In Huanghua Depression In The Cenozoic Igneous Petrology And Geochemistry Characteristics And Formation Mechanism

Huanghua Depression of the Bohai Bay Basin was developed during the Cenozoic on the basis of the Mesozoic tectonic evolution. By utilizing such research methods as core observation, thin section observation, trace elements analysis and U-Pb dating for zircons,we studied the lithology and lithofacies, geochemistry and geochronology of the Mesozoic acid-basic volcanics and the Cenozoic basic igneous rocks in the Huanghua Depression to trace the tectonic evolution history of the Basin and discuss the petrogenesis and dynamics of the igneous rocks. The result shows that the Cenozoic igneous rocks and associated rocks include volcanic lava, pyroclastic lava, pyroclastic rocks, volcanic clastic sedimentary rocks and hypabyssal intrusive rocks. The Mesozoic volcanic rocks are alkaline and enriched in LILE and LREE but depleted in HFSE and HREE with strong REE fraction from acid to basic rocks. The intermediate and acid rocks are characterized by low Ti. P, Fe and Mg and alkali-rich. On the contrary, the basic rocks are characterized by high Ti, Fe and Mg and intermediate alkali. Geochemistry features indicate that the acid rocks should be a product of partial melting of the crust but contaminated by the mantle; the basic rocks display the enriched mantle characteristics but might be contaminated by the crustal materials; the intermediate rocks should be a product of fractional crystallization of mafic magma. The Cenozoic igneous rocks, mainly alkaline with some sub-alkaline, are enriched in LILE and LREE, depleted in HFSE and HREE. They are featured by high Ti, Al, alkali and low Nb with some rich in Pb. These geochemical features also indicate an enriched mantle signatures but the enrichment degree is weaker. The Mesozoic andesites were formed at121.7and114.8Ma, which indicates the andesites are product of the late magma activity in the Early Cretaceous. Considering the acid and basic rocks should be later magmatic products and the outcrop layer of the basic rocks belonging to the Early Cretaceous, all the Mesozoic igneous rocks belong to the Early Cretaceous. The major and trace elements analysis showed that the Mesozoic intermediate and acid rocks in the study area were formed in the compressional island arc environment, the Mesozoic and Cenozoic basic igneous rocks formed in an extensional initial rift. This tectonic environments transform occurred in Early Cretaceous.