Journal of Earth Science  2018, Vol. 29 Issue (5): 1026-1039   PDF    
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Metamorphic Characteristics and Tectonic Implications of the Kadui Blueschist in the Central Yarlung Zangbo Suture Zone, Southern Tibet
Guangming Sun1, Xu-Ping Li1, Wenyong Duan1, Shuang Chen1, Zeli Wang1, Lingquan Zhao2, Qingda Feng3    
1. Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China;
2. Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität, Bochum 44780, Germany;
3. Center of Hydrogeology and Environmental Geology Survey, China Geological Survey, Baoding 071051, China
ABSTRACT: The Kadui blueschist is located in the central section of Yarlung Zangbo suture zone (YZSZ), southern Tibet, and has been subjected to the subduction of the Neo-Tethyan Ocean below the Asian Plate and provides important clues for better understanding the evolution of the India-Asia convergence zone. In this paper, the systematical petrographic and mineral chemical studies were carried out on the Kadui blueschist, which reveal a mineral assemblage of sodic amphibole, chlorite, epidote, albite and quartz with accessory minerals of titanite, calcite and zircon. Electron microprobe analyses demonstrate that amphibole shows zoned from actinolite core to ferrowinchite/riebeckite rim composition indicating that the sodic amphibole has formed during a prograde metamorphic event. The protolith of the blueschist is an intermediate-basic pyroclastic rock. The calculated pseudosection indicates a clockwise P-T path and constrains peak metamorphic conditions of about 5.9 kbar at 345℃. This condition is transitional between pumpellyite-actinolite, greenschist and blueschist facies with a burial depth of 20-22 km and a thermal gradient of 15-16℃/km. This thermal gradient belongs to high pressure intermediate P/T facies series and is possibly related to a warm subduction setting of young oceanic slabs. Our new findings indicate that the Kadui blueschist in the central part of YZSZ experienced a rapid subduction and exhumation process as a response to a northward subduction of the Neo-Tethyan oceanic lithosphere during the initial India-Asia collision stage.
KEY WORDS: Kadui blueschist    metamorphism    P-T conditions    accretionary complex    southern Tibet    

0 INTRODUCTION

Blueschist is formed under HP-LT (high-pressure and low-temperature) conditions and has been recognized as an important marker of subduction zones (Maruyama et al., 1996a; Ernst, 1972; Miyashiro, 1961). Thus the spatial and chronological study of blueschists is critical for understanding the metamorphic evolution and tectonic process which occur at convergent plate boundaries (Isozaki et al., 2010; Stern, 2005; Platt, 1993). Based on the protolith and subduction nature, Maruyama et al. (1996a) classified blueschist belts into two types, i.e., collisional type and Cordilleran type. The former one, also called A-type blueschist (Maruyama et al., 1996a), consists of passive-margin lithologies of limestones, bimodal volcanics, peraluminous sediments and granitic gneiss basement rocks, and is normally formed in continental collision process (Maruyama and Liou, 1998; Maruyama et al., 1996b). The Cordilleran type (B-type) blueschist results from oceanic plate subduction and is characterized by the formation of accretion complexes in an active continental margin, typically containing of bedded chert, MORB basalt, ocean-island basalt and seamount fragments derived from oceanic crust (Maruyama, 1997; Maruyama et al., 1996a). Examples are the Sambagawa metamorphic belt in Japan (Kabir and Takasu, 2016; Aoki et al., 2008) and Franciscan mélange in California, USA (Ukar and Cloos, 2014; Ukar, 2012), which are located at areas as part rocks encircled the Pacific Ocean. In comparison with A-type blueschists, B-type blueschists commonly experience lower metamorphic pressure and temperature conditions (Maruyama and Liou, 1998). A mineral assemblage of metabasaltic rock that experiences blueschist facies metamorphism contains sodic amphibole (glaucophane)+lawsonite (or epidote)+ chlorite+albite+quartz±sodic (jadeitic) clinopyroxene±aragonite (Ota and Kaneko, 2010). Depending on the P-T conditions the metabasalt experienced, different further mineral assemblages are possible, which in general range in the area of subgreenschist to blueschist facies conditions.

