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Geochemistry and sedimentary background of the Middle-Upper Permian cherts in the Xiang-Qian-Gui region.

2010; Science Press; Linguagem: Inglês

2095-8927

Zhen Qiu,

Paleontology and Stratigraphy of Fossils

Permian cherts occured extensively in South China.However,their sedimentary backgrounds including their origins and depositinal environments remain controversial.Based on extensive field observations and lithofacies study of the cherts in the Xiang(Hunan)-Qian(Guizhou)-Gui(Guangxi) region,we analyzed and compared major and trace elements of 81 samples from the Middle-Upper Permian cherts outcropping at 14 localities in the Qiangui(Guizhou-Guangxi) basin and the Xianggui(Hunan-Guangxi) basin.Al2O3 contents,TiO2 contents,the ratios of Al2O3/(Al2O3+Fe2O3),∑REE and the values of Ce/Ce* of the Middle Permian cherts are totally low,and show a same distribution tendency in the studied region: They were lower(0.09%~0.37%,0.00%~0.01%,0.22~0.47,2.47×10-6~14.59×10-6,0.23~0.69) in the Qiangui basin and higher(0.01%~5.73%,0.00%~0.23%,0.02~0.71,7.07×10-6~141.0×10-6,0.24~1.22) in the Xianggui basin.Al2O3 contents,TiO2 contents,the ratios of Al2O3/(Al2O3+Fe2O3),∑REE and the values of Ce/Ce* of the Upper Permian cherts are higher than those of the Middle Permian,and they are different from localities,low in the middle area(0%~1.11%,0.00%~0.15%,0~0.42,0.55×10-6~36.59×10-6,0.27~0.83) and high in both eastern(3.78%~21.37%,0.16%~0.91%,0.73~0.83,51.14×10-6~245.4×10-6,0.99~1.10) and western(1.62%~10.67%,0.04%~0.65%,0.41~0.81,23.10×10-6~248.99×10-6 and 0.46~1.39) areas.The weight percentage of non-detrital Fe in total Fe,W(Feauthig),of the Middle Permian cherts are higher than 50% with a few exception.However,W(Feauthig) of the Upper Permian cherts are lower than 50% in most samples except for the samples at the localities of Laibin,Liuzhou and Bama where W(Feauthig) are higher than 50%.In the Middle Permian cherts,the values of(La/Ce)N are 1.61~5.04 in the Qiangui Basin and 0.82~1.94 in the Xianggui basin(2.63~4.90 in Laibin).However,(La/Ce)N of the most cherts in the Late Permian are 0.69~2.47 except for that in Laibin(2.42~4.50).The values of Eu/Eu* of the Middle and Upper Permian cherts both show no regional difference,with negative anomalies in most samples.A few positive Eu anomalies are found in Bama(1.17 in the Middle Permian) and Laibin and Liuzhou(1.04~1.22 in the Upper Permian).With plotting in diagrams of Fe-Al-Mn,(La/Ce)N vs.Al2O3/(Al2O3+Fe2O3),Fe2O3/TiO2 vs.Al2O3/(Al2O3+Fe2O3),the above data suggest that:(1) The Middle Permian cherts are different in origin.Those in the Qiangui Basin are of hydrothermal origin and those in Xianggui basin are biogenic with some hydrothermal influence.Most of cherts from the Upper Permian are of biogenic origin,except those in Bama,Laibin and Liuzhou with a hydrothermal origin.(2) The hydrothermal activity in the Middle Permian was weak,although extensively occurred,by the contract,it was strong but localized.(3) Although the Middle Permian cherts developed in continental marginal seas,the Xianggui basin received terrigenous input while the Qiangui basin did not.During the Late Permian,both the Xianggui basin and the Qiangui basin received significant terrigenous input.Such a dramatic change probably reflects the regional paleo-geographic evolution and implies that regional uplift and hydrothermal activity resulted from a mantle plume upwelling represented by the Emeishan basalt.