2. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China;
3. Sichuan Institute of Geological Survey, Chengdu 610081, China;
4. Center for Global Tectonics, China University of Geosciences, Wuhan 430074, China;
5. Beijing SHRIMP Center, Beijing 100037, China;
6. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China
Cordierite peraluminous granites (CPGs) and muscovite peraluminous granites (MPGs) are both classified as strong peraluminous granites (SPGs), of which the CPGs contain relatively high contents of MgO and FeO, and thus have been called ferromagnesian peraluminous granites (Rossi et al., 2002). The CPGs widely occur in the Lachlan fold belt, southeast of Australia, the southern Alaska convergent margin, and the Hercynian orogenic belt in the Massif Central of France (Kusky et al., 2003; Barbarin, 1999, 1996; Sylvester, 1998). Partial melting for generation of cordierite granitic magma needs higher temperature conditions (> 875 ℃, Sylvester, 1998) than those for muscovite granitic magmas and might be caused by addition of mantle heat, thus often generated in tectonic processes involving break-off of subduction slab, delaminating of lithosphere, ridge subduction, upwelling of asthenosphere mantle, or underplating of mafic magma from the upper mantle (Santosh et al., 2011; Santosh and Kusky, 2009; Clemens, 2003; Kusky et al., 2003; Barbarin, 1999, 1996; Sylvester, 1998; Finger et al., 1997; Pamic et al., 1996; Thompson and Connolly, 1995; Rottura et al., 1991; Wickham et al., 1987). Recently, Santosh and Kusky (2009) and Santosh et al. (2011) outlined a model where high temperature (HT) peraluminous granites can be associated with paired metamorphic belts of high temperature-high pressure/ultrahigh-temperature (HT-HP/UHP) metamorphism related to episodes of ridge subduction along convergent margins. Therefore, study of cordierite granite genesis can provide important information about mantle-crustal interaction, geodynamics of the major tectonothermal events and tectonic evolution of the lithosphere in a given region.
Recent studies suggest that the Qinghai-Tibet Plateau is predominantly made up of the Proto-Tethys Ocean closure- induced Early Paleozoic orogen system, Paleo-Tethys Ocean closure-linked Late Paleozoic–Early Mesozoic orogen system and (Middle) Neo-Tethys Ocean closure-related Late Mesozoic– Cenozoic orogen system (e.g., Li S Z et al., 2017a; Zhang et al., 2015; Pan et al., 2012 and references therein). The Early Paleozoic orogen system, covering the Atyn-Qilian-Qaidam spreads along the northern margin of the Qinghai-Tibet Plateau was resulted from long-run subduction, accretion and closure of oceanic lithosphere of the Proto-Tethys and continental collision (Pan et al., 2012; Şengör and Natal'in, 1996). The Qilian and Qaidam orogens also extend eastwards to the North Qinling orogen system (Li et al., 2016a; Zhang et al., 2002). Disputes remain on the final subduction polarity and timing of closure of the Proto- Tethys Ocean and initiation of the Paleo-Tethys domain (e.g., Li S Z et al., 2017b, 2016a; Zhang et al., 2015; Pan et al., 2012).
A CPG pluton was discovered in the southern margin of the Qaidam Block, the northern Qinghai-Tibet Plateau, which has been called the Jinshuikou cordierite granite (JSKCPG) 15 years ago (Chen et al., 2002a; Lu et al., 2002). The JSKCPG contains enclaves of granulite, amphibolite and quartzofeldspathic gneiss in addition to development of cordierite, garnet, biotite and other ferromagnesian minerals. Formation of the JSKCPG and the granulite enclaves undoubtedly preserve important information about deep lithospheric processes beneath the Qaidam Block and tectonic evolution of Tethys developed in northwestern China. Preliminary studies of petrology, geochemistry, mineralogy and geochronology and the petrogenetic background had been carried out for both the JSKCPG and the granulite enclaves (Meng et al., 2018; Ba et al., 2012; Long et al., 2006; Yu et al., 2005; Zhang et al., 2003; Chen et al., 2002a; Lu et al., 2002; Liu and Ye, 1998). However, although high precision in-situ techniques had been applied for zircon U-Pb dating (Lu et al., 2008; Long et al., 2006; Zhang et al., 2003), reliable age for emplacement and crystallization of the JSKCPG needs further examination partly due to complex internal structure of the dated zircons, and partly because the magmatic zircon rims are too thin to be dated by SHRIMP with a common beam size. In this paper, combined SHRIMP and LA-ICPMS techniques for U-Th-Pb dating have been conducted on zircon and monazite from the JSKCPG and the granulite enclaves, respectively, to provide reliable integrated chronological data for better understanding the tectonic evolution of the Tethys along the southern margin of the Qaidam Block, NW China.1 GEOLOGICAL BACKGROUND AND GEOLOGY OF THE JSKCPG PLUTON
The Qaidam Block developed the Eastern Kunlun orogenic belt and the North Qaidam orogenic belt on the southern and northern margins, respectively. The Eastern Kunlun orogenic belt is composed of the North Kunlun terrane (NKT) and the South Kunlun terrane (SKT), between the two terranes spreads the east Central Kunlun fault (ECKF) as a separating boundary (Fig. 1).
