2. Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
3. National Research Center for Geoanalysis, Beijing 100037, China
High pressure-ultrahigh pressure (HP-UHP) metamorphic belts are important symbols for the ancient convergent plate boundary. They can record a series of dynamic processes of the oceanic and continental materials that experienced deep subduction and exhumation to the earth surface (Liou et al., 2007; Ernst, 2001; Maruyama et al., 1996; Wang and Cong, 1996; Coleman and Wang, 1995; Ernst and Liou, 1995; Carswell, 1990). As one of the most important rock types in the HP-UHP belts, eclogite usually preserves the peak P-T conditions and multi-stage metamorphic evolution, which makes it a useful tool to constrain the P-T-t path of metamorphic rocks and further reconstruct the formation and evolution of a subduction zone. According to the formation environment of eclogite and their rock association, subduction zones are divided into Pacific-type and Alpine-type, or named as oceanic and continental subduction zones (Maruyama et al., 1996; Ernst and Liou, 1995; Ernst, 1988). UHP index minerals are mostly reported in the continental subduction/collision zones, but rare in the oceanic type (Carswell and Compagnoni, 2003; Zheng et al., 2003; Maruyama et al., 1996). In the early 1990s, coesite was found in Zermatt-Saas area of the Western Alps, which indicated that the oceanic crust could also be subducted to mantle depth, underwent UHP metamorphism and then exhumed to the earth surface (Reinecke, 1991). However, the density of deep subducted oceanic crust is often greater than that of the surrounding mantle rocks which makes it difficult to exhume to the earth surface but continually subduct into the deep mantle (Davies and von Blanchenburg, 1995). Therefore, further studies are still needed on the exhumation mechanism of deep subducted oceanic crustal materials.
The Sumdo eclogite belt is located in the eastern part of the Lhasa terrane on Tibetan Plateau (Yang et al., 2006). This belt was proposed to represent a typical oceanic subduction zone, resulted from the closure of a Paleo-Tethys Ocean between the north and the south Lhasa sub-terranes at late Permian time (ca. 260 Ma) (Chen et al., 2009, Yang et al., 2009). The peak P-T conditions of eclogite in the Sumdo eclogite belt are determined, using both conventional geothermobarometric methods and phase equilibrium modeling, to be 450–800 ℃ and 2.5–3.9 GPa (Zhang C et al., 2019a, b; Cao et al., 2017; Li et al., 2017; Cheng et al., 2015, 2012; Huang et al., 2015; Yang X L et al., 2014; Zhang D D et al., 2011; Yang J S et al., 2009), ranging from low-temperature high-pressure (LT-HP) lawsonite eclogite facies to HT-UHP "dry eclogite" facies. It is hard to imagine that the P-T conditions vary so much for rocks from an adjacent area or even from a same outcrop in the Sumdo eclogite belt. In this contribution, detailed petrological studies and phase equilibrium calculation combined with zircon dating are performed on the eclogite and meta-quartzite in the Jilang area to constrain their metamorphic evolution of the Sumdo eclogite belt and further discuss the exhumation mechanism of the subducted Paleo- Tethys oceanic crust materials in the Lhasa terrane.1 GEOLOGICAL SETTINGS
As one of the major units of the Himalayan-Tibetan Orogen, the Lhasa terrane is located at the southern part of Tibet, with Qiangtang terrane to the north and Tethyan-Himalayan Orogen to the south, divided by the Bangong-Nujiang suture zone (BNSZ) and Yarlung Zangbo suture zone (YZSZ), respectively (Yin and Harrison, 2000; Figs. 1a, 1b). The Lhasa terrane is composed dominantly of the underlying Precambrian crystalline basement, Paleozoic to Mesozoic marine strata and arc-type volcanic rocks together with Mesozoic and Cenozoic intrusions (e.g., Zhang Z M et al., 2018; Zhu et al., 2013, 2011, 2009; Pan et al., 2012; Zhang J J et al., 2012; Yin and Harrison, 2000). This trifold tectono- stratigraphy was used to divide the entire Lhasa terrane into three belts called the northern, central and southern sub-terranes, separated by the Shiquan River-Nam Tso fault and the Luobadui- Milashan fault, respectively. The southern Lhasa sub-terrane is dominated by the Cretaceous–Tertiary Gangdese batholiths and the Paleogene Linzizong volcanic succession, with minor Triassic–Cretaceous volcano-sedimentary rocks, indicating a juvenile crust with its underlying Precambrian crystalline basement only preserved locally (e.g., Zhu et al., 2012; Mo et al., 2008). The central Lhasa sub-terrane represents a micro-continent with Precambrian crystalline basement rocks, covered by Cambrian to Permian meta-sediments and Upper Jurassic–Lower Cretaceous sedimentary units with abundant volcanic rocks (Pan et al., 2012; Zhu et al., 2011). The northern Lhasa sub-terrane is traditionally considered to be underlain by a Cambrian or Neoproterozoic crystalline basement as exemplified by the orthogneiss of Ando (Guynn et al., 2006).
Based on the recognition of eclogite in Sumdo and blueschist in Pana, ~80 km west of Sumdo, a new subdivision of the Lhasa terrane was proposed (Liu et al., 2009; Yang et al., 2009). They considered that the Lhasa terrane consists of two discrete crustal fragments: called the south and the north Lhasa sub-terranes. The North Lhasa terrane includes the former central and northern Lhasa sub-terranes as described above. Zhu et al. (2013) also proposed that the Luobadui-Milashan fault represents a Carboniferous–Permian suture zone forming the boundary between the north and the south Lhasa sub-terranes.
