionations of titaniferous magnetite/rutile or apatite are negligible (Figs. 7d, 7e). The constant Eu/Eu* with decreasing MgO indicates minimal plagioclase fractionation (Fig. 7f). The absence of Nb and Ta negative anomalies in the Chabo Co lavas suggests that there was no significant involvement of continental crustal component during magma ascent. Both the Chabo Co andesite and dacite have similar REE and trace element patterns to that of basalt, indicating that the andesite and dacite were evolved from the basaltic magma by fractionation of Mg/Fe minerals.

    Figure 7.  Whole-rock major and trace element variation diagrams of the Chabo Co lavas.

    The slightly enriched zircon Hf isotope of the Chabo Co lavas is different from the strongly depleted Permian mafic rocks with a plume origin, as well as Triassic arc magmas in the north Qiangtang Block with a mantle wedge origin (Fig. 8a). The Sr-Nd isotopes define a line similar to the EMII type enrichment trend as expressed by the Society magmas (Fig. 8b). These isotopic signatures imply that the Chabo Co lavas were derived from a mantle source similar to the Hawaii and the Society magmas (Porter and White, 2009). This inference is also supported by the high Ta/La ratios of the studied lavas as shown in Fig. 8c. The scattered Sr-Nd isotopes are like the results of the heterogeneity of their magma sources. Quantitative modeling of REE abundance variations suggests that approximately 3%–10% melting of a hypothetical mantle source with spinel+garnet (50 : 50) lherzolite would produce rare earth element concentrations similar to the Chabo Co lavas (Fig. 8d).

    Figure 8.  (a) Zircon U-Pb age-εHf(t) plot for the Chabo Co andesite, Triassic arc magmatism and Permian plume related magmatism from the Qiangtang Block (Zhai et al., 2013a, b). (b) Whole rock initial 86Sr/87Sr-εNd(t) plot for the Chabo Co lavas, blueschists and eclogites from the central Qiangtang metamorphic belt (Dan et al., 2018; Zhai et al., 2011a), plume related mafic rocks from the Qiangtang Block (Zhai et al., 2013a), and mafic rocks from the Jinsha suture ophiolite (Liu et al., 2016), data for MORB, St. Helena, Hawaii, Society and Kerguelen are from Porter and White (2009). (c) (Hf/Sm)N-(Ta/La)N diagram of the Chabo Co basalts. Data for N-MORB and OIB are from Sun and McDonough (1989). All other fields are after La Flèche et al. (1998). (d) Sm/Yb-Sm plot for the Chabo Co basalts. Melting curves for spinel-lherzolite (ol0.530+opx0.270+cpx0.170+sp0.030 and ol0.060+opx0.280+cpx0.670+sp0.110) and garnet-lherzolite (ol0.600+opx0.200+cpx0.100+gt0.100 and ol0.030+opx0.160+cpx0.880+ gt0.090), as well as the DM and PM values are after Aldanmaz et al. (2000). Numbers along curves represent the degree of partial melting.

    Temperature and pressure of mantle melting are critical for revealing the geodynamics of magmatism. Here we use the thermobarometer of Lee et al. (2009) based on Si and Mg contents of primary basaltic magma for estimating the temperature and pressure. The result shows that the parental magma of the Chabo Co basalt is likely derived by partial melting of OIB type mantle at high pressures (2.6–3.3 GPa) and potential temperatures of 1 450–1 500 ℃ (Fig. 9). These melting conditions clearly distinct the Chabo Co basalt from the MORB or arc-type ones. Therefore, the above lines of evidences show an OIB origin for the Late Triassic Chabo Co lavas.

    Figure 9.  Temperature (T) and pressure (P) plots of the Chabo Co basalts. Data for Cascades arc, Izu-Boni arc, mid-ocean ridge, Hawaii and Cordillera basalts are from the GEOROC database and Lee et al. (2009). Dashed lines are melt fraction isopleths, near-vertical gray lines are solid mantle adiabats, Tp is the mantle potential temperature.

  • The origin of the CQMB is in debate. It is not clear whether it is the exhumed Jinsha oceanic plate that had been underthrusted beneath the Qiangtang Block, or the in situ LSS that separated the south and north Qiangtang blocks (Dan et al., 2018; Zhai et al., 2007; Zhang et al., 2006b; Kapp et al., 2003, 2000). The exhumed Jinsha mélange model predicts that all the basaltic rocks must be metamorphosed to high-grade facies, because of the subduction and underthrusting of the Jinsha oceanic plate, while the in situ LSS model does not. The OIB type Chabo Co lavas in this study have not been metamorphosed like the blueschists and eclogites in the CQMB, this evidence clearly argues against the exhumed Jinsha mélange model, and suggests that the CQMB and the LSS are remnant of the LSO lithosphere.

