Timing of the Meso-Tethys Ocean opening: Evidence from Permian sedimentary provenance changes in the South Qiangtang Terrane, Tibetan Plateau

Timing of the opening of the Meso-Tethys Ocean, represented by the Bangong–Nujiang Suture Zone on the Tibetan Plateau, remains controversial. Further research is required to understand the breakup of the northern Gondwana margin and the tectonic evolution of the Tethyan realm. In this study, we present petrography, U-Pb dating and Hf isotopic data for detrital zircons from Upper Carboniferous–Upper Permian strata in the South Qiangtang Terrane on the Tibetan Plateau. These data, together with data from previous literature, indicate a youngest detrital zircon age peak of ca. 550 Ma for Upper Carboniferous–Lower Permian strata. This is far older than the depositional age of ca. 300 Ma and indicates a source in the stable Gondwana Continent. Upper Permian strata yield younger ages (490–247 Ma) with peaks at ca. 460, 355, 290 and 260 Ma, indicating a source in the active South Qiangtang Terrane. Combined with the unconformity between the Lower and Upper Permian strata in the western South Qiangtang Terrane, we conclude that a significant change in sedimentary provenance occurred at 280–260 Ma. This provenance change might have resulted from the 300–279 Ma rifting magmatism on the northern Indian margin of Gondwana (e.g., South Qiangtang). The 300–279 Ma magmatism is interpreted to reflect the early stages of rifting, and the subsequent 280–260 Ma sedimentary provenance change is interpreted as the later stage, both of which established a complete Early–Middle Permian (300–260 Ma) rifting process that marks the opening of the Meso-Tethys Ocean.

In this paper, in order to discuss the timing of the Meso-Tethys opening, we examine the Upper Carboniferous-Upper Permian strata (Fig. 1b) in the South Qiangtang Terrane by using a combined approach of detailed petrographic analysis, detrital zircon U -Pb dating, and Hf isotope analysis. The resultant data allow us to identify a significant change in sedimentary provenance during 280-260 Ma in the South Qiangtang Terrane, which is interpreted as the sedimentary and tectonic response to continental rifting, the precursory process of the Meso-Tethys Ocean opening. This work thus establishes an important framework for the timing of opening of the Meso-Tethys Ocean.
The Zhanjin Formation is dominated by grey-green glacial marine diamictite (Fig. 3a)-formed by the Late Carboniferous-Early Permian Gondwanan glaciation (Jin, 2002;Fielding et al., 2008;Zhang et al., 2013;Fan et al., 2015)-sandstone, siltstone and shale. Sakmarian bivalves (e.g., Eurydesma perversum) and solitary corals (e.g., Cyathaxonia and Lophophyllidium; Liang et al., 1983;Liu and Cui, 1983;Zhang et al., 2013) in the sandstone and siltstone also indicate a Late Carboniferous-Early Permian age. Slump structures, convolute bedding and Bouma sequences are common in the Zhanjin Formation, indicating a bathyal to abyssal depositional environment (Fan et al., 2015;Zhang et al., 2019). The overlying Qudi Fomation is dominated by littoral-neritic sandstone in the western South Qiangtang Terrane, and bathyal to abyssal siltstone and shale in the middle South Qiangtang Terrane (Zhang et al., 2012a. This Formation contains fusulines (e.g., Pseudofusulina, Chalaroschwagerina, Pamirina) of Early Permian age (Zhang et al., 2012a. The Lugu Formation in the middle South Qiangtang Terrane is dominated by basalt and littoral-neritic limestone; Early Permian fusulines (Cancellina, Parafusulina and Pseudodoliolina) occur in the basal strata, and Middle Permian Neoschwagerina and Verbeekina occur in the upper strata of the formation (Nie and Song, 1983a;Zhang et al., 2012aZhang et al., , 2013Zhang et al., , 2019. The Tunlonggongba and Longge formations in the western South Qiangtang Terrane are both dominated by littoral-neritic limestone. The Tunlonggongba Formation contains the fusuline Monodiexodina, indicating a late Early Permian age (Nie and Song, 1983b). The Longge Formation contains the coral Iranophyllum, and the fusulines Neoschwagerina, Dunbarula, Sumatrina, Chusenella and Kahlerina of late Middle Permian age (Liang et al., 1983;Nie and Song, 1983c;Zhang et al., 2013). The nature of the stratigraphic contact between the Tunlonggongba and Longge formations is unclear, because the Longge Formation occurs as 'blocks' in the western South Qiangtang Terrane . The Jipuria Formation is dominated by littoral-neritic conglomerate, sandstone, siltstone and limestone, with minor andesite and tuff (Figs. 2, 3b;Liang et al., 1983;Xia and Liu, 1997;Mou et al., 2010;Zhang et al., 2013Zhang et al., , 2019. This formation overlies the Tunlonggongba Formation with angular unconformity in the western South Qiangtang Terrane, whereas it overlies the Lugu Formation with parallel unconformity in the central South Qiangtang Terrane (Fig. 2). In the western South Qiangtang Terrane, the Jipuria Formation contains the fusulines Codonofusiella, Reichelina and Palaeofusulina, the corals Waagenophyllum and Lophophyllidium, and the brachiopods Permophricodothyris and Leptodus, all indicating a Late Permian age (Wu and Lan, 1990;Zhang et al., 2013). Due to an absence of fossils, the age of the Jipuria Formation in the middle South Qiangtang Terrane remains unconstrained.

