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
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
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).
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.
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).
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.; AlⅥ=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++AlⅥ) 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., AlⅥ=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) > 0.95.4 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).
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).
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.
Ague, J. J., Carlson, W. D., 2013. Metamorphism as Garnet Sees It:The Kinetics of Nucleation and Growth, Equilibration, and Diffusional Relaxation. Elements, 9(6): 439-445. DOI:10.2113/gselements.9.6.439
Aitchison, J. C., Badengzhu, Davis, A. M., et al., 2000. Remnants of a Cretaceous Intra-Oceanic Subduction System within the Yarlung-Zangbo Suture (Southern Tibet). Earth and Planetary Science Letters, 183(1/2): 231-244. DOI:10.1016/s0012-821x(00)00287-9
An, W., Hu, X. M., Garzanti, E., et al., 2014. Xigaze Forearc Basin Revisited (South Tibet):Provenance Changes and Origin of the Xigaze Ophiolite. Geological Society of America Bulletin, 126(11/12): 1595-1613. DOI:10.1130/b31020.1
Ao, A., Bhowmik, S. K., 2014. Cold Subduction of the Neotethys:The Metamorphic Record from Finely Banded Lawsonite and Epidote Blueschists and Associated Metabasalts of the Nagaland Ophiolite Complex, India. Journal of Metamorphic Geology, 32(8): 829-860. DOI:10.1111/jmg.12096
Aoki, K., Itaya, T., Shibuya, T., et al., 2008. The Youngest Blueschist Belt in SW Japan:Implication for the Exhumation of the Cretaceous Sanbagawa High-P/T Metamorphic Belt. Journal of Metamorphic Geology, 26(5): 583-602. DOI:10.1111/j.1525-1314.2008.00777.x
Bédard, É., Hébert, R., Guilmette, C., et al., 2009. Petrology and Geochemistry of the Saga and Sangsang Ophiolitic Massifs, Yarlung Zangbo Suture Zone, Southern Tibet:Evidence for an Arc-Back-Arc Origin. Lithos, 113(1/2): 48-67. DOI:10.1016/j.lithos.2009.01.011
Bhowmik, S. K., Ao, A., 2016. Subduction Initiation in the Neo-Tethys:Constraints from Counterclockwise P-T Paths in Amphibolite Rocks of the Nagaland Ophiolite Complex, India. Journal of Metamorphic Geology, 34(1): 17-44. DOI:10.1111/jmg.12169
Cai, F. L., Ding, L., Leary, R. J., et al., 2012. Tectonostratigraphy and Provenance of an Accretionary Complex within the Yarlung-Zangpo Suture Zone, Southern Tibet:Insights into Subduction-Accretion Processes in the Neo-Tethys. Tectonophysics, 574/575: 181-192. DOI:10.1016/j.tecto.2012.08.016
Connolly, J. A. D., 2005. Computation of Phase Equilibria by Linear Programming:A Tool for Geodynamic Modeling and Its Application to Subduction Zone Decarbonation. Earth and Planetary Science Letters, 236(1/2): 524-541. DOI:10.1016/j.epsl.2005.04.033
Cui, Y., Wang, G., Yao, Y., et al., 2017. Polyphase Deformation and Fluid Inclusion Characteristic of Pervasive Syntectonic Quartz Veins in the Mayer Kangri Indosinian Accretionary Complex, Tibet. Acta Geologica Sinica, 91(2): 384-399.
