Early Paleozoic Tectonic Nature In SW Yunnan: Constraints From Magmatism And Sedimentation

Early Paleozoic igneous rocks, ranging in age from 525 Ma to 460 Ma, exposed within the Himalayan Orogen Belt in NW India, Nepal, South Tibet and SW China. In the early Paleozoic, these regions constituted part of the northern margin of Gondwana facing the proto-Tethys ocean. Two tectonic models have been proposed for the early Paleozoic orogenic event involving:(a) Pan-African orogeny associated with either the breakup of an earlier supercontinent or the final assembly of Gondwana; and(b) Andean-type orogeny following Gondwana assembly, caused by subduction of the proto-Tethyan Ocean beneath the Indian Craton and its adjacent micro-continental blocks. However, the detailed geological evolution of these regions during this time has poorly been established due to the hinder of late Paleozoic disruption and Cenozoic tectonothermal. Southwest Yunnan is one of the important branches of the eastern Tethyan tectonic belt. The Tethyan–Alpine orogenic system has a change in direction from the Himalayan segment(WNW-trending) to the Southeast Asian segment(northerly-trending) in SW Yunnan. In this paper, we selected early Paleozoic gneissic granites, metamorphic volcanic rocks and clastic rocks from the Tengchong, Baoshan and Shan-Thai Blocks in southwestern Yunnan Province for petrology, zircon U-Pb geochronology, whole-rock element and Sr-Nd-Hf isotopic study. The combined geochemical and geochronological data are used to probe into the petrogenesis and geodynamic setting of these rocks. Based on our results, along with the available data rebuilt the paleogeographic position of the micro-continents during early Paleozoic and proposed the tectonic evolution model.The main conclusions are shown as following.(1) Zircon U-Pb and Lu-Hf isotopic data, along with whole-rock elemental and Sr-Nd isotopic data, for early Paleozoic granitoids from the Tengchong, Baoshan and Shan-Thai Blocks revealed that they originally formed along the northern margin of Gondwana. Twelve analyzed samples yield zircon U-Pb crystallization age of 492-460 Ma. These granitic rocks have CIPW-normative corundum and are strongly peraluminous with A/CNK of 1.10-1.39, similar to those of S-type granites. They are characterized by high Si O2, Rb/Sr and Rb/Ba but low Al2O3, Mg O, Ti O2, Fe Ot and Ca O/Na2 O ratios, and are enriched in LILEs and depleted in Nb, Sr, P, Eu and Ti.Initial 87Sr/86 Sr ratios range from 0.7084 to 0.7230 and εNd(t) values from-6.6 to-8.1with Nd model ages of 1.7-2.8 Ga, consistent with those of the published synchronousgranitic rocks in South Tibet. Zircons with early Paleozoic magmatic ages have εHf(t)values ranging from-0.37 to-14.1 and Hf model ages from 1.49 to 2.35 Ga. Their petrogenesis can be interpreted as melting of an ancient metapelite-dominated crustal source with the residual mineral assemblages of plagioclase ? hornblende ? garnet ?zircon. The Ordovician granitoids in SW Yunnan represent the southward continuation of the early Paleozoic granitic belt that extended along the northern margin Gondwana.(2) Zircon U-Pb ages of 462±6 Ma and 454±27 Ma for two representative samples from the Huimin Formation, previously-defined Lancang Group, indicate the eruption of the volcanic rocks in the Late Ordovician. Geochemical data for the metavolcanic rocks together with other available data indicate a calc-alkaline affinity with high Al2O3(13.04~18.77 wt.%) and low Ti O2(0.64~1.00 wt.%). They have mg-numbers ranging from 62 to 50 with Si O2 of 53.57-69.10 wt.%, compositionally correspond to the high-mg andesitic rocks. They display enrichments in LREEs and LILEs with significant Eu negative anomalies(δEu=0.20~0.33), and depletions in HFSEs, similar to those of arc volcanic rocks. Their 87Sr/86 Sr ratios range from0.721356 to 0.722521 and εNd(t) values from-7.63 to-7.62 with Nd model ages of2.06-2.10 Ga. Integration of ages and geochemical data with available geological observations, we propose the presence of Ordovician magmatism related to proto-Tethyan evolution in SW Yunnan, which formed in an island-arc setting. They were part of a regional accretionary orogen that extended along the northern margin of Gondwana during Neoproterozoic to early Paleozoic period.