Sandstone petrographic analysis
Sandstone samples from the Upper Carboniferous-Lower Permian Zhanjin Formation and the Upper Permian Jipuria Formation in the South Qiangtang Terrane were prepared and studied using petrographic analysis. Modal analysis was carried out on Upper Permian samples that exhibit minor metamorphism. Approximately 300 grains were identified and counted in each sample, following the Gazzi-Dickinson method (Dickinson, 1985); crystals or grains larger than ~60 μm in diameter within rock fragments were counted as single minerals (Ingersoll et al., 1984). The results are presented in Supplementary Table S1.

Zircon U-Pb dating
Based on field work, four sandstone samples were selected for U-Pb dating: one from the Upper Carboniferous-Lower Permian Zhanjin Formation in the Jiaco area (sampled S19T21, 33  were used for instrumental calibration. The Pb correction method of Anderson (2002) was applied, with analytical details following those described by Yuan et al. (2004). Reported uncertainties for the age analyses are given as 1σ values with weighted mean ages at the 95% confidence level. Isotopic data were processed using the GLITTER (version 4.4) and Isoplot/Ex (version 3.0) programs (Ludwig, 2003). Reported ages are 206 Pb/ 238 U ages for grains < 1000 Ma and 207 Pb/ 206 Pb ages for grains > 1000 Ma. For statistical purposes, zircon ages with <10% discordance are used in our discussion.

