Geochemical and Si-O isotopic characteristics and sedimentary environment of the siliceous rocks in the Longsheng area of the northern Guangxi,China
Based on systematically detailed geochemical and Si-O isotopic analyses of siliceous rocks outcropped in the Longsheng area of the northern Guangxi in this paper,we have explored the genesis,material sources and significance of the depositional environment of the Longsheng siliceous rocks.The results suggested that the Longsheng siliceous rock samples have SiO2 contents ranging from 92.09％to 99.27％,Si/Al ratios ranging from 412.93 to 3841,Al2O3/(Al2O3+Fe2O3)ratios ranging from 0.03 to 0.58 Al/(Al+Fe+Mn)ratios ranging from 0.04 to 0.66,MnO/TiO2 ratios ranging from 0.43 to 26.64,∑REE contents ranging from 0.20 to 2.37 LREE/HREE values ranging from 1.76 to 11.18,and U/Th ratios ranging from 1.31 to 18.52.There are negative Eu anomalies and relatively strong positive Ce anomalies in their chondrite-normalized REE distribution patterns,but relatively strong positive Eu anomalies and relatively weak negative Ce anomalies in their flat NASC(North American shale composition)-normalized and PAAS(Post-Archean Australian shale)-normalized REE patterns.The serious depletion of incompatible trace elements,such as Th,U,Pb,Nb,Ta,Hf,Rb,etc.,suggests that the Longsheng siliceous rocks are of the hot-water depositional origin.The Longsheng siliceous rocks have δ30Si values varying from-0.36‰ to 0.78‰ and δ18O values varying from 17.42‰ to 18.24‰,indicating that they are the hot-water sedimentary siliceous rocks deposited in the semi-abyssal environment of the continental margin.Comprehensive analysis shows that the Longsheng siliceous rocks were deposited at 765～760Ma of the Neoproterozoic era.The large-scale magmatic activities and the formation and evolution of rift basins during the same period imply that the Longsheng siliceous rocks were formed by the hot-water deposition and were deposited in the area between the continental marginal sea and the semi-abyssal basin.
siliceous rockgeochemistrySi-O isotopessedimentary environmentLongsheng area
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NSTL
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