Dai, J. G., Wang, C. S., Hébert, R., et al., 2011. Petrology and Geochemistry of Peridotites in the Zhongba Ophiolite, Yarlung Zangbo Suture Zone:Implications for the Early Cretaceous Intra-Oceanic Subduction Zone within the Neo-Tethys. Chemical Geology, 288(3/4): 133-148. DOI:10.1016/j.chemgeo.2011.07.011
Dai, J. G., Wang, C. S., Li, Y. L., 2012. Relicts of the Early Cretaceous Seamounts in the Central-Western Yarlung Zangbo Suture Zone, Southern Tibet. Journal of Asian Earth Sciences, 53: 25-37. DOI:10.1016/j.jseaes.2011.12.024
Diener, J. F. A., Powell, R., White, R. W., et al., 2007. A New Thermodynamic Model for Clino-and Orthoamphiboles in the System Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O-O. Journal of Metamorphic Geology, 25(6): 631-656. DOI:10.1111/j.1525-1314.2007.00720.x
Ding, L., Kapp, P., Wan, X. Q., 2005. Paleocene-Eocene Record of Ophiolite Obduction and Initial India-Asia Collision, South Central Tibet. Tectonics, 24(3): TC3001. DOI:10.1029/2004tc001729
Dong, S. B., 1989. The General Features and Distributions of the Glaucophane Schist Belts of China. Acta Geologica Sinica, 13(1): 101-114.
Ernst, W. G., 1972. Occurrence and Mineralogic Evolution of Blueschist Belts with Time. American Journal of Science, 272(7): 657-668. DOI:10.2475/ajs.272.7.657
Geng, Q. R., Pan, G. T., Wang, L. Q., et al., 2006. Isotopic Geochronology of the Volcanic Rocks from the Yeba Formation in the Gangdise Zone, Xizang. Sedimentary Geology and Tethyan Geology, 16(1): 1-7.
Ghose, N. C., Singh, R. N., 1980. Occurrence of Blueschist Facies in the Ophiolite Belt of Naga Hills, East of Kiphire N.E. India. Geologische Rundschau, 69(1): 41-48. DOI:10.1007/bf01869022
Groppo, C., Rolfo, F., Sachan, H. K., et al., 2016. Petrology of Blueschist from the Western Himalaya (Ladakh, NW India):Exploring the Complex Behavior of a Lawsonite-Bearing System in a Paleo-Accretionary Setting. Lithos, 252/253: 41-56. DOI:10.1016/j.lithos.2016.02.014
Guillot, S., Mahéo, G., de Sigoyer, J., et al., 2008. Tethyan and Indian Subduction Viewed from the Himalayan High-to Ultrahigh-Pressure Metamorphic Rocks. Tectonophysics, 451(1/2/3/4): 225-241. DOI:10.1016/j.tecto.2007.11.059
He, S. D., Kapp, P., DeCelles, P. G., et al., 2007. Cretaceous-Tertiary Geology of the Gangdese Arc in the Linzhou Area, Southern Tibet. Tectonophysics, 433(1/2/3/4): 15-37. DOI:10.1016/j.tecto.2007.01.005
Hébert, R., Bezard, R., Guilmette, C., et al., 2012. The Indus-Yarlung Zangbo Ophiolites from Nanga Parbat to Namche Barwa Syntaxes, Southern Tibet:First Synthesis of Petrology, Geochemistry, and Geochronology with Incidences on Geodynamic Reconstructions of Neo-Tethys. Gondwana Research, 22(2): 377-397. DOI:10.1016/j.gr.2011.10.013
Hey, M. H., 1954. A New Review of the Chlorites. Mineralogical Magazine, 30: 277-292. DOI:10.1180/minmag.1954.030.224.01
Holland, T. J. B., Baker, J., Powell, R., 1998. Mixing Properties and Activity-Composition Relationships of Chlorites in the System MgO-FeO-Al2O3-SiO2-H2O. European Journal of Mineralogy, 10(3): 395-406. DOI:10.1127/ejm/10/3/0395
Holland, T. J. B., Powell, R., 1998. An Internally Consistent Thermodynamic Data Set for Phases of Petrological Interest. Journal of Metamorphic Geology, 16(3): 309-343. DOI:10.1111/j.1525-1314.1998.00140.x
Honegger, K., Le Fort, P., Mascle, G., et al., 1989. The Blueschists along the Indus Suture Zone in Ladakh, NW Himalaya. Journal of Metamorphic Geology, 7(1): 57-72. DOI:10.1111/j.1525-1314.1989.tb00575.x
Hu, J. R., Sun, Z. L., Chen, G. J., 2004. New Results and Major Progress in Regional Geological Survey of the Xigaze City Sheet. Geological Bulletin of China, 23: 463-470.
