2. China University of Geosciences, Beijing 100083, China;
3. Key Laboratory for Sedimentary Basin and Oil and Gas Resources, Ministry of Land and Resources, Chengdu 610081, China;
4. Chengdu Center of China Geological Survey, Chengdu 610081, China
The evolution of the Tethys Ocean in the Qiangtang region has been the focus of controversy in the recent several decades (Ding et al., 2017; Pan et al., 2012; Gehrels et al., 2011; Wang J et al., 2009, 2004; Pullen et al., 2008; Wang C S et al., 2001, 1987; Zhao et al., 2001; Yin et al., 2000; Li et al., 1995; Wang et al., 1987). It is commonly believed that the rifting of central Qiangtang is closely associated with the immense break-up event that occurred along the northeastern margin of the Gondwana supercontinent during the Late Paleozoic (Wang et al., 2018; Zhang and Zhang, 2017; Torsvik and Cocks, 2013; Zhai et al., 2013; Zhang et al., 2013; Zhu et al., 2010; Metcalfe, 2006; Şengör, 1979). A recent survey has shown the presence of large quantities of Permian continental basalts throughout the Qiangtang Basin, which is consistent with the timing of the rifting of Gondwana. It is generally accepted that there occured a failed rift in the central Qiangtang from Early to Late Permian (Niu et al., 2011; Wang J et al., 2009; Duan et al., 2006; Wang Y L et al., 2001; Deng et al., 1996; Pearce and Mei, 1988; Hu et al., 1985; Zhang et al., 1985), since the Permian basalts were first reported in Qiangtang area (Hu et al., 1985; Zhang et al., 1985). Although the view of Permian rift has been put forward for a long time, there is still no complete and accurate spatial and temporal evolution for the Permian rift. However, previous studies focused mainly on the Early Permian basic rocks (Ma et al., 2017; Song et al., 2017; Zhang and Zhang, 2 017; Xu et al., 2016; Zhai et al., 2013; Li L et al., 2009; Li S P et al., 2008; Duan et al., 2006; Li C et al., 2004), the Middle Permian rock association remains unclear.
Our petroleum geology investigations in the Qiangtang Basin during the past two decades, along with high-precision geophysical data, have shown the presence of two large magnetic interfaces in the central Qiangtang, composed of the Middle Permian Lugu Formation and the Late Triassic Nadigangri Formaition (Tan et al., 2016). The Lugu Formation has a suit of volcanic-sedimentary association comprising basalt, siliceous clastics, carbonate and minor clastic rocks, of which the basalt and siliceous clastics may be related to the development of the rift in the central Qiangtang during the Permian. The Lugu Formation has been regarded as the stratigraphic equivalent to other Middle Permian strata on the basis of dating of fossils and rock associations throughout Qiangtang Basin, e.g., the Longge Formation in the west and the Nuoribagaribao Formation in the east of Qiangtang Basin, respectively. The siliceous clastics of the rock association was dated between the Roadian and Capitanian stage to the Middle Permian by the radiolarian (Li et al., 1997). Our field observations show that there are a small amount of detrital zircons within the siliceous clastics that consists of minor volcanic debris and biochemical components, which can define the maximum depositional age and may record the evidence of the tectonic evolution since the siliceous clastics was deposited in central Qiangtang. Moreover, the geochemical characteristics of the coeval basalt can reflect the tectonic setting during the formation of the rock association.
