LA-ICP-MS zircon U-Pb dating of schist from the Dalangshan Group in Suizhou City, Hubei Province, and its implications
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    Abstract:

    Located in the southern margin of South Qinling belt, the Dalangshan Group is a series of volcanic sedimentary rocks and exposed well in Xinyangdain area of Suizhou City, Hubei Province. To find out the diagenetic age of the Dalangshan Group and provide credible time information on the materials in “Qinling microplate", the authors carried out a detailed fieldwork on Dalangshan Group and collected representative schist samples to study petrography and LA-ICP-MS zircon U-Pb geochronology. The schist mainly consists of quartz (60%~65%), muscovite (30%~35%) and plagioclase (~3%). The cathodoluminescence (CL) images reveal that most zircons from the schist are euhedral with oscillatory internal zoning, but a few grains display no zoning/weak zoning suggesting metamorphic origin. LA-ICP-MS zircon analyses show that 2 in 30 spots of these zircons have relatively high content of Th (820×10-6, 888×10-6) and U (1024.47×10-6,1146.97×10-6), but others have low content of Th (25×10-6~481×10-6) and U (34.60×10-6~752.10×10-6). Except for six points with Th/U ratios ranging from 0.22~0.39, Th/U ratios all range from 0.42 to 1.41. Most zircon U-Pb ages from the schist are between early and middle Neoproterozoic (884~758 Ma) and could be divided into four groups, indicating that at least four tecto-thermal events occurred in the south Qinling region during this period. The first group of 6 spots yield 206Pb/238U ages between 758 Ma to 769 Ma with a 206Pb/238U weighted average age of 765±6 Ma (MSWD=0.31); the second group of 8 spots yield 206Pb/238U ages between 781 Ma to 802 Ma with a 206Pb/238U weighted average age of 791±6 Ma (MSWD=1.4), suggesting that both of them may be relevant to the breakup of Rodinia supercontinent. The third group of 9 spots yield 206Pb/238U ages between 818 Ma to 840 Ma with a 206Pb/238U weighted average age of 831±6 Ma (MSWD=1.2); the fourth group of 4 spots yield 206Pb/238U ages between 870 Ma to 884 Ma with a 206Pb/238U weighted average age of 879±9 Ma (MSWD=0.52), implying that both of them may be relevant to the convergence of Rodinia supercontinent. The scattering of LA-ICP-MS zircon U-Pb ages suggest that the zircons from the schist of Dalangshan Group are detrital zircons and have not been obviously influenced by later greenschist-facies metamorphism. All of the four groups of zircon ages are concentrated in the Neoproterozoic, indicating that the formation of the schist in the Dalangshan Group should not be earlier than Neoproterozoic. A spot with 2500 Ma age implies a possible tecto-thermal event in the South Qinling region during late Neoarchean to early Paleoproterozoic period. The LA-ICP-MS zircon U-Pb ages of schist in combination with the stratigraphy suggest that the Dalangshan Group was formed in early Sinian.

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李福林,李益龙,周国华,徐士元,李祖刚,周汉文,2010,湖北随州大狼山群片岩中碎屑锆石的U-Pb年龄及其意义[J].岩石矿物学杂志,29(5):488~496. LI Fu_lin, LI Yi_long, ZHOU Guo_hua, XU Shi_yuan, LI Zu_gang, ZHOU Han_wen,2010,LA-ICP-MS zircon U-Pb dating of schist from the Dalangshan Group in Suizhou City, Hubei Province, and its implications[J]. Acta Petrologica et Mineralogica,29(5):488~496.

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