Blueschists are widespread along the Alpine-Himalayan orogenic zones (Miyashiro, 1961). Although the Himalayan blueschists occur in an intracontinental fold belt, they belong to the B-type, because blueschist facies metamorphism occurred prior to continent-continent collision of India and Eurasia (Maruyama et al., 1996a). The Kadui blueschist, located in the central portion of the YZSZ (Figs. 1a, 1b), is considered to be a part of the Yarlung Zangbo blueschist belt (Li et al., 2007). This blueschist in South Tibet has been formed due to the northward subduction of the Neo-Tethyan oceanic crust and forms a conspicuous lithological belt of the YZSZ (Fig. 1c, Shen and Geng, 2012; Li et al., 2007; Xiao et al., 1988, 1983; Xiao and Gao, 1984). It is, however, poorly studied due to limited outcrops which are hard to access. The mineral assemblage of blueschist-facies within the central section of YZSZ was first reported by Xiao and Gao (1984), which contains sodic amphibole, winchite, epidote, chlorite, phengite and stilpnomelane with minor aragonite and pumpellyite. The blueschist in the Kadui area was first described as quartz-biotite-glaucophane schists by Hu et al. (2004). On the basis of field investigation and petrographic studies, it was interpreted to have formed due to progressing metamorphism from sub-greenschist facies to blueschist facies. The Kadui blueschist yields a mineral assemblage of sodic amphibole+chlorite+epidote+albite+quartz (Li et al., 2007). In spite of its significance for analyzing and understanding the regional tectonic evolution of South Tibet, the P-T conditions and tectono-metamorphic evolution of the Kadui blueschist are still unclear. Because of the low variation of mineral assemblage and the presence of disequilibrium textures under such low P-T conditions, it is difficult to constrain the accurate metamorphic P-T conditions of the blueschist facies by using conventional geothermobarometers

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Figure 1. (a) Schematic tectonic map of the Himalaya-Tibetan Plateau (modified after Huot et al., 2002). MBT. Main boundary thrust; MCT. main central thrust; YZSZ. Yarlung Zangbo suture zone. (b) Schematic tectonic map of southern central Tibet (modified after Dai et al., 2012). GCT. Great counter thrust, YZMT. Yarlung Zangbo mantle thrust; ZGT. Zhongba-Gyangze thrust. (c) Blueschist belt in the central part of the YZSZ (modified after Xiao and Gao, 1984). (d) Simplified geological map of the Kadui area showing distribution and sample location of the blueschist-bearing block (modified after Zhang et al., 2016).

During the last decade, great progress has been achieved in phase equilibria modeling, such as P-T, T-X, P-X, X-X pseudosection techniques and different diagrams of important compositional variation in the rocks, which became a powerful and routine approach to constrain P-T conditions and to elucidate the phase relations and evolution of metamorphic rocks (e.g., Groppo et al., 2016; Zhang et al., 2015; Ao and Bhowmik, 2014; Wei et al., 2010, 2009a, b; Wei and Song, 2008). The pseudosection modeling approach was utilized in this study in order to obtain more accurate quantitative data related to the metamorphic P-T conditions the Kadui blueschist experienced and thus to a better understanding their petrogenesis and metamorphic evolution. The objectives of this paper are: (ⅰ) to present a detailed mineral chemical study on the basis of representative metamorphic minerals of the Kadui blueschist; (ⅱ) to work out the P-T path using thermodynamic calculations; (ⅲ) to discuss the geological implications of the Kadui blueschist and further interpret its role in respect of the tectonic evolution of the central part of YZSZ during the India-Asia collision.

1 GEOLOGICAL SETTING AND FIELD RELATIONSHIPS

The Tibetan Plateau consists of a collage of several terranes, including, from north to south, the Kunlun, Qiangtang and Lhasa terranes (Wu et al., 2017; Zhang et al., 2016; Li et al., 2015a, b; Pan et al., 2012). They are separated from each other by four major suture zones, which are a result of the closure of the Paleo- and Neo-Tethyan oceans (Li et al., 2017; Liu et al., 2016; Zhu et al., 2013) (Fig. 1a). The YZSZ is the southernmost and youngest one of the four suture zones, which separates the Lhasa terrane (Asian convergent margin) to the north and the Tethyan Himalaya (Indian passive margin) to the south in southern Tibet (Wang et al., 2017; Ding et al., 2005) (Figs. 1a, 1b). From north to south, the YZSZ can be basically divided into four lithotectonic units, which are all W-E oriented at the southern margin of the Lhasa terrane. These are the Gangdese arc, the Xigaze forearc basin, the Yarlung Zangbo ophiolitic belt and the accretionary complex (Hébert et al., 2012; Dai et al., 2011; Ding et al., 2005; Ratschbacher et al., 1994) (Fig. 1b). The Gangdese arc, located at the southern margin of the Lhasa terrane, is mainly composed of Jurassic, Early Cretaceous and Paleogene volcanic rocks and contains a Late Triassic to Cenozoic granitoid batholiths (Ma et al., 2017; Kang et al., 2014; Zhu et al., 2011; He et al., 2007). The Xigaze forearc basin is next to the Gangdese arc in the south, consists of Cretaceous to Lower Eocene sediments which are flysch, marine limestone and sandstone (Wang et al., 2017; Orme et al., 2015; An et al., 2014; Wan et al., 2002). The Yarlung Zangbo ophiolitic belt, representing the most prominent remnant of the Tethyan Ocean floor, comprises large amounts of ultramafic units with minor mafic rocks, including mantle peridotite, gabbro, dolerite dike and pillow basalt (Liu et al., 2014; Bédard et al., 2009; Wang et al., 1987). The accretionary complex, dominated by Triassic to Eocene sedimentary-matrix mélange with a small amount of serpentinite mélange, is in bordered by the Xigaze ophiolite belt in the north, by Yarlung Zangbo mantle thrust (YZMT), and with Tethyan Himalayan strata in the south by Zhongba-Gyangze fault (ZGT) (Cai et al., 2012; Zhu et al., 2006; Pan et al., 2004). Previous studies indicated that the accretionary complex at the southern margin of the YZSZ was formed during the northward subduction of Neo-Tethyan oceanic lithosphere underneath the Lhasa terrane (Wang et al., 2017; Cai et al., 2012; Ziabrev et al., 2004; Searle et al., 1987).

