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Volume 30 Issue 5
Oct.  2019
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Late Triassic-Cenozoic Thermochronology in the Southern Sanjiang Tethys, SW China, New Insights from Zircon Fission Track Analysis

  • Corresponding author: Wanming Yuan, ywm010@sina.com
  • Received Date: 2018-12-08
  • The Sanjiang Tethys orogenic belt is located in the southeast side of the Qinghai-Tibet Plateau. It has undergone the opening and closing movements in different periods of Tethys oceans, complex accretive orogeny and strong mineralization from Paleozoic to Mesozoic. Using zircon fission track (ZFT) thermochronology, this study reveals the Sanjiang Tethys has experienced multi-stage tectonic activities during the Late Triassic-Cenozoic. The 15 ZFT ages with their decomposition components obtained from Sanjiang Tethysian region range from 212 to 19 Ma, which not only shows 6 age groups of 212, 179-172, 156-133, 121-96, 84-70 and 50-19 Ma, but also constrains the age limit of the tectonothermal events. These age groups recorded the Paleo-Tethys main and branches ocean opening/closure time. The age-elevation plot indicates the Sanjiang region had differential uplifting and exhumation and fast uplifting times of ca. 133, 116 and 80 Ma, coinciding with the age groups mentioned above. These results show new geochronological evidences and viewpoints.
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Late Triassic-Cenozoic Thermochronology in the Southern Sanjiang Tethys, SW China, New Insights from Zircon Fission Track Analysis

    Corresponding author: Wanming Yuan, ywm010@sina.com
  • Science Research Institute, China University of Geosciences, Beijing 100083, China

Abstract: The Sanjiang Tethys orogenic belt is located in the southeast side of the Qinghai-Tibet Plateau. It has undergone the opening and closing movements in different periods of Tethys oceans, complex accretive orogeny and strong mineralization from Paleozoic to Mesozoic. Using zircon fission track (ZFT) thermochronology, this study reveals the Sanjiang Tethys has experienced multi-stage tectonic activities during the Late Triassic-Cenozoic. The 15 ZFT ages with their decomposition components obtained from Sanjiang Tethysian region range from 212 to 19 Ma, which not only shows 6 age groups of 212, 179-172, 156-133, 121-96, 84-70 and 50-19 Ma, but also constrains the age limit of the tectonothermal events. These age groups recorded the Paleo-Tethys main and branches ocean opening/closure time. The age-elevation plot indicates the Sanjiang region had differential uplifting and exhumation and fast uplifting times of ca. 133, 116 and 80 Ma, coinciding with the age groups mentioned above. These results show new geochronological evidences and viewpoints.

0.   INTRODUCTION
1.   GEOLOGICAL SETTING
  • The Sanjiang Paleo-Tethys orogen, as one of the important branches of the eastern Tethyan tectonic belt, preserves the evolutionary history of the Tethyan belt in Southeast Asia. The main units of the Paleo-Tethys in the Sanjiang region include the Changning-Menglian main ocean, and the Jinshajiang, Ailaoshan and Garzê-Litang branch oceans (Deng et al., 2014).

    The study area is located in the southern Sanjiang Tethys belonging to southeastern segment of the eastern Paleo-Tethys tectonic domain and the eastern margin of the Tibet Plateau in southwestern China, with latitude 28°40'N-32°40'N and longitude 96°E-100°E. It is run over by the Jinshajiang-Ailaoshan, Lancangjiang, and Nujiang tectonic belts. This region is the collage of Paleozoic arc terranes and Gondwana-derived micro- continental blocks (Mo et al., 1994). The Sanjiang orogenic belt varies in orientation from the WNW-ENE trending Himalayan-Tethyan segment of the belt to the N-S trending southeastern Asian segment (Zhu et al., 2016; Deng et al., 2014). The southern part of Sanjiang region consists of the Simao-Indochina Block and Sibumasu major continental block. In the northern part of the Sanjiang region are the Songpan- Garzê fold belt, East Qiangtang, West Qiangtang and Lhasa three micro-continental blocks (Fig. 1). The Study area consists of Paleozoic meta-sedimentary rocks, which were deformed and metamorphosed during the Early Triassic collision between the Qiangtang Block and the Yidun arc along the Jinshajiang suture zone, the rocks are overlain by Triassic clastic and intercalated volcanic rocks of the Qugasi and Tumugou formations (Deng et al., 2014).

