2. School of Earth and Space Sciences, Peking University, Beijing 100871, China;
3. Nanjing Institute of Geology and Mineral Resources, Chinese Academy of Geological Sciences, Nanjing 210016, China;
4. Geology Bureau for Nonferrous Metals of Guangdong Province, Guangzhou 510080, China;
5. Hangzhou Mechanical Design Research Institute of Ministry of Water Resources, Standard & Quality Control Research Institute of Ministry of Water Resources, Hangzhou 310012, China
High-pressure pelitic granulite is characterized by the key associated kyanite-K-feldspar in felsic bulk composition (OʼBrien and Rötzler, 2003), and is a result of continental collision and amalgamation leading to the significant burial and crustal thickening (Zhang et al., 2018, 2000; Anderson et al., 2012; Jöns and Schenk, 2011; Santosh et al., 2009; Brown, 2007; Zhang and Meng, 2005; OʼBrien and Rötzler, 2003; Gilotti and Elvevold, 2002; Schaller et al., 1999; Hiroi et al., 1994). It is an important indicator to define the past block convergence boundary and understand the history of a tectonic zone.
Ailaoshan-Jinshajiang tectonic zone, located at the northeastern margin of the Indochina Block (Fig. 1a), was formed by subduction and collision between the Indochina and South China blocks in Permo–Triassic (Wang B D et al., 2018; Qi et al., 2016; Jian et al., 2009; Yumul et al., 2008; Owada et al., 2007; Trung et al., 2006; Lepvrier et al., 2004; Liu et al., 2000; Wang X F et al., 2000; Mo et al., 1993; Sengör, 1984), followed by the southeastward extrusion of the Indochina Block and strike slip-shearing overprinted on the sutures during the northward subduction of the India Plate in the Cenozoic (Liu et al., 2007; Morley, 2007; Zhang et al., 2006, 1999; Burchfiel and Wang, 2003; Wang et al., 2001; Leloup et al., 1995; Leloup and Kienast, 1993; Tapponnier et al., 1990, 1982). From northwest to southeast, this region is composed of four narrow NW-trending high grade metamorphic belts: the Xuelongshan, Diancangshan and Ailaoshan in Yunnan, and the Day Nui Con Voi (Song Ma) in Vietnam. Due to Cenozoic strike-slip shearing overprints, it is difficult to find the key marker to determine whether this tectonic zone is a suture of Indochina and South China blocks. Recently, high-pressure granulites from Song Ma in Vietnam have been reported, and regarded as a product related to a continental collision setting followed by crustal subduction between the Indochina and South China blocks (Nakano et al., 2008, 2004; Osanai et al., 2004). Ailaoshan metamorphic belt is one of the largest metamorphic belts in the northeastern Indochina Block. The high pressure granulite is one of the key markers to define it as the block convergence boundary. Here, we estimate the pressure/temperature conditions and ages of the high-pressure pelitic granulite in the Ailaoshan metamorphic belt, and then elucidate the relationships between the high- pressure pelitic granulite and events of subduction and collision.1 GEOLOGICAL SETTING
The Ailaoshan metamorphic belt, 10 to 30 km in width and more than 500 km in length, is located at the southeast of Ailaoshan-Jinshajiang tectonic zone in West Yunnan, China (Fig. 1b), and extends to the northern Vietnam. The Ailaoshan metamorphic belt consists of Xiaoyangjie and Along formations. The Xiaoyangjie Formation is composed of schists and gneisses with minor amphibolite. The Along Formation includes biotite-sillimanite-garnet-bearing amphibolite facies and a few granulite facies metapelites, granitic gneiss, marble and amphibolite. The granitic gneiss commonly contains large feldspar porphyroclasts, similar to those in augen gneisses, which have undergone various degrees of migmatization. The granitoids intrusion mainly occurred in the Mid-Neoproterozoic and Triassic.
The Indochina Block is separated from the Yangtze Block to the east by the Ailaoshan-Jinshajiang (Ailaoshan-Red River) strike-slip fault (e.g., Leloup et al., 1995; Leloup and Kienast, 1993; Schärer et al., 1990; Tapponnier et al., 1990), and from the Baoshan Block to the west by the Lancangjiang strike-slip fault (Fig. 1). From northeast to southwest, an NW-trending high grade metamorphic belt, a low grade metamorphic fold belt and a Mesozoic fold belt crop out in this region (e.g., Qi et al., 2010). The high grade belt comprises amphibolite facies rocks, while the low grade belt is composed of greenschist facies rocks (Leloup et al., 1995; Leloup and Kienast, 1993). Paleozoic and Mesozoic strata are exposed with minor Cenozoic sedimentary rocks, which have been folded and intruded by magmatic units.
