Mafic granulite and amphibolite in the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt, northwest China, southernmost Central Asian orogenic belt, occur as lenses within the matrix of metapelite and marble, exhibiting typical block-in-matrix fabrics of tectonic mélange. Three stages of metamorphic mineral assemblages were identified in these lenses. Clockwise metamorphic pressure-temperature (P-T) paths were obtained through geothermobarometry and thermodynamic pseudosection modeling, passing from 656 °C and 10.9 kbar through 830 °C and 16.5 kbar to 657 °C and 4.9 kbar for the mafic granulite, and from 564–645 °C and 3.2–9.6 kbar through 634–727 °C and 6.1–14.2 kbar to 615–664 °C and 3.2–4.2 kbar for the amphibolites, respectively. Metamorphic peak P-T conditions in the metapelitic country rocks were estimated to be 635–675 °C and 6.0–6.9 kbar. The metamorphic peak of the mafic granulite approaches the high P-T facies series, indicative of a subduction zone. Secondary-ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of metamorphic zircons suggests that the metamorphic event occurred between ca. 420 and 372 Ma. These data further certify that the subduction of the continental margin and subsequent uplift of the Dunhuang orogenic belt represent a long-lived tectono-metamorphic event in the Paleozoic.
Mélange, which is an important component of orogenic belts, consists of native or exotic blocks of different ages and origin that are commonly embedded in a (meta-)sedimentary or serpentinite matrix exhibiting intense strata disruption and/or internal disruption and characteristic block-in-matrix fabrics (e.g., Silver and Beutner, 1980; Kusky and Bradley, 1999; Harlow et al., 2004; Wakabayashi and Dilek, 2011; Festa et al., 2012; J.P. Wang et al., 2013a). Tectonic mélange is an important type of mélange that forms in a subduction channel (Festa et al., 2012), and it is represented by a chaotic mixture of metamorphic rocks of varying metamorphic grade. From the Precambrian to Cenozoic, tectonic mélange can be found in almost all orogenic belts, and the oldest of such rocks have been dated as early Precambrian (e.g., J.P. Wang et al., 2013a; Wan et al., 2015). As outlined by H.Y.C. Wang et al. (2017a), tectonic mélange can be identified by interruption of metamorphic strata, chaotic gathering of metamorphic lenses of discontinuous metamorphic facies, sharp contacts of schistosity and/or gneissosity between different metamorphic rocks, or a high angle of schistosity and/or gneissosity between the metamorphic lenses and the matrix. One of the most prominent features of tectonic mélange is the huge diversity of metamorphic peak pressure-temperature (P-T) conditions or metamorphic ages of the metamorphic rocks possibly amalgamated during tectonic exhumation. One such typical tectonic mélange is exposed in the Hongliuxia block, southern Dunhuang orogenic belt, southernmost Central Asian orogenic belt, northwest China (H.Y.C. Wang et al., 2017a).
Tectonic mélange is an ideal research target to use to decipher the tectono-metamorphic evolution of an orogenic belt, and therefore it is necessary to further explore the metamorphic evolution of tectonic mélanges. In the 660-km-long Paleozoic Dunhuang orogenic belt, northwest China, only two localities of tectonic mélange have been observed. One is the tectonic mélange exposed in the Hongliuxia block (H.Y.C. Wang et al., 2017a), and the other is exposed in the Dongbatu and Mogutai blocks. In this paper, we report the metamorphic P-T paths and geochronology of the tectonic mélange exposed in the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt. Our data further suggest that the Dunhuang region was not a stable craton formed in the Precambrian as previously believed, but it is instead a Paleozoic orogenic belt formed during the Silurian to Devonian.
The Central Asian orogenic belt is located between the Baltica and Siberia cratons to the north and the Tarim and North China cratons to the south (Fig. 1A) and is known as one of the largest and longest-lived accretionary orogenic systems in the world (Şengör et al., 1993; Windley et al., 2007; W.J. Xiao et al., 2015a, 2015b). The Central Asian orogenic belt, in general, grew southward from the Siberian craton in the Neoproterozoic (ca. 1000 Ma) and terminated in the Permian–Triassic (ca. 250–220 Ma), finally giving rise to the Beishan orogenic collage and the South Tianshan and Solonker sutures (Khain et al., 2002; Song et al., 2015; Tian et al., 2013, 2015; W.J. Xiao et al., 2008, 2010, 2015b). The NE-striking Beishan orogen, as the southern segment of the Central Asian orogenic belt, contains several arcs, ophiolites (e.g., W.J. Xiao et al., 2010, 2015b), and Ordovician eclogites, which are found in the southernmost Liuyuan ophiolitic mélange (Liu et al., 2011; Qu et al., 2011). These rocks document the convergence and suturing between the south Central Asian orogenic belt and the Tarim and North China cratons (W.J. Xiao et al., 2015b).