    Figure 1.  The regional geographical location of Sanjiang region.

    The southern parts of Sanjiang region are separated by the Ailaoshan and Changning-Menglian sutures formed by closure of the Tethys oceans. The northern part is separated by the Longmu Tso-Shuanghu, the Jinshajiang and the Bangong- Nujiang sutures formed by closure of the Tethys oceans, respectively (Deng et al., 2014; Metcalfe, 2013; Zhu et al., 2013).

2.   SAMPLES AND METHODS
  • Fifteen rock samples were collected from the Yidun arc, and the location information of the samples, including altitude, latitude and longitude coordinates, was obtained by the portable GPS (see Table 1). Their altitude is from 1 564 to 2 977 m (Fig. 2).

    Sample Longitude/latitude Elevation (m) Lithology No. of grains (n) ρs (Ns) (105/cm2) ρi (Ni) (105/cm2) ρd (Nd) (105/cm2) P(χ2) (%) Central age (±1σ) (Ma)
    Y40 27°04.399′N/ 99°20.931′E 2 492 Killas/Pt3J 36 74.3 (4 689) 105.311 (6 646) 12.564 (5 958) 3.5 40±2
    Y42 26°51.882′N/ 99°23.937′E 2 102 killas/Pt3J 36 139.41 (3 796) 59.385 (1 617) 12.956 (5 958) 0 137±8
    Y43 26°48.516′N/ 99°25.060′E 2 229 Schist/Pt3J 36 107.942 (7 714) 54.335 (3 883) 13.348 (5 958) 0 120±6
    Y44 26°26.432′N/ 99°23.432′E 2 365 Schist/Pt3J 36 103.655 (6 267) 49.045 (2 965) 13.739 (5 958) 0 137±9
    Y46 26°26.927′N/ 99°20.521′E 2 765 Schist/Pt3J 36 93.364 (5 993) 40.879 (2 624) 14.131 (5 958) 0 146±9
    Y47 26°27.066′N/ 99°19.204′E 2 977 Killas/Pt3J 36 87.762 (4 348) 39.4 (1 952) 12.662 (5 958) 0 128±7
    Y48 26°28.401′N/ 99°17.932′E 2 735 Killas/Pt3J 35 91.382 (5 758) 38.121 (2 402) 10.606 (5 958) 0 120±7
    Y49 26°28.710′N/ 99°16.449′E 2 543 Killas/Pt3J 36 114.838 (6 728) 62.147 (3 641) 10.998 (5 958) 0 91±6
    Y50 26°30.129′N/ 99°15.135′E 2 077 Killas/Pt3J 36 81.475 (5 334) 41.272 (2 702) 11.39 (5 958) 0 100±7
    Y51 26°30.519′N/ 99°13.960′E 1 919 Schist/Pt3J 36 123.229 (4 058) 63.953 (2 106) 11.781 (5 958) 0 101±7
    Y52 26°29.205′N/ 99°12.591′E 1 765 Schist/Pt3J 36 123.168 (5 751) 67.784 (3 165) 12.173 (5 958) 0 100±5
    Y54 26°26.987′N/ 99°08.605′E 1 564 Schist/Pt3J 36 119.135 (4 052) 74.18 (2 523) 12.564 1 (5 958) 0 95±6
    Y56 25°54.557′N/ 99°08.721′E 1 942 Schist/Pt3J 15 55.161 (1 117) 73.532 (1 489) 12.858 (5 958) 0.2 45±3
    Y57 25°53.855′N/ 99°07.388′E 2 363 Schist/Pt3J 36 55.245 (4 540) 143.346 (11 780) 13.152 (5 958) 0 23±1
    Y58 25°53.061′N/ 99°06.157′E 2 603 Schist/Pt3J 36 82.531 (4 170) 182.381 (9 215) 13.641 (5 958) 1.0 28±1
    Note: ρs, ρi and ρd are fossil track density, induced track density and dosimeter track density respectively. Ns, Ni and Nd are fossil track, induced track and dosimeter track. P(χ2) is probability (Galbraith, 1981).

    Table 1.  Geographic position and observed results of ZFT and relevant calculated data of the samples

    Figure 2.  (a) Location of the study area in the Sanjiang area modified after Deng (2014). Blue rectangle in the inset map marks the study area and outline of the geological map (b) Locations of ZFT samples from this study in the Sanjiang area.