The Sumdo eclogite belt is mainly composed of the lower Ordovician Sumdo Group which includes Chasagang, Mabuku and Leilongku formations. The rock association of the Sumdo Group is schist, marble, quartzite, minor eclogite and amphibolite. Eclogite of the Sumdo Group occurs mainly in four areas, which are Xindaduo, Sumdo, Bailang and Jilang from west to east. The eclogite lenses are hosted by country rocks of epidote-amphibolite, garnet-bearing mica schist, serpentinite and quartzite. The investigated samples are located in the Jilang area to the east part of the Sumdo eclogite belt (Fig. 1c). Eclogites occur as lens in quartzite, about 10 m in diameter (Fig. 2a). Some retrograde minerals occurred at the edge of eclogite lenses. Thus, pristine eclogite samples are only collected from the core of eclogite lenses. Previous studies on a phengite- bearing eclogite in Jilang showed that the P-T conditions, were 3.4–3.8 GPa at 753–790 ℃ using conventional geothermobarometry (Cheng et al., 2012), being a MT-UHP type. Shen et al. (2018) have recovered the metamorphic P-T path of the Jilang eclogite with phase equilibrium calculation and got a peak condition of 2.4 GPa at 563 ℃, being a LT-HP type. The metamorphic ages of eclogite are 265.9±1.1 Ma from a garnet- omphacite-whole rock Lu-Hf isochron and 261.2±3.1 Ma from zircon U-Pb dating. The age of retrograde stage is determined at 238.1±3.2 Ma also by zircon U-Pb dating (Cheng et al., 2012).2 ANALYTICAL METHOD
Garnet and other rock-forming minerals were analyzed by a JEOL JXA-8230 electron microprobe micro-analyzer (EPM) at the State Key Laboratory of Marine Geology, Tongji University, Shanghai. Operating conditions were 15 kV acceleration voltage, 10 nA beam current and 0–5 μm beam diameter. Natural and synthetic mineral standards (SPI) and ZAF correction were used to calibrate all quantitative analyses. Representative mineral compositions are listed in Table 1.
The X-ray fluorescence (XRF) analysis was obtained at the China National Research Centre for Geoanalysis. The bulk-rock composition is analyzed by XRF. The calculated molecular ratios for phase equilibrium modeling of the eclogite samples are shown in Table 2.
Zircon crystals were obtained from crushed rocks by combining heavy liquid and magnetic separation techniques. Individual zircon crystals without (or only a few) cracks but including mineral inclusions were handpicked and mounted in epoxy resin. The grain mount was then polished to expose zircon cores. Transmitted- and reflected light-optical micrographs were collected, providing information on the shape of the zircon grains and their relative position in the grain mount. To reveal their internal microstructure and to guide spot locations for further isotope analysis, the zircons were also imaged by cathodoluminescence (CL), using a nano SEM400 field emission scanning electron microscope at Institute of Geology, Chinese Academy of Geological Sciences, with operating conditions of 15 kV and 20 nA.
Zircon U-Pb isotopic analysis was carried out at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University, using a Agilent 7500ce LA-ICP-MS equipped with a 193 nm ArF laser ablation system. After each five measurements were performed on a single sample, a suite of zircon and glass standards, i.e., GJ-1, Plesovice and NIST SRM 612 were analyzed for quality control. Each analysis was comprised approximately 20 s background acquisition and 40 s sample data acquisition. Off-line selection and integration of background and analytical signals, time-drift correction and quantitative calibration for trace element analyses and U-Pb dating were performed by ICPMSDataCal (Liu et al., 2010) and Isoplot (Ludwig, 2003). Common Pb was corrected according to the method of Andersen (2002). NIST SRM 612 glass was used to calculate the contents of Pb, Th and U in zircon. The LA-ICP-MS results are listed in Table 3.
The Jilang eclogite shows granoblastic and massive fabric. A representative sample (15SD123) contains garnet (30%), omphacite (10%), phengite (5%), amphibole (10%), rutile (5%), epidote (10%), quartz (10%) and minor amount of biotite (Figs. 3a, 3c, 3d). Garnets are coarse-grained of 0.5–1 mm in diameter. Some garnet grains are wrapped by pargasite+plagioclase corona (Figs. 3c, 3d). Garnet has a "dirty" core with abundant mineral inclusions and a "clear" rim with less mineral inclusions. Omphacites mainly exist as grains of 100–300 μm in the matrix and a few as inclusions in garnet rims. They are mostly replaced by the symplectites consisting of diopside, hornblende and plagioclase (Fig. 3c). Phengites occur as flakes in the matrix or as inclusions in garnet. Some flakes in matrix are replaced by biotite in their rims. On the basis of the output position of amphibolite, they can be divided into four groups: garnet inclusions, symplectites after omphacite, coronas of garnet, and in the matrix. Amphiboles including glaucophane or barroisite are mainly enclosed in the cores of garnet. Other types of amphibole include sodium-rich amphibole as winchite, taramite and calcium-rich amphibole as hornblende and pargasite, which occur in the symplectites after omphacite and coronas around garnets, respectively, being products of retrogression. Epidotes are found in the matrix and garnet inclusions, some of them grow around the rim of garnet (Fig. 3d).
Quartzites distribute more than 95% in the Jilang area. It consists of quartz (~85%), muscovite (~5%), minor plagioclase and carbonate minerals (Fig. 3b). Detrital zircons are separated from a quartzite sample 13SD72 to constrain its depositional ages.3.2 Mineral Chemistry
Garnet from the Jilang eclogite contains almandine (Alm) of 0.54–0.67, pyrope (Prp) of 0.07–0.16, grossular (Grs) of 0.21–0.26 and spessartite (Sps) of 0.01–0.05 (Fig. 4a). It commonly preserves growth zoning with Sps decreasing from core to rim. Grs is nearly homogeneous in the core but a slightly decrease in the rim (Fig. 5), whereas Prp and Alm share the opposite changes, with Alm increasing while Prp decreasing from core to mantle followed by Alm decreasing and Prp increasing in the rim (Fig. 5).
Omphacite in the matrix contains jadeite (Jd) of 27 mol%– 42 mol%, and the omphacite inclusions in garnet rims have Jd of 32 mol%–35 mol%. Whereas the diopside from symplectites shows much lower Jd of ~20 mol% (Fig. 4b).