The NKT comprises Precambrian metamorphosed strata mainly composed of the Jinshuikou Group and the Binggou Group (Bureau of Geology and Mineral Resources of Qinghai Province, 1997). The Jinshuikou Group consists of the Archean or Mesoproterozoic Baishahe Formation at the lower part and the Mesoproterozoic Xiaomiao Formation at the upper part, with lithologically meta-basic volcanics and psammites with interlaid carbonates (Chen Y X et al., 2011; Lu et al., 2009; Chen N S et al., 2006a, 1999; Wang et al., 2004; Wang and Chen, 1987). They underwent tectonothermal events during the Late Mesoproterozoic to Early Neoproterozoic, intruded by 950–810 Ma granitoids (Chen Y X et al., 2015; Meng et al., 2013a; Zhang J M et al., 2012; Chen N S et al., 2008, 2006b; Lu et al., 2006; Tan et al., 2004) and suffered from ~0.9–1.0 Ga regional metamorphism (He et al., 2016; Wang et al., 2004). During the subsequent tectonothermal event the Jinshuikou Group underwent regional amphibolite-facies metamorphism with local granulite and eclogite facies metamorphism at 510–400 Ma (He et al., 2016; Qi et al., 2014; Meng et al., 2013b; Lu et al., 2009; Chen et al., 2008, 2007b; Li et al., 2006; Long et al., 2006; Zhang et al., 2003) and intrusion of granitoids at 450–420 Ma (Tian et al., 2016; Jiang et al., 2015; Wang G et al., 2014; Liu et al., 2013, 2012; Wang X X et al., 2012; Mo et al., 2007). The molasse-like Devonian Maoniushan Formation was unconformably deposited on the Precambrian and the Early Paleozoic metamorphic rocks in the regional post orogeny (Yin et al., 2003; Bureau of Geology and Mineral Resources of Qinghai Province, 1997), with zircon ages of 423–370 Ma (Liu et al., 2016; Lu et al., 2010; Zhang Y L et al., 2010). The Late-Paleozoic to Early-Mesozoic tectonothermal events took place at 310–200 Ma along with the consumption of the Paleo-Tethys ocean basin (Chen et al., 2007b, 2006b; Zhang et al., 2003; Lu et al., 2002), resulting in formation of wide-spread arc-affinity batholiths along the southern margin of the Qaidam Block (SMQB) and the Eastern Kunlun orogenic belt. The Central Kunlun fault sporadically exposes ultrabasic rocks, gabbro, diabase and basic volcanics. They occur as tectonic slices and masses within the Precambrian metamorphic units and Early Paleozoic Nachitai Group and thus has been considered to be of association of ophiolites (Meng et al., 2015; Cui et al., 2011; Feng et al., 2010; Zhu et al., 2006, 2002, 1999; Wang et al., 1999; Chen and Wang, 1996; Pan et al., 1996; Yang et al., 1996; Gu et al., 1994; Gao et al., 1988; Xiao et al., 1986), to be the Proto-Tethys remnants formed at 520–420 Ma (Meng et al., 2015; Cui et al., 2011; Zhu et al., 2006; Yang et al., 1996).
The northern margin of the Qaidam Block develops the same Mesoproterozoic metamorphosed strata named the Sha-liuhe Group which has been also undergone the Early Neoproterozoic tectonothermal event with intrusion of ~0.9–0.8 Ga S-type granitoids (Song et al., 2013; Xu et al., 2011; Chen et al., 2007b; Lu et al., 2006, 2002), and the intensively affected by the North Qaidam subduction-collision processes that resulted in a tectonic mélange zone. The North Qaidam subduction-collision tectonic mélange zone exposes high- to ultrahigh pressure eclogites and garnet peridotites. The oceanic subduction occurred at 535–486 Ma and developed an arc volcanic rocks and intrusions (Zhu et al., 2014; Shi et al., 2004; Yuan et al., 2002; Li et al., 1999). The ultrahigh pressure metamorphism took place at 460–423 Ma in response to the oceanic and continental subduction processes (Zhang J X et al., 2015, 2010, 2008; Song et al., 2014a, 2006, 2005; Zhang G B et al., 2013, 2009a, b, 2008; Xiong et al., 2012, 2011; Zhang C et al., 2012, 2011; Chen et al., 2009), coevally accompanying intrusion of I-type and S-type granitoids within the mélange zone. Furthermore, adakitic tonalites and granites intruded at 436–415 Ma and ~390 Ma in the Dulan region, eastern portion of the North Qaidam Block (Song et al., 2014b; Wang M J et al., 2014; Yu et al., 2014, 2012). The molasse-like Devonian Maoniushan Formation was also developed within the North Qaidam subduction-collision tectonic mélange zone with unconformably deposited relationship, with zircon U-Pb ages of 430–370 Ma (Kou et al., 2017; Li J B et al., 2017; Hu et al., 2016; Feng et al., 2015; Zhang et al., 2015).
The JSKCPG pluton outcrops in the Jinshuikou area about 30 km south of the Nomuhong Town, circa 160 km east of Golmud City. The pluton intrudes the medium to low-pressure upper amphibolite-facies-granulite-facies Baishahe Formation of the Jinshuikou Group (Fig. 2). The Baishahe Formation was also intruded by other I-type and S-type granitoids at 410–390 Ma (Tian et al., 2016; Liu et al., 2013, 2012) in the post orogenic stage during the Early Paleozoic (Tian et al., 2016) and they were in turn intruded by granitoids in the Mesozoic (Ma et al., 2015; Huang et al., 2014).