The high-pressure pelitic granulite, the focus of this study, traverses the high-grade metamorphic belt at the southeastern margin of the Ailaoshan metamorphic belt, and crops out as a slice of about 500–700 m in thickness, and is separated from the marble lens to the northwest, and from the paragneiss to the southeast by the thrusts respectively (Fig. 2). This granulite is strongly mylonitized with the foliation parallel to that of the paragneiss.2 PETROLOGY
The high-pressure pelitic granulite is composed of a kyanite-K-plagioclase-rutile inclusions-rich melanocratic layer with quartz-feldspar-rich layer of about 2–50 mm in width. Mineral foliation and lineation of the granulite defined by direction arrangement of sillimanite, biotite and ribbon quartz (Fig. 3a), show that the rock had undergone ductile deformation later.
The melanocratic layer in high-pressure pelitic granulite contains porphyroclastic garnet (25%), sillimanite (17%), feldspar (18%), flaky biotite (13%) and ribbon quartz (27%) with accessory kyanite, sapphirine, spinel (hercynite), rutile, ilmenite, zircon, monazite and apatite (Fig. 3). Furthermore, muscovite, sericite and hydromica in the layer belong to the late secondary minerals (Figs. 3e, 3g). The melanocratic layer shows high SiO2 (62.43 wt.%–63.44 wt.%), FeOT (FeO+Fe2O3, 7.19 wt.%–8.70 wt.%), CaO (0.81 wt.%–1.14 wt.%), K2O (3.11 wt.%–3.96 wt.%), TiO2 (0.86 wt.%–1.01 wt.%) and low MgO (2.76 wt.%–3.07 wt.%) contents (Table 1).
The dominant mineral assemblage of the quartz-feldspar- rich layer is garnet (18%), sillimanite (17%), feldspar (23%), quartz (37%) and biotite (5%). Accessory minerals include rutile, ilmenite, zircon and spinel. Compared with the melanocratic layer, the quartz-feldspar-rich layer is characterized by high SiO2 (66.21 wt.%–66.89 wt.%), FeOT (6.98 wt.%–7.44 wt.%), CaO (1.08 wt.%–1.49 wt.%) and Na2O (1.57 wt.%–1.87 wt.%), and low Al2O3 (14.89 wt.%–15.32 wt.%), MgO (2.46 wt.%–2.59 wt.%) and K2O (2.58 wt.%–2.65 wt.%) (Table 1).
Garnet is one of the main porphyroblasts with sizes varying from 0.5 to 4 mm, and contains numerous monophase and multiphase fine-grained inclusions of sillimanite (0.05–0.3 mm), biotite (0.03–0.2 mm), plagioclase (0.05–0.5 mm), K-feldspar (0.03–0.5 mm), rutile (0.01–0.03 mm), monazite (0.01–0.03 mm), zircon (0.01–0.03 mm), quartz (0.01–0.1 mm), and sapphirine (0.03 mm). The multiphase inclusions are composed of fine-grained aggregates of sillimanite+plagioclase+biotite, plagioclase+quartz+kyanite+ spinel, biotite+quartz and sillimanite+sapphirine. Moreover, a series of sub-parallel fractures within the garnet are obliquely aligned to the major foliation of the granulite, and filled by biotite (or hydromica) or biotite+quartz, biotite+quartz+plagioclase assemblages (Fig. 3h).
Biotite (Bt) shows three modes of occurrences in granulite. Bt-1 exist as flaky subhedral to anhedral inclusions within garnet and feldspar porphyroblasts, or multiphase inclusions coexist with sillimanite, quartz, suggesting it was formed at the same time with garnet and feldspar porphyroblast; Bt-2 occurs as coarse-grained subhedral to anhedral flakes together with garnet, sillimanite and feldspar (Figs. 3a, 3b); alternatively, Bt-2 exist as small anhedral flakes crystal with feldspar, quartz, sillimanite and ilmenite in matrix (Figs. 3a, 3b, 3f, 3g), which may be directly linked to deformation showed later; Bt-3 is distributed as anhedral crystals along the fractures within garnet (Fig. 3h), suggesting it is related to liquid formed in retrograde metamorphism, and in equilibrium with garnet in fractural margin. In each of them, three generations can be marked: the first (Bt-1) formed together with garnet and feldspar porphyroblasts; the second (Bt-2), the biotite formed in the first generation was modified and re-equilibrated with the porphyroblasts in the late metamorphism; and the third (Bt-3), coexisted with quartz and plagioclase sometimes, formed in superimposed metamorphism.