Phengite is heterogeneous in composition and generally has a core-rim zoning. Its Si content in the core (Si=3.51 p.f.u.) is higher than that in the rim (Si=3.41 p.f.u.). The Si content shows a positive correlation with Mg+Fe (Fig. 4c), dominant of the substitution of 2Al3+=Si4++(Mg, Fe)2+.
The amphiboles included in garnets and in the matrix are sodic, including winchite, barroisite and/or taramite with Na (M4) > 0.5. While the amphiboles in the symplectites of omphacites are hornblende and from the coronas around garnet if pargasite with Na (M4) < 0.5 (Fig. 4d).
Plagioclases show An (Ca/(Ca+Na+K)) of 3 mol%–5 mol% from the garnet inclusions but much higher An of ~21 mol% in the coronas around garnets (Table 1). Epidotes show Fe3+/(Fe3++Al) value of 0.11–0.51 and mostly concentrating around 0.24.3.3 Generations of Mineral Assemblages
On the basis of the textural relation and mineral compositions mentioned above, three generations of mineral assemblages are constrained as follows: (1) prograde stage is characterized by garnet core and mantle with mineral inclusions of amphibolite, epidote, plagioclase and quartz; (2) near peak stage with mineral assemblage of garnet rim, omphacite and phengite with high Si content in the garnet rim and matrix; (3) retrograde stage mineral assemblage composed of garnet with high Grs in the rim, diopsite+plagioclase symplectite after omphacite and hornblende+plagioclase corona around garnet.4 PHASE EQUILIBRIUM CALCULATION
A P-T pseudosection was calculated for the Jilang eclogite using DOMINO/THERIAK (de Capitani and Petrakakis, 2010; de Capitani and Brown, 1987) and the internally consistent thermodynamic database tcdb55c2d of Holland and Powell (1998). Fluid was set to be pure water and in excess. Modeling was performed in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O- O(Fe2O3) (NCKFMASHO) system with the following activity- composition relationships for solid-solution phases: hornblende (Diener et al., 2007), chlorite (Holland et al., 1998), clinopyroxene (Green et al., 2007), epidote (Holland and Powell, 1998), garnet (Wei and Powell, 2004), phengite (Coggon and Holland, 2002) and plagioclase (Holland and Powell, 2003). Lawsonite, and quartz/coesite are treated as pure end-member phases. The bulk rock composition used for modeling was obtained from the XRF analysis (Table 2).
The calculated P-T ranges of the pseudosection are 1.0–3.0 GPa and 450–700 ℃ (Fig. 6). The mineral assemblages containing lawsonite are stabilized at pressures greater than 2.0 GPa, while the epidote-bearing assemblages stabilize at pressures lower than 2.0 GPa. Diopside and sodic- or calc-amphibole appear in the assemblages with pressure below 1.8 GPa. Plagioclase is stable below 1.4 GPa. The isopleths of XPrp (e.g., P0.08) in garnet are steep and increase with temperature rising, being a good indicator of temperature. The isopleths of XGrs (e.g., gr0.20) in garnet are gently sloped and decrease in XGrs value with pressure increasing within the lawsonite stability fields. The contours of Si in phengite (e.g., S3.50) show moderately positive slopes and the Si content increases with increasing pressure.
The inferred prograde assemblage involving Grt+Omp+ Phn+Tr+Gln+Qz+Lws is predicted to cover the LT-HP fields (2.1–2.6 GPa and 520–550 ℃). The measured garnet core compositions are plotted to indicate a P-T condition of 2.2–2.4 GPa and 530–550 ℃. The inferred near peak assemblage of Grt+Omp+Phn+Qz/Coe+Lws is predicted to be stable in the P-T range of 2.4–3.0 GPa and 550–600 ℃. This field is consistent with the projection of the measured garnet rim compositions, which limit a highest P-T condition of ~2.85 GPa and 575 ℃. Because of the modification of the garnet rim compositions during late retrogression, the real peak metamorphic conditions of the Jilang eclogite could be up to 600–630 ℃ because of the 605–625 ℃ at 2.05–2.35 GPa P-T conditions obtained by some of the garnet outer rim compositions during retrogression. The Si contents (3.41–3.51 p.f.u.) in phengite plots yield a pressure range of 2.0–2.3 GPa at 605–625 ℃, which is in the same P-T area of the early retrograde stage defined by the garnet outer rim. The late retrograde metamorphism is recorded by the mineral assemblage of Di+Pl+Ep+Hb+Bio+Qt, which equilibria in 1.0–1.2 GPa, 510–590 ℃, which make a near isothermal decompression P-T path during retrogression.5 ZIRCON U-PB DATING OF QUARTZITE
Zircon grains from a quartzite sample 13SD72 show prismatic to well-rounded shapes of 100–300 μm across, suggesting a detrital origin with relatively long distance transportation. They are complex in internal texture with both oscillatory and irregular zones (Fig. 7). All the 39 analyses yield concordant ages ranging from 418 to 2 922 Ma (Fig. 8a and Table 3). The relative probability plot of the concordant 206Pb/238U ages shows a major age peak at 583 Ma and two minor peaks at 911 and 1 134 Ma (Fig. 8a), which are consistent with the results previously reported for the detrital zircons from central Lhasa terrane (Fig. 8b, Zhu et al., 2013).6 DISCUSSION 6.1 Metamorphic Evolution of Eclogite in the Jilang Area
As one of the major eclogite occurrences in the Sumdo eclogite belt, Jilang area is mainly composed of quartzite. Eclogites are enclosed in quartzite as lenses or blocks, which may share parts of metamorphic evolution with the surrounding quartzite. In combination with the petrological investigations and phase equilibrium calculation on the Jilang eclogite, three stages of metamorphism have been recognized as follows.6.1.1 Prograde metamorphic stage
The mineral inclusions in the cores of garnets, such as sodic amphibole, quartz, plagioclase and epidote, represent the assemblages of the prograde metamorphic stage (Fig. 5). Pyrope in garnet increases while grossular slightly decreases from the garnet core to rim, defining 2.2–2.4 GPa, 530–560 ℃ as the P-T conditions during prograde metamorphic stage.6.1.2 Near peak metamorphic stage
A rim composition of garnet yields the near peak P-T conditions of the Jilang eclogite at 2.85 GPa, 575 ℃. Because of the garnet rim compositional exchange during retrogression, this ultrahigh pressure metamorphic P-T condition may not represent the real peak metamorphism during subduction but approaching it. The 2.05–2.35 GPa, 605–625 ℃ retrograde P-T conditions obtained by the outer rim of garnet indicates that the real peak metamorphic conditions of the Jilang eclogite could be up to 600–630 ℃ with a mineral assemblage of Grt+Omp+ Phn+Coe (Qz)±Lws. The phase equilibrium calculation shows that the eclogite in the Jilang area is in the stability field of lawsonite from the prograde metamorphism stage to the peak stage, but lawsonite or its pseudomorph has not been found in this study. Cheng et al. (2015) reported that the lawsonite and its pseudomorph were discovered in eclogite from the adjacent Bailang area and the peak P-T conditions of the eclogite were ca. 2.6 GPa and 465–503 ℃. Due to the similar peak metamorphic conditions, we deduced that the appearance of lawsonite in the mineral assemblage is reasonable. However, it may also be affected by the late stage of retrograde metamorphism, making it difficult to be preserved (Wei and Clarke, 2011).