Hu, X. M., Wang, J. G., An, W., et al., 2017. Constraining the Timing of the India-Asia Continental Collision by the Sedimentary Record. Science China Earth Sciences, 60(4): 603-625. DOI:10.1007/s11430-016-9003-6
Hu, X. M., Wang, J. G., BouDagher-Fadel, M., et al., 2016. New Insights into the Timing of the India-Asia Collision from the Paleogene Quxia and Jialazi Formations of the Xigaze Forearc Basin, South Tibet. Gondwana Research, 32: 76-92. DOI:10.1016/j.gr.2015.02.007
Huot, F., Hébert, R., Varfalvy, V., et al., 2002. The Beimarang Mélange (Southern Tibet) Brings Additional Constraints in Assessing the Origin, Metamorphic Evolution and Obduction Processes of the Yarlung Zangbo Ophiolite. Journal of Asian Earth Sciences, 21(3): 307-322. DOI:10.1016/s1367-9120(02)00053-6
Isozaki, Y., Aoki, K., Nakama, T., et al., 2010. New Insight into a Subduction-Related Orogen:A Reappraisal of the Geotectonic Framework and Evolution of the Japanese Islands. Gondwana Research, 18: 82-105. DOI:10.1016/j.gr.2010.05.009
Kabir, M. F., Takasu, A., 2016. Jadeite-Garnet Glaucophane Schists in the Bizan Area, Sambagawa Metamorphic Belt, Eastern Shikoku, Japan:Significance and Extent of Eclogite Facies Metamorphism. Journal of Metamorphic Geology, 34(9): 893-916. DOI:10.1111/jmg.12198
Kang, Z. Q., Xu, J. F., Wilde, S. A., et al., 2014. Geochronology and Geochemistry of the Sangri Group Volcanic Rocks, Southern Lhasa Terrane:Implications for the Early Subduction History of the Neo-Tethys and Gangdese Magmatic Arc. Lithos, 200/201: 157-168. DOI:10.1016/j.lithos.2014.04.019
Leake, B. F., Woolley, A. R., Arps, C. E. S., et al., 1997. Nomenclature of Amphiboles:Report of the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. The Canadian Mineralogist, 35: 219-246.
Lee, H. Y., Chung, S. L., Wang, Y. B., et al., 2007. Age, Petrogenesis and Geological Significance of the Linzizong Volcanic Successions in the Linzhou Basin, Southern Tibet:Evidence from Zircon U-Pb Dates and Hf Isotopes. Acta Petrologica Sinica, 23(2): 493-500.
Li, C., Hu, J. R., Zhai, Q. G., et al., 2007. New Evidence of India-Eurasia Collision and Its Timing:Ar-Ar Dating of the Kardoi Blueschist in Xigaze, Tibet, China. Geological Bulletin of China, 26(10): 1299-1303.
