Magmatic Evidence Of Two Tectonic Extension Events During The Mesozoic In Hainan Island & Geodynamic Implications

The Hainan Island is located at the convergence of Eurasian plate, India-Australian plate and the Philippine plate, it is also the overlap between Paleo-Tethys tectonic domain and Pacific tectonic domain. Because of its unique geotectonic position, it is important to study the Hainan Island for understanding the structural evolution of Indo-China peninsula and continental segment of south China undoubtedly. As the Mesozoic mafic intrusive rocks in Hainan Island are less exposed, therefore, the research on Triassic Cretaceous basic rocks is very little. Based on the field geological survey, through petrology, petrography, geochemistry, isotope geochemistry and LA-ICP-MS zircon U-Pb dating methods, a more systematic study of the Hainan Island include Early Mesozoic magmatic rocks and the late Mesozoic mafic intrusive rocks and dyke swarms was done in this paper. The petrogenesis, nature of source, forming time and tectonic background of the rocks were studied in detail, and then the continental dynamics implications of the Mesozoic magmatism were dicussed, mainly on the Paleo-Tethys Ocean extending to Hainan Island and the Late Mesozoic background of continental dynamics of southeastern China.The conclusions are as follows:(1) LA-ICP-MS zircon U-Pb dating results and rock characteristics show that two stages of magmatism related with extension were devoleped during Mesozoic in Hainan Island. The first stage magmatism formed in the 240～230Ma, the representative rocks were:Xinglong bimodal intrusive rocks, Wanning gabbro and diabase veins, Fenjiezhou syenite. The second stage magmatism formed in 100～90Ma, the representive rocks were:Wenshi, Sanya, Chahe mafic dykes and granodiorite and its dioritic bodies of Qianjia rock, Tunchang rock and Baoting rock.(2) The early Mesozoic basic rocks belong to alkaline series, and are enriched in LREE, LILE (K, Rb, Ba, Th) and Ce, depleted in high field strength elements Nb, Ta,Ti, some further lock of Zr, Hf. The granite and syenite of early Mesozoic both show the characteristic of A-type granite. Sr-Nd isotope analysis showed that the source of the early Mesozoic magmatic rocks were mixed by depleted mantle and EMⅡ-type mantle. The early Mesozoic rocks from Hainan Island were formed in post-collison intraplate extension setting, and there source were influenced by dive liquid replacement. The formation mechanism of the rocks might be underplating of basaltic magma.(3) The late Mesozoic rocks belongs to weak alkaline or subalkaline series, show significant enrichment of LREE and LILE (Sr, K, Rb, Ba, Th) and Ce, deplet in high field strength elements Nb, Ta, Zr, Hf and Ti. Sr-Nd isotope analysis showed that the source area were mixed by depleted mantle and EMⅡ-type mantle. The late Mesozoic rocks were formed in an intraplate entensional tectonic envirornment, Mafic dyke swarm might be derived from partial melting of mantle garnet phase, and the mantle source has been inflenced by the subduction fluid, the magma increased rapidly in the extensional tectonic environment to form mafic dykes. The intrusive rocks were formed by high temperature mafic magma from mantle inject into the more acidic magma from lower crust, and experienced magma mixing.(4) Comparative analysis the predecessors study on magmatism and tectonic evolution of Hainan Island and its adjacent areas in the early Mesozoic, we concluded that the 240-230Ma magmatism in Hainan Islang were raleted with the collison of South China and Indo-China plate, The closure of Eastern Paleo-Tethys Ocean in the late Permian-early Triassic led to Sanya block and Qiongzhong block collided along the jiusuo-lingshuii fracture.240-230Ma may be the the extension phase after the collision, The tectonic evolution characteristic of Hainan Island in early Mesozoic controlled by the Tethyan tectonic domain.(5) Comparative analysis the predecessors study on late Mesozoic magmatism in the southeast coast, we concluded that the 100-90Ma magmatism in Hainan Island were related with the arc extension caused by the retreatment of pacific plate subduction. The dike swarms from Hainan Island at about 90Ma and the coevality dikes from Guangdong, Fujian province constitute dikes strip which is NNE orientated at about 90Ma of SE China, and they have the same cause of formation, show that it was at strong lithosphere extension stage at about 90Ma. The SE was in the arc extensional phase caused by retreatment of pacific plate.