Characteristics,origin and significance of strata-bound chalcedonization siliceous rocks of the Jixianian Luonan Group in Luonan Country,Shaanxi Province
Qu Hong-Jun1,2, Li Wen-Hou1,2, Yao Tian-Xing1,2, Wu Long-Fa1,2, Wang Yan-Xin1,2, Hu Jia-Sen2, Cheng Yi-Shan2
1 State Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China; 2 Department of Geology,Northwest University,Xi'an 710069,China
Abstract：Quasi-lamellar siliceous chalcedonies and lenticular flints developed at the top of the Xunjiansi Formation of Mesoproterozoic Luonan Group of the Jixian System,in northern Luonan of Shaanxi Province,southern margin of the North China Plate. The siliceous chalcedonies and flints have become jade of gay colors and fine texture,with high-grade quartzite jade level,which are claimed as the national gemstone minerals,budding in the gemstone market,with great economic development potential. At present,no research has been conducted on the characteristics,origins and diversity of jade in this siliceous layer. This paper used petrological and geochemical analyses to study the siliceous layer and grained the following findings. (1)The quasi-lamellar siliceous chalcedonies and lenticular flints were found in the fuchsia and sage green lamellar arenaceous pelitic slate at the bottom of the Xunjiansi Formation,with a thickness of 0.5~4 m;the quasi-lamellar chalcedonies are mostly fuchsia,sage green,smoke gray and colorless;most of the lenticular flints are ring-shaped with agate lines in fuchsia and sage green,and part of lenticular flints are brecciated and in sage green. (2)The main components of the silica chalcedonies are quartz;they have crypto-to microcrystalline or blocky structures,fine and hard texture,glassy luster,translucent-slightly transparent. Based on the geochemical analysis of Al/(Al+Fe+Mn)(0.62 on average),Zr/Hf(34.83 on average),Sr/Ba(0.50 on average,except an abnormal sample),V/Ni(0.37 on average),ΣREE(5.11 on average),δCe(1.02 on average),δEu(0.86 on average),and LaN/CeN(0.85 on average),the results suggest that the depositional environment is freshwater settings,and the silicon is mainly terrigenous origin. (3)The quasi-lamellar siliceous chalcedony layer in the Luoguan Group of the Jixian System is a hardgroud surface of seabeds or a siliceous crust formed by fresh-water leaching and siliceous metasomatism caused by rapid regression. Lenticular flints with agate lines and colorful quasi-lamellar siliceous chalcedonies formed during diagenesis under changes of redox conditions during deposition and post-deposition. Transformation of purple and green coloring trace element was caused by late hydrothermal invasion and baking. Brecciated flints resulted from tectonic brecciation of structural breakup zone formed by the soft and hard difference and crack deformation between the upper and lower strata of the quasi-lamellar siliceous chalcedony layer due to the strong tectonic movements of the Qinling Mountains.
Qu Hong-Jun,Li Wen-Hou,Yao Tian-Xing et al. Characteristics,origin and significance of strata-bound chalcedonization siliceous rocks of the Jixianian Luonan Group in Luonan Country,Shaanxi Province[J]. JOPC, 2021, 23(4): 651-667.
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