| Nyenchen Tanglha subzone subordinated to the structure-magmatite belt of Gangdise-Nyenchen Tanglha. structure-magmatite belt of Gangdise,that is,Gangdise landmass,here had experienced a large mount of Earth's tectonic evolution.and always been a hot topic in geological study in the world.Tibet Plateau has intensive magmatic activity.During the rise of the Tibet Plateau,accompanied with different magmatic activity,it's development associate with magmatism throughout all the ages.Gangdise tectonic belt of magmatism is the most intensive region of plateau magmatism.National Mineral Resources Potential Evalution of Tibet Autonomous Region 1:250000 scale complied geological sheet in the western section of Nyenchen Tanglha subzone, Structure-magmatite belt of Gangdise-Nyenchen Tanglha.Mainly focus on the 12 sheets,there are as followings:LhazêCounty sheet, Sangsang sheet,Coqên area sheet,Huo'erba sheet,Selepug sheet, Cuomai area sheet, Coqên County sheet, Rabka sheet, Damxung sheet,Lhasa sheet, Shigatse sheet, Xainza sheet.Frequent magma activity occurred in the western section of Nyenchen Tanglha subzone, Structure-magmatite belt of Gangdise-Nyenchen Tanglha's Mesozoic,with varying degree of magmatism activity from the Triassic period to the Cretaceous period.Our study area lies to Nyenchen Tanglha magmatic subzone in the middle of Gandise mountain.,from west to east,from south to north,more detailed statistical and analyze intrusive rock's definition of the stratigraphic age, lithostratigraphic classification, stratigraphic size and modes of occurrence in the whole subzone.Triassic intrusive rocks of distribution within the subzone not so much,exposed in Xiabo and kangbowuri in the south of Xainza area,ocurrence in batholith shape,the area of outcrop about 292.94 km~2, K-Ar isotopic age about 172.0Ma,the age of older Zircon U-Pb about 217.0±3.4Ma. lithological belongs to Giant spot granodiorites,associate with relatively unitary.In structure-magmatite belt of Gangdise,not only Jurassic intrusive rock exposure have grown dramatically, but granitic activity aslo increasing.lithology characters appears diversity.Shallow intrusive rocks mainly composed of diorite porphyrite,which occurs in Cuomai & Rabka area;which mainly distributed among in Cuomai & Namuding,northwest of Xuru Co and amujiongle.the time to primary Magma intrusion activity about Mid-Late Yanshannian,intermediate-acid rock dominant,very few mafic rocks is avaliable so that not discussed in this paper.Intrusive rock distributes zonally which extended in linearly from the North West West.There is no Early-Middle jurassic intrusive rock body found in the study area,which primarily appears Late Jurassic intrusive rock,rock formations are batholith,sill,stock,and boss,etc.outcrop area about 3000km~2,the age of older Zircon U-Pb about 152±3.8Ma, Intrusive rock types belongs to biotite adamellite,vein rocks mainly are diorite porphyrite,Most sample rocks converged in A-type Granite area,some of appears in I-type and S-type Granite area,it illustrate Granite magma not only drrived from mantle but from crustal bottom.From late Jurassic-early cretaceous,accompanying with sustainable development of magmatism in the whole Gandise belt,and Yarlung-Zangbo subduction to north continuously.By the end of Crataceous,the Yarlung-Zangbo began to close, collision and orogeny occurred after the closure of the Yarlung-Zangbo ocean.Cretaceous intrusive rocks exposed in a large area,which distributed in Coqên area, Rabka area,, Selepug area, Xainza area and Shigatse area,which cross related with mafic in Yarlung-Zangbo River-ultramafic rock subbelt-Nyenchen Tanglha arc lava subbelt. Magma intrusion activity mainly occurred in the Late Yanshannian, intermediate-acid rock dominant,very few mafic rocks is avaliable so that not discussed in this paper. Intrusive rock distributes zonally which extended in linearly from west to east, extended in linearly from the North West West dirction. Intermediate-acid rock mainly distributed in the middle and western of Nyenchen Tanglha subzone, north-west of Xuru Co and amujiongle.In the study area,intrusive rock of Early Cretaceous Epoch,most mainly outcrop intrusive rock belongs to Upper Cretaceous, rock formations are batholith,sill,stock,and boss,etc.outcrop area about 3000km~2,the age of older Zircon U-Pb about 68±3.8Ma Ma, Intrusive rock types belongs to muscovite-granite, biotite diorite-granite, plagioclase granite,two-mica granite, granite porphyry, granodiorite-porphyry, Black mica syenogranite,Black mica granodiorite, granodiorite, quartz monzodiorite.In the Late Triassicepoch epoch,Gandise area already uplifted,magma activity is developmented.The age of Granodiorite with huge phenocryst in the nature of Volcanic arc granite characteristics about 217Ma,this is first time to determine late Triassic epoch neutral-acid intrusive rocks existed in this point.Besides,it aslo found Lower Triassic is composed of clastic rock and interlining with thin layer calcarenite,overlying with limestone(included coral,etc) interlining with fine grained detrital rock which sediments of intertidal zone-barrier gulf environment,preliminarily defined time is Late Triassicepoch epoch.This discovery has been fully proved Gandise intraplate exist spreading centers of undeveloped ocean basin in Late Triassicepoch epoch,which put on a firm basis for the development of Jurassic Period-the Cretaceous period back-arc basin.In Early Jurassic Period,this area cannot proceeding in deposition, unconformity contact with upper Triassicseries.Middle-late Jurassic- Early Cretaceous Epoch,which formation period of back-arc basin.At the same time,Marine&River in New Tethys was peak time which spreading and subduction to northern, Bangong Lake Nujiang River ocean segment in the closing process.Affected by two ocean evolution process, the belt of Shiquanhe-Yongzhu-Namucuo appears arc spreading,formed back-arc basin with a large scale which distributed in an east-west direction.Because of the closing of theYajiang Ocean,and constinuous subduction to northern.In the southern area, developed a large amount of granitoid batholith;In the central area developed calc-alkaline lava system,and finally result in the closing of back-arc basin,finished the history of transgression.Upper Cretaceous,Himalayan plates and Gangdise-Nyanqêntanglha plate occurred in continent-continent collision,lead Gandise Nyanqêntanglha area uplift into landmass,forming large-scale fold and several branches of thrust fault developed on the north side.Because of continental crust shorten&thicken,fold deformation,Linzizong group underlying layers has forming regional angular unconformity.Affected subduction and anataxis process, fault structures in the back arc of Gandise Volcanic magma arc developed large scale Cal-alkaline volcanic eruption&intermediate-acidic magmatic intrusive activity. Upper Cretaceous- Eocene volcanic-the peak period of magmatic intrusion,it is thus clear that regional volcanic eruption and Eruption discontinuous, intermediate-acidic magmatic intrusive rock in Upper Cretaceous are seven types. Pulsatile of volcano-magmatic activities, anastomosis in nonlinerity uplifting of Gandise area. |
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