In situ zircon Hf isotope analysis
Twelve zircons from the Upper Permian sandstone samples (D18T16, D18T17, and B19T17) were analyzed for Hf isotopic compositions. The same dating spots were used for Hf analysis. The Hf isotope data were collected using a NEPTUNE Plus multi-collector (MC)-ICP-MS at the Beijing Createch Testing Technology Co., Ltd., Beijing, China. A single spot ablation mode with a spot size of 44 μm was used to acquire the data. Each measurement consisted of 20 s of background signal acquisition followed by 50 s of ablation signal acquisition, with analytical processes following those described by Hu et al. (2012). Off-line selection, signals integration of analyte, and mass bias calibrations were performed using the ICP-MS DataCal program (Liu et al., 2010). The analyzed 176 Hf/ 177 Hf ratios for the zircon standard (91500) were 0.282299 ± 31 (2σ n , n-40), which are in agreement with the recommended value within error ( 176 Hf/ 177 Hf ratios of 0.282302 ± 8 at 2σ; Goolaerts et al., 2004;Woodhead et al., 2004).
Sandstone samples from the Upper Permian Jipuria Formation in the Ritu area are dominated by fine-grained (<0.1 mm) quartz grains (77%-83%), feldspar (8%-12%) and lithic fragments (9%-14%; Table S1, Fig. 3f). Polysynthetic twinning is common in the feldspar (Fig. 3f), and lithic fragments composed predominantly of metamorphic and volcanic detritus (Table S1). Fig. 4. Summary of detrital zircon age distributions of the Carboniferous-Permian sandstones of this study and previous work (Gehrels et al., 2011;Liang et al., 2020) in the South Qiangtang Terrane. Similar main age peaks between Carboniferous and Permian strata are shown in grey bands, whereas different age peaks are shown in green bands. n = total number of analyses. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Zircon U-Pb dating
Representative cathodoluminescence (CL) images of detrital zircons are presented in Fig. S1, and age data are presented in Tables S2-S4. Detrital zircon ages from one Upper Carboniferous-Lower Permian sandstone sample from the Zhanjin Formation from the Jiaco area (S19T21; Fig. 1c) range from 3944 to 498 Ma, with two main peaks at ca. 958 and ca. 530 Ma (Fig. 4c). These age distributions are in good agreement with those of detrital zircons from Carboniferous-Lower Permian strata in other areas of the South Qiangtang Terrane (Fig. 4d-e).
Detrital zircon ages from two Upper Permian sandstone samples from the Jipuria Formation from the Jiaco area (Fig. 1c) yield a similar range of ages from 3630 to 247 Ma, with five main peaks at ca. 945, 528, 463, 350, and 260 Ma (Fig. 4b). Detrital zircon ages from one Upper Permian sandstone sample from the Jipuria Formation in the Ritu area (B19T17; Fig. 1d) range from 2664 Ma to 247 Ma, with five main peaks at ca. 1870, 456, 363, 290, and 256 Ma (Fig. 4a). These age distributions are significantly different from those of the Carboniferous-Lower Permian strata in the South Qiangtang Terrane (Fig. 4).

Zircon Hf isotope data
Zircon Hf isotope data are presented in Table S5

Age of the Jipuria Formation in the South Qiangtang Terrane
The age of the Jipuria Formation in the Jiaco area of the middle South Qiangtang Terrane is currently unconstrained, owing to a lack of fossils. Andesite and pyroclastic rocks in the Jipuria Formation indicate magmatic eruptions occurred during deposition of the formation (Liang et al., 1983;Xia and Liu, 1997;Mou et al., 2010;Fig. 2); therefore, the depositional age of the Jipuria Formation should be close to the youngest zircon age (Malusa et al., 2011;Cawood et al., 2012;von Eynatten and Dunkl, 2012). To reasonably constrain the depositional age of the Jipuria Formation, we used the mean age of the youngest three or more grains that overlap in age at 2σ (YC2σ). This method has proved effective in sandstones from the Colorado Plateau (Dickinson and Gehrels, 2009). In the Jiaco area, sandstone samples from the Jipuria Formation yield Late Permian YC2σ ages of 259 ± 11 Ma (n = 5) and 257 ± 11 Ma (n = 5), which are similar to those of the Jipuria Formation in the Ritu area (YC2σ = 255 ± 8 Ma, n = 10; Fig. S2). These YC2σ ages (259-255 Ma), together with Late Permian fossils reported in the Ritu area (Wu and Lan, 1990;Zhang et al., 2013), provide strong evidence that the Jipuria Formation in the South Qiangtang Terrane is of Upper Permian age.