The Dunhuang orogenic belt lies to the east of the Tarim craton, to the south of the Beishan orogen and to the north of the Altyn Tagh fault (Fig. 1B). This orogenic belt consists of several discrete blocks separated by Tertiary strike-slip faults. These blocks consist of medium- to high-grade metamorphic rocks, including metapelite, felsic gneiss, quartzite, marble, amphibolite, and mafic granulite, as well as subordinate tonalite-trondhjemite-granodiorite (TTG)–like gneiss (Lu et al., 2008; Zong et al., 2012). These rocks were named the Dunhuang Group by the Bureau of Geology and Mineral Resources of Gansu Province (BGMG) in 1989. Based on limited isotopic data (Li, 1994; Mei et al., 1998; Lu et al., 2008), the metamorphic complex was originally considered as the Archean–Paleoproterozoic basement of the Tarim craton. Recently, late Paleoproterozoic (ca. 1.85 Ga; Zhang et al., 2012, 2013) and Paleozoic (ca. 440–430 Ma; Zong et al., 2012; He et al., 2014) high-pressure (HP) mafic granulites have been found. Some researchers consider the Dunhuang orogenic belt to be part of the basement of the Tarim craton (Li, 1994; Mei et al., 1998; Long et al., 2014), but other workers believe it is the western part of the Alxa block of the North China craton (Zhang et al., 2012, 2013; Yu et al., 2014). Other researchers, however, regard it as the southern part of the Central Asian orogenic belt (Meng et al., 2011; Zong et al., 2012; He et al., 2014; Peng et al., 2014; Z.M. Wang et al., 2014; H.Y.C. Wang et al., 2016, 2017a, 2017b; Y. Zhao et al., 2016). Recent research indicates that the Hongliuxia block to the south and the Qingshigou block to the southwest of the Dunhuang orogenic belt underwent high-pressure metamorphism during the Paleozoic; the Paleozoic high-pressure mafic granulites and amphibolites enclosed by metasediments all record clockwise pressure-temperature-time (P-T-t) paths, typical of orogenic metamorphism (H.Y.C. Wang et al., 2016, 2017a, 2017b). The Dunhuang orogenic belt was previously considered to be a tectonic mélange belt formed in a subduction zone, and to possibly represent the southernmost Central Asian orogenic belt (H.Y.C. Wang et al., 2016, 2017a, 2017b). Nevertheless, most of the other blocks (Dahongshan, Mingshashan, Mogutai, and Dongbatu) remain poorly explored, which in turn limits our understanding of the tectonic evolution of the Dunhuang orogenic belt in the Paleozoic.
In this contribution, detailed examination of the metamorphic reaction textures and mineral assemblages, geothermobarometry, thermodynamic pseudosection modeling, and secondary ion mass spectrometry (SIMS) and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) dating of zircon were conducted to reconstruct the metamorphic P-T-t paths of the Dongbatu and Mogutai blocks, middle Dunhuang orogenic belt (Figs. 1B and 1C), in order to provide more data to constrain the Paleozoic tectono-metamorphic evolution of the southernmost Central Asian orogenic belt.
The Dongbatu and Mogutai blocks are located in the middle Dunhuang orogenic belt (Fig. 1C), and the area is about of 30 × 5 km2. The metamorphic rocks are intruded by massive, locally weakly deformed, late granitic bodies (ca. 355 Ma; Fig. 1C). The high-pressure mafic granulite and amphibolite in the Dongbatu and Mogutai blocks commonly occur as lenses or stripes within the strongly deformed and folded matrix of amphibolite-facies metasedimentary rocks (Figs. 2A–2C; e.g., Zong et al., 2012). The lenses range from centimeters to meters, including high-pressure mafic granulite, garnet clinopyroxene amphibolite, garnet-biotite amphibolite, and garnet-free amphibolite. Thin, gray-white symplectites enclosing the garnet are observed in the granulite and amphibolite (Figs. 3D and 3E). Such symplectic texture was given the nickname “white-eye socket” (Ma and Wang, 1994) in the Chinese literature. Garnet biotite gneiss, felsic gneiss, mica schist, and marble make up the matrix. The Paleozoic high-pressure mafic granulites recording clockwise metamorphic P-T trajectories were first reported in the Mogutai block, and the metamorphic event was dated as ca. 440–430 Ma (Zong et al., 2012; He et al., 2014). However, most of the outcrops in the Dongbatu and Mogutai blocks remain unexplored.
PETROGRAPHY AND METAMORPHIC STAGES
The metamorphic stages of different rocks were examined in detail to reconstruct the metamorphic evolution of the Dongbatu and Mogutai blocks in the Dunhuang orogenic belt, based on micropetrographic observations of representative samples. Metamorphic mineral assemblages formed at the prograde, metamorphic peak, and retrograde stages are designated as M1, M2, and M3, respectively. Mineral abbreviations are after Whitney and Evans (2010).
High-Pressure Mafic Granulite
Three generations of mineral assemblages were found (sample G62). The prograde mineral assemblage (M1) consists of fine-grained, randomly distributed inclusion minerals in garnet porphyroblasts, including hornblende (Hbl1), clinopyroxene (Cpx1), plagioclase (Pl1), quartz (Qz1), and minor epidote (Ep1) and biotite (Bt1); see Figures 3A and 3B. The peak assemblage (M2) consists of ∼20% garnet porphyroblasts (Grt2) in a matrix consisting of ∼30% clinopyroxene (Cpx2) + ∼20% plagioclase (Pl2) + ∼25% quartz (Qz2). Zircon, rutile, and ilmenite are present as accessory minerals.
