Songpan-Ganzi Block And The Longmen Mountains Clastic Sedimentary Rocks Geochemistry

The Ganzi-Songpan block and the Longmen mountains connect the eastern margin of the Tibetan Plateau and western margin of the Yangtze block, respectively, which is the junction of the Central-China Orogenic Chain and the Sichuan-Yunnan-Helan Tectonic Belt. Therefore the tectonic affinity and relations with the Yangtze and the North China blocks have been debated. Clastic sedimentary rocks of the Precambrian to Triassic are studied systematically in this paper.The rocks are classified into graywacke and arkose, whose material source should be a mixture of mafic rocks, felsic rocks and quartzs+calcites. The total REE contents of the rocks increase gradually and the negative Eu anomaly become more evident from the Precambrian to Triassic rocks, which may imply intensifying crustal fractionation. The ratios of trace elements show affinity with Yangtze block and Qinling Orogeny.The Nd isotopic study shows that the Nd model ages range between 1.6Ga and 2.4Ga, peaked at 2.0Ga. The histogram of Nd model ages is similar to that of the Yangtze craton, Southern Qinling and Northern Qinling and quite different from that of the North China craton. Therefore, the sedimentary rocks in the Ganzi-Songpan block and Longmen mountains were mainly from the western margin of the Yangtze craton. Spatial distribution of initial Nd isotopic compositions indicates that denudating areas were in east and north, and depositing areas of deep water were in west and south of the Ganzi-Songpan basin during Triassic.The detrital zircons from the sedimentary rocks were dated through SHRIMP and LA-ICP-MS. The result shows that the detrital zircon U-Pb ages concentrate in 1400 to 600Ma, especially 900-700 Ma,which was the most intensive crustal growth stage in western margin of the Yangtze block. But, tectonic and magmatic events of 1000-700Ma have not been identified yet in North China block. Dated oldest detrital zircons indicate that the Yangtze block started forming its framework during period of 2800-2200Ma. Because the basement of the Yangtze block experienced numerous intra-crustal processes, the oldest basement components only account for ~8% of the provenance components. The detrital zircon ages of 2200-1400Ma indicate contribution of the Qinling micro-plate. The Neoproterozic to Triassic ages of detrital zircon grains suggest derivation from the Qinling Orogeny and the western and northern margin of Yangtze block. The Triassic tectonic movement was the most intensive and extensive, i.e. the Indosinian collisional orogeny movement, which led to exhumation of basements of the Yangtze block and nascent crustal components and their shedding off to present-day west of the Yangze block. Furthermore, the Qinling Orogen formed in Middle Triassic, and it was impossible to get sediment from the North China craton.The Ganzi-Songpan block and the Longmen Mountains are part of the Yangtzeblock. The western and northern margins of the Yangtze block as well as the Qinling micro-plate were the sources of sediments in the Songpan-Ganzi block and the Longmen Mountains.