The Late-Paleozoic Volcanic Rocks In The Gangdise Zone In Tibet: Petrology, Geochemistry And Tectonic Implications

As an important Mesozoic and Cenozoic magmatic arc, the Gangdise has attracted attentionsof many geologists.The widely- exposed Mesozoic and Cenozoic magmatic rocks have been wellstudied. Geologic survey and investigations in recent years have confirmed that there were volcaniceruptions in Carboniferous and Permian times in the Gangdise region. The late Paleozoic volcanicrocks however, still lack for petrological study. The author, funded by a geologic survey project anda "973" project, carded out investigations on the late Paleozoic rocks in the Gangdise zone. Basedon geologic, data of geologic mapping at 1:250,000 and 1:200,000 and preyiously published articles,the author measured the volcanic-sedimentary geologic sections in Ranwu of Basu County, Bomi,and Linzhou for Carboniferous and Permian volcanic rocks. After systemic sampling anddescription, the author did geochemical analysis with advanced apparatuses such asICP-MS, ICP-AES(TJA IRIS), MAT-262 to obtain major element, trace element, REE andSr-Nd-Pb isotopic components. By comprehensive geochemical study and comparison with thevolcanic and sedimentary sequences around the realm of paleo-Tethys, the author obtained primaryimprovements for the Permo-Carbiniferons volcanic rock distributions, petrochemistry and theirtectonic settings.(1) This paper described for the ftrst time the regional distribution geochemistry and tectonicsetting for Gangdise Carboniferous volcanic rocks. The Nuocuo Fm (C₁), Laigu Fm (C₂) and LagaFm (C₂) have volcanic rock interbeddings in marine clastic rocks. Major rock types aremeta-basalts, dacite and rhyolite, as a bimodal series. Major element geochemical study suggestthat the Carboniferous volcanic rocks are characterized by tholeiite with lower MgO and higherTiO₂,Al₂O₃,P₂O₅ contents compared with MORB. These are different from the arc basalts andsimilar to continental basalts. The Carboniferous silicic volcanic rocks belongs to sub-alcali serieswith similar geochemistry to continental rhyolite. Petrochemistry of volcanic rocks and paleo-faciescharacteristics of sedimentary rocks suggest that Carboniferous volcanic rocks erupted inextensional environment at marginal riffs. As parts of northern Gondwanaland, the Gangdise and northem Himalayas have similar extensional situation. Carboniferous rifting in Gangdise.Carboniferous volcanic activities lasted for long period but formed only a small quantity of lavacompared with those in volcanic rifted margins. Therefore, the gangdise zone did not form avolcanic rifted margin in Carboniferous but just a passive rifted margin. The Carboniferous riftingincluded double phases with hi-model volcanic activities. Post-rift thermal uplift appear as in mostcases of volcanic rifted margins. It was an incomplete or immature process of rift evolution. Theregion of Gangdise was affected by both the intensional Variscan orogenic regime and theextensional regime situated within the passive margin of the paleo-Tethys. This could be the reasonfor the typical marginal rifting process in the Gangdise.(2) Sr, Nd and Pb isotopic geochemical studies suggested that the source region situated inthe gamet zone and spinal zone of upper mantle with 60-100 km depth. The mantle source of thebasalts in the Nuocuo Formation in Ranwu might have experienced crustal assimilation beforemelting. The basalts Nuocuo Formation in Bomi and that in Laga Formation in Cuoqin haverelatively lower degrees of crust involvements in source regions. Source regions have experiencedcomplex mixing process. The primary mantle, EMâ…¡and lower crust have been involved in themixing process in Ranwu, while in Bomi, primary mantle and upper crust were involved. TypicalDupal has been recognized that could be resulted by crust recycling into the old mantle.