Discovery of the Early Ordovician strata in Xianza County, Tibet and its significance

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There is no any sediments contain fossils which formed earlier than Middle Ordovician in northern Tibet yet. It is very important whether have developed the Cambrian to Early Ordovician sedimentary strata. During geological mapping of Xainza County in 2000, Tetragraptus approximatus, the typical one of graptolite belts which cosmepolitically distributed in the Arenigian of Early Ordovician, was collected from the so-call Pre-Sinian Nianqingtanggula Group. So there is developped the Early Ordovician sediments and we have established Zakang Formation and Taduo Formation which perfected the Plaeozoic sequences in northern Tibet. According to these, we can realize tectonic control, palaeogeographrc pattern as well as palaeobiogeographic distribution of the Qinghai-Tibet Plateau in the early Paleozoic and also provided the materials to the cosmepolitical biostratigraphic contrast.

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... Cephalopods are the most common macrofossils found in the Jiaqu Formation, including Sinoceras chinense, Michelinoceras elongatum, Archigeisonoceras elegatum and Troedssonella nyalamensis ( Fig. 10G-I; Chen, 1984). Many of these taxa are age diagnostic and also reported from the Sandbian-Katian of South China and the Lhasa terrane, e.g., the Pagoda and Gangmusang formations (Chen and Zou, 1984;Chen, 1986;Cheng et al., 2005;Fang et al., 2018). ...
... The Zhakang Formation was established by Zhang et al. (2004) (also see Cheng et al., 2005) for the weakly to moderately metamorphosed succession consisting in the lower part of siltstone and fine sandstone and in the upper part of quartzitic sandstone and slate with a combined thickness of ca. 263 m. ...
... overlies the Precambrian metamorphics (schist and phyllite). It consists of a basal quartzitic conglomerate, lower quartzite, middle siltstone and sandy mudstone with minor crystalline limestone interbeds and the upper crystalline limestone intercalated with mudstone and siltstone Cheng et al., 2005). The unit is weakly to moderately metamorphosed and the only fossils found were trilobite fragments in the upper part. ...
The Ordovician stratigraphy of southern-central Xizang (Tibet) has been revised based on new conodont data recovered from 43 samples in four stratigraphic units and their integration with existing nautiloid and graptolite data. The Histiodella holodentata and Pygodus serra biozones have been identified respectively in the Alai and Jiaqu formations of the Chiatsun Group exposed near Alai village in Nyalam County within the Himalayan terrane, and the Yangtzeplacognathus foliaceus Subbiozone (lower part of the Pygodus serra Biozone) in the Sangqu Formation exposed at the Guyu section within Zayu County in the Lhasa terrane. Recognition of these biozones has increased the precision of correlation of the middle-upper Darriwilian strata in the region. Regional reassessment of the Ordovician stratigraphy permitted by new biostratigraphic data has allowed revised definitions for the Chiatsun and Keerduo groups and the Sangqu and Xainza formations. The Chiatsun Group is defined herein to include three lithologically distinctive formations in descending order, the Jiaqu, the Alai and the Adang formations. The stratigraphic age for the Jiaqu and Alai formations in the type area ranges from the middle Darriwilian (Histiodella holodentata Biozone) to middle Katian (Hamarodus brevirameus Biozone), but the age of the Adang Formation remains less certain.
... The Ordovician strata composed of conglomerate, pebbly sandstone, feldspathic quartz sandstone, lithic sandstone, fine sandstone, siltstone, sandy slate and limestone (Fig. 9). The fossils of EarlyeLate Ordovician nautiloids and graptolites are presented in the siltstone, limestone and slate (Cheng et al., 2005). In the Zhaqian area, the basal conglomerate is 20e30 m thick, which mainly consists of 2e8 cm quartzite, slate, rhyolite, calceschist, limestone, granite and muscovite quartz schist gravels in arenopelitic matrix. ...
