Late Mesozoic Volcanism In The Zhenqian Region Of Fujian Province

Zhenqian area in northern Fujian is tectonically located in the junction of the NorthwestFujian rifting belt and the East Fujian volcanic fault depression belt. Detailed investigation andresearch have been carried out on the Late Mesozoic volcanic rocks in this area, with focuses onclassification of volcanic sequences, cycles and facies，determination of formation time ofvolcanic rocks, recognition of volcanic apparatuses and structures, and analysis of regionalvolcanism process. Some significant results and achievements are obtained in this thesis.The Late Mesozoic volcanic strata in the area can be divided into four formations,including Late Jurassic Changlin and Nanyuan Formations and Early Cretaceous Huangkeng andZhaixia Formations. Based on their lithological association, strata sequence and contact relation,the strata of the four formations can be subdivided into12members. Some fossils occur in theChanglin, Nanyuan and Huangkeng Formations.On the basis of its composition and texture, lava with broken phenocrysts in the LateJurassic Nanyuan Formation can be classified into7types, including acidic granular with breccia,acidic felsic, acidic cryptocrystalline, intermediate to acidic granular, intermediate to acidicgranular with breccia, and intermediate to acidic cryptocrystalline ones. The acidic felsic lavawith broken phenocrysts occurs as marginal facies covering on the fourth rhyolitic welded tuffwith crystal fragments, with transition between different types of the lava. The U-Pb ages forvolcanic rocks are148.06±0.33Ma for the lava with broken phenocrysts, and150.70±0.38Ma forthe rhyolitic welded tuff with crystal fragments.The volcanic activities in the area can be divided into3sedimentary-eruptive cycles. Theproducts of the volcanic activities can be classified into7volcanic facies, including explosive,explosive-overflow, eruptive-overflow, extrusive-overlfow, volcanic conduit, eruptive-sedimentary, and subvolcanic facies. Four assemblages of volcanic facies can be recognized.There exist at lest5gradelV volcanic structures such as volcanic eruption basin andsub-belt, and44grade V volcanic structures such as caldera，dome，cone, and stratovolcano, ofwhich35volcanic eruptive centers are newly recognized. The volcanic edifices are spatiallycombined as beaded, superimposed, dissected, and satellite-like forms.The Late Jurassic volcanic rocks and structures are distributed in the NE direction andconstitute Shoulin-Meilin NE volcanic eruptive belt, indicating that the Late Jurassic volcanicactivities were mainly controlled by NE basement fault. Evidently influenced by NE and NEEbasement faults, the Early Cretaceous volcanic rocks and structures occur in the NW direction asbeaded form and constitute Songxi-Ningde NW volcanic eruptive belt, showing that the EarlyCretaceous volcanic activities were mainly controlled by combination of NW faults with NE andNEE basement faults.