The Basin Evolution Studies Of The Changning-Menglian Zone In The Gengma Area,Southweatern Yunnan

Changning-Menglian zone always is hot area, where oversea and Chinese geologists study tectonic evolution of Paleo-Tethys ocean. By the long term researching, study degree is evidently advanced because of new discovery and ideal is continuously brought forward. But there are many different ideals and comprehensions among different scholars and experts about tectonic feature, scale and evaluative history of sedimentary prototype basin in Changning-Menglian zone. In particular the Changning-Menglian zone is the back-arc basins, or is the margin rift of the Baoshan landmass, or is the vestigium of the the Paleo-Tehys ocean basin, or is the boundary between the Gondwana and the Eurasia Continent in ate Paleozoic Era, as well as the multitudinous dive-collision juncture zone in west Yunnan-the Mojiang collision juncture zone, the Jinsha River collides unifies the belt, the Gaoligongshan collision juncture zone and Changning-Menglian collision juncture zone, which represented the main branches of the Paleo-Tehys ocean also to have the different understanding has been a controversial focus.Study of holostratotype section of the Nanpihe Group indicates that Four types of sedimentary sequence can be recognized:Frist is the Bouma sequence representing turdidity current deposit; Second is channel depositional sequence which is consists of mudstone and lens-shaped lithic quartz sandstone or gray wake; Third is low density turiblity sequence composed of flaggy sandstone and mudstone; Fourth sequence is a rhythemic interbed composed of thin-bedded siliceous rock and siliceous mudstone. First time, the stratum collected graptolite Monograptus cf. yukonensis-Neomonograptus himalayensis zone of the Lower Devonia Nagaolingian-Yujiangian Stages, plant Zosterophyllum yunnanicum-Drepanophycus spinaeformis assemblages of the Lower Devonia Yujiangian-Shipaian Stages, Taeniocrada sp., Psilophytopsida sp., Psilophytes sp. Of the Middle Devonia and Protopteridium cf. minutum Halle、Dimeripteris sp.、cf. Hamatophyton sp. Upper Devonia and so on, which indicates that the age being Lower-Upper Devonian and should not be restricted to Lower Devonia.According to lithological association, sedimentary enviroment and regional stratigraphic correlation, it ought to be compared with the Devonian Wenquan Formation, which is deposisted on the passive continental margin, and it is the outcome of the Devonian successively spreading.Plenty of conodontes were discovered in siliceous rock of the Nanpi river Formation, which are Palmatolepis gigas zone of the Upper Devonian Frasnian and Palmatolepis triangularis zone of the Upper Devonian Famennian, it determined that is Middle-Upper Devonian Manxin Formation (Dm). Relations with the Wenquan Formation is phase change in horizontal direction, but non-simple about up-down superposition relations.For the first time on the Huiai section at Hepai Gengma and northern Cangyuan Country Paqiu, in the middle and upside silicon rock has discoved volcanic rock band in the Manxin Formation. The basalt has pillow structure, and the SiO2content47.59-51.92%, entire alkali=2.83～6.75, m/f=0.8～1.39, the rich iron (Fe) tendency is obvious. In diagram of FeO/MgO-TiO2, all plotted points fall the ocean island basalt area; In diagram of TiO2-MnO-P2O5, all plotted points fall the ocean island alkalinity island basalt area.EE=121.33-223.68ppm, the REE distribution pattern is characterized by rightinclining type with enrichment of light lanthanon; CeN/YbN=5.94～9.25, which is enrichment of light lanthanon, LaN/SmN=1.93～2.70, GdN/YbN=2.57-3.31, comparison between fractionation of light lanthanon and heavy lanthanon is not obvious.8Eu=0.99-1.10, with positive Eu anomaly, the geochemistry nature is characterized by ocean island basalt (OIB); In standardization chart diagram of trace element mid ocean ridge basalt (MORB), sample curve shape basic consistent, as a whole, incompatible elements Rb, Ba, Th, Ta, Ti and so on are enriched in the rock, compatible elements Cr, Ni, Co and so on are negative anomaly; In didgram of Zr/Y-Zr and Zr/Nb-Zr, all samples fall in enriched mantle neighbor, it indicated that activity of mantle plume has formed the overflow basalt in the Upper Devonian, the position for mantle plume activity is in the continental margin.On the Manxin Formation Silicon rock, majority of rock Al/(A1+Fe+Mn) ratio is between0.44and0.70, the MnO/TiO2value is low, which is less than0.5; The silicon rock has very weak abnormal Ce value, the Ce/Ce*ratio between0.8560and0.9662, belongs to biology origin of the ocean environment; In diagram of silicon rock major elements ratio plot, LaN/CeN-Al2O3/(Al2O3+Fe2O3), standardized REE distribution pattern for the Silicon rock and North America Shale (NASC) and so on, the overall is characterized by the continental margin silicon rock, the partial samples is characterized by similar with oceanic ridge silicon