Li, X.-P., Chen, H. K., Wang, Z. L., et al., 2015a. Spinel Peridotite, Olivine Websterite and the Textural Evolution of the Purang Ophiolite Complex, Western Tibet. Journal of Asian Earth Sciences, 110: 55-71. DOI:10.1016/j.jseaes.2014.06.023
Li, X.-P., Kong, F. M., Chen, H. K., et al., 2015b. Rodingite in the Purang Ophiolite and Its Geological Implication, Southwest Tibet. Acta Geologica Sinica-English Edition, 89(Suppl. 2): 41-42. DOI:10.1111/1755-6724.12308_29
Li, X.-P., Duan, W. Y., Zhao, L. Q., et al., 2017. Rodingites from the Xigaze Ophiolite, Southern Tibet-New Insights into the Processes of Rodingitization. European Journal of Mineralogy, 29(5): 821-837. DOI:10.1127/ejm/2017/0029-2633
Liou, J. G., Tsujimori, T., Zhang, R. Y., et al., 2004. Global UHP Metamorphism and Continental Subduction/Collision:The Himalayan Model. International Geology Review, 46(1): 1-27. DOI:10.2747/0020-6822.214.171.124
Liou, J. G., Wang, X. M., Coleman, R. G., et al., 1989. Blueschists in Major Suture Zones of China. Tectonics, 8(3): 609-619. DOI:10.1029/tc008i003p00609
Liu, C. Z., Zhang, C., Yang, L. Y., et al., 2014. Formation of Gabbronorites in the Purang Ophiolite (SW Tibet) through Melting of Hydrothermally Altered Mantle along a Detachment Fault. Lithos, 205: 127-141. DOI:10.1016/j.lithos.2014.06.019
Liu, T., Wu, F. Y., Zhang, L. L., et al., 2016. Zircon U-Pb Geochronological Constraints on Rapid Exhumation of the Mantle Peridotite of the Xigaze Ophiolite, Southern Tibet. Chemical Geology, 443: 67-86. DOI:10.1016/j.chemgeo.2016.09.015
Mahéo, G., Fayoux, X., Guillot, S., et al., 2006. Relicts of an Intra-Oceanic Arc in the Sapi-Shergol Mélange Zone (Ladakh, NW Himalaya, India):Implications for the Closure of the Neo-Tethys Ocean. Journal of Asian Earth Sciences, 26(6): 695-707. DOI:10.1016/j.jseaes.2005.01.004
Maruyama, S., 1997. Pacific-Type Orogeny Revisited:Miyashiro-Type Orogeny Proposed. The Island Arc, 6(1): 91-120. DOI:10.1111/j.1440-1738.1997.tb00042.x
Maruyama, S., Cho, M., Liou, J. G., 1986. Experimental Investigation of Blueschist-Greenschist Transition Equilibria:Pressure Dependence of Al2O3 Contents in Sodic Amphiboles-A New Geobarometer. Geological Society of America Memoir, 164: 1-16. DOI:10.1130/mem164-p1
Maruyama, S., Liou, J. G., 1998. Initiation of Ultrahigh-Pressure Metamorphism and Its Significance on the Proterozoic-Phanerozoic Boundary. The Island Arc, 7(1/2): 6-35. DOI:10.1046/j.1440-1738.1998.00181.x
Maruyama, S., Liou, J. G., Terabayashi, M., 1996a. Blueschists and Eclogites of the World and Their Exhumation. International Geology Review, 38(6): 485-594. DOI:10.1080/00206819709465347
Maruyama, S., Kadarusman, A., Kaneko, Y., et al., 1996b. On-Going Exhumation of A-Type Blueschist Belt, Timor-Tanimbar Region, Eastern Indonesia. EOS Transaction of American Geophysical Union, 77: 779.
Miao, L. C., Zhang, F., Jiao, S. J., 2015. Age, Protoliths and Tectonic Implications of the Toudaoqiao Blueschist, Inner Mongolia, China. Journal of Asian Earth Sciences, 105: 360-373. DOI:10.1016/j.jseaes.2015.01.028
Miyashiro, A., 1961. Evolution of Metamorphic Belts. Journal of Petrology, 2(3): 277-311. DOI:10.1093/petrology/2.3.277
Nakajima, T., Banno, S., Suzuki, T., 1977. Reactions Leading to the Disappearance of Pumpellyite in Low-Grade Metamorphic Rocks of the Sanbagawa Metamorphic Belt in Central Shikoku, Japan. Journal of Petrology, 18(2): 263-284. DOI:10.1093/petrology/18.2.263
