Petrogenesis And Dynamic Significance Of The Funiu Granites Pluton In South Margin Of The North China Plate

Granites,whose petrogenesis involves the formation and evolution of tectonics,are an important part of orogenic belt research.The Funiu pluton,on which the research has a great significance to the study of evolution of Qinling orogenic belt,is located in the southern margin of the North China plate.By methods of the petrology,zircon geochronology,geochemistry and zircon Hf isotopes geochemistry,this thesis analyzes the petrogenesis and tectonic evolution process of the Funiu pluton.The following main conclusions are drawn:The Funiu pluton is a complex pluton formed by multi-stage magma invasions.The coarse red granites are the main component of this pluton.Zircon geochronology study finds that Funiu pluton formed in the Jurassic-Cretaceous Period,and experienced three times of magmatic activities.The first stage of magmatic activity occurred at around 145Ma,mainly forming porphyry red I-type granites,with arc magmatic geochemical characters.According to Hf isotopes analysis,zircon ?Hf(t)average value is-16.4 and(TDM2)average age is 2654Ma.It indicates that the source rock is continental crust.The second stage magma activity occurred at about 130Ma,forming medium coarse and I-type granites.The Hf isotopes of zircon ?Hf(t)average value is-14.1 and(TDM2)average age is 2125Ma.It shows that the source rock is Palaeoproterozoic continental crust and is similar to the source rock of the first phase of granites.The third stage magma activity occurred at 110?120Ma,forming coarse red granites and medium-fine gray granites.This stage is the formation stage of Funiu Mountain.These two kinds of granites are I-type and have similar geochemical properties.The Hf isotopes of the medium coarse zircon ?Hf(t)average value is-9.2,and(TDM2)average age is 1761 Ma.It shows that the source rock,which has late Palaeoproterozoic continental crust and a little of juvenile crust,is different from former two phases of granites.However,the Hf isotopes of medium-fine gray granites zircon ?Hf(t)average value is-3.4,and(TDM2)average age is 1370Ma.It indicates that the source rock is different from others'.The source has Mesoproterozoic juvenile crust and Paleoproterozoic continental crust.Based on field geological survey,this paper has completed:1)zircon U-Pb chronological mapping.It is found that the first phase of granites is distributed in mid-east of the pluton,while the second is mainly distributed in the mid-west.The third phase of granites is the main lithology of the pluton.2)zircon Hf isotopic mapping,it is found that the mainly source rock of pluton is the mixture of Paleoproterozoic continental crust,while the source rock of mid-western part of plution is Mesoproterozoic juvenile crust,blended with continental crust.It is considered that the Pacific plate subducted in the direction of SSW from 150 to 125Ma,and had long-range effects on the Qinling orogenic belt.Therefore the North China plate and the Yangtze plate experienced tension,which led to the reactivity of the fault structures in the Qinling orogenic belt and partial melting of the crust to form granitic magma.In the period of 120Ma?110Ma,the Pacific plate was subducted to NWW direction and retreated.The North China plate and Yangtze plate belt formed stretching tectonic environment along the Qinling Mountains orogenic belt.As a result,a huge amount of upwelling basic magma below lower crust formed a huge amount of I-type granitic magma,and source rock included juvenile crust and continental crust.Meanwhile,The North China plate and the Yangtze plate experienced expansion and formed cracks.The granite magma follows cracks to surface and form Funiu pluton.