Petrogenesis And Implications Of The Late Mesozoic Intrusive Rocks From Dikou Area,Fujian Province

The petrogenesis and geodynamic background of Late Mesozoic volcanic rocks in southeastern China have been controversial for many years.Dikou area of Fujian Province is located at the junction of Zhenghe Dapu fault zone and Nanping Ninghua tectonic belt.A large amount of intermediate-acid intrusive rocks are exposed along the fault,which intrude into the Jian’ou tectonic mélange.In the study,combined zircon U–Pb geochronological,whole-rock geochemical and isotopic studies were carried out on the Dikou intermediate-acid intrusive rocks,in order to discuss the petrogenesis of the intrusive rocks and tectonic evolution of these SE China.Zircon U-Pb dating results show that the Dikou intermediate-acid intrusive rocks were formed during late Early Jurassic to early Early Cretaceous.The ages of the Dikouzhen diorite porphyrites and Zhukeng quartz diorites are 178±2 Ma and 158±2 Ma,respectively.Geochemical characteristics show that they were derived from the magma formed by crust-mantle interaction.Compared with quartz diorites,the source region of the diorite porphyrites is relatively depleted.The Dikouzhen syenogranites were formed at 146±2 Ma.Geochemical characteristics show that they originated from partial melting of metamorphic igneous rocks and metamorphic sedimentary rocks.Dikouzhen diorite porphyrite,quartz diorite and syenite were formed in island arc or syn-collisional compression environment on the tectonic discrimination diagrams,which may be caused by the continuous subduction and compression of the Pacific plate.Qiankengcun granite porphyries and Luocun granites were formed in the Early Cretaceous,with ages of 141±2 Ma and 132±2 Ma,respectively.Geochemical characteristics show that they all originated from the magma formed by crust-mantle interaction.The wide range of theirε_Hf（t）indicates the difference of sources and the extent of crust-mantle interaction in different periods.The Qiankengcun granite porphyries and Luocun granites were formed in an extensional environment on the tectonic discrimination diagrams,which may be caused by lithospheric extension after the peak of Pacific plate compression.Based on the above results and previous published data,the Late Mesozoic volcanic activities of Southeast China can be divided into four cycles:Early Jurassic（195～175 Ma）,Middle-Late Jurassic（173～146 Ma）,the early period of Early Cretaceous（145～115 Ma）and the late period of Early Cretaceous-Late Cretaceous（113～82 Ma）,corresponding to the four Mesozoic tectonic evolution stages in southeast China.And the tectonic evolution process of Late Mesozoic magmatism in Dikou area could be divided into two stages.The late Early Jurassic-Middle Jurassic intrusive rocks were formed in the compression environment caused by subduction of the Pacific plate.The early Cretaceous intrusive rocks were formed in lithospheric extensional environment.