The JSKCPG is an irregularly-shaped pluton in the SMQB, which is locally covered by modern deserts along the southern edge of the Qaidam Basin (Fig. 2). Rocks of the JSKCPG generally display stripped and/or gneissic structures (Fig. 3a), and have major minerals of quartz, plagioclase and potassium-feldspar, with subordinate cordierite (Fig. 3b), garnet, biotite, and accessory minerals of zircon, monazite, sillimanite and andalusite etc., and thus are named as garnet-cordierite monzogranite and considered to be strong peraluminous S-type granite (Ba et al., 2012; Yu et al., 2005). The garnets consist of euhedral to subhedral and inclusion-free magmatic type and anhedral inclusion-rich peritectic type (Meng et al., 2018). The granite has εNd(t) values of -9–-13, responding to two-stage model ages of 1.9–2.2 Ga, suggesting Paleoproterozoic source materials for pluton (Ba et al., 2012). Early studies yielded 401±17 Ma (Zhang et al., 2003) and 411±17 Ma (Long et al., 2006) for zircons from the garnet, suggesting its emplacement age. Locally, about 50 cm-wide massive leuco-granitic veins invade the JSKCPG (Fig. 3c) and cut the gneissic foliation of the JSKCPG but exhibit extremely fuzzy boundaries with the JSKCPG, suggesting a short time interval between their emplacement and simultaneous formation of the veins and pluton. These veins are free or rare of ferromagnesian minerals with major minerals of potassium-feldspar, plagioclase and quartz, and thus are named as leuco-monzogranite. The granulite, amphibolite and quartzofeldspathic enclaves inside the JSKCPG range from centimeter to meter sizes, exhibiting rounded to striped shapes (Fig. 3d). The granulite enclaves might be divided into two groups. The first group shows brownish dark in color and is composed of fine-grained hypersthene+plagioclase±hornblende. Some of these granulite enclaves developed centimeter-wide reaction rims consisting coarser-grained association of garnet+hypersthene+ plagioclase, suggesting high-temperature (≥900 ℃) for their precursor magma (see Ba et al., 2012). The other group of granulite-enclaves displays dark green color and comprises of mafic minerals of Opx, hornblende, plagioclase and minor Cpx, some of the mafic minerals usually occurring as porphyroblasts with diablastic texture, suggesting rapid growth due to addition of a high-heat flow. These granulites differ from the 460 Ma sillimanite-bearing pelitic granulites reported by Zhang et al. (2003) in which they are of mafic composition and occur as enclaves. The JSKCPG pluton and its granulite enclaves were once considered to be associated with Achean TTG (tonalite-trondhjemite-granodiorite) gneisses and a member of the Archean Tiantaishan Group, respectively (Liu and Ye, 1998 and references therein). Metamorphic ages for these rocks was previously constrained in the range from 2 000 to 1 600 Ma according to the conventional ID-TIMS U-Pb dating on zircon and Rb-Sr isochron dating on whole-rock samples (Yin et al., 2003; Chen et al., 2002a; Lu et al., 2002; Bureau of Geology and Mineral Resources of Qinghai Province, 1997).2 SAMPLE SELECTION AND ANALYTICAL METHODS
About 1 kg of whole-rock samples was collected for both the JSKCPG and the massive leuco-monzogranite vein (MLGV), and about 10 kg for a granulite enclave. Zircon grains were separated from samples of the JSKCPG, the monzonitic leuco-granite vein and the granulite enclave and monazite grains from sample of the JSKCPG, employing heavy liquid and magnetic techniques and then hand-picked under a binocular microscope, enclosed in epoxy resin and polished to about half their thickness. The grain mounts were photographed in both reflected and transmitted light; the cathodoluminescence (CL) images for zircons and the backscatter electronic (BSE) images were taken at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, and were used to investigate their structure, and to choose points for in-situ U-Pb analyses.
In-situ analysis of Th, U and Pb isotopes for the zircon from both the JSKCPG and the granulite enclave, and for the monazite from the JSKCPG was performed using the SHRIMP Ⅱ at the Beijing SHRIMP Center; analytical procedures followed Williams et al. (2004) and Williams (1998). The standard zircon used, analytical conditions, and interference deductions were the same as those described by Wan et al. (2005). In-situ determination of Th, U and Pb isotopes for zircons from the MLGV was determined by LA-ICP-MS instrument at the State Key Laboratory of Continental Dynamics, Northwestern University, Xi'an, China, with 10 Hz frequency, 80 mJ intensity and 32 μm beam size, and zircon 91500 as external standard. Common Pb was corrected using Anderson's method (Andersen, 2002). Detailed analytical procedure and data processing method are described by Yuan et al. (2004). The age calculation was performed with the ISOPLOT program (Ludwig, 2003).3 RESULTS 3.1 Cordierite Granite (Sample 03JSK-2)
Seventeen analyses were made on 12 zircon grains (Table 1), the concordia plot is shown in Fig. 5a. Zircons from the cordierite granite show idiomorphic crystals with a clear core-rim structure (Fig. 4a). The zircon core occupies a larger volume of the total crystal and shows irregular shape with resolved margins, thus is considered as inherited in origin; the zircon rim displays as a very thin shell enclosing the core, confining the idiomorphic shape, representing a magmatic origin. All the zircon rims have low luminescence and low 232Th/238U (≤0.07), yielding a minimum 206Pb/238U age of 407.3±9.8 Ma. The average of the 5 youngest ages (points 1, 2, 4, 9 and 10) is 412.0+8.0/-5.0 Ma with MSWD=0.1 (Fig. 5a). The inherited zircon cores have high luminescence, 232Th/238U=0.09–1.31, showing two-groups that are concordant or near concordant with ages of ~2.9 Ga (point 17) and ~2.8 Ga (point 12); the other ages vary from ~1.68 to ~0.45 Ga, seemingly defining a poorly constrained discordia with an upper intercept age of about 1.7 Ga (not shown in Fig. 5a).