Feldspar (Fsp) shows three modes of occurrence in the granulite: Fsp-1 occur as subhedral to anhedral mineral inclusions (0.01–0.3 mm) in garnet, which is dominated by plagioclase, or coexists with quartz sometimes; Fsp-2 occur as anhedral porphyroblast (0.5–2 mm), surrounded with fine-gained matrix minerals. It is composed of plagioclase, K-feldspar, perthite (per) and anti-perthite (anti-per) (Fig. 3c). The albite occurs as fine-veined lamella in K-feldspar as perthite, and the K-feldspar occurs as schistic lamella in plagioclase as anti-perthite, which can be derived from the exsolution. They occur mainly around fine grained quartz, suggesting the possible presence of a quartz-feldspathic melt. Fsp-3, comprising K-feldspar and plagioclase, occur as fine grained anhedral crystal together with quartz, biotite and minor sillimanite in the matrix and fractures within garnet.
Sillimanite appears mainly as needle-shaped aggregate, partly as a lath-shaped crystal (Figs. 3a, 3b, 3g) with kyanite pseudomorph, and as inclusions within garnet (Figs. 3b, 3e). As shown in Fig. 3g, it is in contact with porphyroclastic garnet, K-feldspar and plagioclase, although most of them are surrounded by fine-grained matrix minerals, indicating they are in equilibrium with them. Kyanite preserves as inclusion within plagioclase (Fig. 3d), or as porphyroblast surrounded by needle-shaped sillimanite and sillimanite+biotite which show the kyanite was replaced by sillimantite (Fig. 3f). The kyanite within feldspar is partially replaced by the spinel (hercynite)+ quartz (Fig. 3d), which suggests the following common decompression reaction: Ky+Grt→Spl+Qtz. The sapphirine occurs as a small spheroid (25 µm) or droplet (5 µm) included in sillimanite and garnet (Fig. 3e). Given the phase relationships in thin sections studied, garnet has most likely supplied Fe and Mg for the formation of the sapphirine.
To sum up, the high-pressure pelitic granulite is composed of porphyroblast and matrix; the matrix was formed by exhausting pre-existing feldspar, quartz, biotite and sillimanite, and which is thought to be directly linked to deformation. This texture shows that garnet, kyanite, feldspar, biotite and rutile inclusions are pre-existing minerals in equilibrium, and belong to the first generation of mineral assemblages (e.g., Dong et al., 2018). Then they experienced modification to get re-equilibrium during decompression at isothermal or increasing temperature condition. The garnet was re-crystallized, and its composition zone disappeared. The kyanite was converted to sillimanite, or to spinel+quartz except a few inclusions in plagioclase, or to sapphirine by reaction with garnet. The biotite was partly dehydrated to sillimanite+liq (melt). The feldspar is rarely involved in the reactions and usually in equilibrium with other minerals. The mineral assemblage of garnet, sillimanite, spinel, sapphirine, K-feldspar, plagioclase, biotite, quartz, rutile, as porphyroblast or in equilibrium with porphyroblast, belongs to the second generation. Together with the first generation, they formed in the first stage. After that, liquid extracted and crystallized during superimposed metamorphism, Grt+liq (melt)→Bt (Bt-3)+Pl (Fsp-3)+Qtz, Sil+liq→Ms. The assemblage of garnet, biotite, muscovite, plagioclase, quartz within fractures of garnet and matrix belongs to the third generation formed in the second metamorphic stage.3 ANALYTICAL METHODS
Whole-rock major elements were analyzed at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing. Major element oxides were determined using ICP-AES (PE8300). The analytical uncertainty was < 0.5%. Analyses of chemical compositions of the constituent minerals were performed on JEOL JXA-8100 wavelength-dispersive electron microprobe at the State Key Laboratory of Continental Tectonics and Dynamics, Beijing. The accelerating voltage was 15 kV, and the beam current was 20 nA.