The metamorphic belt within the central part of the YZSZ referred to as HP-LT metamorphic belt (Xiao et al., 1988; Xiao and Gao, 1984) can be divided into the blueschist belt in the north and greenschist belt in the south (Fig. 1c). The blueschist belt, along the south side of Yarlung Zangbo ophiolite belt, discontinuously extends W-E from Zhongba to Qusong for more than 400 km (Shen and Geng, 2012; Xiao et al., 1988, 1983; Xiao and Gao, 1984). The blueschists occur as blocks in the serpentinite mélange within the Upper Jurassic to Lower Cretaceous metasediments and Upper Triassic turbidites (Maruyama et al., 1996a; Xiao et al., 1983). The Kadui serpentinite mélange is located on the south side of the Xigaze ophiolite belt, about 2.5 km southeast of the Kadui Town. It extends about 140 m E-W, and is oriented parallel to the regional structural deformation of the major lithostratigraphic units (Fig. 1d). The Kadui serpentinite mélange comprises predominantly of blueschist and micaschist as well as deformed and metamorphosed mantle peridotite with minor diabase, pillow basalt, rodingite, and sedimentary siliceous protoliths (Li et al., 2007). It is in fault contact with the Triassic Langjiexue flysh series to the north and Triassic Nieru terrane to the south (Hu et al., 2004). The blueschists occur as small lenses or irregular layers within Kadui serpentinite mélange (Hu et al., 2004). According to analysis on regional geological condition, the blueschists of the YZSZ have been considered to be Paleocene to Eocene in metamorphic age (Xiao et al., 1988). Sodic amphiboles from metamorphic pyroclastic rocks in Kadui area are reported to yield an 40Ar/39Ar age of 59 Ma, which is commonly attributed to the closure of the Neo-Tethyan Ocean and the first "contact" between the Indian and Asian plates (Li et al., 2007).

2 PETROGRAPHY AND BULK-ROCK COMPOSITION

The investigated blueschist facies rocks within the Kadui serpentinite mélange are heterogeneous with regards to mineral assemblage and are mainly composed of variable lithological types, including albite-chlorite-epidote-sodic amphibole schist, stilpnomelane-bearing quartz schist, chlorite-quartz schist and phengite-quartz schist. These schists show similar foliated structure, derived from different types of protoliths, but experienced a joint metamorphism and deformation. Amphibole-bearing rocks of this study have a pyroclastic precursor (Table 1), and display a foliation/lineation and nematoblastic texture (Fig. 2). The foliated structure is characterized by an interlayering of light colored albite-quartz layers and dark layers of amphibole, chlorite, epidote and titanite (Fig. 2a). Three representative blueschist samples from the Kadui mélange were selected to investigate mineral assemblages and mineral compositions in order to obtain petrogenetic information. The sample localities are shown in Fig. 1d. All three samples exhibit a similar mineral assemblage of amphibole, epidote, chlorite, albite, and quartz with accessory titanite (Fig. 2).

Table 1 Bulk-rock compositions (wt.%) of Kadui blueschist, southern Tibet
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Figure 2. Photomicrographs (in plane-polarized light; (a)-(d)) and back-scattered electron images (e), (f) of samples 13KD23 (a), (b), (d), (f) and 15KD10-1 (c), (e). (a) Blueschist showing a banded structure defined by preferred orientation of amphibole, epidote, chlorite, albite and titanite. (b), (c) and (e) Zoned amphibole with actinolite core and ferrowinchite/riebeckite rim. (d) Amphiboles showing sharp boundary between actinolite and ferrowinchite/riebeckite. (f) Fractured clinopyroxene enveloped and partially replaced by actinolite. Mineral abbreviations and end-member abbreviations are after Whitney and Evans (2010).