Provenance analysis: a 280-260 Ma sedimentary provenance change in the South Qiangtang Terrane
Prior to provenance analysis, it is necessary to consider the paleoposition of the South Qiangtang Terrane during the Carboniferous-Permian. The glacial marine diamictites (Fig. 3a) that formed as a result of the Late Carboniferous-Early Permian Gondwanan glaciation (ca. 300 Jin, 2002;Fielding et al., 2008;Zhang et al., 2013;Fan et al., 2015) are widespread in the South Qiangtang Terrane (Fan et al., 2015). This indicates that the South Qiangtang Terrane was located near the Gondwana Continent during the Late Carboniferous-Early Permian period. The distinctive ca. 950 Ma age peak observed in the Carboniferous-Lower Permian strata (Fig. 4) in the South Qiangtang Terrane is consistent with the emplacement of the 990-900 Ma granitoids of the Indian margin of Gondwana (Zhu et al., 2013). These observations, together with the similarities between the Early Permian fossils (Zhang et al., 2012a(Zhang et al., , 2012b(Zhang et al., , 2014b and magmatic activity (e.g., the 300-260 Ma mafic magmatism; Zhai et al., 2013;Wang et al., 2019) of the South Qiangtang Terrane and the northern Indian margin of Gondwana (i.e., the Himalayas; Shellnutt et al., 2014) indicate that the South Qiangtang Terrane was part of the northern Indian margin of Gondwana during the Carboniferous-Early Permian (Zhang et al., 2012a;Zhu et al., 2013;Metcalfe, 2013;Zhai et al., 2013;Liao et al., 2015;Chen et al., 2017).
Paleogeographic analysis indicates that the Indian margin of Gondwana was an erosional zone, and the South Qiangtang Terrane was in a passive margin depositional setting during the Late Carboniferous-Early Permian (Fan et al., 2015). The Indian margin of Gondwana may therefore be the source of the Upper Carboniferous-Lower Permian deposits in the South Qiangtang Terrane (Fan et al., 2015). The youngest zircon age peak of ca. 550 Ma in the Upper Carboniferous-Lower Permian strata is far older than the depositional age (ca. 300 Ma;, and thus provides strong evidence for the stable Indian margin of Gondwana source. The presence of abundant angular to subangular volcanic (e.g., basalts) and sedimentary (e.g., limestones) lithic fragments in the Upper Permian sandstone of the South Qiangtang Terrane (Fig. 3d-e) indicate that their provenance lies in a tectonically active rather than stable setting such as the Indian margin of Gondwana. In addition, these samples plot in the recycled orogen sector in quartz-feldspar-lithic fragment (QFL) and monocrystalline quartz-feldspar-total lithic fragments (QmFLt) discrimination diagrams (Fig. 6). Moreover, many detrital zircons in the Upper Permian sandstone of the South Qiangtang Terrane yield ages  with peaks at ca. 460, 355, 290 and 260 Ma, which are not observed in the age spectra of the Carboniferous-Lower Permian sandstones (Figs. 4, 7). These observations provide evidence that a significant sedimentary provenance change occurred between the Carboniferous-Early Permian and the Late Permian in the South Qiangtang Terrane.
Lithic fragments in the Upper Permian sandstone are mostly poorly sorted, and are angular to subangular in shape (Fig. 3d-  J.-J. Fan et al. sandstone in the Ritu area mostly exhibit weak and broad zoning in CL images (Fig. S1), and the detrital zircon grain with an age of 285 Ma has a εHf (t) value of +6.9 (Table S5), both of which are similar to those of the 300-279 Ma mafic rocks (e.g., mafic dike swarms, +4.2 to +15.8; Fig. 1b; Zhai et al., 2013;Wang et al., 2019) in the South Qiangtang Terrane. In the Upper Permian sandstone, the 490-445 Ma zircon ages with a peak at ca. 460 Ma, and the 384-334 Ma zircon ages with a peak at ca. 355 Ma (Fig. 7) indicate Ordovician and Late Devonian-Early Carboniferous magmatism occurred in the source region. This corresponds with the magmatism in the South Qiangtang Terrane (Fig. 1b); for example, magmatism occurred at 500-450 Ma in a 300 km-long belt from Bensongco in the east to Dawashan in the west ( Fig. 1b; Hu Xie et al., 2017;Liu et al., 2019;Xu et al., 2020), and the 360-350 Ma magmatism occurred in the Gangmuco area in the South Qiangtang Terrane (Fig. 1b; Wang et al., 2015a). These observations, together with the basal unconformity of the Upper Permian Jiapuria Formation (Fig. 2) that indicates uplift and erosion of the South Qiangtang Terrane, provide strong evidence that the source of the Upper Permian sandstone is derived from the erosion of sedimentary and magmatic rocks in the South Qiangtang Terrane.
In conclusion, the source of the Upper Carboniferous-Lower Permian strata in the South Qiangtang Terrane lies in the stable Indian margin of  Gondwana, whereas the Upper Permian strata are derived from the active South Qiangtang Terrane (Fig. 7). The sedimentary provenance changed significantly between the Late Carboniferous-Early Permian and Late Permian periods. The angular unconformity between the Lower Permian Tunlonggongba and Upper Permian Jipuria formations in the Ritu area ( Fig. 2; Liang et al., 1983;Zhang et al., 2019) indicate that the western South Qiangtang Terrane must have been uplifted after deposition of the Lower Permian Tunlonggongba Formation, which marks the point at which the provenance changed (Figs. 2, 7). The same observations (Figs. 2, 7) further suggest that the provenance change may have started in the Early Permian (ca. 280 Ma), and continued into the Middle Permian (273-260 Ma).
The 280-260 Ma sedimentary provenance change in the South Qiangtang Terrane closely follows the 300-279 Ma rifting magmatism in time and space. We infer that the 280-260 Ma sedimentary provenance change in the South Qiangtang Terrane was caused by the widespread 300-279 Ma rifting magmatism. This process resulted in uplift of the northern Indian margin of Gondwana (e.g., the South Qiangtang) and a change of depositional environment from marine to terrestrial at 280-260 Ma (Figs. 2,7). This uplift resulted in erosion of Ordovician, Late Devonian-Early Carboniferous and Permian magmatic and sedimentary rocks, which changed the sedimentary provenance signature of the area significantly.
The rifting magmatism at 300-279 Ma may represent the early stage of rifting, and the 280-260 Ma sedimentary provenance change may represent the late stage (Fig. 8). The rifting magmatism and subsequent sedimentary provenance change represent a complete Early-Middle Permian (300-260 Ma) rifting process on the northern Indian margin of Gondwana.