    Bulk rock samples were first crushed and pulverized with a disk mill. Zircons were separated from the washed and sieved pulverized samples using standard heavy-liquid and magnetic separation techniques, then grains were hand-picked under a binocular microscope at the Langfang Laboratory.

    The ZFT analyses were performed at the Institute of high energy physics Chinese Academy of Sciences (Beijing). The zircon grains were mounted in glass slides, heated and covered by fluorinated ethylene propylene teflon sheets. Their external prismatic surfaces were ground and polished. Etching duration was about 20-35 h with NaOH/KOH (=1 : 1) eutectic etchant at 210 ℃ (Yuan et al., 2007). Uranium-free mica sheet as external detectors were packed together with sample grain mounts and CN2 uranium dosimeter glass irradiated in the well thermalized (Cd for Au > 100) hot-neutron nuclear reactor (Yuan et al., 2009). The induced fission tracks were recorded in a muscovite detector during the irradiation, after irradiation, the muscovite external detectors were detached and etched in 40% HF for 20 min at 25 ℃ to reveal the induced fission tracks. Fission track densities for both natural and induced fission track populations were manually measured 100× dry objectives and 10× eyepieces under the AUTOSCAN. Fission track ages (FTAs) were calculated using the IUGS-recommended zeta calibration approach, age errors are quoted at the 1σ confidence level and were derived by the conventional method (Green et al., 1986). The zeta values (ξ) used in this study have been determined from repeated measurements of standard apatites (Hurford, 1990; Hurford and Green, 1983). The weighted mean zeta value obtained in this paper is 90.9±2.8 a/cm2 for zircon.

3.   ZFT RESULTS
  • Fifteen samples from Yidun arc of the Sanjiang Tethys yielded ZFT ages ranging from 19±1 to 212±13 Ma (Table 1). The χ2-test was used to quantify age homogeneity; 15 ZFT ages all failed χ2 test, ranging from 0 to 3.5%, implying the presence of multiple age populations for each sample (Brandon, 1996). For samples that failed the P(χ2) test, the dataset was decomposed into the best-fitting grain-age components using RadialPlotter, labelled as peak 1, 2 (in Ma with uncertainty at 1σ level) (Vermeesch, 2009). The deriving ages of the relative samples are given in Figs. 3 and 4.

    Figure 3.  Y40-Y50 ZFT dating results are shown as radial plots with ages for each grain population using the RadialPlotter program (Vermeesch, 2009).

    Figure 4.  Y51-Y58 ZFT dating results are shown as radial plots with ages for each grain population using the RadialPlotter program (Vermeesch, 2009).

4.   DISCUSSION
  • These fifteen single-grain ages which show wider distribution suggest that the samples experienced a complicated thermal event after deposition. Due to the variable retentivity of zircons, ZFT results commonly consist of multiple grain-age populations with the younger populations comprising low-retentive zircons with a high U+Th content and older populations comprising high-retentive zircons with a low U+Th content (Figs. 3 and 4) (Galbraith, 1981).

    It is shown in a diagram between the sample ages and the distances from the Jinshajiang fault zone (Fig. 5) that the samples 40, 42 and 43 distribute at lower left and the other samples at upper right, indicating that the two groups of samples have the significant negative correlation separately in Fig. 5. Combined with the sample locations, we consider that the samples 40, 42 and 43 are mainly controlled by Jinshajiang suture zone, and the other samples are closely related to Shuanghu- Menglian suture zone.

    Figure 5.  The relation of age and increasing distance from the fault.

    The ZFT thermochronology studies show a wide distribution of the age data that can be divided into 6 age groups of 212, 179-172, 156-133, 121-96, 84-70 and 50-19 Ma (Fig. 6). These ZFT ages are younger than the protoliths and all dated samples are inferred at temperatures higher than 250 ℃ caused by different tectonothermal events before or during Late Triassic. Thus, our new ZFT ages provide evidences of Sanjiang Tethys tectonic evolution and propose a new model for the tectonic evolution of the Sanjiang region.

    Figure 6.  Histogram of zircon fission track age.

    We established the age-elevation relationship and found that an apparent correlation between elevation and zircon fission track ages from the Sanjiang area, zircon apparent ages are plotted against elevation in Fig. 6.