Cheng et al. (2012) have calculated the P-T conditions of the Jilang eclogite using garnet-clinopyroxene-phengite (GCP) geothermobarometry (Krogh Ravna and Terry, 2004), giving the peak conditions of 753–790 ℃ at 3.4–3.8 GPa, being a MT-UHP condition. Using phase equilibrium modeling, Shen et al. (2018) have obtained a peak condition of 563 ℃ and 2.4 GPa, which is 200 ℃ and 1.0–1.4 GPa lower than that calculated by the conventional geothermobarometry, indicating a low temperature high pressure (LT-HP) metamorphism of the Jilang eclogite. Our P-T result is located in the LT eclogite facies field with P-T condition of 2.85 GPa and 575 ℃, which is similar to the recalculated P-T conditions (2.9 GPa, 610 ℃) modeled by Theriak/Domino on the same sample from Cheng et al. (2012) (Zhang et al., 2019a).
The P-T conditions modeled by phase equilibrium calculation from Jilang eclogite are lower than those from conventional geothermobarometry. One explanation is that the differences may be caused by the calculation methods. The P-T conditions calculated by the conventional geothermobarometric techniques are often higher than those calculated by phase equilibrium calculations. A good example of it is illustrated by the work of Wei et al. (2009), who published pseudosections calculated for an eclogite with a MORB bulk rock composition. This work simulated the intersection point between the GCP geothermobarometer and thus to test the applicability of it on eclogite with different mineral assemblage. The results clearly demonstrated that the position in P-T space of the isopleth lines of Si (in phengite) and Ca, Mg, Fe (in garnet and omphacite) were strongly controlled, not only by the P-T conditions, but also by the bulk rock composition and/or assemblages. As for the GC geothermometer, at least ten different calibrations have been applied to calculate the temperature for eclogite (Krogh Ravna and Terry, 2004; Krogh Ravna, 2000; Krogh, 1988; Powell, 1985; Ellis and Green, 1979). The application of all these geothermometric techniques results relatively big error (100–200 ℃) on the same sample. Powell and Holland (2008) considered that the biggest problem with the application of the garnet-clinopyroxene geothermometer to eclogite was that the initial experiments were carried out under a temperature range of 600–1 500 ℃, making it suitable for application in eclogite formed under medium to high temperature conditions (i.e., kyanite-bearing eclogite (Wei et al., 2009)), but not for LT eclogite. Another explanation is that Jilang eclogites record both MT and LT eclogite facies metamorphism by different samples. The differences of the peak P-T conditions are caused by the mixture of exhumed eclogites sharing different depth with various peak conditions. What kind of exhumation mechanism could cause this phenomenon will be discussed in the next section.6.1.3 Retrograde metamorphic stage
This stage is a process of near isothermal decompression and followed by a P-T decreasing at shallow depth. Omphacite and garnet gradually become unstable with chemical exchange at their rims and further decomposed. Due to the limited local fluids, omphacite transforms into symplectite (diopside+plagioclase). The rim of garnet is wrapped by pargasite+plagioclase forming corona structure. When the pressure drops to about 1.2 GPa, mineral assemblages of Di+Pl+Ep+Hb+Bio+Qz record the retrograde metamorphism during exhumation process.6.2 Exhumation Processes of the Subducted Oceanic Crust in the Jilang Area
Oceanic crust rocks can transform into HP-UHP metamorphic rocks when they are subducted to the mantle depths. The dehydration of the mafic oceanic crust during deep subduction process leads to an increase in density and continually subduct to the mantle depth forming eclogite. The eclogite is usually denser than the surrounding mantle rocks, making it hard to exhume by its own buoyancy (Yamato et al., 2007). However, some oceanic eclogites are exhumed to the earth surface in several oceanic subduction zones (e.g., Zhai et al., 2011; Agard et al., 2009). Some of them are even coesite-bearing UHP eclogites that were exhumed from depth more than 90 km (Guillot et al., 2009; Lü et al., 2008). The occurrence of natural HP and UHP oceanic eclogites indicates that at least parts of the subducted oceanic crust are detached from the down-going slab and were exhumed back to the earth surface.
By phase equilibrium modeling of the deep subducted oceanic crust with a MORB composition, Chen et al. (2013) found out that the rock density is mainly controlled by the stability of water-bearing minerals. During the process of cold subduction, the hydrous minerals remain stable when the subduction depth is less than 110 km and the eclogite density is less than that of the surrounding mantle materials. In other words, when the pressure was not more than ca. 3 GPa, the oceanic eclogite can keep a potential to exhume to the earth surface relying on its own buoyancy.