    Most of the blueschists and eclogites from the CQMB have geochemical compositions comparable to OIB type basalt, the gabbros and basalts within the LSS exhibit N-MORB to E-MORB characteristics (Figs. 5e, 5f). The OIB-type blueschists and eclogites (Figs. 5g–5j) in the CQMB are likely formed by the subduction and exhumation of seamounts/ oceanic islands. Based on the linear distribution of the OIB-type blueschists and eclogites between Gangma Co and Shuanghu (Fig. 1a), we propose that the OIB lavas could have built a chain of seamounts on the floor of the LSO, and the strike of this seamount chain was oblique to the trench during the subduction of the LSO. When these seamounts meet the trench, a large-scale mélange with seamount fragments will be formed in the accretionary prism (Clarke et al., 2018; Safonova et al., 2016), and therefore resulted in the formation of the CQMB (Fig. 10a). This mechanism is similar to the south Central American (Clarke et al., 2018; Buchs et al., 2011, 2009) and Japan (Sakai et al., 2019; Safonova et al., 2015; Okamura, 1991) accretionary complexes containing fragments of seamounts and oceanic islands.

    Figure 10.  Tectonic model showing the formation process of the central Qiangtang metamorphic belt.

    Depending on the sizes of seamounts and the geometry of the subduction systems, some of the seamounts may have been subducted and metamorphosed up to blueschist- or eclogites-facies and then exhumed to shallow levels in the subduction channel. The sedimentary covers had also been metamorphosed to bedded cherts or marble that associated closely with the blueschists and eclogites in the CQMB. In contrast, some seamounts, such as the Late Triassic Chabo Co, had not been subducted because of their large size, or because they were detached from the subducting slab, and thus did not experience any high-pressure metamorphism (Fig. 10b).

    The metamorphic ages for the CQMB blueschists and eclogites determined using Ar-Ar method on syn-metamorphism potassium bearing minerals show a prolonged period (282–211 Ma) for the HP/LT metamorphism (Dan et al., 2018; Liang et al., 2017; Zhang et al., 2017; Pullen et al., 2011; Zhai et al., 2011b), indicating the successive subduction and exhumation of seamounts of the chain during the subduction of the LSO lithosphere beneath the north Qiangtang Block. In this case, metamorphism with old and young ages correspond to early and late deep subduction of individual seamount of the chain, respectively. In addition, the HP/LT metamorphism shows an overall younging to the east trend, with 282–275 and 219 Ma near Gangma Co in the west, 244–230 Ma near Rongma in the middle as summarized by Zhang et al. (2017), and 227–211 Ma near Shuanghu Country in the east (Liang et al., 2017). This spatial pattern of metamorphic ages suggests that the seamount chain in the LSO trends NW, and the timing for seamounts subduction and exhumation shows a SE-wards younging trend. The 219 Ma HP/LT metamorphism near Gangma Co may be related to the subduction of an isolated seamount that did not belong to this seamount chain.

  • The metasedimentary xenoliths carried from depths of 30–50 km to the surface by the shoshonitic lavas (3 Ma) implies a metasediment-bearing lower crust for the north Qiangtang Block (Lai et al., 2011; Hacker et al., 2000). How and when did the sediments incorporate to the lower level of the crust is critical to understand the processes of continental crustal growth.

    It was initially proposed that the sediments had been carried to the lower crustal level by the underthrusted Jinsha oceanic plate beneath the Qiangtang Block. However, the Chabo Co seamount that lacks high-grade metamorphic rocks and its spatial relationship with the eclogites- and blueschists-bearing CQMB argue against this model, instead, the evidences in the above sections are in good agreement with the in situ model for the CQMB. We do not know the lateral extent of the metasediment bearing lower crust in the north Qiangtang Block, because of the scarcity of the xenolith. Nevertheless, if it extends along the whole north Qiangtang Block, then the incorporation of the sediments in the lower crust of the north Qiangtang Block may occur later than the closure of the Longmu Co-Shuanghu Paleo-Tethyan Ocean, possibly during the Cenozoic south-dipping underthrusting of the Asian lithosphere beneath the Tibetan Plateau (Zhao et al., 2010).

  • Late Triassic (ca. 229 Ma) volcanic rocks and limestones, which crop out near the Chabo Co within the southern margin of the central Qiangtang metamorphic belt possess characteristic lithologies of a seamount. Unlike other metabasalts (eclogites and blueschists) in the CQMB, the Chabo Co volcanic rocks are OIB-type lavas without any high-pressure metamorphic overprint. These evidences argue for an in situ origin for the CQMB.

  • This study was financially supported by the National Natural Science Foundation of China (Nos. 41702212, 41672054) and the Fundamental Research Funds for Central Universities, China University of Geosciences (Wuhan) (Nos. CUGL170816 and CUGQYZX 1745). We thank Prof. Hans-Peter Schertl and an anonymous reviewer for useful comments and the editors for editorial handling. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1250-9.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-019-1250-9.

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