The monazite grains from the JSKCPG show idiomorphic or hypidiomorphic features with dim zoning in CL images (Fig. 4c).
Fourteen analyses were conducted on 14 monazite grains (Table 2), the concordia plot is shown in Fig. 6a. The monazites have Th=48 395 ppm–102 597 ppm, which is one order or more magnitude higher than U (851.0 ppm–9 899 ppm), 232Th/238U= 7.66–88.3. Their common Pb content is low (< 0.6%), thus the effects on age calculation is negligible. The 207Pb/206Pb ages range from 465±100 to 147±130 Ma with a comparatively large variation and low precision, while the 206Pb/238U and 208Pb/232Th ages range from 402±21 to 328±17 Ma and 404±23 to 330±18 Ma, respectively, with a small variation and higher precision. The weighted average 206Pb/238U and 208Pb/232Th ages are 359±10 (MSWD=1.2) and 371±15 Ma (MSWD=1.4) (Figs. 6b, 6c), respectively, and are identical to each other within error.
Zircons from the granulite enclave show no zoning or nearly structure-free features in their CL images (Fig. 4b). Ten measurements have been made on 10 grains of the zircon separates (Table 1). The results show 232Th/238U=0.16–0.29, the 207Pb/206Pb ages ranging from 506±51 to 223±59 Ma with a comparatively large variation and low precision, however, the 206Pb/238U ages nearly identical. A total 10 measures yielded a weighted average age of 377±10 Ma with an MSWD value of 1.4 (Fig. 5b).3.3 MLGV (Sample 04JSK-1)
Zircons from the MLGV show grain sizes ranging from about 50 to 200 μm or more, and the coarser grains almost all display idiomorphic features and show core-rim or core- mantle-rim structures (Fig. 4d). Most zircon cores have irregular edges and display high- to moderate-luminescence with bright CL imaging and with or without strong oscillatory zoning, suggesting a magmatic- and metamorphic-inherited detrital origin. The zircon rims have regular or idiomorphic shapes and show low-luminescence with dark or dark-gray CL imaging. The zircon mantles show irregular shapes (grain 5 in Fig. 4d) or regular shapes (grain 7 in Fig. 4d), and display moderate- luminescence with gray CL imaging and weak oscillatory zoning, also suggesting an inherited detrital origin.
Twenty five measures were made on 19 grains of the zircon (Table 3). The inherited zircon cores yielded three groups of concordant ages (Fig. 7a). The oldest concordant age group is ~1.60 Ga (points 7 and 22) with 232Th/206U=0.57–0.16. The second old concordant age group is ~1.40 Ga (points 10 and the 17) with 232Th/206U=0.99–0.45. The youngest concordant age group is ~1.0 Ga (points 2 and 12) with 232Th/206U=0.27–0.44. At ranges from 950 to 380 Ma, in addition to point 16 (232Th/206U=0.01) having a concordant age of ~0.47 Ga, all the other points (points 4, 23, 25) yielded a wide range of discordant ages. Two measurements for zircon mantles (points 5 and 8) yielded highly discordant ages. The 12 measurements on the zircon rims are nearly concordant and partly highly discordant in U-Pb ages, however, their 206Pb/238U ages vary in range from 378±3 to 322±3 Ma with a weighted average of 352+18/-20 Ma (96.1% conf.). If we only consider those 5 points (spots 1, 3, 14, 15 and 24) on or nearly on the concordia (conf. of 100±10%), we have got two high concordant ages of 378±3 (spot 15) and 327±3 Ma (spot 3) (Table 3), which are nearly parallel with the two peak ages according to relative probability plot for the 12 measurements (Fig. 7b).