Amphiboles are sodic, sodic-calcic and calcic species, which are riebeckite, winchite/ferrowinchite and actinolite (Figs. 3a, 3b), respectively. Actinolite occurs as small subhedral and prismatic crystals with a preferred orientation; its grain size is up to 0.3 mm in length. Riebeckite, winchite, ferrowinchite are light blue and occur as prismatic to needle-like crystals in the matrix of the rocks (Figs. 2b, 2c, 2d, 2e). Two textural relationships of coexisting amphiboles have been recognized. Amphiboles either show actinolite cores which are surrounded by later generations of ferrowinchite to riebeckite (Figs. 2b, 2c) or occur as individual elongated and unzoned euhedral crystals of actinolite and ferrowinchite to riebeckite (Fig. 2d). Epidote appears as tiny anhedral rounded crystals, 0.03 mm in dimension. Chlorite occurs as scaly shapes (up to 0.2 mm in diameter) (Fig. 2a). Albite occurs as small subhedral to anhedral grains with a size up to 0.05 mm. Small highly fractured relicts of igneous clinopyroxene make up less than 1% of the modal proportion and are overgrown by thin rims of amphiboles (Fig. 2f). Note that the matrix contains very small amounts of calcite.

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Figure 3. (a) Si vs. NaM4 p.f.u. and (b) Fe3+/(Fe3++Al) vs. Fe2+/(Fe2++Mg) diagrams of analyzed amphibole compositions for Kadui blueschist samples (after Leake et al., 1997). (c) Si vs. Na/(Na+Ca) and (d) glaucophane (ferroglaucophane)-magnesioriebeckite (riebeckite)-actinolite ternary diagram of zoned amphiboles (on the basis of Gln/Fg)=Al/(Al+Fe3+)×Na/(Na+Ca)B, Mrbk/Rbk=Fe3+/(Al+Fe3+)×Na/(Na+Ca)B, Act=(2-Al)/2×Ca/(Na+Ca)B) (modified after Ota et al., 2004). For further explanations see text.

The bulk-rock compositions were determined by X-ray fluorescence (XRF) at Ruhr-Universität Bochum, Germany. Fe2+ and Fe3+ contents were determined by titration. Loss on ignition (LOI) was measured by standard weight-loss techniques after devolatilization and therefore represents the total volatile content. As shown in Table 1, the Kadui blueschist is mafic to intermediate in composition with SiO2 contents ranging from 55.97 wt.% to 58.49 wt.%. The LOI of the samples varies from 1.27 wt.% to 2.36 wt.%, reflecting different stages of weak alteration. The samples are characterized by relatively low amounts of Al2O3 (9.31 wt.%-9.34 wt.%), high TiO2 (2.67 wt.%-2.94 wt.%), high Na2O (5.62 wt.%-6.35 wt.%) and low K2O (0.21 wt.%-0.25 wt.%). Together with the previous studies in Li et al. (2007), these geochemical characteristics indicate that the protolith of blueschist is an intermediate-basic pyroclastic rock.

3 MINERAL COMPOSITION

Mineral compositions were obtained using a JXA-8100 electron microprobe in wavelength-dispersive mode at State Key Laboratory for Mineral Deposits Research of Nanjing University. Operating conditions were: acceleration voltage 15 kV, beam current 15 nA, counting time 20-30 s. The beam diameter was set to 1 μm for all phases. Natural minerals served as analytical standards. The representative mineral compositions are presented in Table 2. Amphibole formulae are calculated on the basis of 23 oxygens, using the program AX (Holland; http://www.esc.cam.ac.uk/research/research-groups/holland/ax).