Zircons were separated from whole-rock samples by a combination of density techniques and handpicking under an electron microscope. The grains were mounted in epoxy, polished to about half of their thickness, and photographed under transmitted and reflected light. Cathodoluminescence (CL) imaging of the zircon grains was carried out in a scanning electron microprobes at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences. The U-Pb dating of the zircons was conducted synchronously by laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) at the State Key Laboratory of Geological Processes and Mineral Resources, Faculty of Earth Sciences, China University of Geosciences (Wuhan) using Agilent 7500a. The GeoLas 2005 laser ablation system was used after the analytical methods and procedures by Hu et al. (2012) and Qi et al. (2012).4 MINERAL CHEMISTRY 4.1 Garnet
The garnet in Ailaoshan pelitic granulite is a solid solution of almandine (Alm)-pyrope (Pyr)-grossular (Grs)-spessartine (Sps), with almandine component always more than 60 mol% and spessartine component less than 5 mol%. Compositional profiles were measured across representative crystal of garnets. The profiles exhibit compositional plateaus with relatively high Pyr and low Alm values encompassing most of the interior crystals with Alm59–60Pyr34–35Grs4–5Sps1–2, and relatively high Alm and low Pyr values in the rim (including the fractural rim) of the crystal with Alm61–71Pyr22–33Grs4–5Sps2–3 (Table 2, Fig. 4). It shows the garnet had been homogenized at high temperature metamorphism.
The first two generations of biotite (Bt-1 and Bt-2) are characterized by high TiO2 content (3.51 wt.%–5.40 wt.%) and low MgO content (9.41 wt.%–14.29 wt.%). The XMg values [XMg=(Mg2+/(Mg2++Fe2+)] range from 0.47 to 0.65. On the contrary, biotite of the third generation (Bt-3) has lowest TiO2 contents (0.56 wt.%–0.75 wt.%) (Table 2).4.3 Feldspar
K-feldspar in the occurrences of porphyroblast, matrix and lamella in plagioclase shows similar compositional characteristic of high Or content as Ab9–13An0–1Or87–91 (An. anorthite) (Table 3). As porphyroblast and matrix, plagioclases have slightly lower anorthite content with Ab65–68An31–34Or0–1 compared to that as inclusions with Ab58–62An37–42Or0–1. The plagioclase lamella in K-feldspar has higher albite content as Ab91–98 An1–2Or1–7 (Table 3).
Spinel is hercynite-rich (XMg=0.27–0.28) with negligible amounts of Cr2O3 and MnO (Table 4), and its ZnO content reaches up to 4.96% (XZn=0.12–0.13), together with quartz in contact with the kyanite within plagioclase (Fig. 3e). Sapphirine is characterized by high FeO contents (20.37 wt.%–24.72 wt.%) and low MgO contents (MgO=2.79 wt.%–3.37 wt.%, XMg=0.19–0.21) (Table 4). The sapphirine shows the high Si contents (0.94–1.84 p.f.u.) consistent with that in association with quartz (Tsunogae and Santosh, 2006; Ouzegane and Boumaza, 1996). Sillimanite is nearly homogeneous content in terms of Al2O3 and SiO2 with minor FeO and Cr2O3 for all occurrences (Table 4).
Several geothermobarometers based on the experimental, empirical and thermodynamic parameters using different composition-activity models for the various minerals are applicable for high-grade metamorphic rocks due to the presence of a number of ferromagnesian minerals, such as garnet-biotite geothermometer (Holdaway, 2000), garnet-Al silicate-plagioclase (GASP) geobarometer (Koziol and Newton, 1988), and garnet- biotite-plagioclase-quartz (GBPQ) geothermobarometer (Wu et al., 2018, 2004).