... The Ordovician strata represent the littoraleneritic shelf and turbidite deposition. The Early Ordovician Floian stage graptolites fossils (Cheng et al., 2005) and youngest rhyolite (497 Ma; this study) in the Zhaqian area indicate that the hiatus of the CambrianeOrdovician unconformity in LH may be~20e25 Myr (Fig. 9). ...
... 9. Stratigraphic sections of CambrianeOrdovician strata in Tibet Plateau (modified from Myrow et al., 2016). The data are from the South Qiangtang (Yang et al., 2014;Xie et al., 2017), Lhasa (Cheng et al., 2005;Li et al., 2010;Hu et al., 2013), Himalaya Zhou et al., 2004;Myrow et al., 2006aMyrow et al., , 2006bMyrow et al., , 2016, and East Tibet Zhang et al., 2015;Mao, 2016;Zhao et al., 2017bZhao et al., ). 2002Zhu et al., 2003;Zhou et al., 2004;Myrow et al., 2006aMyrow et al., , 2006bMyrow et al., , 2016. ...
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The central–south domain of the Tibet Plateau represents an important part of the northern segment of Gondwana during the early Paleozoic. Here we present zircon U–Pb, Lu–Hf isotope, and whole–rock geochemical data from a suite of early Paleozoic magmatic rocks from the central Tibet Plateau, with a view to gain insights into the nature and geotectonic evolution of the northern margin of Gondwana. Zircon grains in four granitic rocks yielded ages of 532−496 Ma with negative εHf(t) values (−13.7 to −0.6). Zircon grains in meta–basalt and mafic gneiss yielded ages of 512 ± 5 Ma and 496 ± 6 Ma, respectively. Geochemically, the granitic rocks belong to high–K calc–alkaline and shoshonitic S–type granite suite, with the protolith derived from the partial melting of ancient crustal components. The mafic gneiss and meta–basalt geochemically resemble OIB (Oceanic Island Basalt) and E–MORB (Enriched Mid–Ocean Ridge Basalt), respectively. They were derived from low degree (∼5–10%) partial melting of an enriched mantle (garnet and spinel lherzolite) that was contaminated by upper crustal components. The parental magmas experienced orthopyroxene–dominated fractional crystallization. Sedimentological features of the Cambrian–Ordovician formations indicate that the depositional cycle transformed from marine regression to transgression leading to the formation of parallel/angular unconformities between the Cambrian and Ordovician strata. The hiatus associated with these unconformities are coupled with the peak of the early Paleozoic magmatism in Tibet Plateau, indicating a tectonic control. We conclude that the Cambrian–Ordovician magmatic suite and sedimentary rocks formed in an extensional setting, and we correlate this with the post–peak stage of the Pan–African orogeny. The post–collision setting associated with delamination, orogenic collapse or lithospheric extension along the northern margin of Gondwana, can account for the Cambrian–Ordovician magmatism and sedimentation, rather than oceanic subduction along the external margin. We thus infer a passive margin setting for the northern Gondwana during the Early Paleozoic.
... Ordovician cephalopods of the Xainza region, North Tibet, were first reported by the Tibet Geological Bureau General Survey Team (1980) during mapping of the Shigatse geological map sheet. Subsequently, many Ordovician cephalopods from this region were described by Lai (1982a, b), Chen (1986Chen ( , 1987, Li and Cheng (1988), and in a series of works by Cheng et al. (2005aCheng et al. ( , b, c, 2006. However, most of the recorded taxa were from the Upper Ordovician Keerduo and Gangmusang formations, which are both suggested as of Sandbian-Katian age by Xia (1983; Fig. 1.1), based on the outcrops mainly in the Xainza region. ...