Newton, R. C., Charlu, T. V., Kleppa, O. J., 1980. Thermochemistry of the High Structural State Plagioclases. Geochimica et Cosmochimica Acta, 44(7): 933-941. DOI:10.1016/0016-7037(80)90283-5
Oh, C. W., Liou, J. G., 1998. A Petrogenetic Grid for Eclogite and Related Facies under High-Pressure Metamorphism. The Island Arc, 7(1/2): 36-51. DOI:10.1046/j.1440-1738.1998.00180.x
Okay, A. I., 1980. Sodic Amphiboles as Oxygen Fugacity Indicators in Metamorphism. The Journal of Geology, 88(2): 225-232. DOI:10.1086/628493
Omori, S., Kita, S., Maruyama, S., et al., 2009. Pressure-Temperature Conditions of Ongoing Regional Metamorphism beneath the Japanese Islands. Gondwana Research, 16(3/4): 458-469. DOI:10.1016/j.gr.2009.07.003
Orme, D. A., Carrapa, B., Kapp, P., 2015. Sedimentology, Provenance and Geochronology of the Upper Cretaceous-Lower Eocene Western Xigaze Forearc Basin, Southern Tibet. Basin Research, 27(4): 387-411. DOI:10.1111/bre.12080
Ota, T., Kaneko, Y., 2010. Blueschists, Eclogites, and Subduction Zone Tectonics:Insights from a Review of Late Miocene Blueschists and Eclogites, and Related Young High-Pressure Metamorphic Rocks. Gondwana Research, 18(1): 167-188. DOI:10.1016/j.gr.2010.02.013
Ota, T., Terabayashi, M., Katayama, I., 2004. Thermobaric Structure and Metamorphic Evolution of the Iratsu Eclogite Body in the Sanbagawa Belt, Central Shikoku, Japan. Lithos, 73(1/2): 95-126. DOI:10.1016/j.lithos.2004.01.001
Pan, G. T., Ding, J., Wang, L. Q., 2004. Geological Map of Qinghai-Tibet Plateau and Adjacent Regions. Chengdu Map Publishing Company, Chengdu (in Chinese)
Pan, G. T., Wang, L. Q., Li, R. S., et al., 2012. Tectonic Evolution of the Qinghai-Tibet Plateau. Journal of Asian Earth Sciences, 53: 3-14. DOI:10.1016/j.jseaes.2011.12.018
Peacock, S. M., Rushmer, T., Thompson, A. B., 1994. Partial Melting of Subducting Oceanic Crust. Earth and Planetary Science Letters, 121(1/2): 227-244. DOI:10.1016/0012-821x(94)90042-6
Peacock, S. M., Wang, K., 1999. Seismic Consequences of Warm versus Cool Subduction Metamorphism:Examples from Southwest and Northeast Japan. Science, 286(5441): 937-939. DOI:10.1126/science.286.5441.937
Phillips, G., Hand, M., Offler, R., 2010. P-T-X Controls on Phase Stability and Composition in LTMP Metabasite Rocks-A Thermodynamic Evaluation. Journal of Metamorphic Geology, 28(5): 459-476. DOI:10.1111/j.1525-1314.2010.00874.x
Platt, J. P., 1993. Exhumation of High-Pressure Rocks:A Review of Concepts and Processes. Terra Nova, 5(2): 119-133. DOI:10.1111/j.1365-3121.1993.tb00237.x
Ratschbacher, L., Frisch, W., Liu, G. H., et al., 1994. Distributed Deformation in Southern and Western Tibet during and after the India-Asia Collision. Journal of Geophysical Research:Solid Earth, 99(B10): 19917-19945. DOI:10.1029/94jb00932
Rolland, Y., Pêcher, A., Picard, C., 2000. Middle Cretaceous Back-Arc Formation and Arc Evolution along the Asian Margin:The Shyok Suture Zone in Northern Ladakh (NW Himalaya). Tectonophysics, 325(1/2): 145-173. DOI:10.1016/s0040-1951(00)00135-9
Searle, M. P., Windley, B. F., Coward, M. P., et al., 1987. The Closing of Tethys and the Tectonics of the Himalaya. Geological Society of America Bulletin, 98(6): 678-701. DOI:10.1130/0016-7606(1987)98<678:tcotat>2.0.co;2
Shen, Q. H., Geng, Y. S., 2012. The Tempo-Spatial Distribution, Geological Characteristics and Gensis of Blueschist Belts in China. Acta Geologica Sinica, 86: 1407-1446.
