Regional Geological Characteristics Of Qiangtang In The Tibetan Plateau

This paper is basised on the preparation of the Qiangtang area of 1/100 000geological map.The study area,Qiangtang,which belongs to the north of Tibet giant orogenic belt,between Ruolagangri-Litang-Jinsha River suture zones to Bagonglake-Nu river suture zones,is the main part of Changdu-Qiangtang landmass.The study area discussed by this article contains the north of Qiangtang landmass,Kaixinridge-Zaduo volcanic rift fault fold belt,middle of Qiangtang suture zones and south of Qiangtang landmass from north to south.This paper mainly discusses therock formations,tectono-stratigraphy,intermediate-acid intrusive rock and regional metamorphite,Finally,the author summarizes the characteristic of the two suture zones of north and south and the main landmass in Qiangtang,andstudies the evolution of Qiangtang area again.The main results obtained as follows:?1?There is found more complete lower palaeozoic strata in the north of Rongma township,with the characteristics of biological cold water biota,and the littoral-neritic faciesclastic rock are discovered in the Devonian and middle-upper Jurassic strata of Gangmacuo and Chasang,Richly Containing biota of South-type warm phase.Showing that the basement of Qiangtang area is composed by the landmasses ofDifferent Cathaysian type and Gondwana type.?2?thetectono-stratigraphy of study area mainly contains seven superficial Structural rock,two medium-superficial Structural rock and two deeper Structural rock,allofthemhaverelationshipwithTheIndosinian orogeny.thetectono-stratigraphy appears mainly in middle of Qiangtang,Similar-Wuqi–Zuogong and Ruolagangri-Litang-Jinsha River suture zones.?3?the yanshanian granite of Qiangtang have Dual band characteristics:the south band,which is located in Duolong-Sewa,belongs to I type granite band of subduction origin,and with diorite-granodiorite-granite combination,the times is mainly in the late Jurassic and early Cretaceous,with porphyry Cu-Au ore deposit,and it belongs to continental margin magmatic arc belt.the north band,which is located in Gangmacuo-Buruocuo,belongs to S type granite band of shell source origin,and with monzonitic granite-granite combination,the times is mainly in the cretaceous,and it belongs to back arc magmatite belt.?4?Ruolagangri-Litang-Jinsha River suture zones,which has gone through twoorogenic cycle of expansion-collision from Hercynian,early Indo-Chinese epoch to late Indo-Chinese epoch,and ends in the orogenic belt of late Indo-Chinese epoch which is closed in the end of the Norian Age of late Triassic.Corresponding,Contaminated rock of Nuolagangri in the north of the study area is not the foreland flysch,but ophioliticmelange.?5?North of Qiangtang landmass is composited by fore-arc bathyal-secondary bathyalturbiditeclasticrock?Tibetan Xiahe group,T₃z?,marine faciesisland arc volcano-sedimentary rocks(Tuzhonghu group,T_2-3t)and back-arc carbonate platform?Juhuashan group,T₃jh?,formed a complete arc basin system,because of the expansion and southward subduction of oceanic basin in Nuolagangri.?6?In the Mesozoic era,the south of Qiangtang had a transition process from passive continental margin to activitive continental margin.The stratums of late Indo-Chinese epoch and yanshanian are superficial in north and deep in south,with the evolutive characteristic from shallow shelf facies to the deep-sea slope facies,and ending up with abyssal facies in the Bangong lake and the Nu river from nouth to south.In addition,neutral-basic volcano rocks mainly belongs to tholeiite which were formed in tension of mainland.Showing that south of Qiangtang has the characteristic of passivecontinental margin.Its formation was related to class anger oceanic basin.Tiegelong group(K_1-2t),coverring in Sewa group(J_1-2s)and other strata with angular unconformity relationship,is land facies neutral-acidic volcano rock with clasticrock,andhasthecharacteristicofcontinental margin volcanorock.Southof Qiangtang was transformatfrom a passive continental margin to active continental margin volcanoarc,is the product of Bangonglake and the Nu riveroceanic crust subductedfrom northward to the southern Qiangtang massif.?7?The middle Qiangtangsuture zones is the abutment zones,because of divisional evolution betweenGondwanan landmasses and Guthrie Gangdeseocean basin of cathaysian typemainland,and is formed by Gondwana typeterrane and Cathaysian type Old terrane.TheGondwana type terrane,ocean islandterrane,and island arc volcano rock show that the middle Qiangtangsuture zonesare formed during the process of passive continental margin and active continental margin collision orogenic.?8?Kaixinridge-Zaduois volcanic rift fault fold belt in Paleozoic.neutral-basic volcano rocks is in the lower clastic rock of Zaduogroup?C₁Z?,showing that the rift faultaffect had been starting to happen.From the Ribao group we can know that Kaixinridge–Zaduo area were formed in the environment of intercontinental rift to continental rift evolution.Jiapeila group in Triassic and Yanshiping group in Jurassic are up the stratums of carboniferous-permian,showing that the band is the uplift belt which divides the Qiangtang basin and Changdubasin in the Mesozoic.