The JSKCPG pluton and its granulite enclaves were once considered to be Archean TTG gneisses and the rock association of the Archean Tiantaishan Group, respectively (Liu and Ye, 1998 and references therein). Metamorphic ages for these rocks were once considered to range from 2 000 to 1 600 Ma according to conventional ID-TIMS U-Pb dating on zircon and Rb-Sr isochron method on whole-rock samples (Yin et al., 2003; Chen et al., 2002a; Lu et al., 2002; Bureau of Geology and Mineral Resources of Qinghai Province, 1997). However, our data indicates that the JSKCPG pluton can not be older than ~470 Ma with regards to the concordant age for the inherited zircon cores (Table 3, point 16). Zhang et al. (2003) obtained an age of 402±6 Ma for the JSKCPG pluton using SHRIMP zircon U-Pb method. Long et al. (2006) reported a zircon SHRIMP U-Pb age of 411±17 Ma for the JSKCPG pluton in a later investigation. Our current study also show a weighted mean age of ~410 Ma for the magmatic zircon rims based on five measurements (Fig. 5a), consistent with that obtained by Long et al. (2006). However, it is not reasonable to consider crystallization of the JSKCPG pluton to have occurred at ~410 Ma. As shown in Fig. 4a, the magmatic zircon rims are too narrow (< 24 μm) to be analyzed by the beam size used during the SHRIMP analysis. Any spot-analysis on such narrow rims can result in mixing of isotopic composition of the real magmatic zircon rims with the Pre-Early Paleozoic zircon cores, leading to overestimation of the magma crystallization age. Accordingly, our intergrated U-Pb data of the zircon from both the granulite enclave and the MLGV, and the U-Th-Pb data of the monazite from the JSKCPG pluton will better constrain the crystallization age for the JSKCPG.
Zircon grains from the granulite enclave have 232Th/238U values lower than 0.4 (0.16–0.29) and show no oscillatory zoning (Fig. 4b), suggesting characteristics of metamorphic origin under high-temperature (granulite-facies) conditions (Belousova, 2002; Vavra et al., 1999). Therefore, the U-Pb dating of these zircons constrains the timing of the low-pressure granulite facies metamorphism that induced growth of the fine-grained granulite-facies mineral assemblage in the enclave at ~380 Ma (377±10 Ma), definitely confining the intrusion and crystallization age of the JSKCPG pluton at ≤~380 Ma. Most zircon grains from the MLGV developed magmatic rims several times wider than the beam size used in the LA-ICPMS experiment, except for spot 15 which shows a clear position on boundary between the detrital core and the magmatic rim (Fig. 4d) and yielded an age of 378±3 Ma, thus no isotope compositional mixing is expected during the measurement and the age of 370–330 Ma might suggest a long interval for crystallization of the MLGV (Fig. 7b). Thus the ~370 Ma should be considered as the timing of the intrusion and earliest crystallization for the MLGV, then crystallization for JSKCPG pluton should be terminated earlier than ~370 Ma according to the cross- cutting relationships (Fig. 3c). Therefore, our comprehensive constraint indicates that emplacement and crystallization of the JSKCPG pluton occurred at ~380–370 Ma.
Some of the monazite grains from the JSKCPG pluton display euhedral crystals (Fig. 4c) and have characteristics of magmatic origin, thus their age information can also be used for constraining the age of the JSKCPG pluton. The U-Pb age of 359±10 Ma and the Th-Pb age of 371±15 Ma for the monazite are identical within error, indicating the timing of the magma intrusion and crystallization. However, the Th-Pb age of ~370 Ma might be considered as more reasonable timing of the near peak-crystallization of the magma because the monazite is enriched in Th with a far higher content than that of U (see Table 2) and can result in more accuracy and precision for isotope determination. This estimation is coincident with the initial crystallization time (~370 Ma, within error) for the MLGV, but reasonably slightly younger than the metamorphic age for the granulite enclaves, probably representing the termination time for crystallization of the JSKCPG pluton.4.2 Thermal Events in the Source Provenances and Their Natures
The source rocks for the JSKCPG magmas were previously considered to be derived from continental crust with a residence time interval of 1.8–2.0 Ga according to Nd isotopic composition (Ba et al., 2012; Yu et al., 2005), and were suggested to have been deposited in the Mesoproterozoic (Ba et al., 2012; Chen et al., 2007a, 2006b). However, little has been known about the nature of the thermal events occurring in the provenance areas or source rocks of the JSKCPG and the MLGV. The age and origin (magmatic or metamorphic origin) of inherited cores of zircon grains can provide further information about the tectonothermal events in the provenance areas for the magmatic source rocks, and can also provide constraints for depositional ages of the source rocks.
The inherited detrital zircon cores of the JSKCPG pluton record 2.8–2.9 and ~1.7 Ga magmatic events (Fig. 5a), however, those of the MLGV record magmatic events at ~1.6 and ~1.4 Ga, and magmatic and metamorphic events at ~1.0 Ga that might correlate with the intrusion of the Early Neoproterozoic S-type granitic gneiss pluton and metamorphism in deep crustal levels (rocks not exposed) in the Qaidam Block (Lu et al., 2008, 2002; Chen et al., 2007a, 2006a, b). The Early Neoproterozoic thermal event was regarded to result from the convergent processes in response to the assembly of the global Rodinia supercontinent (Lu et al., 2008, 2002; Chen et al., 2007a, 2006a). The metamorphic event at ~0.47 Ga is consistent with the granulite-facies metamorphism recorded in the Baisahei Formation of the Jinshuikou Group (e.g., Zhang et al., 2003) (see Tables 1, 2 and Figs. 5a, 7a). Therefore, the magmas of the JSKCPG might derive from a source without materials younger than those for the magmas of the MLGV.4.3 Petrogenesis of the JSKCPG
The cordierite granitic melt was usually generated in low-pressure conditions, i.e., the shallow level of the crust by partial melting of biotite-bearing metamorphosed argillaceous greywacke or a mixture of melt from partial melting of meta-pelites with mafic magma. The cordierite granitic magma temperature is generally considered to be higher than 875 ℃ (Sylvester, 1998), so the melting temperature conditions should be even higher. René et al. (2008) conducted a low-degree melting experiment, showing that melting temperature is higher than 850 ℃ for initial decomposition of biotites with Mg# (=Mg/(Mg+Fe2+))=0.5.