The retrograde (M3) assemblage is defined by the “white-eye socket” coronitic symplectites consisting of vermicular hornblende (Hbl3) + plagioclase (Pl3) ± clinopyroxene (Cpx3) + quartz (Qz3), rimming the garnet porphyroblast (Fig. 3). These intergrowths were possibly developed at the expense of the garnet rims and the adjacent matrix minerals, and they represent retrograde metamorphism. Another retrogression phenomenon involves hornblende (Hbl3) rimming on clinopyroxene (Cpx2; Figs. 3A and 3B).
Garnet-Clinopyroxene Amphibolite and Garnet Amphibolite
Three distinct metamorphic assemblages (M1, M2, and M3) were also recognized in the amphibolites (Figs. 4A–4E). The M1 assemblage is observed as fine-grained inclusions within garnet porphyroblasts, including hornblende (Hbl1), plagioclase (Pl1), and ilmenite (Ilm1). The M2 assemblage consists of ∼15%–20% garnet porphyroblast (Grt2) + ∼30%–35% hornblende (Hbl2) + ∼10%–15% clinopyroxene (Cpx2) + ∼15%–20% plagioclase (Pl2) + ∼15%–20% quartz (Qz2) in the garnet-clinopyroxene amphibolite (samples WH18 and WH84), and ∼15% Grt2 + ∼50% Hbl2 + 20% Pl2 + ∼15% Qz2 in garnet amphibolite (sample G89). Ilmenite and zircon are present as accessory minerals.
Coronitic “white-eye socket” symplectite (M3) is also observed in these amphibolites and is characterized by mineral intergrowths rimming the garnet porphyroblasts (Figs. 4A, 4C, and 4E). The symplectite assemblage (M3) is mainly composed of very fine-grained hornblende (Hbl3) + plagioclase (Pl3) + quartz (Qz3) ± ilmenite (Ilm3). Minor symplectic clinopyroxene (Cpx3) is found in the high-pressure amphibolite WH84 (Fig. 4A). In addition, hornblende occurs as rims on matrix clinopyroxene (Fig. 4B).
Two metamorphic mineral assemblages were recognized in the garnet biotite amphibolite. The M1 assemblage consists of hornblende (Hbl1) + biotite (Bt1) + plagioclase (Pl1) + quartz (Qz1) + ilmenite (Ilm1) as inclusions within garnet porphyroblast (Fig. 4F). The M2 assemblage is composed of ∼15% garnet porphyroblast (Grt2) and matrix minerals, ∼30% hornblende (Hbl2) + ∼20% biotite (Bt2) + ∼20% plagioclase (Pl2) + ∼15% quartz (Qz2). Zircon, apatite, and ilmenite occur as accessory minerals.
The M2 assemblage of metapelite sample G59 consists of garnet porphyroblast (Grt2) plus matrix biotite (Bt2) + plagioclase (Pl2) + quartz (Qz2), along with accessory zircon and ilmenite. No prograde assemblages were recognized in this sample.
Two stages of metamorphic mineral assemblages (M1 and M2) were recognized in the garnet metapelite sample G81. The M1 assemblage occurs as inclusion minerals (Pl1 + Bt1 + Qz1 + Ilm1) within garnet porphyroblasts (Fig. 4G). The M2 assemblage consists of garnet porphyroblast (Grt2) plus matrix biotite (Bt2) + plagioclase (Pl2) + quartz (Qz2). Zircon, magnetite, and ilmenite occur as accessory minerals.
Compositional analyses of the representative metamorphic minerals were determined using the electron microprobe at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing (IGGCAS), and School of Resources and Environmental Engineering, Hefei University of Technology, China. LA-ICP-MS U-Pb dating and trace-element measurements of zircon were conducted at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. SIMS U-Pb dating of zircon and whole-rock major-element measurements were done at IGGCAS. The details are listed in Supplemental Item A1, and the analytical results are listed in Supplemental Item B2 (Tables S1–S3).
X-ray compositional mapping of garnet porphyroblasts was conducted to investigate the compositional zonation (Fig. 5). Electron microprobe analysis (EPMA) compositional traverses (red line with arrow in Fig. 4) were performed on the representative garnet porphyroblasts. All garnet in all samples is dominated by almandine (XFe), grossular (XCa), and pyrope (XMg), with minor spessartine (XMn; Fig. 6; Table S1 [see footnote 2]).
X-ray mapping and chemical profiling of two garnet porphyroblasts in the high-pressure mafic granulite (G62) showed typical growth zoning (Figs. 5A, 6A, and 6B). The Fe# [= Fe/(Fe + Mg)] and XMn [= Mn/(Fe + Mg + Ca + Mn)] values decrease outward from the core (Fe# = 0.71–0.73, XMn = 0.052–0.056) to the inner rim (Fe# = 0.51–0.44, XMn = 0.011–0.005), forming typical bell-shaped, prograde zonation (Spear and Florence, 1992). This type of zoning is frequently found in metapelite (Spear et al., 1990; Kohn and Spear, 2000; L.L. Xiao et al., 2011), but rarely in mafic granulite or amphibolite. At the outermost rim, Fe# increases to 0.56–0.61, and XMn increases to 0.009–0.013. This is interpreted to result from resorption and diffusional Fe-Mg exchange during retrograde metamorphism (Spear, 2014; Spear and Florence, 1992).