In this paper, we present U-Pb geochronological data of detrital and magmatic zircons from the siliceous clastics and basalt in the Yaqu area of central Qiangtang, as well as geochemical data for the basalt. These new data, combined with the related geological events are used to provide reliable constraints on the formation age of Lugu Formation and to discuss the rift evolution of the central Qiangtang from the Early Permian to Late Permian.1 GEOLOGICAL SETTING AND SAMPLING
The Tibetan Plateau, consists of four main terranes: the Songpan-Ganzi, the Qiangtang, the Lhasa, and the Himalaya, which are separated by the Kekexili-Jinsha suture zone (KJSZ), the Bangong-Nujiang suture zone (BNSZ), and the Indus-Yarlung Zangbo suture zone (IYZSZ), respectively (Fig. 1a, Yin and Harrison, 2000). The Qiangtang Basin is located in the north of the Tibetan Plateau, eastern Tethyan realm, which is a Mesozoic marine sedimentary basin that developed on a Precambrian crystalline basement (Tan et al., 2016, 2009). The north of the basin is connected to the Kekexili-Jinsha suture zone, the south is connected to the Bangong-Nujiang suture zone, and the center is an uplift (Fig. 1b, the west is a physical uplift, while the east is a hidden one). Overall, the basin presents a tectonic framework whereby an uplift is sandwiched between two depressions (Wang and Fu, 2018). Our study area is located in the middle of the central uplift, more than 100 km east of the county of Shuanghu and adjacent to the Xianshui River. Permian strata are intermittently exposed in this area (Fig. 1c), overlain unconformably by the Upper Triassic Xiaochaka Formation which have undergone epimetamorphism through low-temperature hydrothermal transformation. The Lugu Formation is dominated by volcanic rocks, interbedded siliceous clasticss, limestones and sandstones, but the Xiaochaka Formation mainly comprise sandstones. We selected a section with a full set of volcanic-sedimentary sequence from the Lugu Formation on the north bank of the Xianshui River, of which an upper and lower sequence could be clearly distinguished. The lower sequence primarily comprised volcanic rocks containing thin layers of siliceous clastics, and the upper section primarily comprised limestone (Fig. 2). The siliceous clastics samples with grayish-green stripes are dense and hard (Fig. 3b). The basalt samples exhibit porphyritic textures dominated by basic plagioclase and pyroxene. Basic plagioclase is in the shape of thin needles and pyroxene is in the shape of a short cylinder, which form a pilotaxitic texture (Fig. 3e). These minerals are also distributed unevenly throughout the matrix (Fig. 3f).2 ANALYTICAL METHODS
Samples crushing and zircon selection were performed at the Langfang Institute of Regional Geological Survey, Hebei Province. To accurately analyze the zircons' internal structure and determine the dating domains, cathodoluminescence images (CL) were obtained using the electron microprobe at Beijing Geoanalysis Company Limited. LA-ICP-MS zircon U-Pb dating was performed in the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan), using Agilent 7500a ICP-MS and GeoLas 2005 equipped with a 193 nm gas laser. The spot diameter of the beam was 32 μm. For U-Pb isotope dating, 91500 zircon served as the external standard for fractional isotope distillation calibration, wherein two 91500 zircon samples were analyzed for every five sample points. GJ-1 zircon target samples served as blind samples for monitoring and testing the stability. Detailed operating conditions for the laser and the ICP-MS instrument can be found in Liu et al. (2008). ICPMSDataCal 10.2 was used for data analysis and age frequency histograms were obtained using Isoplot 3.75 (Ludwig, 2012; Liu et al., 2010).
The basalt samples were associated with the siliceous clastics. Fresh samples without almonds were collected and grounded to form thin sections. Using an electronic microscope, nine samples were selected for geochemical analysis. First, the weathered surface of the rock samples was removed using a blade, and they were polished until a fresh surface was exposed. Then, they were powdered to 200 mesh in an agate mill to avoid contamination. Geochemical analysis and testing were performed at the Southwest China Supervision and Inspection Center of Mineral Resources, Ministry of Land and Resources. Bulk rock major and trace-element compositions were analyzed by X-ray fluorescence spectrometer (Axios-X) and ICP-MS (Element 2), respectively. The analytical accuracy was generally better than 1% for major elements and within 5% for trace elements. The analysis and test procedures were based on Chen et al. (2010). The geochemical illustrations of the samples, including the major and trace elements, were created using Geokit (Lu, 2004).3 RESULTS 3.1 Zircon U-Pb Age
The zircon U-Pb dating results for the siliceous clastics sample (16R4) and the basalt sample (16R5) are listed in Supplementary Table S1.
Fifty-seven analytical spots from 57 zircon grains were chosen from the siliceous clastics samples for U-Pb dating. These grains generally range in size from 50 to 150 μm with length/width ratios of 1 : 1 to 3 : 1. Most of the analyzed zircons exhibit typically oscillatory zoning in CL images, although some are rounded-to-subrounded in shape (Fig. 4). Therefore, the zircon grains are categorized into two types based on their appearances and internal textures. The first type is rounded and significantly worn, suggesting that the detrital zircons may have migrated long distances from a broad source area. The second type is angular with complete crystalline texture suggesting that these grains derive from near igneous rock sources instead of sediments from distant sources.
The Th and U contents in the zircons are 46 ppm–3 567 ppm and 85 ppm–3 344 ppm, respectively. The Th/U values range between 0.3 and 2.1. The 206Pb/238U (<1 000 Ma) and 207Pb/206Pb (>1 000 Ma) ages of the siliceous clastics range between 257 and 2 039 Ma, but the majority fall in clusters of 257–270 Ma. The minimum cluster of 19 concordant grains yielded a weighted mean 206Pb/238U age of 265.4±2.6 Ma (MSWD = 2.7) (Fig. 5a). In addition, there are 3 subpopulations, showing 3 major peaks at ca. 450,700–800 and 1 800 Ma, as well as one significantly older age of 2 039 Ma (Fig. 5a).