Table 2 Representative microprobe analyses of minerals of blueschists (samples 13KD23 and 15KD10-1)

Using the classification of Leake et al. (1997), sodic, sodic-calcic and calcic amphiboles are riebeckite, winchite/ferrowinchite and actinolite, respectively (Figs. 3a, 3b). Sodic and sodic-calcic amphiboles are not distinguishable using the polarizing microscope, their chemical compositions range gradually between riebeckite and winchite/ferrowinchite (Table 2). Sodic amphiboles (NaM4=1.59-1.69 a.p.f.u.; Al=0.29-0.51 a.p.f.u.) are grouped as riebeckite with variable Fe2+/(Fe2++Mg) ratios between 0.52 and 0.62 and Fe3+/(Fe3++Al) between 0.59 and 0.85. The compositions in the discrimination plots are close to the boundary between riebeckite and magnesioriebeckite (Fig. 3b). Sodic-calcic amphiboles with Si=7.71-8.00 a.p.f.u., NaM4= 0.52-1.35 a.p.f.u., Al=0.19-0.34 a.p.f.u., Fe2+/(Fe2++Mg)= 0.43-0.67 are winchite to ferriwinchite (Fig. 3a). Calcic amphibole is actinolite with Si=7.72-7.99 a.p.f.u., NaM4=0.02-0.48 a.p.f.u., Fe2+/(Fe2++Mg)=0.38-0.52. Zoned amphiboles increase in Al, NaM4 and Fe2+ from core to rim while Fe3+, Ca and Mg accordingly decrease (Figs. 3c, 3d).

Chlorite analyses from the Kadui blueschists which yield Fe2+/(Fe2++Mg) ratios between 0.48 and 0.54 plot within the pycnochlorite field (Fig. 4) and are slightly higher in iron than those from blueschists of Ladakh (western Himalya) and Nagaland (eastern Himalaya). The pistacite component Fe3+/ (Fe3++Al) of epidote is in the range of 0.26-0.35. The plagioclase is pure albite with Na/(Na+Ca) > 0.95.

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Figure 4. Chemical composition of chlorites in the Kadui blueschist of the current study, plotted in Hey's diagram (Hey, 1954). Chlorite compositions of the Ladakh blueschist (Honegger et al., 1989) and Nagaland blueschist (Ao and Bhowmik, 2014) are given for comparison.
4 PHASE EQUILIBRIA MODELING AND P-T CONDITIONS

In order to investigate the phase relationships and constrain the P-T evolution of the studied blueschists, P-T pseudosections were constructed in the model system NCFMASHTO on the basis of observed whole rock compositions as well as mineral assemblages and measured compositions. The P-T pseudosections were computed employing the Perple_X computer program package (Connolly, 2005; version 6.6.6, updated 2012) with the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2002). The following solid solution models were used: garnet (White et al., 2007, 2005), amphibole (Diener et al., 2007), clinopyroxene (Holland and Powell, 1998), plagioclase (Newton et al., 1980), chlorite (Holland et al., 1998), epidote (Holland and Powell, 1998) and ilmenite (White et al., 2000). Quartz and the fluid phase are assumed to be pure SiO2 and H2O, respectively and to occur in excess. Lawsonite is treated as a pure end-member phase. The subtypes of amphibole used in pseudosections are glaucophane (Gln) as sodic amphibole, actinolite (Act) as calcic amphibole and amphibole (Amp) refers to sodic/calcic amphibole beyond the stability field of this study.

4.1 P-T Pseudosection

The P-T pseudosection presented in Fig. 5 was calculated for the blueschist sample 13KD23 within a P-T range of 1-12 kbar and 200-500 ℃. This sample is representative for a typical albite-chlorite-epidote-blueschist. The P-T pseudosection shows the field of mineral associations with boundaries, derived from the analyzed constituents (sodic amphibole, actinolite, epidote, albite, chlorite, quartz and titanite). The stability of chlorite for the related mineral assemblage of the blueschist is restricted to an upper temperature limit of 390 ℃ and upper pressure limit of 7.7 kbar. Sodic amphibole can be stable over a wide range of pressures at low-T conditions and coexists with actinolite in most cases. The prograde P-T path shown in Fig. 5 is constructed using the isopleths of Al and XNa of the zoned amphiboles, defined by actinolite core and ferrowinchite to riebeckite rim compositions (Figs. 2b, 2c, 3c, 3d).

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Figure 5. P-T pseudosection for sample 13KD23 in the system NCFMASHTO (Qz and H2O in excess) and using a bulk-rock composition from Table 1 normalized on the basis of mole-proportions as: SiO2=61.54, Al2O3=5.79, CaO=7.23, MgO=7.33, FeO=8.74, Na2O=5.73, TiO2=2.11, Fe2O3(O)=1.52. For amphiboles, the isopleths for Al are shown by blue dashed lines and for XNa in purple dashed-dotted lines. The calculated P-T conditions on the basis of the zoned amphiboles and their Al- and XNa-contents (Fig. 3 and Table 2) are shown as orange stars in the P-T field related to the assemblage Gln+Act+Ep+Chl+Ab+Ttn. The geothermal gradient is abstracted from Zhang et al. (2015).