    It is shown that the ZFT ages decrease from ca. 137-120 Ma in the northeast to 45-23 Ma in the southwest (Fig. 2b), which are closely related to the Tethysian evolution because the Tethysian oceans closed from the northeast to southwest. The earlier closing the ocean, the earlier uplifting the block and thus the higher age the sample. Meanwhile, the sample ages were controlled by active fault zones. The sample in the fault zone has lower age and the age becomes higher with increasing distance from the fault generally (Fig. 5). The reason is that the active fault was a thermal resource so that the sample could be annealed and the resetting age became lower.

    The age-elevation plot obtained (Fig. 7) shows 3 development lines although the right one is less controlled by insufficient points that provides much information for the tectonic and uplifting events. Same district has 3 development lines in the age-elevation plot, indicating differential uplifting and exhumation that could be related to the samples from different blocks and fault zones. Each vertical line reflects a fast uplifting time, corresponding to ca. 133, 116 and 80 Ma respectively and coinciding with the age groups mentioned above. The upper oblique lines represent the exhumation time with a slow rate of ca. 0.01 mm/a.

    Figure 7.  Plot of zircon fission track ages versus elevation. This plot suggests that the Sanjiang region was vertically exhumed as different blocks with a slow rate of ca. 0.01 mm/a.

    It was suggested that the episodic evolution and stages of the tectonic movement in the Sanjiang Tethys are the far-field responses to a series of Gondwana-derived continental blocks, amalgamation and continental collision subduction from Paleozoic to Mesozoic (Deng et al., 2014). Meanwhile, researchers are still debating over the time of the tectonic evolution. The age of the Late Triassic (212 Ma) is consistent with the timing of the formation of Cu deposits and emplacement of voluminous porphyritic intrusions within the southern Yidun arc (Chen et al., 2017); Yang et al. (2018) reported zircon U-Pb ages indicate that two-stage magmatism and related mineralization, i.e., Late Triassic porphyries were emplaced at ~225 and ~215 Ma respectively. Late Triassic Yidun arc-type volcanic rocks are interpreted to have resulted from Garzê-Litang oceanic crust westwards subduction (Hou et al., 1993). Some volumes of igneous rock dates have been reported, and the huge granitic batholiths were probably formed under an extension environment, and zircon U-Pb dating results indicate there is a dominant peak in magmatism at ~216 Ma, i.e., Ganluogou dioritic complex ages are Late Triassic (230-213 Ma) that was associated with the closure of the Garzê-Litang Ocean (Wu et al., 2016). A number of the porphyritic rocks and associated volcanic rocks probably resulted from the same tectonic and magmatic event. In a word, we conclude that the final closure of the Garzê-Litang segment of the Paleo-Tethys Ocean occurred prior to 212 Ma. The age group of 212 Ma most probably records cooling event during the Late Triassic, which reflects the tectonic activity of the final stages of the Garzê-Litang closure.

    The 179-172 Ma episode, being recorded by the Amdoophiolite Gabbro and plagiogranite with ages of 188-164 Ma (Wang et al., 2016; Sun et al., 2011), reflects the formation of the Bangonghu-Nujiang Ocean. Some researchers have suggested that the Bangong-Nujiang Tethyan Ocean expanded and developed through the Jurassic (Zhu et al., 2013). Others have suggested that Bangong-Nujiang Tethyan Ocean was subducted northwards under the South Qiangtang subterrane during the Middle Jurassic on the basis of 150-185 Ma I-type intrusive rocks along the southern margin of the South Qiangtang subterrane (Liu et al., 2017; Li et al., 2014). According to the widespread Early-Middle Jurassic ophiolites and the coeval magmatism in the Bangong-Nujiang suture zone, we conclude that Bangong-Nujiang Tethyan Ocean was an intra-ocean arc- backarc basin complex beginning no later than the Early Jurassic. It is possible that the ZFT age groups reflecting additional tectonic thermal events times were confirmed using zircon FT methods although further research should be done in the future.