The low-density materials in the oceanic subduction zone mainly include metamorphic sediments, subducted felsic continental crust associated with oceanic crust and serpentinized ultramafic rocks. Based on the different buoyancy sources, the peak P-T conditions and the occurrence patterns of metamorphic rocks, three types of exhumation mechanism have been proposed: (1) the accretionary wedge type, (2) the exhumed continental crust type and (3) the serpentinized subduction channel type (Guillot et al., 2009).
The accretionary wedge type of exhumation in the oceanic subduction zone with shallow depth is common. The low density sedimentary rocks in the accretionary wedge are mostly variable metamorphic shale and greywacke with deepest subducted depth of ca. 20 km in the present-day subduction zones and barely more than 40–60 km. The eclogites enclosed in the accretionary wedge usually occur as lens or blocks in the meta-sediments and the pressure is mostly less than 2.0 GPa (Agard et al., 2009; Guillot et al., 2009). This type of exhumation mechanism is applicable to the blueschist with shallow subduction depth.
Felsic continental materials are forced to subduct due to the hauling of subducted oceanic crust to the mantle depth of more than 200 km (Ye et al., 2000; van Roermund and Drury, 1998). Some of the oceanic crust material can be trapped into the subducted continent and exhumed with the low density continental crust (Song et al., 2014). The continental type of exhumation is usually fast, short-lived and occurs at the transition zone from oceanic subduction to continental subduction. The main power in the continental type of exhumation is buoyancy forces and asthenospheric return ﬂow (Guillot et al., 2009).
Subduction channel is a strong deformation zone that formed by soft materials between the lower and upper flakes from the plate convergent boundaries (Shreve and Cloos, 1986). The low density serpentinite causes strong buoyancy, thus providing a mechanism for the exhumation of deeply subducted rocks. The serpentinized subduction channel exhumation model is widely accepted to explain the exhumation process of the deep subducted eclogite in the serpentine-bearing mélange region, such as Alps, Cuba, Dominica and the Tianshan area of China (Guillot et al., 2009 and references therein). The HP-UHP terranes exhumed from subduction channels generally have the features of (1) different rock types mixed together (eclogite occur as lens in metapelites or serpentinites); (2) big P-T intervals between different rocks; (3) the peak metamorphic ages spread within a certain range; (4) usually with complex P-T trajectory and pressure cycling (Li et al., 2016; Zheng et al., 2015; Hacker and Gerya, 2013).
The Sumdo eclogite-bearing (U)HP metamorphic belt formed in a Permian–Triassic oceanic subduction zone between the south and the north Lhasa sub-terranes (Yang et al., 2009), leading to the reinterpretation of the tectonic evolutionary of the Lhasa terrane in the Tibetan-Himalayan orogeny. With detailed field investigations, we found out that the country rocks of the Sumdo eclogite varies with different locations (Zhang et al., 2019b). Such as in the Xindaduo area, the country rocks are mainly garnet-bearing mica schist, epidote amphibolite and serpentinite. However, it is quartzite in the Jilang area. Eclogites are wrapped as lens in the country rocks, which is consistent with the characteristics of the subduction channel. Furthermore, the peak P-T conditions of the Jilang eclogite are limited by both recalculated conventional geothermobarometer and phase equilibrium calculations, yielded the peak metamorphic conditions of 2.9 GPa, 610 ℃ (Zhang et al., 2019a; Cheng et al., 2012) and ca. 2.5 GPa, 550 ℃ (Shen et al., 2018 and this study), respectively. The P-T interval may be caused by the mixture of eclogites from different depth in the subduction channel. Then the subduction channel process could be part of the exhumation mechanism of the Jilang eclogite. However, the lack of serpentinite and metapelite but only quartzite in Jilang area makes this "subduction channel" less buoyancy and hard to exhume to the earth surface, which is different from the situation in the Xindaduo and Sumdo area in the adjacent region. According to the zircon age peaks of 583, 911, and 1 134 Ma from quartzite in the Jilang area, it is comparable with the results from the detrital zircons from the north Lhasa terrane (Zhu et al., 2013). It means that part of the continental crust may be involved in the subduction and exhumation process. The zircons from quartzite do not record the Permian–Triassic age during the subduction may indicate that the continental crust does not subduct to big depth, giving little time for zircon to crystallize. And the exhumation mechanism of the Jilang eclogite is deduced to be exhumed to medium crust level by subduction channel and then trapped by the shallow subducted continental crust and exhumed to the earth surface. If this is the case, that one of the long and pending question "subduction polarity of the Sumdo eclogite belt" could be constrained as from north to south according to the oceanic and continental crust distribution signature.7 CONCLUSION
In combination of the detailed field and petrological investigations with phase equilibrium calculations on a phengite- bearing eclogite in the Jilang area from the Sumdo eclogite belt, three stages of metamorphism have been recognized: (1) prograde stage represented by the core of garnet and mineral inclusions therein; (2) near peak stage represented by the rim of garnet, omphacite, phengite, lawsonite, and quartz; and (3) retrograde stage characterized by symplectites after omphacite and coronas rimmed garnet. The near peak metamorphic P-T conditions are recovered to be ca. 575 ℃ at 2.85 GPa and the P-T path is clockwise with isothermal decompression, indicating a fast subduction and exhumation processes. LA-ICP-MS zircon age dating for a country quartzite of eclogite yields age peaks of 583, 911, 1 134 Ma, indicating a continental crust involved lifting process at shallow depth after the exhumation along the subduction channel and may further infer that the subduction polarity of the Sumdo eclogite belt is from north to south.ACLNOWLEDGMENTS
Prof. Chunjing Wei and two anonymous reviewers are thanked for their comprehensive, constructive suggestions to improve the content of the manuscript. We thank Dr. Lingmin Zhang from Tongji University for the help in electron microprobe analysis. This research was financially supported by the National Natural Science Foundation of China (Nos. 41572051, 41630207, 41872067 and 41703053), and Chinese Academy of Geological Sciences (No. YYWF20 1702). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0894-9.