In the compatibility field of Gl+Act+Ep+Chl+Ab+Ttn (+Qz), the Al-isopleths in sodic amphibole show low slopes, which increase reaching the compatibility field of Amp+Act+Ep+Ab+Ttn at higher temmperatures. Thus at lower temperatures, relevant for the studied samples, the Al-content of sodic amphibole can be used as a geobarometer (e.g., Zhang et al., 2015; Maruyama et al., 1986). The slope of the isopleths of XNa in sodic amphibole slightly increases with increasing as temperature and pressure. Electron microprobe analyses indicate that the sodic amphiboles have relatively high Al- and XNa-values. It is strongly supported by the zonal textural sequence of amphibole compositions that the crystallization of riebeckite and ferrowinchite is later than that of actinolite during the prograde metamorphic process. The intersection of the measured maximum Al of 0.51 and the corresponding XNa-value of 0.84 in riebeckite fixes a P-T constraint of 5.9 kbar/345 ℃ for the peak condition. Two further compositions measured in sodic amphibole with lower Al and XNa values lead to P-T conditions of 5.8 kbar/340 ℃ and 5.1 kbar/310 ℃, respectively (Fig. 5). Plots of the measured Al and XNa in ferrowinchite constrain a P-T range of 3.5-4.3 kbar/300-310 ℃. These estimates define a part of the prograde P-T path of sample 13KD23, which is characterized by the increase in both P and T from 3.5-4.3 kbar/300-310 ℃ to peak condition at about 5.9 kbar/345 ℃.

4.2 P-Fe2O3 Pseudosection

In order to further evaluate the influence of Fe2O3 on the compatibility fields of mineral assemblages, and on the relationships between Al in sodic amphibole and related pressure conditions, a P-Fe2O3(O) diagram is constructed at a constant temperature of 350 ℃ with a Fe2O3 range of 1.0-3.0 (Fig. 6). As Fe2O3(O) increases, the stability field for the observed mineral assemblage of Gln+Act+Ep+Chl+Ab+Ttn decreases towards lower pressures. In this compatibility field, the Al isopleths of sodic amphibole have flat slopes (sub-parallel to the x-axis) and show increasing pressures with increasing amounats of Al. On the other hand, in the higher pressure chlorite-absent compatibility field Gln+Act+ Ep+Ab+Ttn, the Al-isopleths show steep slopes and change significantly at variable Fe2O3(O) values. Therefore, the Al-content in sodic amphibole can be used either as a geobarometer or geothermometer, dependent on the limiting mineral assemblage (e.g., Zhang et al., 2015; Phillips et al., 2010).

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Figure 6. P-Fe2O3(O) pseudosection calculated for sample 13KD23 in the NCFMASHTO system at 350 ℃ where Fe2O3=1.52 (the measured value for 13KD23) is shown as a black dashed line. Isopleths of Al in amphibole are given in blue dashed lines and those of Mg in green dashed-dotted lines. The arrow connects compositions of ferrowinchite and riebeckite. For further explanations see text.

In Chl-bearing assemblages, the Mg isopleths in amphibole show moderately negative slopes; the Mg-values increase with increasing bulk Fe2O3 contents. There is a minor discrepancy between the observed Mg-values derived from EMP-measurements with those predicted by the modeling, which indicates that the mineral reaction to form sodic amphibole was possibly not yet completed. Ague and Carlson (2013) for instance showed, that non-equilibrium states during metamorphic processes are not uncommon, especially for the low-grade metamorphic rocks such as for blueschist or subgreenschist-facies conditions (Phillips et al., 2010). The measured Al-contents increase with decreasing Mg-values to form sodic amphibole from sodic-calcic amphibole (Table 2), reflecting a segment of the prograde P-T path. It is known from phase assemblages which include sodic amphibole from other localities, that amphibole compositions may have changed as a response to changing oxygen fugacity conditions during metamorphism (Okay, 1980). This seems to be also very probable for the current study, documenting a prograde growth of sodic amphibole during proceeding subduction, concurrently accompanied by a significant decrease of bulk Fe2O3-contents.