Opening of the Meso-Tethys Ocean
The opening of the Meso-Tethys Ocean was genetically the rifting of the South Qiangtang Terrane from the Indian margin of Gondwana (Yin and Harrison, 2000;Metcalfe, 2013;Zhai et al., 2013;Liao et al., 2015;Chen et al., 2017). The Early-Middle Permian (300-260 Ma) rifting process on the northern Indian margin of Gondwana (eg., the South Qiangtang) may represent the initial opening of the Meso-Tethys Ocean. This interpretation is also supported by the following three lines of evidence.
(1) The parent material of the Late Carboniferous-Early Permian glacial marine diamictites and sandstones (ca. 300 Ma) in the South Qiangtang Terrane was derived directly from the Indian margin of Gondwana ( Fig. 7; Fan et al., 2015).  (Fig. 8), which provides strong evidence that the Meso-Tethys Ocean opened during the Early-Middle Permian (300-260 Ma). (3) Previous paleontological studies have shown that a significant paleobiogeographic change from a peri-Gondwanan to transitional affinity (the Tethyan Cimmerian subregion) occurred in the South Qiangtang Terrane from the Artinskian to the Kungurian (Zhang et al., 2012b(Zhang et al., , 2014bShen et al., 2016). This transition was the result of the effects of the northward drift of the South Qiangtang Terrane (Zhang et al., 2012b), which provides further evidence that the opening of the Meso-Tethys Ocean occurred during Early-Middle Permian (300-260 Ma).

Conclusions
(1) A significant change in sedimentary provenance occurred between 280 and 260 Ma in the South Qiangtang Terrane of the Tibetan Plateau.

Declaration of competing interest
We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work; there is no professional or other personal interest of any nature or kind in any product; service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled.
Grant No. 2019QZKK0703), the National Science Foundation of China (Grant Nos. 41972236, 41702227), and Self-determined Foundation of Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources of China (DBY-ZZ-18-04). All data are provided in the present manuscript and supporting information. The latter includes Figs. S1-2 and Tables S1-5 and is available at https ://zenodo.org/deposit/3754981 (DOI: https://doi.org/10.5281/zenod o.3754981).  Torsvik and Cocks, 2013;Metcalfe, 2013;Zhai et al., 2013;Liao et al., 2015;Chen et al., 2017;Wang et al., 2019). H, Himalayas; L-T, Lhasa-Tengchong; S, Sibumasu; SQ, South Qiangtang; GI, Greater India. Red bars show the cross sections. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)