Agard, P., Yamato, P., Jolivet, L., et al., 2009. Exhumation of Oceanic Blueschists and Eclogites in Subduction Zones: Timing and Mechanisms. Earth-Science Reviews, 92(1/2): 53-79. DOI:10.1016/j.earscirev.2008.11.002
Andersen, T., 2002. Correction of Common Lead in U-Pb Analyses that do not Report 204Pb. Chemical Geology, 192(1/2): 59-79. DOI:10.1016/s0009-2541(02)00195-x
Cao, D. D., Cheng, H., Zhang, L. M., et al., 2017. Post-Peak Metamorphic Evolution of the Sumdo Eclogite from the Lhasa Terrane of Southeast Tibet. Journal of Asian Earth Sciences, 143: 156-170. DOI:10.1016/j.jseaes.2017.04.020
Carswell, D. A., 1990. Eclogite Facies Rocks. Blackie, New York. 396
Carswell, D. A., Compagnoni, R., 2003. Introduction with Review of the Definition, Distribution and Geotectonic Significance of Ultrahigh Pressure Metamorphism. In: Carswell, D. A., Compagnoni, R., eds., EMU Notes in Mineralogy, vol. 5, E tv s Lorànd University Press, Budapest. 3–9
Chen, S. Y., Yang, J. S., Li, Y., et al., 2009. Ultramafic Blocks in Sumdo Region, Lhasa Block, Eastern Tibet Plateau: An Ophiolite Unit. Journal of Earth Science, 20(2): 332-347. DOI:10.1007/s12583-009-0028-x
Chen, Y., Ye, K., Wu, T. F., et al., 2013. Exhumation of Oceanic Eclogites: Thermodynamic Constraints on Pressure, Temperature, Bulk Composition and Density. Journal of Metamorphic Geology, 31(5): 549-570. DOI:10.1111/jmg.12033
Cheng, H., Zhang, C., Vervoort, J. D., et al., 2012. Zircon U-Pb and Garnet Lu-Hf Geochronology of Eclogites from the Lhasa Block, Tibet. Lithos, 155: 341-359. DOI:10.1016/j.lithos.2012.09.011
Cheng, H., Liu, Y. M., Vervoort, J. D., et al., 2015. Combined U-Pb, Lu-Hf, Sm-Nd and Ar-Ar Multichronometric Dating on the Bailang Eclogite Constrains the Closure Timing of the Paleo-Tethys Ocean in the Lhasa Terrane, Tibet. Gondwana Research, 28(4): 1482-1499. DOI:10.1016/j.gr.2014.09.017
Coggon, R., Holland, T. J. B., 2002. Mixing Properties of Phengitic Micas and Revised Garnet-Phengite Thermobarometers. Journal of Metamorphic Geology, 20(7): 683-696. DOI:10.1046/j.1525-1314.2002.00395.x
Coleman, R. G., Wang, X., 1995. Ultrahigh-Pressure Metamorphism. Cambridge University Press, New York. 528
Davies, J. H., von Blanckenburg, F., 1995. Slab Breakoff: A Model of Lithosphere Detachment and Its Test in the Magmatism and Deformation of Collisional Orogens. Earth and Planetary Science Letters, 129(1/2/3/4): 85-102. DOI:10.1016/0012-821x(94)00237-s
de Capitani, C., Brown, T. H., 1987. The Computation of Chemical Equilibrium in Complex Systems Containing Non-Ideal Solutions. Geochimica et Cosmochimica Acta, 51(10): 2639-2652. DOI:10.1016/0016-7037(87)90145-1
de Capitani, C., Petrakakis, K., 2010. The Computation of Equilibrium Assemblage Diagrams with Theriak/Domino Software. American Mineralogist, 95(7): 1006-1016. DOI:10.2138/am.2010.3354
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
Ellis, D. J., Green, D. H., 1979. An Experimental Study of the Effect of Ca upon Garnet-Clinopyroxene Fe-Mg Exchange Equilibria. Contributions to Mineralogy and Petrology, 71(1): 13-22. DOI:10.1007/bf00371878
Ernst, W. G., 1988. Tectonic History of Subduction Zones Inferred from Retrograde Blueschist P-T Paths. Geology, 16(12): 1081-1084. DOI:10.1130/0091-7613(1988)016<1081:thoszi>2.3.co;2
Ernst, W. G., Liou, J. G., 1995. Contrasting Plate-Tectonic Styles of the Qinling-Dabie-Sulu and Franciscan Metamorphic Belts. Geology, 23(4): 353-356. DOI:10.1130/0091-7613(1995)023<0353:cptsot>2.3.co;2
Ernst, W. G., 2001. Subduction, Ultrahigh-Pressure Metamorphism, and Regurgitation of Buoyant Crustal Slices—Implications for Arcs and Continental Growth. Physics of the Earth and Planetary Interiors, 127(1/2/3/4): 253-275. DOI:10.1016/s0031-9201(01)00231-x
Green, E., Holland, T., Powell, R., 2007. An Order-Disorder Model for Omphacitic Pyroxenes in the System Jadeite-Diopside-Hedenbergite-Acmite, with Applications to Eclogitic Rocks. American Mineralogist, 92(7): 1181-1189. DOI:10.2138/am.2007.2401
Guillot, S., Hattori, K., Agard, P., et al., 2009. Exhumation Processes in Oceanic and Continental Subduction Contexts: A Review. In: Lallemand, S., Funiciello, F., eds., Subduction Zone Geodynamics. Springer, Berlin, Heidelberg. 175–205
Guynn, J. H., Kapp, P., Pullen, A., et al., 2006. Tibetan Basement Rocks near Amdo Reveal "Missing" Mesozoic Tectonism along the Bangong Suture, Central Tibet. Geology, 34(6): 505-508. DOI:10.1130/g22453.1
Hacker, B. R., Gerya, T. V., 2013. Paradigms, New and Old, for Ultrahigh-Pressure Tectonism. Tectonophysics, 603: 79-88. DOI:10.1016/j.tecto.2013.05.026
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., Powell, R., 2003. Activity-Composition Relations for Phases in Petrological Calculations: An Asymmetric Multicomponent Formulation. Contributions to Mineralogy and Petrology, 145(4): 492-501. DOI:10.1007/s00410-003-0464-z
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
Huang, J., Tian, Z. L., Zhang, C., et al., 2015. Metamorphic Evolution of Sumdo Eclogite in Lhasa Block of the Tibetan Plateau: Phase Equilibrium in NCKMnFMASHTO System. Geology in China, 42(5): 1559-1571.
