Characteristics Of Intermediate-Acid Volcanic Rock’s Lithofacies Of Qingshan Group In Southeastern Jiaolai Basin

The Early Cretaceous intermediate-acid volcanic rocks are widely exposed in the southeast margin of the Jiaolai Basin and the interior of the Sulu orogenic belt.However,the study of volcanic lithofacies has not been carried out in detail.In addition,there is a controversy about the properties of magma source.In this paper,the characteristics of volcanic rocks’lithofacies of Bamudi and Shiqianzhuang Formations in Jiaozhou and Huangdao areas are investigated that may supply a certain reference for volcanic oil and gas exploration in the similar surrounding area.Based on the delicate field survey and microscopic identification,simultaneously integrating previous researches and relevant geological data,the characteristics of volcanic lithologies and facies association are researched,then the regularity of volcanic activity being analyzed.Additionally,the tectonic environments of volcanic rocks in these two areas are compared through geochemical analysis of major,trace and rare earth elements so as to further explore the differences of magma source properties.The obtained main results is as follows:（1）In Jiaozhou area,the lithology of volcanic rocks mainly includes basaltic andesite,andesite and trachyandesite in Bamudi Formation.Rhyolite and dacite are developed in Shiqianzhuang Formation,which belong to K-rich and calc-alkalic volcanic rocks.In Huangdao area,the Si O₂ content of Bamudi Formation volcanic rocks ranges from 56.41%to60.89%,the lithology mainly includes trachyandesite,trachyte and dacite.The Si O₂ content of Shiqianzhuang Formation volcanic rocks ranges from 71.74%to 81.37%,the lithology is mainly rhyolite,which belongs to K-rich,calc-alkalic and alkalic volcanic rocks.（2）In Jiaozhou area,the diagenetic mode of the pyroclastic rocks of Bamudi Formation includes welded,unwelded and compacting,the main diagenetic mode of the pyroclastic rocks of Shiqianzhuang Formation is welded compaction.Moreover,there are few pyroclastic rocks in Shiqianzhuang Formation in Huangdao area.（3）The explosive facies and volcanic sedimentary facies in Jiaozhou and Huangdao areas are divided into a series of subfacies and microfacies.In Jiaozhou area,the development of airfall subfacies and pyroclastic flow（block and ash flow,scoria and ash flow）of Bamudi Formation indicates that the early volcanic activity was more intense,and the volcanic activity showed a periodic change of intermission-eruption-effusion-intermission.A large scale of pyroclastic flow（pumiceous pyroclastic flow）with welded phenomenon developed in Shiqianzhuang Formation,indicating that the volcanic eruption intensity is higher than that of Bamudi Formation,and the volcanic activity shows a periodic change of eruption and effusion.In Shiqianzhuang Formation of Huangdao area,there are base surge（ground surge）with oblique bedding,and extensive rhyolites with effusive facies,indicating that the volcanic eruption intensity is weaker than that of Jiaozhou area as a whole.（4）The consistence of REE and trace element distribution curves and the analysis of related geochemical elements show that the volcanic rocks in Jiaozhou and Huangdao areas were formed by similar magmatic process.The magma source was produced by partial melting of enriched lithospheric mantle reformed by metasomatism.The magma underwent partial melting and crystallization differentiation during upwelling.The volcanic rocks in Jiaozhou and Huangdao area are all obviously affected by the delamination of lithospheric mantle.However,the occurrence of alkalic rock series as well as the adakite characteristics suggest that the magma source of volcanic rocks may be deeper in Huangdao area.