The basalt samples contain fewer zircons, but the CL images display clear oscillatory zones (Fig. 4). All analyzed ages range from 263 to 1 790 Ma, and 6 youngest grains give a weighted mean 206Pb/238U age of 266.6±2.8 Ma (MSWD = 0.55) (Fig. 5b), which can be interpreted as the eruption age of the basalt sample.3.2 Geochemistry
The analytical results and relevant parameters of the major and trace contents of the nine basalt samples are listed in Supplementary Table S2. SiO2 varies between 47.8 wt.% and 55.63 wt.%, with an average of 51.38 wt.%. TFeO varies between 6.73 wt.% and 9.04 wt.%, with an average of 7.38 wt.%. Na2O varies between 2.92 wt.% and 5.38 wt.%, with an average of 3.91 wt.%. K2O varies between 0.28 wt.% and 1.06 wt.%, with an average of 0.65%. TiO2 varies between 1.5 wt.% and 2.94 wt.%, with an average of 1.93 wt.%. Mg# values (molar×100 MgO)/(MgO+TFeO) varies between 52 and 69, with an average of 56. Most of the samples are distributed in the basalt field on the total alkali-silica diagram, with one sample plotted in the basaltic trachy-andesite field (Fig. 6a). In the Zr/TiO2 vs. Nb/Y diagram, all the samples were distributed in the sub-alkaline basalt field (Fig. 6b).
The total rare-earth element content of the basalt is between 63 ppm and 199 ppm, with an average of 104 ppm. Consequently, all of the analyzed samples are strongly enriched in light rare-earth element (LREE) relative to heavy rare-earth element with no Eu anomalies (0.92–1.02, Fig. 6c). In primitive mantle-normalized trace-element spidergram (Fig. 6d), the basalts display enrichments in high-field-strength elements such as Th, Ta, and Hf, but show relative depletion in large ion lithophile elements such as Ba, K, and Sr, as well as slight depletion in Nb, which might be the result of contamination by continental crust. Moreover, the significant negative anomaly of P may be caused by the crystallization separation of apatite.4 DISCUSSION 4.1 Constraints on the Formation Age of the Lugu Formation
The frequency distribution histograms of detrital grains of the siliceous clastics sample display 5 clusters, including 5 peak ages at ca. 267 Ma (257–270 Ma, n = 19), ca. 442 Ma (422–476 Ma, n = 7), ca. 724–815 Ma (724–917 Ma, n = 10), ca. 1 725 Ma (1 693–1 802 Ma, n = 9), and 2 039 Ma (n = 1), respectively (Fig. 5b). The predominant population of 19 concordant grains give a weighted mean 206Pb/238U age of 265.4±2.6 Ma (n = 19, MSWD = 2.7), which can be interpreted as the maximum depositional age of the siliceous clastics sample. Furthermore, the crystallization age of the basalt in this paper is 266.6±2.8 Ma (n = 6, MSWD = 0.55), which is consistent with 267±1.7 Ma (n = 22, MSWD = 0.18) obtained from the QZ-5 well drilled in the Jiaomuchaka area (unpublished data). According to the latest international chronostratigraphic chart, these ages are all in the Middle Permian. As mentioned above, there are a small amount of radiolarian fossils in the siliceous clastics, including Latentistula sp., Pseudoalbaillella longtangensis Sheng and Wang, Pseudoalbaillella fusiformis, Pseudoalbaillella sakmarensis Kozur, and Pseudoalbaillella sp. The fossils were dated between Kungurian and Capitanian stage during the Middle Permian (CIGMR, 2012). In summary, all data indicate that the rock association in the study area formed during the Middle Permian, as well as the Lugu Formation.