Researches suggested that Al2O3/TiO2 values can provide information about partial melting temperature conditions because biotite and ilmenite minerals will be more easily decomposed than garnet, plagioclase and other aluminosilicate minerals do during temperature gradually increasing, leading more Ti to be dissolved into the melts. Partial melting temperature conditions are usually higher than 875 ℃ when Al2O3/TiO2 value is < 100 but will be or lower than 875 ℃ when Al2O3/TiO2 value is > 100 (Sylvester, 1998). The JSKCPG has whole-rock Al2O3/TiO2 values < 100, suggesting parting melting temperature conditions higher than 875 ℃. Furthermore, the whole-rock values of TFeO+MgO+TiO2 vs. SiO2 of granite can also be useful indicators for partial melting temperature conditions of its protoliths. For paragneiss and biotite-gneiss protoliths, the values of TFeO+MgO+TiO2 and SiO2 show positive and negative correlation trends, respectively to their partial melting conditions. Ba et al. (2012) showed that the JSKCPG has CaO/Na2O values higher than 0.3 (0.49–1.12), suggesting greywacke protolith for the magma generation, and that all whole-rock data plot in a domain near the greywacke trend line showing melting temperature conditions higher than 950 ℃, suggesting very high partial melting temperature conditions for magmas of the JSKCPG.
The fine-grained granulite enclaves reacted with the JSKCPG magma to form the coarser-grained hypersthene+ garnet+plagioclase assemblage rims, supporting magma temperatures higher than 700–850 ℃, which indicates a lower limit for temperature conditions of the lower granulite-facies metamorphism. Development of sillimanite-andalusite minerals and low-pressure granulite enclaves in the JSKCPG indicate partial melting and crystallization of the magma in a low-pressure field, equivalent to a depth < 10 km. In such a shallow level, extra heat is needed because radioactive decay of certain elements in crust level shallower than 50 km could not produce enough heat for partial melting of meta-greywacke (Thompson and Connolly, 1995).
Upwelling of the asthenosphere mantle and intrusion into or underplating of mafic (basaltic) magma beneath the continental crust are the two significant ways for supplying such extra heat (Clemens, 2003; Finger et al., 1997; Pamic et al., 1996; Thompson and Connolly, 1995; Rottura et al., 1991; Wickham et al., 1987). Break-off and delamination of the subduction slab of oceanic crust after continental-continental collision (Davies and von Blanckenburg, 1995; Black and Liegeois, 1993) are main processes that can instigate such mantle upwelling, as well as crustal thinning and mountain collapse in post-orogenic processes are generally invoked to explain such upwelling (Clemens, 2003). When the asthenosphere mantle ascends to continental levels less than 50 km depth, partial melting will occur to generate the basaltic magma that will inject into or underlay beneath (underplating) the thinning crust, or cause partial melting of large voluminous continental crust to produce peraluminous granitic magma (Davies and von Blanckenburg, 1995). An alternative model by Santosh and Kusky (2009) suggested that the heat can be produced before collision, during ridge subduction episodes where a slab window opens and brings hot asthenospheric mantle into contact with hydrous accretionary wedge material, generation of high-temperature peraluminous melts. However, since there are no association of coeval ultra-high temperature and pressure rocks (Santosh et al., 2011; Santosh and Kusky, 2009) in the region, and most importantly, none of a specific narrow belt has identified with coeval plutons crystallized of magmas directly derived from partial melting of the ridge mantle, such as that being discovered in the Central Asia orogenic belt (Geng et al., 2011), the ridge subduction model is not considered here for the JSKCPG, but it can not be completely ruled out.
