Abstract The siliceous and dolomitic rocks of the Permian Fengcheng are the most significant oil source rocks in the Junggar Basin. However,there is a debate on the origin of dolomitic and siliceous rocks. Base on the core observation and geochemical analyses including major elements,trace elements and silica isotopes,a more detailed study was made on the origin of siliceous rock. The results indicate the SiO2 content of siliceous rocks ranges from 55.15% to 85.61%,which indicates the rocks are not purely siliceous and contain a relatively high content of salt minerals formed in an alkaline lake and minerals formed in hydrothermal fluids. The siliceous rock is mainly distributed around the hydrothermal vents and usually interbedded with dolomitic rocks. Boron is enriched in siliceous rock,ranging from 90×10-6 to 10 000×10-6 with an average of 680×10-6,which might be related to the searlesite and reflect an alkaline lake depositional environment the Sr/Ba of siliceous is larger than 1,and the Th/U and Sm/Nd values range from 0.62 to 2.06 and 0.20 to 0.24,respectively. REE distribution pattern of siliceous rock shows Eu negative anomaly and Ce negative anomaly or weak positive anomaly and significant enrichment in LREE. The δ30Si values range from 0.39‰ to 1.88‰. Trace elements and δ30Si values are different from typical hydrothermal siliceous origin. Both geological occurrence and geochemical characteristics of siliceous rocks indicate that the siliceous rocks of Members 2 and 3 of Fengcheng Formation are formed in an alkaline lacustrine involves hydrothermal activities. The formation of siliceous rock is controlled by many factors,such as the location of hydrothermal vents,the hydrothermal exhalative period and the temperature of each hydrothermal vents. During the hydrothermal exhalative period,the hydrothermal fluids mix with the alkali lake water to form the siliceous rocks that contain salt and dolomite. The sedimentary environment of siliceous rocks is a relatively closed environment with high salinity and alkaline reduction. During the intermittent stage between hydrothermal exhalative activity,the dolomite rocks containing salt minerals are the major products.
Fund:Financially supported by the National Science and Technology Major Project of China(No.2016ZX05002-002)
Corresponding Authors:
About the corresponding author Wang Guo-Zhi,born in 1964,is a professor and doctoral supervisor. He is mainly engaged in sedimentology and petroleum geology. E-mail: 87409459@qq.com.
About author: About the first author Gao Yuan,born in 1980,graduated from Chengdu University of Technology in 2011 with her Ph.D. degree. Now,she is a lecturer and is mainly engaged in petrology teaching and research. E-mail: yuanyuan400406@126.com.
Cite this article:
Gao Yuan,Wang Guo-Zhi,Li Na. Geochemical features and origin of siliceous rocks of the Permian Fengcheng Formation in the northwestern margin of Junggar Basin[J]. JOPC, 2019, 21(4): 647-660.
Gao Yuan,Wang Guo-Zhi,Li Na. Geochemical features and origin of siliceous rocks of the Permian Fengcheng Formation in the northwestern margin of Junggar Basin[J]. JOPC, 2019, 21(4): 647-660.
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