Clinopyroxene in the high-pressure mafic granulite (sample G62) occurs as inclusion (Cpx1) within garnet, matrix (Cpx2), and symplectite (Cpx3; Fig. 3). All clinopyroxene generations can be classified as diopside (Fig. 7A; Morimoto et al., 1988). Cpx2 contains a higher jadeite component (XJd = 7.3%, IVAl = 0.12) than Cpx1 (XJd = 3.66%, IVAl = 0.03) and Cpx3 (XJd = 3.69%, IVAl = 0.05).
Hornblende occurs as inclusions in garnet (Hbl1), as a matrix phase (Hbl2), and in symplectites (Hbl3) in the granulite and amphibolite samples (G62, WH18, WH82, WH84, and G89), except for samples G62 and WH82, which contain no Hbl2 or Hbl3, respectively. The hornblende contains similar CaO content (10.6–12.6 wt%; Table S1 [footnote 2]) and is ascribed to the calcic group (Figs. 7B and 7C; Leake et al., 1997). The TiO2 contents of the hornblende formed during the M1, M2, and M3 stages have ranges of 0.31–1.43 wt% (Hbl1), 1.17–1.57 wt% (Hbl2), and 0.95–1.47 wt% (Hbl3), respectively.
Plagioclase occurs as inclusions (Pl1) within the garnet, in the matrix (Pl2), and as symplectite (Pl3) rimming the garnet, in the vast majority of samples studied. The matrix-type plagioclase (Pl2) in the high-pressure mafic granulite (sample G62) exhibits compositional zoning, with XAn increasing from the core to the rim (Fig. 7D). No compositional zonation of plagioclase was found in the other samples. The nomenclature for plagioclase is depicted in Figure 7E. The systematic increase in XAn from Pl2 to Pl3 in the granulite and amphibolite (Fig. 7D; Table S1 [footnote 2]) suggests that the symplectic plagioclase (Pl3) was formed by breakdown of the garnet rims and associated matrix minerals.
Biotite was found in the garnet-biotite amphibolite (sample WH82) and metapelite (samples G59 and G81), either as inclusions (Bt1) within the garnet or in the matrix (Bt2). The matrix-type biotite (Bt2) contains higher TiO2 contents (2.7–3.1 wt%) than the inclusion-type biotite (Bt1; 1.5–2.0 wt%), which is interpreted to reflect a temperature increase during prograde metamorphism. No compositional heterogeneity was observed.
METAMORPHIC P-T PATHS
Three kinds of thermodynamic methods were used to derive metamorphic P-T paths of the metamorphic rocks, i.e., mono-equilibrium geothermobarometry, multi-equilibria geothermobarometry, and forward pseudosection modeling. These methods yielded similar results for the metamorphic rocks of the Dongbatu and Mogutai blocks.
Multi-Equilibria Geothermobarometry: TWQ Computation
Metamorphic peak (M2) P-T conditions of the representative samples were first estimated using the multi-equilibrium program TWQ (thermobarometry with estimation of equilibrium state; versions 1.02 and 2.32; Berman, 1991; updated 1997). The metamorphic P-T conditions were determined by multi-equilibria computation using the thermodynamic data of Berman (1988), Berman et al. (1995), and Berman and Aranovich (1996) for end-member phases. The activity models of garnet, plagioclase, amphibole, clinopyroxene, and biotite used in this multi-equilibrium approach were from Berman (1990), Fuhrman and Lindsley (1988), Mäder et al. (1994), Berman et al. (1995), and Berman et al. (2007), respectively. For a given assemblage, good convergence of equilibria curves suggests equilibrium among minerals, whereas divergence suggests that one or more phases were improperly involved in the assemblage (e.g., G.C. Zhao et al., 1999). An average P-T condition is estimated using the INTERSX program after discarding intersections with standard deviation outside of 1.5s (Berman, 1991). The metamorphic peak (M2) P-T conditions were obtained by perfect independent equilibria curves, and they showed good intrasample convergence (Fig. 8). The TWQ P-T conditions are listed in Table 1.
Forward Pseudosection Modeling
In order to more accurately decipher the metamorphic evolution of the representative samples, phase equilibria modeling was performed using the Perple_X program (Connolly, 2005, version of 6.7.7) based on the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2002). In the forward pseudosection modeling, the respective activity models used were garnet (White et al., 2007), orthopyroxene (Holland and Powell, 1998, Powell and Holland, 1999), clinopyroxene (Holland and Powell, 1996), plagioclase (Newton et al., 1980), amphibole (Diener et al., 2007), biotite (Tajčmanová et al., 2009), chlorite (Holland et al., 1998), cordierite (Holland and Powell, 1998), and melt (White et al., 2007).