The JSKCPG pluton in the southern margin of the Qaidam Block contains enclaves of granulite and amphibolite rocks, which might imply a magma activity from the upper-mantle. If this interpretation is reasonable, the tectonic processes undertook in the region in the Middle- to Late-Devonian might be included break-off of the subducted slab or delamination of the lower crustal base, upwelling of the asthenosphere mantle, crustal thinning and partial melting of the upper mantle and intrusion and underplating of the generated basaltic magma and continental-continental collision and mountain building and collapse at ~380 Ma ago. Successive injection and/or underplating of the basaltic magma led to metamorphism of the earlier intruded basaltic dykes or sheets to low-pressure granulite- facies rocks and partial melting of the biotite-bearing meta- graywacke at shallow crustal level, producing strongly peraluminous granitic magma at ~380 Ma. These peraluminous magmas ascended along with entrapment of the broken mafic granulite masses as enclaves, emplaced into the Baishahe Formation of the Jinshuikou Group and crystallized at ~380–370 Ma. Further work should be done in order to better understand the igneous-metamorphic history of the southern margin of the Qaidam Block.4.4 Tectonic Implications
Süess (1893) first introduced "Tethys" to name an envisaged seaway developed from the Alps to Himalaya during the Paleozoic and Mesozoic (c.f., Tozer, 1989). The concept of Tethys is now generally considered to represent the westward narrowing triangular oceanic realm that existed between the Eurasia and Gondwana since the Paleozoic (e.g., Metcalfe, 2006; Stampfli et al., 2000; Hsü et al., 1995; Tozer, 1989; Şengör et al., 1984). The evolution of Tethys involves northward drift of numerous continental fragments detached from Gondwana (e.g., Metcalfe, 2011; Blakey, 2008; Yin and Harrison, 2000; Şengör and Nata'lin, 1996; van der Voo, 1993; Zonenshain et al., 1985), and subsequent collision with the Eurasia, leading to opening, consuming and closing of two major Tethys oceans: Paleo-Tethys and Neo-Tethys (Stampfli et al., 2000; Şengör et al., 1984; Stöcklin, 1984; Şengör and Kidd, 1979). The Paleo-Tethys and Neo-Tethys were separated by an elongate continental sliver (sensu stricto, a collage of continental terranes including Qiangtang and IndoChina blocks, etc.) called "Cimmerian Continent", which rifted from the northern margin of Gondwana during the Late Carboniferous to Late Permian (Metcalfe, 2006; Stampfli and Borel, 2002; Stampfli et al., 2000; Şengör et al., 1984; Şengör and Kidd, 1979). Sandwiched by Paleo- and Neo-Tethys, existence of a relatively small-scale oceanic basin, Meso-Tethys, was also envisaged between the Lhasa and Qiangtang blocks (Gehrels et al., 2011; Metcalfe, 2011, 2006, 1999, 1994). Opening of the Paleo- Tethys was considered to be in the Late-Ordovician to Devonian (e.g., Metcalfe, 2013, 2006, 1996, 1988; Ferrari et al., 2008; von Raumer and Stampfli, 2008; Stampfli et al., 2000), and might progressively reached its maximum extent in the Late Carboniferous to Permian before the opening of Neo- Tethys (e.g., Angiolini et al., 2003; Stampfli and Borel, 2002; Garzanti and Sciunnach, 1997; Stampfli et al., 1991).
Some researches believed that there existed an older ocean before the Paleo-Tethys, named the Proto-Tethys in the North Tibet (Li S Z et al., 2017b, 2016a, b, c, d, e; Yu et al., 2015; Zhang et al., 2015; Zhao et al., 2015; Pan and Fang, 2010; Mattern and Schneider, 2000; Pan and Kong, 1998; Pan et al., 1996; Pan, 1996, 1994) and proposed a three-belt division of the north, middle and south for the Tethys in the Qinghai-Tibet Plateau, pertaining to the Proto-Tethys, Paleo-Tethys and Neo-Tethys in the early time. The Proto-Tethys Ocean is an ancient ocean that developed between Gondwana and Siberia during the Neoproterozoic–Early Paleozoic Period, with numerous continental blocks, such as the Yangtze Block and Tarim Block and micro-continental blocks or masses such as the Qaidam Block, the central Qilian Block and the Qinling Block settling within and being separated by different scale oceanic basins (Li S Z et al., 2017a, b, 2016a). Complex subduction and closure processes of the oceanic basins between each two neighboring micro- blocks during the Early Paleozoic (480–420 Ma) generated a number of high- to ultrahigh pressure metamorphic zones and/or ophiolite mélange belts. The Paleo-Tethys starts from the Keke- xili, Bayanha and Qiangtang ranges in the western portion, going to eastwards into dichotomous branches, with one stretching along the South Kunlun to the South Qinling Mountains, and another one along the Jinshajiang, Ganzi-Litang then turning southerly to the Changning-Menglian belt and Ailaoshan Mountains in the western Yunnan, and further southerly into the Burma and running southeasterly through the Red River into the South China Sea. The Neo-Tethys mainly locates in the Himalayan Mountains and Lhaza Block, and possibly spreading into the Burma and Indonesian region.
Zhang et al. (2002) suggested that the Kunlun (Qilian)- Qinling-Dabie tectonic belts once belonged to a part of the north branch of the Tethys, experienced Proto-Tethys and Paleo-Tethys evolution cycles, and recorded the interaction history between the southern margin of Laurasia and the northern margin of Gondwana (Schwab et al., 2004; Xiao et al., 2003; Yin and Harrison, 2000; Dewey et al., 1988; Molnar et al., 1987; Chang et al., 1982). However, an opposite viewpoint was recently proposed that subduction and collision of the continental fragments of the East Asia occurred southwards to the Gondwana continent during 450–400 Ma (Li et al., 2016b, c, d, e; Yu et al., 2015; Zhang et al., 2015; Zhao et al., 2015). Our geochronological data from both the zircon and the monazite suggest that the JSKCPG pluton, the granulite enclaves and the MLGV were the product of a complex tectonic process in southern margin of the Qaidam Block in the Late-Devonian (Fig. 8). In the Early Paleozoic Period, the Proto-Tethys