5 DISCUSSION AND CONCLUSIONS 5.1 Metamorphic P-T Conditions of the Kadui Blueschists

Prior to this study, the peak metamorphic conditions for the HP/LT subduction zone related rocks of the central segment of YZSZ have tentatively ascribed to be at the transition between greenschist and blueschist facies (Xiao et al., 1988; Xiao and Gao, 1984). The results of our P-T pseudosection modeling constrains the peak metamorphic conditions to be at about 5.9 kbar (20-22 km depth) and 345 ℃ (sample 13KD23). Such P-T conditions are located in the border region of pumpellyite-actinolite, greenschist and blueschist facies (e.g., Liou et al., 2004; Oh and Liou, 1998). The estimated temperature is higher than the pumpellyite stability field, which is consistent with our petrographic observations. With the increase of metamorphic grade during subduction, albite and chlorite become unstable, and the released Na and Al results in an increase of these elements in amphibole. The amphibole compositions calculated in the P-T pseudosection can be used for an estimation of P-T paths. Maruyama et al. (1986) proposed that the Al2O3 content in sodic amphibole is buffered by epidote; the amount of actinolite, chlorite, albite and quartz in the rock systematically decreases with decreasing pressure. As shown in the P-T pseudosection (Fig. 5), the Al isopleths are mainly pressure dependent in the observed assemblage of Gln+Act+Ep+Chl+ Ab+Ttn. Therefore, the Al-content can be considered an excellent geobarometer for epidote-blueschist (e.g., Phillips et al., 2010; Aoki et al., 2008). Compared to pressure, the temperature is more difficult to determine. The reaction Pump+Chl+Qz= Ep+Act, constrains a low temperature limit of 270-320 ℃ at pressures of 4-7 kbar (e.g., Xu et al., 2001; Winkler, 1979; Nakajima et al., 1977), however in the studied samples pumpellyite is absent. The conditions of the above reactions coincide with the results of the P-T pseudosection modeling for Kadui blueschist. The petrographic feature and P-T pseudosection calculation indicate that the Kadui blueschist recrystallized outside of the lawsonite stability field (Fig. 5).

Maruyama et al. (1986) demonstrated that the mineral assemblage of metabasite in blueschist-greenschist facies transition is composed of sodic amphibole+epidote+actinolite+ chlorite+albite+quartz. A similar mineral assemblage (and compositional features) is derived from our study and suggests that the Kadui blueschist in the central YZSZ experienced greenschist to subgreenschist facies metamorphic conditions, close to the low-P limit of blueschist facies. This estimation is consistent with data from Xiao and Gao (1984), calling for a high P/T metamorphic belt along the southern boundary of Yarlung Zangbo ophiolite zone. The current results demonstrate that due to a combined study of texture, mineral composition, metamorphic reactions and phase equilibrium modeling, the metamorphic P-T evolution of Kadui blueschist can be qualitatively (in parts also quantitatively) reconstructed. Fractured clinopyroxene relics are replaced by amphibole, which demonstrates that igneous precursors experienced deformation and metamorphism during subduction. The prograde metamorphic texture in zoned amphiboles with actinolite core and ferrowinchite to riebeckite rim is well reserved, suggesting the Kadui blueschist had a short time in peak stage and then followed by a rapid exhumation and uplift process.

5.2 Tectonic Implications

It is generally believed that the blueschist is formed under the condition of a low geothermal gradient (Maruyama et al., 1996a; Ernst, 1972; Miyashiro, 1961) which are achieved in subduction zones (e.g., Omori et al., 2009). The blueschists along the YZSZ are mostly interpreted as related to paleo-accretionary prisms formed in response to the northward subduction of the Neo-Tethyan lithosphere beneath the southern margin of Asian Plate. They are thus crucial in understanding the evolution of the India-Asia convergence zone during the closure of the Neo-Tethyan Ocean (Guillot et al., 2008). Previous studies by Li et al. (2007) and Hu et al. (2004) concluded that the protolith of Kadui blueschists belongs to pyroclastic rocks rather than basic magmatic rocks such as basalts, on basis of field investigation and petrographical study. Our obtained major element compositions suggest that the protolith of Kadui blueschists is in general represented by basic to intermediate pyroclastic rocks. The blueschists occur in a serpentinite mélange associated with an accretionary complex, associated with metasediments which indicates that the protoliths were formed in an oceanic crust setting (e.g. Zhao et al., 2017; Miao et al., 2015). In contrast with tectonic mélange within accretionary wedge, the characteristic assemblage of subduction channel consists of typical HP-LT blueschist-facies to eclogite-facies metamorphic rocks at deep depths (Zheng et al., 2015), usually indicating a rather low thermal gradient of 8-10 ℃/km belonging to high P/T metamorphic facies series (Cui et al., 2017). The obtained tectonic and metamorphic architecture of Kadui blueschist (locates in tectonic mélange, relatively high thermal gradients of 15-16 ℃/km, shallow burial depth of 20-22 km, transitional peak condition between sub-greenschist, greenschist and blueschist facies), combined with previous studies, suggest that the blueschist belt within central YZSZ composing of intensely deformed and low-grade metamorphic rocks was a remnant of a fossil accretionary wedge which formed during the northward subduction of the Neo-Tethyan Ocean crust beneath the Asian Plate.