rock because of influence to hydrothermalism.Researching result indicates that study area was located in an oceanic basin environment adjacent to the continental margin on the Middle-Upper Devonian, and that the Changning-Menglian Paleo-Tethys oceanic basin with features of ocean crust had already formed on the Upper Devonian.In the Gengma, eastern Nongba area, it outcropped a set of volcanic rock-silicon rock series, in which attains Lower Carboniferous conodont Scaliognathus anchoralis zone, Lower Gnathodus bilineatus bilineatus zone, radiolarian Albaillella paradoxa zone, Albaillella cartalla assemblage, Latentifistula assemblage and Middle-Upper Permian radiolarian Nazarovella-Ishigaum assemblage, Entactinia itsukaichiensis assemblage, and newly built Carboniferous-Permian Guangse Formation.(CPg) Main rock type for Guangse Formation volcanic rock has olivine basalt, massive basalt, amygdaloidal basalt, amygdaloidal olivine-pyroxene porphyrite, picrite, olivine diabase, dolerite, amphibole basalt and so on. In diagram of A-F-M, plotted points fall the boundary between tholeiite series and calc-alkaline basalt series, which is characterized by transitional-type basalt. In diagram of FeO/MgO-TiO2, volcanic rock Ti-V, plotted points falls in the mid-ocean ridge basalt area.ΕREE=35.27～41.16ppm, and the REE partition curve is of even type with inclines toward the right, CeN/YbN=1-67～2.07, which is enrichment of light lanthanon, LaN/SmN=1.02-1.12, GdN/YbN=1.11～1.23, fractionation of light lanthanon and heavy lanthanon is not obvious; δEu=1.12～1.32, has positive Eu anomaly. On the web diagram of trace element ratio with midocean ridge basalt standardization, which is characterized by transitional-type midocean ridge basalt, K, Rb, Ba, Th, P, Ti, Sc, Cr enrichment and with Sr, Ta, Nb, Ce, Zr, Yb deficiency, is similar to transitional-type midocean ridge basalt and low potassium tholeiite. In diagram of geochemical-tectonic discriminition of trace and major elements, the plotted points falls the oceanic tholeiite area or the midocean ridge basalt area and neighbor. The silicon rock SiO2content above80%, MnO/TiO2mean value is0.58, Al/(A1+Fe+Mn) mean valuers approximately0.48, Ce/Ce*mean value is0.91, which contrast with the geochemical characteristics for known geotectonics background silicon rock, indicates that is silicon rock near continental margin ocean basin type.The ocean crust slices is composed of the Guangse Formation volcanic rock and radiolarian silicon, which is the Paleo-Tethys median ridge magmatic activity. Thus, further confirmed that the Changning-Menglian zone has truly existed representative the ocean median ridge volcanic eruption-ocean basin deposition series who is composed of the early Carboniferous volcanic rock-silicon rock, the result is Paleo-Tethys to expand fast.In Ankang and Xiang zhulin, we see the Yutangzhai Formation carbonate rocks to assume the isolated island shape conformity above the early Carboniferous Pingzhang Formation ocean island volcanic rock, from bottom to top, which is composed of the Carboniferous series, Pingzhang Formation ocean island basalt, Yutangzhai Formation carbonate collapse breccia/calcareous breccia, bed Baoma sequence which is consists of light gray purple gravel sandstone-siltstone-shale, and Yu tangzhai Formation carbonate platform deposition, have typical two-layer structure with oceanic island-type basement of volcanic rock and cap of carbonate rock, is similar with the modern Pacific Ocean’s multitudinous seamounts as well as the ancient ocean island stratal configuration and make up of typical seamount sequence.In the Gengma county Dalongtang, Nongba area, outcrop a set of fluvial facies which is composed of fine grain lithic quartz sandstone, muddy siltstone, greyish-green thin-bedded mudstone, greyish-green thin-medium bedded silty mudstone with micro-grained sand and sand, and greyish-green silty mudstone, it is in contact with underlying Devonian Wenquan Formation and Manxin Formation by unconformability of dip. Above the contact surface with the Manxin Formation can see the massive french grey argillaceous silicon breccia and partially honeycomb-shaped ferruginous beded, which is characterized by ancient weathering crust. Through the palynological studies of several position mudstone in Dalongtang section, sporopollen assemblage, pteridophytes spore content goes far beyond the gymnosperm pollen content. It mainly is acyclic trilete spores in pteridophytes spore。