High-Pressure Mafic Granulite
The bulk-rock composition of the high-pressure mafic granulite sample G62 (in wt%) was SiO2 = 46.49, TiO2 = 0.53, Al2O3 = 14.82, TFe2O3 = 8.78, MnO = 0.16, MgO = 11.16, CaO = 15.51, K2O = 0.14, Na2O = 1.48, and H2O = 2.35, where TFe2O3 indicated total Fe and water content value was restricted just saturating the subsolidus. A P-T pseudosection was calculated in the MnO-Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2 (Mn-NCFMASHT) model system over a P-T range of 5–20 kbar and 500–900 °C (Fig. 9). The corresponding prograde mineral assemblage (M1) is Grt + Cpx + Pl + Amp + Qz + Rt. The prograde (M1) metamorphic P-T conditions were quantitatively determined to be 10–12 kbar and 690–730 °C, using the isopleths XMg [= Mg/(Mn + Mg + Fe2+ + Ca)] = 0.28–0.38 in the garnet mantle and XAn [= Ca/(Ca + Na)] = 0.50–0.56 in the plagioclase (Pl1; Table S1 [footnote 2]) enclosed by garnet mantle (Fig. 9). It was not possible to quantitatively estimate the metamorphic peak (M2) P-T conditions in the pseudosection modeling, possibly because the effective bulk-rock composition had been strongly modified due to the growth of chemical zonation in the matrix minerals. Nevertheless, a prograde process (dashed red line with arrow in Fig. 9) from M1 to M2 (Grt + Pl + Cpx + Qz + Rt) was inferred.
During retrograde metamorphism, the effective bulk composition also changed, because only the garnet rim and neighboring matrix minerals were effectively involved in the retrograde reactions. Therefore, the effective bulk composition (in wt%) for P-T pseudosection modeling of the retrograde stage (M3) was estimated to be SiO2 = 47.68, TiO2 = 0.45, Al2O3 = 22.94, TFeO = 3.96, MnO = 0.03, MgO = 7.11, CaO = 14.2, K2O = 0.13, and Na2O = 2.52, according to the mineral compositions of the phases within the “white-eye socket” microdomain and their volume proportions (Cpx = 4.7%, Pl = 50.3%, and Hbl = 45.0%). Proportions of Fe3+ to Fe2+ were retrieved from Fe-bearing minerals, for which Fe3+/Fe2+ was estimated by the stoichiometric and charge balance method of Droop (1987). The P-T pseudosection of M3 was calculated over the P-T range of 2–8 kbar and 600–800 °C (Fig. 10) and showed that garnet is unstable below pressures of ∼6–7 kbar. The corresponding retrograde mineral assemblage (M3) is Cpx + Pl + Amp + Qz + H2O. Therefore, a retrograde process (dashed red line with arrow in Fig. 10) with possible rapid decompression was inferred.
The bulk-rock composition of the amphibolite sample G89 (in wt%) was SiO2 = 53.40, TiO2 = 2.84, Al2O3 = 11.76, TFe2O3 = 18.27, MnO = 0.23, MgO = 4.25, CaO = 6.60, K2O = 0.38, Na2O = 2.46, and H2O = 0.2. The P-T pseudosection was calculated in the Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-TiO2 (NCFMASHT) model system. The P-T pseudosection was calculated over a P-T range of 3–15 kbar and 500–800 °C (Fig. 11). The isopleths XMg [= Mg/(Mg + Fe2+ + Ca)] = 0.09 and XCa [= Ca/(Mg + Fe2+ + Ca)] = 0.26–0.27 in the inner rim composition of garnet define an intersection at ∼9.3 kbar and 610 °C, which represent the metamorphic peak (M2) P-T conditions. The proportion of clinopyroxene under the peak P-T conditions was estimated to be ∼8%–12% (green dashed line in Fig. 11), which probably accounts for the absence of clinopyroxene locally in the rock. A retrograde process (dashed red line with arrow in Fig. 11) from Grt + Pl + Cpx + Amp + Qz + Ilm (M2) to Pl + Amp + Qz + Ilm (M3) was inferred.
The bulk-rock composition of the metapelite sample G81 (in wt%) was SiO2 = 56.84, TiO2 = 1.34, Al2O3 = 17.87, TFe2O3 = 8.19, MnO = 0.14, MgO = 3.25, CaO = 5.80, K2O = 2.88, Na2O = 1.48, and H2O = 1.18. Ferrous oxide content (FeO) was measured by titration of a potassium dichromate standard solution. The P-T pseudosection was calculated in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2 (NCKFMASHT) model system. The P-T pseudosection was calculated over a P-T range of 5–15 kbar and 500–900 °C (Fig. 12). The corresponding peak (M2) mineral assemblage is Grt + Pl + Bt + Qz + Ilm + Melt. The isopleths XMg [= Mg/(Mg + Fe2+ + Ca)] = 0.10–0.16 and XCa [= Ca/(Mg + Fe2+ + Ca)] = 0.10–0.13 in garnet constrain the M2 P-T conditions to be ∼9.5 kbar and 740 °C (Fig. 12).