Here, the P-T conditions were estimated by GBPQ geothermobarometer (Wu et al., 2004) using the compositions of host minerals re-equilibrated at the initial stage of retrograde metamorphism or occurred along fractures within garnet at late stage of retrograde metamorphism. The results show that the P-T ranges of the second and third generations are 8.6–10.4 kbar at 850–919 ℃ and 3.5–3.9 kbar at 572–576 ℃, respectively. The top temperature (920 ℃) for the second generation is slightly higher than the upper limit of the GBPQ geothermobarometery, but it is still within its range, and supported by a high temperature mineral assemblage of garnet-sillimanite-feldspar-biotite with high-TiO2 and FeO contents. According to the maximum temperature and the lower limit of kyanite stability, the pressure is determined to be greater than 12 kbar (Moulas et al., 2013). We therefore suggest the 8.6–12 kbar at 850–920 ℃ and 3.5–3.9 kbar at 572–576 ℃ could represent the P-T condition of the second and third generations of metamorphism respectively.6 GEOCHRONOLOGY
Zircons analyzed in this study were separated from samples (11QHG-3) collected from the melanocratic and leucocratic domains in high-pressure pelitic granulite, respectively. Zircons are euhedral to subhedral, translucent, and prismatic. Most of the zircon grains are 80 to 120 µmm long and have aspect ratios ranging from 1.5 to 2.0. Most grains have a core-mantle-rim, core-rim or core-mantle structure (Fig. 5). The mantles and rims with dark-gray homogenous patterns show that the zircons might be formed in multiple metamorphic stages (Figs. 5b, 5c, 5e, 5f). The cores are subhedral to rounded, varying from 20 to 100 µmm long with oscillatory zoning (Figs. 5a, 5c, 5e), indicating that they are inherited from older detrital zircons. As shown in Table 5 and Fig. 6, 62 spots of 50 zircons from sample 11QHG-3 were conducted. Among these, 32 spots on zircon cores, yield 238U/206Pb ages of 337 to 2 791 Ma with U and Th contents ranging from 33 ppm to 3 881 ppm and 28 ppm to 1 167 ppm, respectively, and higher Th/U ratios varying from 0.13 to 1.33. Part of them, discordant and distributing inside the concordant line (Fig. 6a), yield 206Pb/238U ages of 1 877 to 440 Ma. These indicate metamictization and lead loss, which could not represent the crystallization age of the inherited detrital zircons. Other concordant analyses, yielding 206Pb/238U ages from 1 871 to 337 Ma (Fig. 6a), could represent the crystallization ages of the inherited zircons. We interpret those ages as the crystallization ages of the inherited detrital zircons (Fig. 6), the youngest 238U/206Pb age of 337 Ma as oldest age for the protolith of the pelitic granulite. Eighteen spots on zircon mantles yield 238U/206Pb ages of 66 to 236 Ma with U and Th contents of 167 ppm–2 585 ppm and 1 ppm–125 ppm, respectively, and lower Th/U ratios of 0.01 to 0.05 except spot 39, and distributed on the concorde line (Fig. 6b). Thirteen spots on zircon rims yield 238U/206Pb ages range from 28 to 38 Ma. Among these, 3 spots (spots 12, 37 and 49 yield slightly younger or elderly 206Pb/238U ages of 28±0.5, 37±0.8 and 38±0.4 Ma). Ten spots are concordant, and yield a weighted mean age of 33±0.8 Ma (MSWD=11) (Fig. 6c) with similar U and Th contents and Th/U ratios to zircon mantles.
The detailed petrographic and compositional features of minerals indicate various reactions at both prograde and retrograde metamorphism in Ailaoshan pelitic granulite, such as: (1) sillimanite with kyanite pseudomorph, and minor kyanite as inclusion within plagioclase, illustrated that the metamorphic condition changed from the kyanite to the sillimanite stability fields, and the reaction occurred during prograde metamorphism: Ky→Sil. (2) Kyanites as inclusions within plagioclase were partially replaced by spinel (hercynite)+quartz, suggesting the following common decompression reaction: Ky+Grt→Spl (Her)+ Qtz. (3) Small spherical and drop shaped sapphirine distributed in sillimanite and garnet, shows the reaction: Grt+Sill (Ky)→Spr. (4) As shown in Figs. 3h and 4, the homogeneous compositional section, except the iron enrichment at the rims adjacent to biotite and fractures filled by biotite±quartz±plagioclase, shows the garnet has been homogenized at peak metamorphism, and undergone the Fe-Mg exchange reaction between garnet and biotite or liquid (melt) during retrograde metamorphism, and had re-equilibrated each other with the reaction: Grt+liq (melt)→ Bt±Qtz±Pl. (5) Per/anti-Per during decreasing temperature. These indicate the pelitic granulite has experienced two stages of peak to initial retrograde, and late superimposed metamorphism. Based on the equilibrium mineral assemblage and GBPQ geothermobarometery, we estimate P-T at 8.6–12 kbar, 850–920 ℃ as the initial retrograde metamorphic conditions, which are corroborated by critical UHT assemblages of Grt+Sil+Spl+Spr+ Bt+Rut. At the same time, we take the P-T of 3.5–3.9 kbar, 572–576 ℃ for the condition of final metamorphic stage based on the mineral assemblage of Pl+Bt+Qtz filling in the fractures of garnet.
