Petrography, geochemistry, and geochronology of Lower Jurassic sedimentary rocks from the Northern Tianshan (West Bogda area), Northwest China: Implications for provenance and tectonic evolution

摘要

The Bogda Mountains, situated in the southern part of the Central Asian Orogenic Belt (CAOB), is an important intraplate orogenic belt for studying the Mesozoic tectonic evolution mechanism of the CAOB. The detailed palaeogeographic evolution process of the Bogda area during Early Jurassic times remains poorly understood. The sandstone petrography, sedimentary rock geochemistry, and detrital zircon U-Pb geochronology were integrated to research the provenance of the Lower Jurassic Badaowan and Sangonghe formations clastic rocks from the northern flank of the Bogda Mountains and further constrain the palaeogeographic evolution of Bogda Mountains. The petrological and geochemical data in this study indicate that the Lower Jurassic sediments were derived from a felsic source area. Compared with the Badaowan Formation, the source rocks for the Sangonghe Formation show more mafic characteristics. U-Pb age patterns of the Badaowan Formation have multiple age groups, and most of the zircon ages are concentrated in the Carboniferous and Late Permian-Early Triassic ages. The Late Permian-Early Triassic age group disappears in the U-Pb age spectrum of the Sangonghe Formation, and every peak age is 20-50Ma younger than those of the Badaowan Formation. Combined with the compiled sedimentary facies and paleocurrent data, the felsic-intermediate volcanic rocks and granitoid intrusions in the Harlik arc and the east of the Jueluotage belt were the predominant sources of the Badaowan Formation, and then the dominant source changed to the bimodal volcanic rocks in the Eastern Bogda Mountains during the Sangonghe Formation deposition period. Early Mesozoic palaeogeographic evolution of the Northern Tianshan belt is speculated: (1) the paleotopography of the Jueluotage belt had a differential uplift along its strike, transforming from high in the west during Triassic times to high in the east during early Early Jurassic times and (2) the Eastern Bogda Mountains initiated uplift during