The Feature Of Geochronology,Geochemistry And The Cause Study Of Ehu Composite Plution In Northeastern Jiangxi

Northeastern Jiangxi in China has experienced four stages of complex structural deformation movements since Neoproterozoic period,the four stages can be summarized as Jinning Movement,Caledonian Movement,Indosinian Movement,and Yanshan Movement.In this paper,the author selects Yanshanian medium-coarse biotite monzogranite from Ehu composite plution in Tong Yuan unit,the middle-fine muscovite from Gaoling independent unit,and the granite porphyry as the research object in the Northeastern Jiangxi region.By doing field observation thoroughly and collecting the specification of rock samples,the author learns the knowledge of petrography,petrochemistry,zircon U-Pb geochronology,and in-situ Hf isotopes of zircons.The author also identifies the age and the origin of the Magma and the genetic types of the composite plution.Based on regional geological background,the structural background of the composite plution was discussed.The result of LA-ICP-MS zircon U-Pb shows the formation of The Ehu composite plution is from the Early Cretaceous.The medium-coarse biotite monzogranite from Ehu composite plution in Tong Yuan unit,the middle-fine muscovite muscovite from Gaoling independent unit and thegranite porphyry mean²⁰⁶Pb/²⁰⁸U age of 136±1.3Ma,132±2.0Ma and 128±2.0Ma.The sequence of the Ehu composite plution period from early to late is the Tongyuan unit?the Gaolin independent unit?Granite porphyry.The Hf isotopic composition of Ehu composite plution is show that the value of?Hf(t)is-20.94?-8.94 and the age of crustal varies from 2.48 to 1.73Ga in medium-coarse biotite monzogranite from Tong Yuan unit.The value of?Hf(t)is-16.51?-6.67 and the age of crustal varies from 2.24 to 1.59 Ga in middle-fine muscovite from Gaoling independent unit.The value of?Hf(t)is-16.29?-5.00 and the age of crustal varies from 2.19 to 1.47 Ga in middle-fine muscovite from granite porphyry.The Hf isotopic composition is relatively uniform,?Hf(t)is negative,and the crustal model age is the older Paleoproterozoic.It is indicated that there is no mantle material addition and the interaction of mantle and crust in the granite source region besides the partial melting of the ancient crustal material.The Ehu composite rock mass consists of metamorphic mudstone and a small amount of metamorphic sandstone.The geochemical characteristics of major elements of the medium-coarse biotite monzogranite from Ehu composite plution in Tongyuan unit,acid rock granite porphyry and the middle-fine muscovite from Gaoling independent unit are basically the same.The characteristics of this region are rich in silicon and aluminum elements and lack of calcium and magnesium elements.The aluminum saturation index A/CNK values are all greater than 1.1,which are typical S-type granites;The geochemical characteristics of trace elements are enriched with large-scale ion-lithophilic elements Rb,Th,and U.The elements Ba?Sr?Ti and Yb.The microelement standardization diagram has similar distribution pattern.It reflect the characteristics of the same source.There is a patterns of"seagull"in the standardization curve of rare earth element.The value of LREE/HREE is 6.28?13.63 and the Light rare earths are more abundant.The Ehu composite plution has low Sr,low Yb and obvious Eu anomaly.It reflects the thin crust.Based on the regional geological background,it is think that the Ehu composite plution was formed in the extensional environment where the the crust thinned of the Early Cretaceous.Under the extension,the mantle upwelling resulted in Major faulting activity make the partial melting of the Paleoproterozoic metamorphic sedimentary rocks from the depths of the earth's crust.The reason of the formation of the Ehu composite plution is riseing and intrusion of the magma at the different time.