Temperatures estimated for sapphirine-spinel assemblages have been reported from garnet-sillimanite-spinel quartzofeldspathic gneisses at Skallen and Langhovde in Lützow-Holm Bay (Motoyoshi et al., 1985; Hiroi et al., 1983), Rundsvågshetta (Motoyoshi and Ishikawa, 1997; Motoyoshi et al., 1989), and the Highland Complex in Sri Lanka (Hiroi et al., 1994; Ogo et al., 1992). The experimental results (Bertrand et al., 1991; Hensen and Green, 1972) demonstrated temperatures of 900–1 050 ℃ and pressures of 11–12 kbar for the sapphirine bearing assemblage. However, the phase relations of very small spinel and sapphirine distributed around kyanite and sillimanite, the low pressure spinel with high Zn content (Harley, 2008), and the sapphirine with high FeO content and low XMg values (< 0.21), are different from UHT granulite above. These show the mineral assemblage of the spinel and sapphirine formed by the metastable reaction of garnet and kyanite under the P-T conditions of 5–7 kbar and 580–800 ℃ (Moulas et al., 2013) during the uplift stage of the pelitic granulite. Hence, we can't estimate the peak metamorphic P-T condition with sapphirine and spinel. Detailed investigations on mineral inclusions within porphyroblastic garnet and feldspar are required to evaluate the peak metamorphic condition and understand the P-T path. Zircon LA-ICP-MS U-Pb dating shows that there are three age groups from zircon core, mantle and rim, respectively. Among them, we suggest the concordant ages of the zircon cores as the crystallization ages of the inherited zircons, and the youngest 238U/206Pb age of 337 Ma (spot 54) as the oldest age for the protolith of the pelitic granulite. The second age group from the zircon mantles ranges from 236 to 66 Ma. We interpret the oldest age of 236 Ma as the age of the first metamorphic stage, and the reasons are as follows: (1) The CL images show that the zircon mantles have euhedral shapes and homogeneous patterns with rims of 0–40 µmm in width (Fig. 5), except spot 54, other metamict ones have the same gray rims (Figs. 5a, 5d), and radiogenic Pb loss during retrograde metamorphism, including radiation damage, self-annealing and chemical reaction (Nam et al., 2001; Sano et al., 1999; Pidgeon, 1992). (2) Partial spots containing mantles and rims, yield mixed 238U/206Pb ages (Figs. 5b, 5c, 5e, 5g, 5h). (3) The migmatization represented by an intercalation of felsic leucosomes and restitic melanocratic layers, shows that there are products of liquid or melt, which was related to the dehydration of biotite (Aït-Djafer et al., 2009). Such fluid or melt remaining in communication with the mineral assemblages following UHT has a significant effect on the ages recorded by geochronological systems, even to the extent of producing widespread post-UHT metamorphic zircon (Harley, 2004). (4) The oldest age of 236 Ma is generally consistent with that of 230–270 Ma reported for high pressure granulites and eclogites of the northeastern Indochina in Vietnam (Nakano et al., 2008, 2007, 2004; Osanai et al., 2004; Nam et al., 2001; Lepvrier et al., 1997). We therefore interpret the oldest age of 236 Ma in the zircon mantle as the age for the first stage of metamorphism. The third age group from the zircon rims of 38 to 28 Ma is consistent with the ages of the ductile shearing along the Ailaoshan metamorphic belt (35–21 Ma) estimated by zircon U-Pb and amphibole, muscovite, biotite and K-feldspar Ar-Ar dating (Liu et al., 2015; Tang et al., 2013; Cao et al., 2011a, b ; Liang et al., 2007; Searle, 2006; Leloup et al., 1995). Combining P-T conditions of the second retrograde metamorphic stage similar to the ductile shearing P-T conditions of 480 ℃ at 3 kbar to 650 ℃ at 5 kbar (Wu et al., 2017; Liu et al., 2012; Nam et al., 1998; Leloup and Kienast, 1993), we suggest the weighted mean age of 33 Ma of the zircon rims as the age of the second metamorphic stage.
