| Bengbu uplift is located in the southeast of North China Craton,the southmost part of Jiao-Liao-Ji orogenic belt and records the most ancient crustal material,the geological events and the evolution process.This paper is based on metamorphic petrology,tectonic geology and isotope chronology selected Wuhe-Mingguang geological section as the main research object.System research on metamorphic complex material combination type,the strength of metamorphism,deformation,formation environment,the distribution and change law of chronology,in further understanding the southern margin of north China,the tectonic levels of metamorphic deformation characteristics.Based on the Wuhe-Mingguang geological section,rocks type and characteristic divided 103 lithostromes into 5 rock units: XiGuDui unit,ZhuangZiLi unit,FengShanLi unit,XiaoZhangZhuang unit and YinJian unit.A detailed of analysis of rocks type suggests that the transition from north to sourth of mafic rock to salic rock is characterized by the transition from the base rock to the acidic rock.The results of peak metamorphic PT conditions of representative rocks from 5 petrological tectonic units are T=731?~801? and P=0.90~1.10GPa(XiGuDui unit),T=733?~968?and P=0.92~1.23GPa(ZhuangZiLi unit),T=764?~825?and P=0.5~ 0.9GPa(FengShanLi unit),T=567?~729? and P=0.3~0.5GPa(XiaoZhangZhuang unit),T=555?~700? and P=0.3~0.5GPa(YinJiaJian unit)respectively.XiGuDui unit and ZhuangZiLi unit has similar PT conditions,were granulite-facies metamorphism.The pressure of FengShanLi unit is far below than lower unit and it were granulite-facies metamorphism.XiaoZhangZhuang unit and YinJiaJian unit has similar PT conditions and all of them are amphibolite facies metamorphism.The P-T trail analyzing on garnet pyroxenite from ZhuangZiLi unit reveals Bengbu uplift experienced at least 3 phase and PT conditions are =691 ? ±36Ma and=0.93GPa±0.05(Stage-I),=953?±14 and =1.04GPa±0.03(Stage-II),=649?±25 and =0.86GPa±0.04(Stage-III)respectively.It indicated a rapid heating and cooling process with nearly constant pressure.The research of deformation strength and strain ratio deformation analysis revealed Bengbu uplift overall performance for weak deformation,the ccharacterictics of strain rate faster.From XiGuDui unit,ZhuangZiLi unit to FengShanLi unit deformation become weak.From FengShanLi unite to XiaoZhangZhuang unit and YinJiaJian unit deformation become strong,and XiaoZhangZhuang unit have highest deformation strength grade among 5 units.Furthermore,each layer has a strong deformation zone,and it is mainly controlled by mylonite,rheological zone and fluid.The metamorphic basement chronology analysis revealed that it had multiple stages of growth or metamorphism and the peak ages are 1844 Ma,2084Ma,2480 Ma,2538Ma and 2922 Ma.Among them,XiGuDui unit as the ancient continental crust age of >2800Ma,and from XiGuDui unit to ZhuangZiLi unit and XiaoZhangZhuang of metamorphic basement age by sophisticated new small differences.It is believed that the main stage metamorphic age of 1.8~1.9Ga is associated with tectonic thermal event/orogenic event of ~1.9Ga.The protoliths age of 2.4~2.6Ga was consistent with the main growth period of north China craton of ~2.5Ga.The age of > 2.8Ga from inherited zircon is basically the same as the formation period of palaeocontinental crust.Petrology,metamorphic,deformation and chronology analysis results revealed that Wuhe-Mingguang geological section can be further divided into 3 structural levels:(1)Level-I: consists of mafic rocks from XiGuDui unit and ZhuangZiLi unit,for the overall weak deformation,deformation of local area enhancement,all experienced granulite-facies metamorphism.The PT conditions and rocks type similar to lower crust and medium-lower crust.(2)Level-II: consists of mafic and salic rocks from FesngShanLi unit with less deformation,experienced amphibolite facies metamorphism and granulite-facies metamorphism.It similar to the characteristics of medium-upper crust.(3)Level-III: consists of salic rocks from XiaoZhangZhuang unit and YinJiaJian unit with strong deformation strength(rheological zone).It underwent greenschist metamorphism to amphibolite facies metamorphism,has the feature of material composition and the formation of the environment on the upper-crust.In addition,the features of Bengbu uplift rutile trace elements,Nb/Ta and Cr/Nb ration reveals the crystalline of metamorphic characteristics,mainly divided into 3 stages:(1)Stage-I: Nb/Ta>40 and controlles by Nb,the genetic type may be hydrothermal crystallization.(2)Stage-II: 10<Nb/Ta<20 and controlled by Ta,the genetic type may be hydrothermal crystallization.(3)Stage-III: Nb/Ta<10 and controlled by Ta,the genetic type maybe crust-source.Comprehensive analysis,stage-I and stage-II has hydrothermal characteristics,stage-III is more prone to shell source characteristics,and the original rock type of 3 stages may be felsic or argillaceous granulite. |
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