Discovery of double-peaking potassic volcanic rocks in Langshan Group of the Tanyaokou hydrothermal-sedimentary deposit,Inner Mongolia,and its indicating significance |
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| Authors: | Email author" target="_blank">Peng?Runmin?Email author Yusheng?Zhai Zhigang?Wang Xuefeng?Han |
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| Institution: | 1. Key Laboratory of Lithospheric Tectonics and Lithoprobing Technology, Ministry of Education, China University of Geosciences, Beijing 100083, China
2. Inner Mongolia Geological Prospecting Institute, Ministry of Chemical Industry, Hohhot 010074, China |
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| Abstract: | It is revealed that the protolith of gray-light brown potash-feldspar-leucogranulites and granulites in the 2nd formation
of the LG in Tanyaokou deposit are quartz kerotophyre of synsedimentary eruption based on the following facts and features:
(1) The rocks look compact and homogeneous without obvious crystals with naked eyes; (2) they contain blastoporphyritic or
glomeroporphyritic and blasto-crystalloclastic crystals consisting of quartz with wavy extinction and albite with obvious
alteration and deformation; (3) they also contain radiated and fibrous blasto-microspherulitic texture and swallow-tailed
bifurcate and blasto-hollow-skeleton crystal texture, representing the rapid cooling characteristic of the magma during submarine
volcanic eruption; (4) the major chemical compositions of the rocks are: SiO2 = 70.80%–76.00%, K2O (4.83%–6.22%)>Na2O(2.78%–3.80%), and K2O+Na2O = 8.63%–9.00%; and (5) their petrochemical diagrams indicate that they are volcanic rocks. Together with the characteristic
that they occur in the same sequence with potassic spilite (SiO2 = 46.12%―50.68%, K2O = 4.23%–5.93% >Na2O = 2.15%―3.14%, K2O+Na2O = 6.51%―8.08%), it can be confirmed that the volcanics occurring in the 2nd Formation of the LG in Tanyaokou district are
double-peaking potassic volcanic rocks. The discovery, together with the tuffs with ore minerals and the distribution of lead
isotopic as well as the value of Co/Ni of pyrites >1 showing the obvious endogenic metalization, can prove that the Tanyaokou
deposit is an untypical SEDEX-type deposit formed in the extension fault basin in the Mesoproterozonic aulacogen of the northern
margin of the North China Platform, and its metallogenesis is related to the synsedimentary volcanic activities and the hydrothermal
exhalation, and both the ore-forming material source and volcanics came from mantle or lower crust. These facts mentioned
above, together with the meta-volcanic rocks (double-peaking) found in the Dongshengmiao and Huogeqi districts and the host
stratigraphic sequence of LG, can further prove that the Mesoproterozoic aulacogen of passive continental margin of Langshan-Zhaertaishan
area had been unevenly expanded. This provides some information and new approaches for the study on tectonic-hydrothermal
events in Langshan-Zhaertaishan aulacogen and their evolutionary process, hydrothermal dynamical source the relationship between
the volcanic activities and the ore-forming process, the regional ore-forming regularity as well as for the correlation with
the similar deposits abroad. |
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| Keywords: | extensional rift basin double-peaking potassic volcanic rocks Mesoproterozoic the 2nd formation of LG Tanyaokou deposit Inner Mongolia |
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