(3) Integrated U-Pb age and in-situ Hf isotope analysis on detrital zircons of three meta-sandstone samples from the Ximeng Group and other seven samples from the Lancang Group, SW Yunnan, have been used to identify detrital zircon provenances and crustal evolution process. A total of 394 detrital zircon grains from the samples have the ages range from 3490 Ma to 465 Ma, and 246 detrital zircon grains of them give the εHf(t) values of-35.45~26.3. The youngest ages of those samples are 515±19 Ma, 531±13 Ma, 514±14 Ma, 480±12 Ma, 559±16 Ma, 489±5Ma, 548±7 Ma, 505±20 Ma, 503±14 Ma and 465±4 Ma, respectively. The corresponding samples yield the youngest age-peaks of 568 Ma, 541 Ma, 560 Ma, 506 Ma, 562 Ma, 491 Ma, 551 Ma, 530 Ma, 551 Ma and 614 Ma, respectively. These data suggest that they should be deposited after 491 Ma. The gneissic granites from the Ximeng Group is dated ~460 Ma, and the metavolcanic rocks from the HuiminFormation, the Lancang Group is dated about 454-462 Ma, indicating that the minimum deposition time of those rocks should be Middle Ordovician(454-462 Ma).As a result, the clastic rocks in SW Yunnan should deposit at Late Cambrain to Middle Ordovician, about 491-454 Ma. The Ximeng Group and the Lancang Group are composed of different type of rocks, rather than previously thought Precambrian or Proterozoic strata. U-Pb age data and Lu-Hf isotopic sompositions of these grains suggest that the zircon host rocks have diverse sources, including ancient crustal materials and juvenile components which mainly derived from east Gondwana. Their age spectrum characteristics suggest that the microcontients in SW Yunnan and Tethyan Himalayan orogen shared a common provenance in the Paleozoic and they were correlated paleogeographically with east India during the late Precambrian-Late Ordovician.(4) Integrated the above findings show that the Ordivician high-Mg volcanic rocks, coeval with mafic and granitic rocks are the products of Proto-Tethyan Ocean subducting beneath the Indian Craton and its adjacent micro-continental blocks.Compilation of age data for granitic rocks along the north Gondwana margin suggest a widespread granitic belt that extends from NW Turkey, Central Iran, NW Pakistan,Northwest India and Nepal to South Tibet and SW Yunnan. The age distribution shows a progression along-strike from oldest in the west(Turkey) toward younger ages in the east(Shan-Thai) along the northern margin of Gondwana; 585-543 Ma in NW Turkey, 557-522 Ma in SE Turkey and Central Iran, 516 Ma in NW Pakistan,514-471 Ma with the peak age of 495 Ma from NW India, Nepal to west Tibet and then onto Namche Barwa and Gaoligong(Tengchong), 474-472 Ma in Baoshan(Pingda) and 463-460 Ma in Shan-Thai(Ximeng). The diachronous age range of this orogenic event(~550-460 Ma) along the northern margin of East Gondwana in part overlaps with but extends to younger ages than final assembly of East(Australia,Antarctica, India, Madagascar and Arabia) and West(Africa and South America)Gondwana by the Pan-African orogenic event(~570-510 Ma).(5) The detrital zircon age spectrum characteristics suggest that the microcontients in SW Yunnan and Tethyan Himalayan orogen shared a common provenance in the Paleozoic and they were correlated paleogeographically with east India during the late Precambrian-early Ordivician. A brief outline of the events as proposed in our model is given:(a) Late Cambrian(>490 Ma). It is revealed that only few of the granites were generated during 530 Ma to 500 Ma in South Tibetan andSW Yunnan. At this stage, the Proto-Tethyan Ocean subducted beneath the Himalayan terrane and Indian Craton along the margin of Gondwana during late Cambrian or silightly earlier, following the assembly of the supercontinent, and simultaneously generating an active convergent margin. The partial melting and rollback of the subducted oceanic lithosphere caused mantle convection and coeval mafic magma emplacement below the lower crust. The heat from the mafic magmatic underplating triggered the melting of the crust and emplacement of the early Paleozoic granites.(b)Late Cambrian to early-middle Ordivician(490-450 Ma). In the early phase of this stage, the ongoing subduction of the Proto-Tethyan oceanic lithosphere triggered large scale mantle convection and mafic magmatic underplating. Widespread melting of the crust and granitic magmatism occurred in the Himalayan terrane and Indian Craton.Simultaneously, during the on-going subduction of the Proto-Tethyan oceanic lithosphere, the convergent continental margin setting supplied the tectonic driver for the orogenesis related to the coupling of the plate margin, causing the crustal thickening together with late Cambrian to early-middle Ordovician metamorphism,uplift and erosion. Detrital zircon data show that the clastic rocks in SW Yunnan should deposit at 491-454 Ma. Thus, at about 491 Ma, the micro-continents in SW Yunnan assembled with Gondwana and began to accept depositions. The subducted and break-off of the Proto-Tethyan oceanic slab caused mantle convection and mafic magma underplating into the middle-lower crust, triggering crustal melting and magma emplacement. The early Paleozoic magmatism lasted up to ca. 450 Ma.