The accretionary complex is generally interpreted to have formed in an oceanic subduction zone beneath the southern margin of the Lhasa terrane (Searle et al., 1987). In contrast, Aitchison et al. (2000) considered that the Neo-Tethyan oceanic lithosphere was subducted along both an intra-oceanic subduction zone within the Neo-Tethys Ocean and an oceanic subduction zone beneath the southern margin of the Lhasa terrane. More recently, a wide intra-oceanic subduction system along the whole length of the YZSZ within the Neo-Tethyan Ocean during Late Jurassic to Early Cretaceous was proposed by Dai et al. (2011). However, no apparent geologic and stratigraphic records for the existence of an intra-oceanic island arc within the Neo-Tethyan are present in southern Tibet (Wu et al., 2014). Several studies have demonstrated that Ladakh blueschist in western Himalaya (Groppo et al., 2016; Mahéo et al., 2006; Rolland et al., 2000; Honegger et al., 1989) and Nagaland blueschist from eastern Himalaya (Bhowmik and Ao, 2016; Ao and Bhowmik, 2014; Ghose and Singh, 1980) were formed in an intra-oceanic subduction setting, which provides metamorphic evidence from the western part and eastern extension of the intra-oceanic system along the YZSZ.

Nevertheless, new data for Kadui blueschist obtained from this study point to a thermal gradient in the range of 15-16 ℃/km, which belongs to intermediate P/T geotherms (Fig. 5). This thermal gradient disaccords with an intra-oceanic subduction setting and is much higher than Ladakh blueschist (7-9 ℃/km) and Nagaland blueschist (8-9 ℃/km), which are suggested to belong to the same geotectonic setting. Indeed, a thermal gradient of about 15-16 ℃/km corresponds to subduction of warm lithosphere (e.g., Peacock and Wang, 1999; Peacock et al., 1994), which is expected for a setting involving young oceanic slabs (e.g., Zhang et al., 2015). The absence of lawsonite and garnet and the lower peak P-T conditions for the Kadui blueschist compared to the Ladakh and Nagaland blueschist in western and eastern Himalaya, respectively, indicate that in the central YZSZ a shallower burial depth was reached.

Based on the studies of tectonostratigraphy, petrology and geochronology, Cai et al. (2012) proposed that the accretionary complex within the central section of YZSZ was formed in a single subduction zone within the Neo-Tethyan Ocean during the Cretaceous, and accreted to the Asian margin prior to the India-Asia continental collision. Previous studies, which focused on HP-LT metamorphism of blueschist in the central YZSZ, point towards a Late Cretaceous to Eocene Age (Dong, 1989; Liou et al., 1989; Xiao et al., 1988; Xiao and Gao, 1984). Li et al. (2007) reported sodic amphiboles with an 40Ar/39Ar age of 59 Ma from the Kadui blueschist and regarded this age as the peak metamorphic age. A further available 40Ar/39Ar age for sodic amphiboles of 63 Ma, which was determined for the formation of the blueschist in the Sangsang area (Ding et al., 2005), whose geological setting (i.e., representing a sedimentary-matrix hosted mélange along the south side of Yarlung Zangbo ophiolite belt) is interpreted to be similar to that of the Kadui blueschist. Despite controversial arguments, the latest studies constrain the initial timing of the India-Asia collision between 65 and 55 Ma, determined by a range of independent methods by different authors (Hu et al., 2017, 2016; Zhu et al., 2017, 2015; Wu et al., 2014). These ages are in accordance with the evolution of a northward subduction of the Neo-Tethyan from Late Jurassic to Paleocene (Lee et al., 2007; Geng et al., 2006). According to regional geology and the discussion above, we further interpret that the Kadui blueschist in the central part of YZSZ were produced in a portion of the subduction zone which did not went as deep as in the neighboring Ladakh- and Nagaland-areas and which experienced a higher geotherm. In any event, the Kadui blueschist is a product of the northward subduction of the Neo-Tethyan oceanic lithosphere that formed during the initial India-Asia collision stage.

ACKNOWLEDGMENTS

This paper is dedicated to Professor Zhendong You for his many and manifold contributions to metamorphic geology on the occasion of his 90th birthday. This study was financially supported by the National Natural Science Foundation of China (No. 41572044) and the SDUST Research Fund (No. 2015TDJH101). We thank two anonymous reviewers for their constructive and helpful comments and suggestions to improve the paper. Sincere thanks also go to the journal editor Dr. Yanru Song for her careful editorial handling. We also greatly appreciate Jianxin Pei, Xin Zhang for their helps during the fieldworks and Ms. Wenlan Zhang for her assistance during the mineral analytical process. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0854-9.


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