The age of ca. 1 800 Ma was consistent with the primary diagenesis stage of the crystalline basement in the Qiangtang tarrane (Tan et al., 2009; Deng et al., 2007; Wang et al., 2001). The single zircon age of 2 039 Ma was estimated to be the original rock of the Qiangtang metamorphic basement (Tan et al., 2009). The age cluster of ca. 700–800 Ma were associated with the rifting of the Rodinia supercontinent during the Neoproterozoic (Du et al., 2013). The age of ca. 450 Ma could be associated with the opening of the Paleo-Tethys Ocean (Wang et al., 2008). The minimum peak of ca. 266 Ma in this paper provides sufficient evidence that the rift is still proceeding during the Middle Permian.4.2 Tectonic Implications
The discussion presented in this paper is based on the ratios and diagrams of immobile elements (e.g., Th, Nb, Ta, and Zr) to eliminate the significant effects of the later alterations on mobile elements (e.g., Rb, Ba, Sr, and K). The trace-element compositions of most samples are different from those of typical OIB in terms of enrichments of LREEs and Ti-Nb-Ta relative to similarly incompatible elements, as well as lacking Eu anomalies (Figs. 6c, 6d). In addition, the slight depletion of Nb elements and the relatively low Nb/U ratio (12–17) suggest that a little contamination by continental crust occurred during magma evolution (Zindler and Hart, 1986). The continental basalt and island-arc basalt can be effectively distinguished by examining Zr content and Zr/Y ratios (Pearce and Norry, 1979). The basalt collected on the north bank of the Xianshui River exhibit a higher Zr/Y ratio (48–59), and all the samples are distributed in the within-plate basalt (Fig. 6e). Only slight changes in the Th/Ta and Ta/Hf ratios occurred in the mantle partial melting and in crystallization differentiation processes. Therefore, the variations on the Th/Hf vs. Ta/Hf diagram reflect differences in source composition (Wang et al., 2001). Based on the Th/Hf vs. Ta/Hf discrimination diagram (Fig. 6f), all the samples are plotted in the continental rift basalt, which are also similar to the Early Permian basic rocks in the Qiangtang Basin (Fig. 1).
In addition, the geochemical characteristics (Mn/Ti < 0.5) of the siliceous clastics suggest that the siliceous clastics formed in a semi-deep-sea to land sedimentary environment (CIGMR, 2012; Niu et al., 2011; Murray, 1994). Combined with the results of the detrital zircons in the siliceous clastics, we propose that the Qiangtang terrane was separated into the southern and northern Qiangtang by a continental rift from the Early to Middle Permian. Also, there were a mass of volcanic activities to generate the bimodal volcanics with the proceeding of the rift (Li et al., 2012). As the volcanic activities stopped, thin layers of siliceous clastics were deposited in the semi-deep-sea sedimentary environment of the rift. Consequently, the volcanic debris with magmatic zircons deposited into the siliceous clastics, as well as the southern Qiangtang and northern Qiangtang provided terrigenous detrital sediments with detrital zircons. Therefore, the basalt collected on the north bank of the Xianshui River is the continental basalt formed during the rifting process.4.3 Evolution of the Permian Rift
In the past 20 years, the Qinghai-Tibetan oil and gas project team have proved that the Qiangtang Basin has a Precambrian basement through systemic studies including geological survey, petroleum survey, geophysical data and physical property data (Fu et al., 2016; Tan et al., 2016; Wang et al., 2009). The relative continuity of the sedimentary successions from Ordovician to Permian suggests the stability of the passive continental marginal deposition, implying that the Qiangtang terrane could be a part of the northeast margin of Gondwana during the Early Paleozoic (Wang et al., 2004; Zhao et al., 2001).
Some authors have suggested that the Qiangtang Block is comprised of separate northern Qiangtang has Cathaysian or Laurasian affinities and southern Qiangtang rifted from Gondwana delineated by the Longmucuo-Shuanghu suture (Wu et al., 2017; Li et al., 2016; Zhang et al., 2013). The Qiangtang metamorphic belt currently exposed in an intracontinental setting in the central Qiangtang Block, which is an Early Mesozoic mélange subhorizontally underthrusted southward from Jinsha suture that was exhumed within the central Qiangtang (Pullen et al., 2014, 2011, 2008; Kapp et al., 2003, 2000). In addition, recent results documenting paleomagnetic paleoaltitude, diamicites and detrital zircon age provenance imply that northern and southern Qiangtang both has a Gondwana affinity and the central Qiangtang metamorphic belt is not an in-situ Paleo-Tethys suture (Ding et al., 2017; Song et al., 2017). Therefore, the central Qiangtang should be an intra-continental rift in the Permian, which can give a more explanation for the Permian evolution of central Qiangtang.