Although multi-equilibria geothermobarometry and forward pseudosection modeling have been extensively applied to derive metamorphic P-T paths, the P-T errors of these two methods are not necessarily smaller than that for a carefully calibrated mono-equilibrium geothermometer or geobarometer (Holdaway, 2001). Therefore, various mono-equilibrium geothermobarometers were also used to determine the metamorphic P-T conditions of the M1, M2, and M3 assemblages of the granulite, amphibolite, and metapelite. For the high-pressure mafic granulite and garnet-clinopyroxene amphibolite, the garnet-clinopyroxene (GC) Fe-Mg exchange geothermometer (Ravna, 2000a) in concert with the garnet-clinopyroxene-plagioclase-quartz (GCPQ) geobarometer (Eckert et al., 1991) were applied. For the clinopyroxene-free amphibolite, the hornblende-plagioclase (PH) thermometer (Holland and Blundy, 1994) coupled with the garnet-hornblende-plagioclase-quartz (GHPQ) geobarometer (Dale et al., 2000) and the garnet-hornblende (GH) geothermometer (Ravna, 2000b) coupled with the GHPQ barometer (Kohn and Spear, 1990) were used. When biotite occurs in the matrix, the garnet-biotite (GB) geothermometer (Holdaway, 2000) combined with the garnet-biotite-plagioclase-quartz (GBPQ) geobarometer (Wu et al., 2004) were adopted. As for the metapelite, the GB geothermometer (Holdaway, 2000) combined with the GBPQ geobarometer (Wu et al., 2004) were used. The M1 and M3 assemblages were not likely at equilibrium with the garnet porphyroblasts (Wu et al., 2014); therefore, garnet-absent geothermobarometers, i.e., the PH geothermometer combined with the hornblende-plagioclase-quartz (HPQ) geobarometer (Bhadra and Bhattacharya, 2007), were used. Where clinopyroxene occurred in the symplectite, the clinopyroxene-plagioclase-quartz (CPQ) geobarometer (McCarthy and Patiño Douce, 1998) was also applied.
It should be emphasized that in computing the M2 P-T conditions, the inner rim composition of the garnet with the lowest XSps and Fe# values was adopted (e.g., Spear and Florence, 1992; Kohn and Spear, 2000; H.Y.C. Wang et al., 2017a, 2017b). Ferric iron contents of garnet and clinopyroxene were evaluated by stoichiometric and charge balance criteria (Droop, 1987), while ferric iron content of hornblende was evaluated by the method of Holland and Blundy (1994) and Dale et al. (2000). The metamorphic P-T conditions of the different metamorphic stages of the lenses and metapelite are listed in Table 1. Metamorphic P-T conditions derived from traditional geothermobarometry and the TWQ method (Table 1) show some diversity, possibly due to the inherent errors of these two methods. However, the general P-T trends of any one sample derived by these two methods are similar.
Summary of Metamorphic P-T Paths
Clockwise metamorphic P-T paths were retrieved for the high-pressure mafic granulite, amphibolite, and metapelite (Fig. 13), which are tentatively inferred to record subduction and subsequent tectonic exhumation. The P-T paths are similar, although their metamorphic peak (M2) P-T conditions show large differences (Fig. 13F). Such P-T gaps resemble those found in the Hongliuxia block (H.Y.C. Wang et al., 2017b). The metamorphic peak (M2) P-T conditions of the high-pressure mafic granulite (sample G62; Fig. 13A) lie in the high-pressure granulite-facies field (O’Brien and Rötzler, 2003), or transition zone between the granulite and eclogite facies (Spear, 1993, p. 19), and approach the lower limit of the high-P-T facies series (Miyashiro, 1961). Meanwhile, the metamorphic peak (M2) P-T conditions of the garnet-clinopyroxene amphibolite (samples WH18 and WH84) lie in-between the high- and intermediate-P-T facies series (Figs. 13B and 13C).
SIMS AND LA-ICP-MS U-Pb DATING RESULTS FROM ZIRCON
The SIMS and LA-ICP-MS zircon U-Th-Pb analytical results are presented in Tables S2 and S3, respectively (see footnote 2). Zircon crystals separated from these metamorphic rocks are anhedral, and stubby to ovoid in shape (Fig. 14), with characteristic morphology of high-grade metamorphic zircon (Hoskin and Black, 2000; Hoskin and Schaltegger, 2003). Most grains showed relatively homogeneous luminescence, while a few grains exhibited dark-bright or sector zoning or core-rim structure in the cathodoluminescence images.
Seventeen spots were analyzed on 15 grains (Fig. 14A) separated from the high-pressure mafic granulite (sample G62). Several of these showed core-rim structure. U and Th contents were 1.7–616 ppm and 0.2–722 ppm, respectively. The resulting data define a discordant curve with an upper intercept at 747 ± 71 Ma and a lower intercept at 419 ± 26 Ma (Fig. 15A). Twelve spots, with low Th/U ratios of 0.13–0.38, clustered on the concordia curve and produced a concordia age of 412.2 ± 2.6 Ma (mean square of weighted deviates [MSWD] = 0.066; Fig. 15A). One analytical spot on the overgrowth rim (no. 16 in Fig. 14A) had very low U contents (1.7 ppm) and Th contents (0.2 ppm), and so only a “207-corrected” age is given, 413.8 ± 18.1 Ma. The remaining four discordant data spots analyzed on the cores had much higher Th/U ratios of 0.84–1.2.
Seventeen spots were analyzed on 16 grains (Fig. 14B) separated from the garnet-clinopyroxene amphibolite (sample WH18). Four grains showed dark-bright zoning. U and Th contents were 19–202 ppm and 6–79 ppm, respectively, with low Th/U ratios of 0.22–0.42. All the data points fell on the concordia curve and produced a concordia 206Pb/238U age of 420.1 ± 4.1 Ma (MSWD = 5.5; Fig. 15B). Thirteen spots were analyzed on 13 grains (Fig. 14C) separated from the garnet-clinopyroxene amphibolite (sample WH84). The grains were texturally homogeneous and contained quite low U and Th contents of 2–82 ppm and 0.03–10 ppm, respectively, with Th/U ratios between 0.01 and 0.42. Accordingly, only nine concordant data spots yielded a concordia age of 416.6 ± 2.1 Ma (MSWD = 5.3; Fig. 15C). However, an intercept age of 416.5 ± 4.8 Ma (MSWD = 0.53) was determined from the Tera-Wasserburg concordia diagram (Fig. 15D) using all 13 data spots, and a weighted mean “207-corrected” age of 416 ± 3.8 Ma (MSWD = 0.46) was obtained (Fig. 10D).