It is inferred that the high-pressure pelitic granulite in Ailaoshan Orogen has experienced multiple tectonothermal events. The upper continental crust subducted to the middle-lower crust, and underwent granulite facies metamorphism, then exhumed to the middle-upper crust quickly and initial retrograde metamorphism occurred at 8.6–12 kbar at 850–920 ℃ (about 236 Ma), and final second stage of metamorphism overprinted at the condition of 3.5–3.9 kbar at 572–576 ℃ since 33 Ma (Fig. 6).7.2 Tectonic Implications
The LA-ICP-MS U-Pb ages for zircon cores consist of a Early Paleozoic group and a Precambrian group (Table 5). The Precambrian ages can be further divided into Neoproterozoic and Mesoproterozoic populations. In general, the orderless distribution of these ages shows their detrital zircons origin in the rocks from Mesoproterozoic to Lower Carboniferous, and implying that the protolith of the granulite is a pelitic sedimentary rock formed after Early Carboniferous. The P-T condition of peak metamorphism shows that the granulite formed in the lower continental crust (≥30 km). It means that the pelite had been brought down to the lower continental crust. How and when was it brought to the lower crust? There are two tectonothermal events in Ailaoshan Orogen since Paleozoic: (1) The amalgamation of the South China and Indochina blocks during the Early to Middle Triassic (Jian et al., 2009; Yumul et al., 2008; Owada et al., 2007; Trung et al., 2006; Lepvrier et al., 2004; Sengör, 1984). It was supported by numerous Carboniferous ophiolitic mafic-ultramafic blocks regarded as relics of the Paleo-Tethys oceanic crust (Jian et al., 2008, 1998; Metcalfe, 2002; Zhang et al., 1995, 1994), an island arc volcano- sedimentary zone (Dong et al., 2000; Mo et al., 1993) and the Upper Triassic molasse at the southwestern margin of the Ailaoshan Orogen, China, and HP-UHP granulites and eclogite from northern Vietnam (Nakano et al., 2010, 2008; Lepvrier et al., 2004; Osanai et al., 2004). (2) The large scale sinistral slip- shearing resulted from the convergence between India and Asia, and the southeastward extrusion of Indochina Block related to the South China Block since the Early Cenozoic (Searle et al., 2010; Yeh et al., 2008; Morley, 2002; Leloup et al., 1995; Tapponnier et al., 1990, 1982). The age of the peak metamorphism recorded by the oldest age of zircon mantle at 236 Ma and that of the superimposed metamorphism recorded by a weighted mean age of zircon rim at 33 Ma are consistent with the two tectonothermal events respectively. So we speculate that the sedimentary rock in Ailaoshan metamorphic belt was subducted/underthrusted to the lower continental crust (deeper than 30 km) and experienced granulite-facies metamorphism and partial melting, resulting from the collision of the Indochina and South China blocks during Indosinian, and superimposed metamorphism since the Oligocene, which is demonstrated by sub-parallel fractures within porphyroblastic garnet formed by slip shearing.8 CONCLUSIONS
The high-pressure pelitic granulite has undergone two stages of metamorphism: (1) the peak to initial retrograde metamorphism recorded by the mineral assemblage of garnet, kyanite, K-feldspar, quartz and rutile inclusions and the mineral assemblage of garnet, sillimanite, spinel, sapphirine, K-feldspar, plagioclase, biotite, quartz and rutile, respectively; (2) the superimposed metamorphism recorded by the mineral assemblage of biotite, plagioclase, quartz and ilmenite.
The initial retrograde metamorphism occurred at 8.6–12 kbar at 850–920 ℃ in Indosinian (about 235 Ma), and is related to the amalgamation of the South China and Indochina blocks during the Triassic. The late superimposed metamorphism occurred at the P-T condition of 3.5–3.9 kbar at 572–576 ℃ since Oligocene (33 Ma), and is related to the large scale sinistral slip-shearing since the Early Cenozoic.ACKNOWLEDGMENTS
We thank two anonymous reviewers for their critical and constructive comments. Heartfelt thanks also go to Profs. Zeming Zhang, Jianxin Zhang, and Xiwen Zhou for their discussion and help. Dr. Hua Xiang provided valuable suggestions for the original manuscript. This study was supported by the National Natural Science Foundation of China (Nos. 91755101, 41272219), the Chinese Ministry of Science and Technology (No. Sinoprobe-05-03), and the China Geological Survey (No. DD20160022-07). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-0893-x.
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