During the Early Permian, Gondwana disintegrated into the Qiangtang terrane and others by a mantle plume related to a ca. 290 Ma large igneous province when some terranes were connected by within-plate rifts or limited oceans, while others were separated by the deep ocean with oceanic crust (Zhang and Zhang, 2017; Liu et al., 2016; Stojanovic et al., 2016; Xu et al., 2016; Liao et al., 2015; Duan et al., 2006; Wang et al., 1987). The presence of a sequence of coeval continental basalts confirms that there was a continental rift in the central Qiangtang. The general absence of ophiolite in the central Qiangtang also proves that the continental rift did not continue to develop a complete oceanic crust. However, it does not preclude the possibility that there was an ocean basin in part (Zhang et al., 2016). Petrology and lithogeochemistry demonstrate that the rift crust has the transitional properties (Liu et al., 2002; Wang et al., 1987), indicating that the rift was a relatively large scale. The crust beneath the rift was extremely thin that caused an upwelling of magma from the asthenosphere contaminated by few crustal materials when it rose upward. Subsequently, a series of volcanic-sedimentary sequences, including volcanic rocks, limestone and sandstone were deposited in the rift between the northern and the southern Qiangtang. We assume that the central Qiangtang probably initiated the rift in ca. 290 Ma (Fig. 7a), and developed to the peak during the Middle Permian (ca. 266 Ma). These continental fragments rifted and drifted northwards from Gondwana to Eurasia since the Late Paleozoic, opening the Meso-Tethys oceans behind them and closing the paleo-Tethys on their approach (Metcalfe, 2011; Yin and Harrison, 2000).
However, when and how the rift was closed remain unclear. In recent years, we have found a great deal of available evidence that the rift had been closed in the Late Permian described in another paper (Li et al., 2018). Firstly, the Nayixiong Formation of the Late Permian is composed of a suite of molasse formation, marking the end of amalgamation between the northern and southern Qiangtang. The conglomerate at the bottom of the Nayixiong Formation is generally more than 100 m in thickness, implying a significantly angle unconformity between the Late and Middle Permian. In addition, the volcanic rocks of the Nayixiong Formation also show geochemical characteristics related to subduction (Li et al., 2018; Yang et al., 2011; Zhang et al., 2010; Bai et al., 2004) and the Late Permian strata are marine and continental alternative depositions in extension setting after continental collision. Secondly, our field investigations have shown the presence of widespread paleo-weathering crust at the top of the Longge and Lugu formations (Fu et al., 2009; Wang et al., 2007). It indicates that most parts of the South Qiangtang and the central uplift regions were uplifted into a land and subjected to the weathering and erosion after the amalgamation between the northern and southern Qiangtang in the Late Permian. Thirdly, owing to the absence of Early and Middle Triassic outcrops in Qiangtang Basin, we can not better understand the geological evolution of this period. Recently, we have identified a large number of Middle Triassic volcanic rocks related to subduction along BNSZ, central uplift and JSSZ, suggesting that closure of the rift and uplift of the southern Qiangtang is a continuous process related to the initiation of northward subduction of the Bangong-Nujiang Tethyan Ocean (Chen et al., 2016a, b). All the evidence shows that the rift had ceased to develop or been closed in central Qiangtang during the Late Permian (Fig. 7b).
As mentioned, it was a long and complex rifting process along the northeast margin of Gondwana during the Early to Middle Permian, which caused the central Qiangtang to create the initial rift or limited ocean between the northern and southern Qiangtang. There were no longer large-scale rifts or ocean basins in the Qiangtang region since the Late Permian. The Tethys Ocean mainly existed in the south of the Qiangtang terrane (Fig. 7).5 CONCLUSION
(1) The siliceous clastics and basalt collected in the central Qiangtang, yielded a maximum depositional age of 265.4±2.6 Ma (n = 19, MSWD = 2.7) and a weighted mean 206Pb/238U age of 266.6±2.8 Ma (n = 6, MSWD = 0.55), respectively. The Lugu Formation was formed in the Middle Permian. The basalts display the geochemical features of within-plate basalt, which are similar to the continental basalt. However, the geochemical characteristics of whole-rock that relatively enrich Ta and slightly lack Nb suggest that the basalts originated from partial melting of asthenosphere mantle and were contaminated by continental crust.
(2) The Permian rift in the central Qiangtang had undergone a complete cycle: birth, life and death. The rift opened in the Early Permian, fully developed in the Middle Permian, and closed in the Late Permian.ACKNOWLEDGMENTS
We appreciate Professor Zhaochu Hu for helping with LA-ICP-MS U-Pb analyses. We would also like to express our appreciation to all members of the project team from the Chengdu Center of CGS, for their support and assistance during the fieldwork stage of the study. This study was jointly funded by the National Natural Science Foundation of China (Nos. 41502112, 41702119) and two programs under China Geological Survey (Nos. 1212011221114, DD20160159). The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0954-y.
Electronic Supplementary Materials: Supplementary materi-als (Tables S1, S2) are available in the online version of this article at https://doi.org/10.1007/s12583-017-0954-y.
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