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摘要
This paper is based on the sedimentary characteristics, protolith formation, geochronology and geochemistry of the metamorphic volcanic and clastic rocks of Xuniwusu Formation, to discuss the Early Paleozoic tectonic evolution in Bainaimiao area of Sonid Youqi, Inner Mongolia. Four representative samples of Xuniwusu Formation (metamorphic gravel coarse greywacke, metamorphic dacite crystal tuff and metamorphic feldspar quartz sandstone) are analyzed by LA-ICP-MS zircon U-Pb geochronology and geochemistry. The U-Pb dating rusults show that the age of the two metamorphic dacitic crystal tuffs are 440.9 +/- 1.8Ma (MSWD = 0.10) and 440.9 +/- 1.7Ma (MSWD =0. 15). The zircons form two metamorphic dacitic crystal tuffs are euhedral-subhedral in shape, display fine-scale oscillatory growth zoning and have high Th/U ratios (0.46 similar to 1.59), indicating a magmatic origin. It is indicated that the formation epoch of Xuniwusu Formation is Early Silurian. The detrital zircon ages of the metamorphic gravel coarse-grained greywacke are between 452 +/- 1.3Ma and 535 +/-. 0 1. 0Ma, and the highest peak age is about 490Ma. The detrital zircon ages of the metamorphic feldspar quartz sandstone are between 440 +/-. 15.7Ma and 3268 +/-. 957.7Ma. The minimum concordia age is 440.1 +/- 5.7Ma, and the highest peak age is about 490Ma, but there are also four obvious peak ages (1.0Ga, 1.6Ga, 1.8Ga and 2.5Ga). According to the sedimentary rhythm, Xuniwusu Formation can be divided into three sedimentary cycles. Combining with the dating results, petrological and geochemical characteristics, protolith formation and regional magmatic data, three sedimentary stages were used to represent the evolution of Bainaimiao back-arc basin. The initial stage of rapid accumulation (452. 3 similar to 440.9Ma), formed a suite of coarse clastic rocks with low maturity, belongs to the coastal environment. The provenance is mainly derived from the magmatic rocks of Bainaimiao volcanic arc. The intermediate stage (about 440.9 similar to 440.1Ma), should be the Early Paleozoic subduction orogenic stage, which was dominated by volcanic sedimentation, mainly composed of volcanic clastic rock formation with carbonate rocks and a small amount of clastic rocks interlavers. These clastic materials were mainly derived from volcanic activity. Late stable sedimentary stage (< 440. 1Ma) formed a fine clastic rock formation and carbonate formation, belongs to the unchanging shallow marine sedimentary facies. At this time, the provenance is derived from the North China Craton, Xing-Meng Orogenic Belt and Early Paleozoic volcanic arc. According to the characteristics of sedimentary formation and volcanic formation, combined with the characteristics of the magmatic activity in the volcanic arc of Bainaimiao, we considered that the formation of Xuniwusu Formation is in the Early Paleozoic back-arc basin. At this time, the northern margin of the North China plate was the Andean-type active continental margin.