Carboniferous Nujinshan ophiolite melange is situated from Niujinshan to Mengyong area, where is dominated by Shuangjiang County and Gengma County respectively, southwestern Yunnan Province. The ophiolite melange can be connected with Tongchangjie ophiolite melange further north in Mengyong area. Major petrographic category are amphibolite, schistous amphibolite, layered amphibolite, greenschist. Subordinate are plagioclase granite dike, layered tonalite, tonalitic conglomeration dispersed in amphibolite, ultramafic rock and metabasalt etc..Ultramafic rock can be divided two kinds according to it’s geochemical characteristics. One kind is characterized by high and steady MgO, lower Al2O3and trace CaO, m/f=8.0-8.4, it can be correlated with metaperidotite from some representative ophiolite melange in the world.. Other kind can be considered as ultramafic cumulate, which wide ranged and continuously varied in MgO, Al2O3, CaO and ΕREE, m/f=9.8-3.6.Major elements of metamorphic mafite, characterized by K2O=0.09-0.76%, Ti2O=0.21-2.1%, show some geochemical characteristics of typical mid-ocean ridge basalt (MORB) or low kalium tholeiite (LKT) from ocean bottom. Two REE distributed pattern, LREE-depleted and flat, which display a specific character of typical mid-ocean ridge tholeiite, indicate that primary magma may be derived from near primary or a typical large ion lithophile element-depleted mantle. Trace elements distributed pattern is similar to transitional mid-ocean ridge basalt. Simultaneously, the pattern, which displayed a clear (K、R、Ba、Th、(Th)-enriched.According to occurrence and petrographic data from field, the leucocratic rock in the ophiolite melange can be divided into four kinds as follows:(1)medium-grained biotite tonalite, dispersed in amphibolite as conglomeration,(2)fine-grained layered tonalite and plagioclase granite, which is rhythmic interbed scattered in schistous and layered amphibolite,(3)plagioclase granite dike, intruding in amphibolite.(4)gneissose tonalite and plagioclase granite. Respective lithogenesis may be different, notwithstanding their geochemical characteristics are similar to ocean plagiogranite in whole.This paper studies the zircon Uranium-Lead dating method of amphibolite in the Ganglongtang section, an emplacement age of330.69-329.05Ma was obtained which ought to interpreted as the upper limit of diagenetic age of the Nujinshan ophiolite melange(CNop).All in all, it already exsisted petrographic category in Carboniferous Nujinshan ophiolite melange, their petrographic category and geochemical characteristics can be correlated with representative ophiolite melange in the world.This discovery offered a convincing evidence from petrology, which testified that Changning-Mengliang zone was a vestige of Paleozoic Tethys Sea.To sum up, Changning-Menglian zone Gengma area have comparatively entire preserved the Paleo-Tethys ocean basin deposition and rock records notes in west Yunnan even the whole world, it is composed of a series of relatively stable blocks, passive continental margins, ocean islands, seamount carbonas platforms, ocean basins, midocean ridge etc., which is characterized by Tethyan archipelagic ocean model, and has represented the Paleo-Tethyan Main Ocean of the western Yunnan. Niujingshan ophiolite tectonic melange is a subduction zone due to Paleozoic Tethys Ocean closed in west Yunnan Province, and it closed time is Triassic.In Gengma area, Paleo-Tethys Oceanic basin can be divided into western Gengma passive continental margin, middle oceanic basin and eastern continental margin in middle segment of Changning-Menglian basin. Gengma passive continental margin is consists of Lower-Upper Devonian Wengquan Formation, Middle-Upper Devonian Manxin F., Lower Carboniferous Pingzhang F., Carboniferous-Permian Yutangzhai F., Middle Permian damingshang F. and Upper Permian Shifudong F.; It is passive continental margin-Oceanic Basin sedimentary sequence, Seamount sequence and Carboniferous-Permian carbonate platform sedimentary sequence. In the ocean basin belt carboniferous series the basic volcanic rock by the Pingzhang Formation, the Carboniferous-Permian is the Guangse Formation and Niujingshan ophiolite tectonic melange is composed, belongs by the carbon period primarily oceanic ridge, the accurate oceanic ridge pulls the spot basalt-ocean island volcanic rock series, the ocean island alakli-basalt series and the ocean board internal bracing spot basalt-alakli-basalt series, for raises the Paleo-Tethys antiquely the reason land ocean transition shell curtain source magmatic activity product. The eastern edge of mainland China are mainly from the Devonian-Carboniferous Nanduan Formation, Permian Laba Formation, and representatives of Lincang to the passive continental margin sediments.Author thinks that Paleo-Tethys Ocean successively evolved based on the Proto-Tethys of the Yunnan-Burma-Tailand-Malaysia continental plates in the Changning-Menglian basin, and the basin evolution in this area is marked by inheritance expansed nature. The evolution of Paleo-Tethys Ocean accord with Wilson cycle in west Yunnan Province, it experienced Devonian successively spreading stage(D), Upper Devonian-Lower Carboniferous rapidly spreading and forming oceanic crust stage(D3-C1), Upper Carboniferous-Upper Permian oceanic crust subduction-consuming stage(C2-P3), Upper Permian-Lower Triassic oceanic crust subduction stage(P3-T1)and Middle-Upper Triassic collision-orogenesis stage(T2-T3).