(3) The Gangdise Permian volcanic rocks, stratigraphy geochemistry and tectonic setting aresystematically studied in this paper. The Permian volcanic rocks in the eastem Gangdise zone occurmainly in the Luobadui Formation of lower to middle Permian. To the western part of Gangdise,the middle and upper Permian volcanic rocks Occur in the Xiala Formation, with basalt, dacite andandesite, he volcanic rocks in the Luobadui Formation are basalt belonging to tholeiite series. Thebasalt in the Leqingia section has higher content of Al₂O₃ than those of continental flood basalt(CFB) and close to arc basalt and lower MgO content than that of CFB but close to archigh-aluminum basalt. The Permian volcanic rocks have enriched LREE and LILE contents andprofound Nb, Ta negative anormaly on spider diagrams. Geochemical studies suggest that the lowerand middle Permian Luobadui basalts and upper Permian silicic rocks generated in volcanic arcsituation which were related to the paleo-Tethys southward subduction. Reactivated subductionhappened in early Permian to form a oceanic arc and Tangjiaxiang arc basalt. It evolved tocontinental island arc and Andean type continental in middle and late Permian. The Gangdiseregion evolved to a strip of erosional land in latest Permian and early Triassic. The rifting innorthern Himalayas continued in Permian and the Cimmeride and Sibumasu blocks driftednorthward in middle and late Permian to form the neo-Tethys along northern Gondwanaland.(4) Sr, Nd, and Pb isotopic geochemistry reveal that the Luobadui basalts formed in differentupper mantle regions with Dupal anormaly. A mixing process happened between depleted mantleand lower crust for the Tangjiaxiang basalt at depth of 50 km±. The Leqingla and Cuoqin basaltwere formed by EMâ…¡mantle sources at depth of 50-100 km without evidence for mixing process.(5) This paper summarized the Permo-Carboniferous tectono-magmatic evolution in theGangdise and compared with other parts of the paleo-Tethys realm. Typical features display sharpcontacts at the boundaries of C₂/C₁, P/C and P₂₃/P₁, which correlated with three tectonic events. The northem passive margin of Gondwanaland began initial rifting in early Carboniferous andevolved to strong rifting stage in late Carboniferous with speeding up crust-mantle exchange. ThePermian volcanic rocks with imprints of subduction zone and crustal involvements imply the thirdphase of tectonic evolution in the Gangdise zone that became a volcanic arc.The wastern Tethysclosed in Carboniferous and formed Hercynian Variscan orogeny. It evolved into continental in lateCarboniferous. The eastern Tethys kept as extensional shallow marine environments inCarboniferous time. Paleo-Tethys subduction began in Permian to the north of Gangdise, while theHimalaya kept as extensional rifting period.(6) This paper analysed and compared volcanic-sedimentary sequences around paleo-Tethy.We suggest that the eastern and western parts of paleo-Tethys are different in geologic evolution inlate Paleozioc. In the early Carboniferous, the Laurussia and Gondwanaland began initial contactwhich merged in the composite called Pangea in mid-Carboniferous. The Variscan collisional stressand subsequent uplift felt as far as America, Africa-Arobia and eastern Australia. EarlyCarboniferous magmatism occurred as bi-model volcanic activities in south Europe, north Americaand the region of Gangdise and as alkali intrusions in northwest Hinalaya. These imply a transitionprocess of passive margin to rifting margin. In late Carboniferous, syn-orogenic and post-orogenicgranite intrusions and rhyolite eruption in Europe represents the formation of Pangea continent.Meanwhile, the eastern Tethys still remained an open seaway, where the northern Gondwanalandkeep rifting. The Hercynian continental collide and orogenesis did not happen between northernIndia and Cathaysia continents. In Carboniferous and Permian, the Cathaysia remained asindependent continents to the east of Gondwanaland and Laurussia. The Gangdise as the northernmargin of Gondwanaland occurred as shallow marine environment until middle Permian. It evolvedinto a subduction and continental arc in late Permian. The north Himalaya and Gangdise havesimilar_tectonic setting until Carboniferous. The Gangdise evolved to island arc and Andean typecontinental margin since middle Carboniferous. The north Himalaya however, became aextensional passive margin while the neo-Tethys incipient opening and widening since middlePermian.As part of northern Gandwana, in late Paleozoic time, the Gangdise evolved from passivemargin to continental arc. The Permo-Carboniferous volcanic rocks recorded the process.