Elemental and Sm-Nd isotopic geochemistry of clastic sedimentary rocks in the Garzê-Songpan block and Longmen Mountains

Abstract：Based on systematic studies of major and trace element and Sm-Nd isotopic geochemistry, the Precambrian to Triassic clastic sedimentary rocks in the Garzê-Songpan block and Longmen Mountains have been finished. Ｔhe rocks are determined to be grａywacke and arkose and their material source should be a mixture of mafic and felsic materials and quartz+carbonate. The total REE contents of the rocks increase gradually and negative Eu anomalies become more evident from Precambrian to Triassic rocks, which may imply intensifying crustal anatexis. The tectonic setting was relatively stable during the Late Paleozoic；therefore there appear pronounced negative Ce anomalies. The Nd model ages range between 1.6 and 2.4 Ga；this age range is similar to that of the Yangtze craton, South Qinling and North Qinling and quite different from that of the North China craton. Therefore, the provenance of the sedimentary rocks in the Garzê-Songpan block and Longmen Mountains is the western margin of the Yangtze craton, and the block is a tectonic unit that developed on the basis of the Yangtze craton. The correlation between provenances of the western Yangtze craton shows that the source materials of the sedimentary basin should mainly be the Neoproterozoic tectonic layer and secondarily the Archean and Paleoproterozoic tectonic layers. Triassic clastic sedimentary rocks may contain Late Paleozoic mafic mantle-derived, and the spatial distribution of the Nd isotopic composition indicates that there occurred denudating areas in the east and north and deep-water depositional areas in the west and south in the Garzê-Songpan basin during the Triassic.