Late Mesozoic Tectonic Evolution In Southeastern South China

There are large scale areas of late Mesozoic magmatic province that distribute in Southeastern of South China Block(SCB).The major rock types include granitoid,basanitoid,rhyolite and volcaniclastic rocks.In recent decades,geologists in China and abroad have paied more attention on the age,genesis and geodynamic background of these rocks.In this study,a near N-E trending giant-ring volcanic crater in Yongtai-Yuanzhuang zone,Fujian Province,is selectd as our study subject.We put our study mainly concentrate on petrography,zircon chronology and Hf isotopic characteristics,in order to restrict the age of volcanism in late Mesozoic,then to define the origination of these volcanics.Our study reveals that the rock samples we have collected in study area are crystal tuff(SC54),andesitic crystal tuff(SC55),welded tuff(SC58)and rhyolite(SC64).Zircon U-Pb chronology indicates that the ages of sample SC54,SC55,SC58 are ca.125Ma,ca.128Ma and ca.126Ma,respectively.Meaning they were products of Early Cretaceous volcanism.While the age of sample SC64 is ca.95Ma,suggesting that it was Late Cretaceous volcanic product.In suit zircon Hf isotope analyses of these samples yield sHf(t)values of-12.5 to+0.2(SC54),-3.5 to+0.3(SC55),-5.8 to-1.0(SC58),-0.6 to+2.5(SC64).All tuff samples show negative value of EHf(t),and rhyolite shows positive value of ?Hf(t).The zircons yield Hf model ages(TNCc,TNCc?1.81?0.96Ga,1.24?0.96Ga,1.38?1.06Ga and 0.99?0.79Ga(SC54,SC55,SC58,SC64,respectively).In this study we conclude our research that the paleo-Pacific plate transfered from flat-slab subduction to plate delamination under the plate of SCB during late Mesozoic.At ca.185-160Ma,the flat-slab subductioion gradually stopped,and began to delaminate under gravity.At ca.160-105Ma delamination came to climax,large scale crust-mantle interactions were initiated.At ca.105-90Ma the delamination terminated,and paleo-Pacific plate transferred to normal subduction and roll back.Therefore,the magmatic activity of SCB in later period of late Mesozoic is most likely link to the roll back of paleo-Pacific subduction plate.