Eighteen spots were analyzed on 18 grains (Fig. 14D) separated from the garnet amphibolite (sample G89). Zircon crystals showed weak sector zoning and contained U and Th contents of 93–498 ppm and 0.3–5 ppm, respectively, with quite low Th/U ratios of 0.002–0.012. All the data spots were concordant and produced a concordia age of 394.3 ± 1.3 Ma (MSWD = 1.9; Fig. 15E). Twenty-one spots were analyzed on 21 grains (Fig. 14E) separated from the garnet-biotite amphibolite (sample WH82). Zircon crystals showed weak sector zoning and contained U and Th contents of 91–949 ppm and 0.2–14 ppm, respectively, with quite low Th/U ratios between 0.002 and 0.016. All the analyses clustered on the concordia and yielded a concordia age of 400.1 ± 1.2 Ma (MSWD = 11.3) and a weighted mean 206Pb/238U age of 398.7 ± 3.3 Ma (MSWD = 1.5).
Twenty-four spots were analyzed on 21 grains (Fig. 14F) separated from the metapelite (sample G81). These zircons contained U and Th contents of 28–680 ppm and 16–609 ppm, respectively. Most zircon crystals showed clear dark-bright zoning, an dark domains contained higher U, but no apparent distinction in ages (Fig. 14F; Table S2 [see footnote 2]). All the resulting data define a discordant curve with an upper intercept at 1578 ± 17 Ma and a lower intercept at 370 ± 19 Ma (Fig. 15F). Three data spots dominate the lower intercept point, with Th/U ratios of 0.64–0.9, and these produced a concordia age of 372.4 ± 4 Ma (MSWD = 0.004; Fig. 15G). The remaining 21 spots, with Th/U ratios between 0.16 and 1.27, were distributed on the discordant curve, controlling its upper intercept point.
Twenty-two grains (Fig. 14F) were separated from the metapelite (sample G59) and analyzed using LA-ICP-MS. Zircon displayed obvious core-rim structure, and some cores showed oscillatory zoning (e.g., no. 8 and No. 19 in Fig. 14G). The bright overgrowth rims were too thin for analysis. Dating results exhibited three groups: core ages including a 207Pb/206Pb age of 2944 ± 35 Ma, an upper intercept age of 2263 ± 250 Ma, and another upper intercept age of 1793 ± 43 Ma (Fig. 15H; Table S3 [see footnote 2]). The last group forms an imprecise lower intercept at 416 ± 130 Ma. Three data spots (red ellipse in Fig. 15H) were analyzed on the metamorphic grain or rim (no. 7, no. 16, and no. 17).
For the high-pressure mafic granulite and amphibolite, metamorphic ages were determined to be ca. 420–394 Ma (Figs. 14–15) by analyzing the anhedral, metamorphic zircon grains and/or the metamorphic overgrowth rim of older zircon. As for the metapelite, lower-intercept ages of ca. 420–372 Ma (Figs. 15G–15H) were interpreted to be ages of metamorphism. However, at present, we cannot distinguish among ages corresponding to the prograde (M1), the metamorphic peak (M2), or the retrograde (M3) stages, because the zircon was separated from smashed rocks.
The metamorphic P-T trajectories of high-grade terranes provide important constraints on their tectonic evolution (e.g., England and Thompson, 1984; Thompson and England, 1984; Ernst, 1988; Harley, 1989; Spear, 1993; Barnhart et al., 2012; Parsons et al., 2016). Deep subduction, usually to mantle depth, is required for high-pressure metamorphism to occur (O’Brien and Rötzler, 2003). The peak metamorphic P-T conditions documented by the high-pressure mafic granulite in the Mogutai block are ∼800–830 °C and ∼17–16.5 kbar (Zong et al., 2012; He et al., 2014; this study), suggesting a relatively “cool” (high P-T) gradient of ∼14–15 °C/km, if a lithostatic pressure gradient (Spear, 1993, p. 8) is adopted. This thermal gradient approaches the high-P-T metamorphic series (Miyashiro, 1961). In fact, high P/T ratios are similar to gradients observed in modern subduction zones (Miyashiro, 1961; Ernst, 1988; Spear, 1993, p. 9). The newly discovered Paleozoic (ca. 411 Ma) eclogite in the Hongliuxia area (H.Y.C. Wang et al., 2017b) provides further support for the subduction zone origin for the Dunhuang orogenic belt.