Stern, R. J., 2005. Evidence from Ophiolites, Blueschists, and Ultrahigh-Pressure Metamorphic Terranes that the Modern Episode of Subduction Tectonics Began in Neoproterozoic Time. Geology, 33: 557-560. DOI:10.1130/g21365.1
Ukar, E., 2012. Tectonic Significance of Low-Temperature Blueschist Blocks in the Franciscan Mélange at San Simeon, California. Tectonophysics, 568/569: 154-169. DOI:10.1016/j.tecto.2011.12.039
Ukar, E., Cloos, M., 2014. Low-Temperature Blueschist-Facies Mafic Blocks in the Franciscan Mélange, San Simeon, California:Field Relations, Petrology, and Counterclockwise P-T Paths. Geological Society of America Bulletin, 126(5/6): 831-856. DOI:10.1130/b30876.1
Wan, X. Q., Jansa, L. F., Sarti, M., 2002. Cretaceous and Paleogene Boundary Strata in Southern Tibet and Their Implication for the India-Eurasia Collision. Lethaia, 35(2): 131-146. DOI:10.1080/002411602320183999
Wang, H. Q., Ding, L., Cai, F. L., et al., 2017. Early Tertiary Deformation of the Zhongba-Gyangze Thrust in Central Southern Tibet. Gondwana Research, 41: 235-248. DOI:10.1016/j.gr.2015.02.017
Wang, X. B., Bao, P. S., Deng, W. M., et al., 1987. Xizang (Tibet) Ophiolites. Geological Publishing House, Beijing.
Wei, C. J., Li, Y. J., Yu, Y., et al., 2010. Phase Equilibria and Metamorphic Evolution of Glaucophane-Bearing UHP Eclogites from the Western Dabieshan Terrane, Central China. Journal of Metamorphic Geology, 28(6): 647-666. DOI:10.1111/j.1525-1314.2010.00884.x
Wei, C. J., Song, S. G., 2008. Chloritoid-Glaucophane Schist in the North Qilian Orogen, NW China:Phase Equilibria and P-T Path from Garnet Zonation. Journal of Metamorphic Geology, 26(3): 301-316. DOI:10.1111/j.1525-1314.2007.00753.x
Wei, C. J., Wang, W., Clarke, G. L., et al., 2009a. Metamorphism of High/Ultrahigh-Pressure Pelitic-Felsic Schist in the South Tianshan Orogen, NW China:Phase Equilibria and P-T Path. Journal of Petrology, 50(10): 1973-1991. DOI:10.1093/petrology/egp064
Wei, C. J., Yang, Y., Su, X. L., et al., 2009b. Metamorphic Evolution of Low-T Eclogite from the North Qilian Orogen, NW China:Evidence from Petrology and Calculated Phase Equilibria in the System NCKFMASHO. Journal of Metamorphic Geology, 27(1): 55-70. DOI:10.1111/j.1525-1314.2008.00803.x
White, R. W., Pomroy, N. E., Powell, R., 2005. An in situ Metatexite-Diatexite Transition in Upper Amphibolite Facies Rocks from Broken Hill, Australia. Journal of Metamorphic Geology, 23(7): 579-602. DOI:10.1111/j.1525-1314.2005.00597.x
White, R. W., Powell, R., Holland, T. J. B., 2007. Progress Relating to Calculation of Partial Melting Equilibria for Metapelites. Journal of Metamorphic Geology, 25(5): 511-527. DOI:10.1111/j.1525-1314.2007.00711.x
White, R. W., Powell, R., Holland, T. J. B., et al., 2000. The Effect of TiO2 and Fe2O3 on Metapelitic Assemblages at Greenschist and Amphibolite Facies Conditions:Mineral Equilibria Calculations in the System K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology, 18(5): 497-511. DOI:10.1046/j.1525-1314.2000.00269.x
Whitney, D. L., Evans, B. W., 2010. Abbreviations for Names of Rock-Forming Minerals. American Mineralogist, 95(1): 185-187. DOI:10.2138/am.2010.3371
Winkler, H. G. F., 1979. Petrogenesis of Metamorphic Rocks. Springer-Verlag, Berlin. 1-347
Wu, F. Y., Ji, W. Q., Wang, J. G., et al., 2014. Zircon U-Pb and Hf Isotopic Constraints on the Onset Time of India-Asia Collision. American Journal of Science, 314(2): 548-579. DOI:10.2475/02.2014.04
Xiao, X. C., Gao, Y. L., 1984. Some New Observations on the High P/T Metamorphic Belt along the Southern Boundary of Yarlung Zangbo (Tsangpo) Ophiolite Zone, Xizang (Tibet). Himalaya Geology (Ⅱ). Geological Publishing House, Beijing. 1-16 (in Chinese with English Abstract)
Xiao, X. C., Li, T. D., Li, G. C., et al., 1988. Pandect of Himalayan Lithosphere Tectonic Evolution. Beijing: Geological Publishing House: 1-236.