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Actinocerid nautiloids from the Lhasai Formation in the Xainza region are studied systematically for the first time. The nautiloids are identified as Middle Ordovician in age based on stratigraphic correlations with those from North China, Sibumasu, North Australia (northern Gondwana), and North America (Laurentia). A cluster analysis shows strong affinities between the actinocerid nautiloids of the Lhasa Terrane and those of the Himalaya, North China, and Sibumasu terranes. Our results support Middle Ordovician paleogeographic reconstructions that place North China rather than South China much closer to Australia. Nine species assigned to six genera of Meitanoceratidae, Wutinoceratidae, Armenoceratidae, Ormoceratidae, and Discoactinoceratidae are described in detail: Pomphoceras nyalamense (Chen, 1975), Pomphoceras yaliense (Chen, 1975), Wutinoceras cf. W . foerstei (Endo, 1930), Mesowutinoceras giganteum Chen in Chen and Zou, 1984, Armenoceras tani (Grabau, 1922), Armenoceras teicherti Endo, 1932, Armenoceras xizangense new species, Deiroceras globosom Zou and Shen in Chen and Zou, 1984, and Discoactinoceras cf. D . multiplexum Kobayashi, 1927. UUID:
The tectonics and paleogeography of Ordovician rocks in China record the four major paleoplates-the South China, North China, Tarim, and Xizang (Tibet) Blocks. New paleogeographic maps of South China for the Tremadocian, Darriwilian, Sandbian-early Katian, and late Katian-Hirnatian time intervals display lithofacies and biofacies belts that depict continuous changes from the Yangtze Platform through the Chiangnan (Jiangnan) Slope to the Zhujiang Basin. North China was dominantly a carbonate platform during the Ordovician. Facies belts, particularly the trilobite biofacies belts, change westward from the platform edge to the slope along the west margin of the platform. In Tarim, Ordovician rocks provide the main source and reservoir rocks for oil and gas. The vast expanse of the block was a northward-deepening, shallow-water platform that was fringed by peripheral, deeper water facies belts developed along the northern side of the South Tianshan. Facies analysis indicates that the paleogeographic setting varied in response to eustatic sea level changes. North of the Tibet Block lies a long mobile belt that crosses more than half of China from west to east. The Tibet Block is mainly composed of two units, the north Qiantang region, which remained as a separate, small paleoplate through the late Paleozoic, and the south Gandise-Himalaya region, which persisted as a distinct paleoplate through the late Paleozoic and well into the Mesozoic. Ordovician rocks consisting mainly of carbonates with shelly faunas have been recorded from regions of the Tibet Block.
To better understand the Pan-African-early Paleozoic tectonothermal events of the Nyainrong microcontinent and the constraints on its tectonic evolution, here we report the results of zircon LA-ICP-MS U-Pb dating and geochemical features of Amdo gneiss in the Nyainrong microcontinent. The outcrops of Amdo gneiss is about 30 km south of Amdo County in northern Tibet. The field occurrence, mineral composition, textural characteristics, and whole-rock geochemical features of the four gneiss samples indicate the protolith of the gneisses is intermediate-acid intrusive rock. Gneiss zircon trace element tracing and genetic analysis shows that zircon has typical characteristics of magmatic zircon. The 206Pb/238U concordant age of zircon is 505–517 Ma, corresponding to the Middle-Late Cambrian, which is the formation age of the protolith. The samples have characteristics of high silicon, alkali-rich, alkalic rate AR =1.73–3.7, the differentiation index DI = 70.78–90.28; rock aluminum saturation index ranges from 1.02 to 1.05, FeO / MgO ranges from 2.63 to 4.50, 10000 × Ga/Al ranges from 2.12 to 2.41, and P2O5 and Al2O3 content decreased with SiO2 increasing. Th and Y contents have a good positive correlation with Rb content; the genetic type of protolith of the gneiss is the differentiation of subalkaline over aluminum I-type granite. Combined with regional data, the tectonic setting of the Amdo gneiss protolith is closely related to the collision orogenic process. The preliminary view is that the Middle-Late Cambrian magmatic events developed on the microcontinent could be the result of Andean-type orogeny along the Gondwana super-continental margin after the end of the Pan-African orogeny.
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