Traditional geothermobarometry, TWQ computation, and isochemical phase diagram (pseudosection) modeling yielded similar P-T trajectories (Fig. 13). The metamorphic P-T paths derived from the mafic rocks have relatively tight loops (Fig. 13), which are commonly interpreted as a consequence of subduction and subsequent exhumation (Ernst, 1988). Nevertheless, great discrepancy among metamorphic peak (M2) P-T conditions was found for these higher-grade rocks, ranging from 830 °C and 16.5 kbar (high-pressure granulite puddingstone) to 635 °C and 6.0 kbar (metapelite matrix). These differences are far beyond the typical errors of the geothermobarometers, and thus they indicate significantly different depths reached by these high-grade rocks in a subduction channel (Cloos and Shreve, 1988), and subsequent amalgamation of the lenses and the matrix during the exhumation to form a tectonic mélange (e.g., Kusky et al., 1997; Festa et al., 2012).
As mentioned earlier, the Dongbatu and Mogutai metamorphic complexes exhibit the characteristic block-in-matrix fabrics of mélange (e.g., Silver and Beutner, 1980; J.P. Wang et al., 2013a, 2017; Raymond and Bero, 2015). These rocks are characterized by chaotic mixtures of metasediments, including metapelite, metapsammite, felsic gneiss, and marble (Zong et al., 2012; He et al., 2014; this study) as matrices, and large amounts of exotic amphibolite and granulite as lenses. The metamorphic events for some high-pressure mafic granulite and their metapelitic country rock have been dated as ca. 440–430 Ma, as mentioned earlier herein, representing a Paleozoic orogenic event (Zong et al., 2012; He et al., 2014). Our new zircon ages for the metamorphic lenses and metapelite in the Dongbatu and Mogutai blocks range from ca. 420 Ma to ca. 372 Ma. It is noted that the high-pressure mafic granulite was dated as ca. 412 Ma in this study, compared to the ca. 440–430 Ma high-pressure mafic granulite sampled at the neighboring locality by Zong et al. (2012) and He et al. (2014) in the Mogutai block. This supports a mixing process of rocks metamorphosed at fairly different depths and possibly different times in a subduction channel (e.g., Kusky et al., 1997; Festa et al., 2012). These granulite and amphibolite lenses or interlayers possibly have different origins, and their protolith ages range from Paleoproterozoic to Neoproterozoic (Z.M. Wang et al., 2013b, 2013c, 2014; Y. Zhao et al., 2015, 2016; H.Y.C. Wang et al., 2016, 2017a, 2017b; this study), therefore suggesting a previous accretionary process in this Silurian–Devonian orogenesis. Combined with the previous metamorphic zircon U-Pb and hornblende 40Ar/39Ar data (Zong et al., 2012; He et al., 2014; H.Y.C. Wang et al., 2016, 2017a, 2017b; Y. Zhao et al., 2016), we conclude that the Silurian–Devonian tectono-metamorphic event in the Dunhuang orogenic belt was a prolonged process (from 440 to 365 Ma) that lasted for at least ∼75 m.y. We can now reasonably anticipate that the prograde metamorphic stage (M1) corresponds to the subduction, the metamorphic peak (M2) corresponds to the deepest depth, and the retrograde stage (M3) corresponds to tectonic exhumation.
(1) Three stages of metamorphism were identified in the high-pressure mafic granulite and amphibolite in the Dongbatu and Mogutai blocks of the middle Dunhuang orogenic belt. The metamorphic peak of the high-pressure mafic granulite approaches the high P-T metamorphic facies series. The derived metamorphic P-T paths are similar and clockwise, passing from 656 °C and 10.9 kbar through 830 °C and 16.5 kbar and finally to 657 °C and 4.9 kbar for the high-pressure mafic granulite; from 631–645 °C and 6.7–9.6 kbar through 720–727 °C and 13.6–14.2 kbar to 615–664 °C and 3.2–4.2 kbar for the garnet-clinopyroxene amphibolite; from 564 °C and 3.8 kbar through 653–674 °C and 7.6–8.6 kbar to 618 °C and 4.9 kbar for the garnet amphibolite; and from 584 °C and 3.2 kbar to 634–689 °C and 6.1–7.9 kbar for the garnet-biotite amphibolite. These retrieved P-T trajectories of the mafic granulite, amphibolite, and metapelite are typical of orogenic metamorphism.
(2) SIMS and LA-ICP-MS U-Pb dating of metamorphic zircon from the mafic granulite, amphibolite, and metapelite suggests that the metamorphism in the Dongbatu and Mogutai blocks occurred at ca. 420–372 Ma.
(3) The significant differences in the metamorphic peak P-T conditions and the long duration of the Paleozoic metamorphism (from ca. 440 Ma to ca. 365 Ma) support a tectonic mélange formed in a subduction channel. The Dunhuang orogenic belt represents a Silurian–Devonian subduction zone involved in the Paleozoic accretionary orogenesis at southernmost Central Asian orogenic belt.
We sincerely thank Xian-Hua Li and Qiu-Li Li for guidance with the secondary-ion mass spectrometry experiments and Qian Mao, Yu-Guang Ma, Yong-Hong Shi, and Juan Wang for help with electronic microbe analyses. Both the scientific quality and the English of this paper were greatly improved from reviews by Timothy Kusky and Sean Regan, as well as the editorial review by Mike Williams and Raymond M. Russo. This work was supported by the National Natural Science Foundation of China (41730215), Chinese Academy of Sciences (QYZDJ-SSW-DQC036), National Postdoctoral Program for Innovative Talents (BX201700240), and the State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences (Kai201605).