Provenance Analysis And Tectonic Attribute From The Lower Carboniferous Riwanchaka Formation Of The Central Tibetan Plateau

The Qiangtang block is located at the northern Tibetan Plateau. The Qiangtang block isdivided into the Qiangtang–Baoshan block in the north and the Qiangtang–Changdu plate inthe south, separated by the Longmu Co–Shuanghu–Lancangjiang suture zone. South ofQiangnan shows good affinities to Gongdwana superterrane, whereas north of Qiangtang hasplenty of features similar to Eurasia continent.There are significant differences in the base-ment properties and composition, Palaeozoic sedimentary formations and biogeographicalaffinities, magmatism and metamorphism, and stages of tectonic movement between bothsides of the Longmu Co–Shuanghu–Lancangjiang plate suture zone in the Tibetan Plateau. Inrecent years, the stydy of Longmu Co–Shuanghu–Lancangjiang suture zone has a break-through. Many critical geological problems still remain in the Qiangtang area concentratelly.The mysterious veil of unmanned area is gradually being unraveled. Professor Li Cai likens itto the pearl on the crown of the geology of the Tibetan Plateau.The EW trending Early Carboniferous stratum, i.e., Riwanchaka Formation (C1r), ischaracterized by enrichment in warm water type fossils, and has an exposed area of about100square kilometers at Riwanchaka area in the central segment of the LongmuCo–Shuanghu–Lancangjiang suture zone. Riwanchaka Formation is one of the rare sedimen-tary strata in the central Qiangtang, whose deposition timing was constrained by the fossils.This, together with the recognition of the Yangtze type Guizhouphyllum-Yuanophyllum coralfossil assemblage within this formation, makes it become the focus of attention of manyscholars. However, the development of the Riwanchaka Formation is mainly in the un- manned area of Qiangtang, whereby the circumstances are extremely hard, the fresh water islack and having no traffic. Therefore, its research is still very superficial. More recently, it isfound that the typical Gondwana facies cold-water sedimentary stratum, i.e., the ZhanjinFormation (C2P1z), tectonically crops out to the north and south of the Riwanchaka Forma-tion. The abundant warm-water fossils within Yangtze-type sedimentary strata is sandwichedin the typical Gondwana facies cold-water sedimentary strata, the explanations of that specialgeological phenomenon has great significance for the evolution of this area and even thewhole Longmu Co–Shuanghu–Lancangjiang suture zone.In this paper, on the basis of a large number of field work, I research The RiwanchakaFormation comprehensively by making full use of the predecessors’ research data and results,using the knowledge of petrology, isotopic chronology and structural geology and other dis-ciplines integrally. An analysis of213age measurements of sandstone samples from the Ri-wanchaka Formation indicates that detrital zircons fall into three age ranges:325–400,400–500,500-600,800-1100and>1200Ma.The youngest detrital zircon age is320Ma, con-sistent with fossils portfolio reflect the times, confirms its Early Carboniferous era Visean.These age ranges contain peak ages of360,437and454Ma that reflect the tecton-ic–magmatic events of the initial arc reduction stage in the Longmu Co–Shuanghu–Lancangjiang ocean and the Caledonian Orogeny, implying that the provenance of the clasticrocks was probably the Yangtze continent. Together with palaeontological information, weconclude that the Riwanchaka Formation comprises Carboniferous sedimentary strata of Eu-rasian Yangtze type, and was the product of shallow-water deposition in an island-arc envi-ronment.