Ocean in the Kunlun region closed through a prolonged process (Xiao et al., 2003; Mattern and Schneider, 2000; Pan et al., 1996). First, the Kunlun Ocean was consumed through coevally northwards and southwards subduction of the oceanic plate beneath the Qaidam Block and the South Kunlun terrane in the Early Paleozoic, leading to arc-related low-pressure granulite- facies metamorphism of the Baishahe Formation in the southern margin of the Qaidam Block with a thermal peak at ~460 Ma (Zhang et al., 2003) (Fig. 8a). Secondly, the Ordovician volcanic arc basin (zircon U-Pb age 447.9±4.2 Ma for the felsic volcanic rocks, Chen et al., 2002b) closed in the Silurian (Ar-Ar 426.5±3.8 Ma for metamorphic hornblende), causing medium-pressure epidote amphibolite-facies metamorphism and extensive and intensive shear-related deformation (Chen et al., 2002b) and eclogite facies metamorphism in the depth (Qi et al., 2014; Meng et al., 2013b) (Fig. 8b). The subsequent collision and thickening of both the South Kunlun terrane and the North Kunlun terrane of the Qaidam Block led to break-off of the subduction oceanic slab and delamination of the lower crust and upwelling of the asthenosphere, resulting in generation of basaltic magmas by partial melting of Paleoproterozoic subcontinental lithosphere mantle. The hot basaltic magmas emplaced as underplating at the bottom of the thinned lower crustal continent and injection upwards into the brittle fractures as mafic dykes (Fig. 8c). The increasing addition of basaltic magmas into the base of the more-thinned lower crust, provided heat energy high enough to cause low-pressure granulite-facies metamorphism of the early formed mafic dykes and triggered partial melting of the biotite-bearing gneisses from metamorphosed psammites, generating Fe/Mg-rich strong peraluminous granitic magmas which ascended in short distance along with peritectic garnet from the source rocks and some of the low-pressure mafic granulite enclaves and crystallized at ~380 Ma as the JSKCPG which exposes nowadays on the southern margin of the Qaidam Block (Fig. 8d). The low-pressure granulite-facies metamorphism of the granulite enclaves occurred at ~380 Ma, lagging for ~80 Ma from the low- pressure granulite-facies metamorphism of the Baishahe Formation of Jinshuikou Group at ~460 Ma (Zhang et al., 2003) and ~40–30 Ma from the subduction related eclogite metamorphism at ~430–420 Ma (Qi et al., 2014; Meng et al., 2013b), suggesting no relation to this arc low-pressure granulite-facies metamorphism during plate subduction from Ordovician to Silurian. In summary, generation of strong cordierite peraluminous granitic magmas and low-pressure granulite-facies metamorphism should be linked to a mantle-crustal interaction undertook post orogenic process. Our new data for magmatic and low pressure granulite-facies metamorphism at ~380 Ma and the other study on magmatism at 410–390 Ma in the region (Tian et al., 2016; Liu et al., 2013, 2012; Zhang et al., 2003) as well as deposits of the Maoniushan Formation at 420–370 Ma on both the south and north margins of the Qaidam Block (Kou et al., 2017; Li J B et al., 2017; Hu et al., 2016; Liu et al., 2016; Feng et al., 2015; Zhang J X et al., 2015; Lu et al., 2010; Zhang Y L et al., 2010) together suggest a prolonged post collision process after the final closure of the Proto-Tethys Ocean 420 Ma ago; and intrusion of the massive monzonitic leuco-granite veins at circa 370–330 Ma, probably suggests the initiation of the Paleo-Tethys in southern Kunlun, East Asia (Fig. 8d).5 CONCLUSIONS
(1) Zircon and monazite data from our study provide reliable metamorphic ages for the granulite enclaves and emplacement ages for the cordierite peraluminous granite pluton and the massive leuco-granite vein in the Jinshuikou area, southern margin of the Qaidam Block, as well as the nature of the provenance for the source rocks for these granitoid melts. Low-pressure granulite-facies metamorphism of the granulite enclaves occurred at ~380 Ma ago, followed by emplacement and crystallization of the JSKCGP pluton at ~380–370 Ma, and the MLGV intruded and crystallized at 370–330 Ma.
The source rocks for the JSKCPG magma were derived from provenances that underwent ~2.9, ~2.8, and ~1.7 Ga thermal events and deposited in Mesoproterozoic time from ~1.4 to ~1.0 Ga. However, the source rocks for the MLGV magma was different from those for the JSKCPG in that they were derived from a provenance that suffered from ~1.6, ~1.4, ~1.0, and ~0.47 Ga magmatic or metamorphic events and were deposited during a period from ~0.47 to ~0.37 Ga.
(2) The low-pressure granulite-facies metamorphism and peraluminous granitic magmatism in the southern margin of the Qaidam Block occurred after completion of plate subduction and continental collision and orogeny in southern margin of the Qaidam Block; break-off or delamination of the subducted slab and upwelling of asthenosphere mantle along with mountain collapse took place in the Qaidam Block in the Middle- to Late-Devonian Period.
(3) The closure of the Proto-Tethys Ocean might complete 420 Ma ago, a longer post collision proceeded during ~410 to 330 Ma and initiation of the Paleo-Tethys probably started around 330 Ma.ACKNOWLEDGMENTS
This paper is written in celebration of Prof. Zhendong You's 90th birthday anniversary. Two anonymous reviewers are thanked for their critical and constructive comments and suggestions that greatly improve the original manuscript. Professor Fancong Meng is thanked for his friendly providing author(s) his original drawing of Figs. 1 and 2 for our further modification. This study was supported by the National Natural Science Foundation of China (Nos. 40972042, 40772041, 42072030), and the Open Research Program of the Key Laboratory of Continental Dynamics, Northwest University. The BSE images for monazites and CL images for zircons were performed using a Cameca SX-51 by Doctor Ping Xu and senior engineer Yuguang Ma in the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0853-x.
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