Petrological Characteristics And Genesis Of Permian Granite In Baoting Area,Hainan Province

Hainan Island is located at the junction of the Eurasian Plate,the Indian Plate and the Pacific Plate.It is the superimposed area of the ancient Paleo-Tethys tectonic domain and the Pacific tectonic Belt.The genesis of granite in Hainan Island and its representative tectonic significance have always been one of the hot issues that geologists have paid attention to.The main rock type of Permian granites in Baoting area of Hainan Island is syenite granite rich in biotite.Based on detailed field investigations,this paper attempts to comprehensive petrographic studies,zircon U-Pb chronology,and full-rock geochemical studies are discussed to discuss the Permian granitic magmatism and its tectonic setting in Baoting area,Hainan.Attempt to further reveal the inspiration of the Permian granite genesis to the evolutionary history of the Tethys tectonic.The Permian granite in study area vary greatly in size,and are mainly composed of five Granitic pluton of Tongshi,Zhonglinling,Huangshanshuiku,Xiangnan and Hongfeng team.Available high-resolution zircon U-Pb geochronology results so far demonstrate that the intrusions formed at 252.0-262.7 Ma,which reflects that the set of intrusive rocks is the product of magmatism in the Middle-Late Permian.Rocks with medium to high Si O₂ content(64.74%?75.69%)and relatively high alkali content(Na₂O+K₂O=0.94%?2.84%),have an average A/CNK value of less than 1.1,and belong to quasialuminum-weak overaluminum granite which also have the characteristics of shoshonitichigh-K calc-alkaline series.The total amount of rare earth elements in Permian granite varies greatly,ranging from 77.95 to 450.27?g/g.The rare earth element distribution curves of most granite samples are obviously rightwards,and the light and heavy rare earths are strongly differentiated;with moderate negative europium anomalies,the majority of?Eu is between 0.67 and 0.27.Most rocks are obviously enriched in light rare earth and large ion lithophilic elements(LILE)such as Rb,Th,U,K,and Pb,and deplete high field-strength elements such as Ta,Nb,and Ti.The trace element spidergram are all right-dipping curves.The enrichment of Rb and the loss of Sr,Ba,P,and Ti indicate that the Baoting Permian granite in Hainan experienced the separation and crystallization of plagioclase,biotite,apatite,and ilmenite.The two-stage Nd isotope model age T_2DM of intrusive rocks in the study area is concentrated in the range of 1248?1681Ma,which is basically consistent with the twostage model age of Nd isotope in the Yangtze block(1.4?1.7 Ga),which may reflect thestudy area and the South China Blocks have an affinity.These data indicate that a small amount of Neoproterozoic crustal material may have been added to the source area.The obvious heterogeneity of the magma source area combined with the characteristics of xenoliths suggest that the Middle-Late Permian granites in Baoting area may come from the-crust-mantle mixing source area.The magmatic source of Permian granite belongs to the melting of lower crustal basic materials or the mixed source of crust and mantle,and has the characteristics of homologous magmatic evolution sequence.The rock genetic type belongs to I-type granite,and locally has the characteristics of A-type granite.The Middle Permian granites are mainly co-collision granite,and the Late Permian granites are mainly post-collision granite.In summary,the Permian granites in Hainan Island were formed in the tectonic environment of the Indosinian plate subduction toward the NE margin of the northern margin of the South China block,which was mainly affected by the Paleo-Tethys tectonic domain.It has been transformed from ocean-land subduction to post-collision,and the whole is the product of the tectonic evolution process of squeeze-relaxation.