Petrogenesis Of Intermediate-Acidic Volcanic Rocks Of The Early Cretaceous Volcano,in Baomao,Eastern Zhejiang

In the Late Mesozoic,as the paleo-Pacific plate subducted to the Asia-Europe plate,the southeast coast of China had experienced intense magmatism,which was an important part of the volcanic rocks of pacific rim.The Late Mesozoic volcanic rocks in eastern Zhejiang are particularly developed,with excellent volcanic outcrops and rich types.Therefore,the eastern Zhejiang is the most suitable area to study volcanism,magmatic evolution and deep mantle dynamics.The Bamao volcano is a well-maintained late Early Cretaceous volcanic institution in eastern Zhejiang.Its lithology is a combination of andesitic-siliceous volcanic rocks,and the siliceous volcanic rocks contain rare dark particle enclaves in SE China.At present,the origin of volcanic rocks and enclaves remains controversial.Therefore,this study conducts systematic field geological surveys,petrology,chronology,mineralogy and geochemistry studies on the Bamao andesitic-siliceous volcanic rocks and enclaves,elaborates their genesis and connections,and further discusses the dynamics of the deep part of the earth.Several conclusions were reacher:1.The lithology of the Bamao volcano rocks is trachy andesite and trachy dacite,and the trachy dacite contain dark particle enclaves,which are andesite.The result of zircon LA-ICPMS U-Pb dating for trachy dacite was 107.75±0.89 Ma,showing trachy dacite in Bamao volcano was producted of the late Cretaceous volcanic activity in the region.2.The plagioclase with complex zonal texture commonly in the trachy andesite and trachy dacite.Chemical composition analysis by microprobe and BSE images of the representative plagioclase crystal grains indicate that the cores,mantles and rims of plagioclase have different genesis,implying the process of dacitic and Andesitic magma aggregation in the deep magma chamber and periodic recharge in the shallow reservoir.3.Mineralogy and whole-rock geochemistry studies show that the trachy dacite in Bamao volcano were the products of the invading of basalts and melting of the middle and lower crust.Trachy andesite were formed by mixing of the basaltic magma from mantle and siliceous magma produced by partial melting of the middle and lower crust.Enclaves were formed by locally mixing of the basaltic magma and acidic magma and captured by acidic magma.4.The study of volcanic rocks in the Bamao volcano shows that large-scale crust-mantle material mixing occurred in the deep crust during the Cretaceous in th e southeast coast of China,then the magma experienced periodic recharge in the shallow reservoir.The evolutionary mechanism provides a new paradigm.It provides a new paradigm for the crust-mantle Interactions and the evolution mechanism of the magma reservoir in the shallow crust.