Krogh, E. J., 1988. The Garnet-Clinopyroxene Fe-Mg Geothermometer: A Reinterpretation of Existing Experimental Data. Contributions to Mineralogy and Petrology, 99(1): 44-48. DOI:10.1007/bf00399364
Krogh, Ravna E., 2000. The Garnet-Clinopyroxene Fe2+-Mg Geothermometer: An Updated Calibration. Journal of Metamorphic Geology, 18(2): 211-219. DOI:10.1046/j.1525-1314.2000.00247.x
Krogh, Ravna E., Terry, M. P., 2004. Geothermobarometry of UHP and HP Eclogites and Schists—An Evaluation of Equilibria among Garnet- Clinopyroxene-Kyanite-Phengite-Coesite/Quartz. Journal of Metamorphic Geology, 22(6): 579-592. DOI:10.1111/j.1525-1314.2004.00534.x
Li, P., Zhang, C., Liu, X. Y., et al., 2017. The Metamorphic Processes of the Xindaduo Eclogite in Tibet and Its Constrain on the Evolutionary of the Paleo-Tethys Subduction Zone. Acta Petrologica Sinica, 33(12): 3753-3765.
Li, J. L., Klemd, R., Gao, J., et al., 2016. Poly-Cyclic Metamorphic Evolution of Eclogite: Evidence for Multistage Buria-Exhumation Cycling in a Subduction Channel. Journal of Petrology, 57(1): 119-146. DOI:10.1093/petrology/egw002
Liou, J. G., Zhang, R. Y., Ernst, W. G., 2007. Very High-Pressure Orogenic Garnet Peridotites. Proceedings of the National Academy of Sciences, 104(22): 9116-9121. DOI:10.1073/pnas.0607300104
Liu, Y., Liu, H. F., Theye, T., et al., 2009. Evidence for Oceanic Subduction at the NE Gondwana Margin during Permo-Triassic Times. Terra Nova, 21(3): 195-202. DOI:10.1111/j.1365-3121.2009.00874.x
Liu, Y. S., Gao, S., Hu, Z. C., et al., 2010. Continental and Oceanic Crust Recycling-Induced Melt-Peridotite Interactions in the Trans-North China Orogen: U-Pb Dating, Hf Isotopes and Trace Elements in Zircons from Mantle Xenoliths. Journal of Petrology, 51(1/2): 537-571. DOI:10.1093/petrology/egp082
Ludwig, K. R., 2003. Userʼs Manual for Isoplot/Ex, Version 300: A Geochronological Toolkit for Microsoft Excell. Berkeley Geochronology Center Special Publication, Berkeley. 70
Lü, Z., Zhang, L. F., Du, J. X., et al., 2008. Coesite Inclusions in Garnet from Eclogitic Rocks in Western Tianshan, Northwest China: Convincing Proof of UHP Metamorphism. American Mineralogist, 93(11/12): 1845-1850. DOI:10.2138/am.2008.2800
Maruyama, S., Liou, J. G., Terabayashi, M., 1996. Blueschists and Eclogites of the World and Their Exhumation. International Geology Review, 38(6): 485-594. DOI:10.1080/00206819709465347
Mo, X. X., Niu, Y. L., Dong, G. C., et al., 2008. Contribution of Syncollisional Felsic Magmatism to Continental Crust Growth: A Case Study of the Paleogene Linzizong Volcanic Succession in Southern Tibet. Chemical Geology, 250(1/2/3/4): 49-67. DOI:10.1016/j.chemgeo.2008.02.003
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
Powell, R., 1985. Regression Diagnostics and Robust Regression in Geothermometer/Geobarometer Calibration: The Garnet-Clinopyroxene Geothermometer Revisited. Journal of Metamorphic Geology, 3(3): 231-243. DOI:10.1111/j.1525-1314.1985.tb00319.x
Powell, R., Holland, T. J. B., 2008. On Thermobarometry. Journal of Metamorphic Geology, 26(2): 155-179. DOI:10.1111/j.1525-1314.2007.00756.x
Reinecke, T., 1991. Very-High-Pressure Metamorphism and Uplift of Coesite-Bearing Metasediments from the Zermatt-Saas Zone, Western Alps. European Journal of Mineralogy, 3(1): 7-18. DOI:10.1127/ejm/3/1/0007
Shen, T. T., Zhang, C., Tian, Z. L., et al., 2018. Petrological Studies of Jilang Eclogite in the Lhasa Terrane and Its Constraint on the Subduction and Exhumation Processes of the Paleo-Tethys Oceanic Crust. Petrologica et Mineralogica Sinica, 37(6): 917-932.