    The 156-133 Ma was closing stage of the Bangonghu- Nujiang Ocean and forming process of the Yarlung-Zangbo Ocean. Palaeo-Tethyan Ocean closure was followed by the opening of the Meso-Tethyan Ocean during the Jurassic, marked by the Banhong-Nujiang suture in Tibet and recorded by the 156-133 Ma FT age. Shi et al. (2009) reported the occurrence of Mesozoic intra-oceanic subduction, as recorded in the Indo-Burman Range at ~147 Ma and the coeval formation of the Myanmar jadeitite, as well as the age of serpentinization/ rodingitization (163 Ma) in the host rock. When the Bangonghu-Nujiang Ocean closed at 156-133 Ma, the Yarlung- Zangbo Block separated from Gondwana. After the Yarlung- Zangbo Ocean was formed, the Yarlung-Zangbo oceanic crust subsequently started to subduct during the Early Cretaceous. Felsic magmatic rocks related to this subduction occur as dacitic volcanic-subvolcanic rocks (145-125 Ma) and are associated with Pb-Zn-Ag mineralization in the Dexing District.

    The age group of 121-96 Ma represents the subduction of Yarlung-Zangbo oceanic crust and closing of the Yarlung- Zangbo Ocean. As a result of the convergence between the Lhasa Block and the Qiangtang Terrane during the Cretaceous, the Yarlung-Zangbo Ocean closed, and the associated magmatic arcs were accreted to the southern margin of the Eurasian Continent. The 121-96 Ma ZFT age interval is consistent with ca. 110 Ma volcanic rocks in the Gangdese (Chen, 2014; Zhu et al., 2013). The closure of the Yarlung-Zangbo Ocean marks the beginning of the collision between the Indian Continent and the Lhasa Block of Eurasia, signifying the final stage of the evolution of the Tethys.

    The 84-70 Ma tectonic events are the start of post- orogenic intracontinental extension. The closure of the Yarlung- Zangbo Ocean, the subduction of the Indian Plate under the Eurasian Plate, and the obstruction/interference of the eastern Yangtze Block together led to the uplift of the Tibetan Plateau, with consequently squeezing and shortening of the Sanjiang regional crust. At the same time, magmatism and associated mineralization also occurred (Hou et al., 2006). The Cretaceous A-type granites and skarn-type Sn deposits in the central and southern parts of the Yidun arc indicate an extensional setting. The granites and associated mineralization that developed in the Late Yanshanian (Cretaceous) post-orogenic extensional domain in the Yidun arc occurred mainly around 80 Ma. The mineral deposits include the ~81-77 Ma Hongshan porphyry and its related Cu-Mo deposit, such as a molybdenite deposit with Re-Os ages of 80 and 77 Ma (Yang et al., 2017). The Late Cretaceous A-type granites were emplaced in the Queershan-Cilincuo belt and include the Xiangchengmolong K-feldspar granite with an Ar-Ar age of 77 Ma (Wu et al., 2016; Qu et al., 2002; Lü et al., 1993) and A-type granite in the Techong Block with a zircon U-Pb age of 76-68 Ma as well as associated Sn-W mineralization (Deng et al., 2014; Xu et al., 2012). These granites and mineralization events were associated with regional extension and the remelting of residual oceanic plate. The ~81-76 Ma high-K igneous rocks in the southern Qiangtang Block resulted from lithospheric delamination (Li et al., 2013).

    The ZFT ages of 50-19 Ma record the long-term tectothermal events after the evolution of Sanjiang Tethys that was mainly affected by the India-Asia collision. The detailed events include 50-43, 36-34, 29-27, 23 and 19 Ma according to the ZFT age distribution, belonging to major intracontinental tectonic epochs and will be discussed in a special article in the near future.

5.   CONCLUSION
  • A series of ZFT ages are obtained and range from 212 to 19  Ma. The large spread in ZFT ages implies a complex tectonic activity of Sanjiang Tethys. The results reveal not only 6 episodes of tectonothermal events, i.e., 212, 179-172, 156-133, 121-96, 84-70 and 50-19 Ma, but also time limits for relative epochs. These epochs of events were closely related to the evolution of the Garzê-Litang, Yarlung-Zangbo and Bangong-Nujiang Tethys oceans, as well as intracontinental tectonic activities. This work also uses the zircons fission-track analysis to the research area first.

ACKNOWLEDGMENTS
  • This research was jointly supported by the National Program on Key Basic Research Project (973 Program) (No. 2015CB452606) and the National Natural Science Foundation of China (Nos. 41730427, 41172088). The authors thank Profs. Chuanbo Shen (China University of Geosciences, Wuhan), Ruxin Ding (Sun Yat-Sen University) and Li Li (China University of Petroleum) for discussion and good suggestions. We express our sincere thanks to anonymous reviewers who are greatly appreciated for improving the manuscript. The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1014-6.

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