Xiao, X. C., Wan, Z. Y., Li, G. C., et al., 1983. On the Tectonic Evolution of the Yarlung Zangbo (Tsangpo) Suture Zone and the Adjecent Areas. Acta Geologica Sinica, 2: 205-212.
Xu, B., Charvet, J., Zhang, F. Q., 2001. Primary Study on Petrology and Geochronology of the Blueschist in Sunidzuoqi, Northern Inner Mongolia. Chinese Journal of Geology, 36: 424-434.
Zhang, J. R., Wei, C. J., Chu, H., 2015. Blueschist Metamorphism and Its Tectonic Implication of Late Paleozoic-Early Mesozoic Metabasites in the Mélange Zones, Central Inner Mongolia, China. Journal of Asian Earth Sciences, 97: 352-364. DOI:10.1016/j.jseaes.2014.07.032
Zhang, X., Li, X. P., Wang, Z. L., et al., 2016. Mineralogical and Petrogeochemical Characteristics of the Garnet Amphibolites in the Xigaze Ophiolite, Tibet. Acta Petrologica Sinica, 32(12): 3685-3702.
Zheng, Y. F., Chen, Y. X., Dai, L. Q., et al., 2015. Developing Plate Tectonics Theory from Oceanic Subduction Zones to Collisional Orogens. Science China Earth Sciences, 58(7): 1045-1069. DOI:10.1007/s11430-015-5097-3
Zhu, D. C., Wang, Q., Zhao, Z. D., 2017. Constraining Quantitatively the Timing and Process of Continent-Continent Collision Using Magmatic Record:Method and Examples. Science China Earth Sciences, 60(6): 1040-1056. DOI:10.1007/s11430-016-9041-x
Zhu, D. C., Wang, Q., Zhao, Z. D., et al., 2015. Corrigendum:Magmatic Record of India-Asia Collision. Scientific Reports, 5(1): 14289. DOI:10.1038/srep14289
Zhu, D. C., Zhao, Z. D., Niu, Y. L., et al., 2011. The Lhasa Terrane:Record of a Microcontinent and Its Histories of Drift and Growth. Earth and Planetary Science Letters, 301(1/2): 241-255. DOI:10.1016/j.epsl.2010.11.005
Zhu, D. C., Zhao, Z. D., Niu, Y. L., et al., 2013. The Origin and Pre-Cenozoic Evolution of the Tibetan Plateau. Gondwana Research, 23(4): 1429-1454. DOI:10.1016/j.gr.2012.02.002
Zhu, J., Du, Y. S., Liu, Z. X., et al., 2006. Mesozoic Radiolarian Chert from the Middle Sector of the Yarlung Zangbo Suture Zone, Tibet and Its Tectonic Implications. Science in China Series D:Earth Sciences, 49(4): 348-357. DOI:10.1007/s11430-006-0348-y
Ziabrev, S. V., Aitchison, J. C., Abrajevitch, A. V., et al., 2004. Bainang Terrane, Yarlung-Tsangpo Suture, Southern Tibet (Xizang, China):A Record of Intra-Neotethyan Subduction-Accretion Processes Preserved on the Roof of the World. Journal of the Geological Society, 161(3): 523-539. DOI:10.1144/0016-764903-099