Song, S. G., Niu, Y. L., Su, L., et al., 2014. Continental Orogenesis from Ocean Subduction, Continent Collision/subduction, to Orogen Collapse, and Orogen Recycling: The Example of the North Qaidam UHPM Belt, NW China. Earth-Science Reviews, 129: 59-84. DOI:10.1016/j.earscirev.2013.11.010
Shreve, R. L., Cloos, M., 1986. Dynamics of Sediment Subduction, Melange Formation, and Prism Accretion. Journal of Geophysical Research, 91(B10): 10229. DOI:10.1029/jb091ib10p10229
van Roermund, H. L. M., Drury, M. R., 1998. Ultra-High Pressure (P > 6 GPa) Garnet Peridotites in Western Norway: Exhumation of Mantle Rocks from > 85 km Depth. Terra Nova, 10(6): 295-301. DOI:10.1046/j.1365-3121.1998.00213.x
Wang, Q. C., Cong, B. L., 1996. Tectonic Implication of UHP Rocks from the Dabie Mountains. Science in China Series D: Earth Sciences, 39: 311-318.
Wei, C. J., Powell, R., 2004. Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O) with Application to Natural Rocks. Journal of Petrology, 45: 183-202. DOI:10.1093/petrology/egg085
Wei, C. J., Su, X. L., Lou, Y. X., et al., 2009. A New Interpretation of the Conventional Thermobarometry in Eclogite: Evidence from the Calculated P-T Pseudosections. Acta Petrologica Sinica, 25(9): 2078-2088.
Wei, C. J., Clarke, G. L., 2011. Calculated Phase Equilibria for MORB Compositions: A Reappraisal of the Metamorphic Evolution of Lawsonite Eclogite. Journal of Metamorphic Geology, 29(9): 939-952. DOI:10.1111/j.1525-1314.2011.00948.x
Yamato, P., Agard, P., Burov, E., et al., 2007. Burial and Exhumation in a Subduction Wedge: Mutual Constraints from Thermomechanical Modeling and Natural P-T-t Data (Schistes Lustrés, Western Alps). Journal of Geophysical Research, 112(B7): B07410. DOI:10.1029/2006jb004441
Yang, J. S., Xu, Z. Q., Geng, Q. R., et al., 2006. A Possible New HP/UHP(?) Metamorphic Belt in China: Discovery of Eclogite in the Lhasa Terrane, Tibet. Acta Geologica Sinica, 80(12): 1787-1792.
Yang, J. S., Xu, Z. Q., Li, Z. L., et al., 2009. Discovery of an Eclogite Belt in the Lhasa Block, Tibet: A New Border for Paleo-Tethys?. Journal of Asian Earth Sciences, 34(1): 76-89. DOI:10.1016/j.jseaes.2008.04.001
Yang, X. L., Zhang, L. F., Zhao, Z. D., et al., 2014. Metamorphic Evolution of Glaucophane Eclogite from Sumdo, Lhasa Block of Tibetan Plateau: Phase Equilibria and Metamorphic P-T Path. Acta Petrologica Sinica, 30(5): 1505-1519.
Ye, K., Cong, B. L., Ye, D. N., 2000. The Possible Subduction of Continental Material to Depths Greater than 200 km. Nature, 407(6805): 734-736. DOI:10.1038/35037566
Yin, A., Harrison, T. M., 2000. Geologic Evolution of the Himalayan- Tibetan Orogen. Annual Review of Earth and Planetary Sciences, 28(1): 211-280. DOI:10.1146/annurev.earth.28.1.211
Zhai, Q. G., Zhang, R. Y., Jahn, B. M., et al., 2011. Triassic Eclogites from Central Qiangtang, Northern Tibet, China: Petrology, Geochronology and Metamorphic P-T Path. Lithos, 125(1/2): 173-189. DOI:10.1016/j.lithos.2011.02.004
Zhang, C., Bader, T., van Roermund, H. L. M., et al., 2019a. The Metamorphic Evolution and Tectonic Significance of the Sumdo HP-UHP Metamorphic Terrane, Central-South Lhasa Block, Tibet. Geological Society, London, Special Publications. https://doi.org/10.1144/sp474.4
Zhang, C., Bader, T., Zhang, L. M., et al., 2019b. Metamorphic Evolution and Age Constraints of the Garnet-Bearing Mica Schist from the Xindaduo Area of the Sumdo (U)HP Metamorphic Belt, Tibet. Geological Magazine. https://doi.org/10.1017/s001675681800033x
Zhang, D. D., Zhang, L. F., Zhao, Z. D., 2011. A Study of Metamorphism of Sumdo Eclogite in Tibet, China. Earth Science Frontiers, 18(2): 116-126.
Zhang, J. J., Santosh, M., Wang, X. X., et al., 2012. Tectonics of the Northern Himalaya since the India-Asia Collision. Gondwana Research, 21(4): 939-960. DOI:10.1016/j.gr.2011.11.004
Zhang, Z. M., Ding, H. X., Dong, X., et al., 2018. High-Temperature Metamorphism, Anataxis and Tectonic Evolution of a Mafic Granulite from the Eastern Himalayan Orogen. Journal of Earth Science, 29(5): 1010-1025. DOI:10.1007/s12583-018-0852-y
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 Science, 58(7): 1045-1069. DOI:10.1007/s11430-015-5097-3
Zheng, Y. F., Fu, B., Gong, B., et al., 2003. Stable Isotope Geochemistry of Ultrahigh Pressure Metamorphic Rocks from the Dabie-Sulu Orogen in China: Implications for Geodynamics and Fluid Regime. Earth-Science Reviews, 62(1/2): 105-161. DOI:10.1016/s0012-8252(02)00133-2
Zhu, D. C., Mo, X. X., Niu, Y. L., et al., 2009. Zircon U-Pb Dating and in-situ Hf Isotopic Analysis of Permian Peraluminous Granite in the Lhasa Terrane, Southern Tibet: Implications for Permian Collisional Orogeny and Paleogeography. Tectonophysics, 469(1/2/3/4): 48-60. DOI:10.1016/j.tecto.2009.01.017
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., 2012. Cambrian Bimodal Volcanism in the Lhasa Terrane, Southern Tibet: Record of an Early Paleozoic Andean- Type Magmatic Arc in the Australian Proto-Tethyan Margin. Chemical Geology, 328: 290-308. DOI:10.1016/j.chemgeo.2011.12.024
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