Petrogenesis Of Mesozoic Granite In Songjianghe Region From Dunhua,Yanbian Korean Autonomous Prefecture

As an important part of continental crust,granite is one of the main objects to reveal the dynamic process of crustal evolution.Therefore,the study on the granite petrogenesis has attracted much attention from scholars for a long time at home and abroad.The study area is Songjianghe region from Dunhua City,Yanbian Autonomous Prefecture of Korea,Jilin Province and it is located in the middle of the terrain collage zone between the eastern section of the northern margin of the North China Platform and the southern part of the eastern section of the Xing Meng Orogenic Belt.This region records numerous tectono-magmatic events and its tectonic history of the region is dominated by the evolution of the Paleao-Asian Ocean,the orogeny of Xing Meng orogenic belt,the subduction of Paleo-Pacific in Mesozoic and supershell fracture in Cenozoic.Obviously,it is significance to systematically study the diagenetic age,elemental geochemistry and tectonic setting of granites for revealing the geological evolution history in the region.We choose medium-fine grained porphyritic monzogranite,medium grained porphyritic monzogranite,fine grained biotite monzogranite and medium grained biotite monzogranite as research objects,based on field investigation in the study area.The geology,geochronology and geochemistry researches have been carried out on them,and we obtain following conclusions:The geology,petrography and zircon LA-ICP-MS U-Pb dating reveal that the magmatism of medium grained porphyritic monzogranite?169.6±1.8 Ma?,fine grained biotite monzogranite?168.0±1.0Ma?and medium grained biotite monzogranite?162.3±2.1Ma?occurred in the the early Middle Jurassic?162.3¹69.6Ma?.Based on emplacement relations and the U-Pb geochronology results that the diagenetic time order are medium-fine grained porphyritic monzogranite,medium grained porphyritic monzogranite,fine grained biotite monzogranite and medium grained biotite monzogranite.The petrography and geochemistry of granites indicate that medium-fine grained porphyritic monzogranite,medium grained porphyritic monzogranite,fine grained biotite monzogranite and medium grained biotite monzogranite belong to I type granitic rocks.The geochemistry features of them are similar,although some differences in mineral content,texture and structure.Furthermore,these granites in the mineral assemblages are mainly consist of alkali feldspar,plagioclase,quartz and biotite.They are enriched in Si?range from 68.54%to 73.32%?and alkali?the content of Na2O+K2O range from 7.82%to 8.63%,Na2O>3.2%,?,lower Mg?range from0.34%to 0.81%?,w?K2O?/w?Na2O?greater than 1,A/CNK less than 1.1.The REE of them are characterized by enrichment of LREE(range from 68.78×10^-6 to143.54×10^-6),loss of HREE(range from 4.93×10^-6 to 6.85×10^-6),weak positive Eu anomaly and?REE range from 73.71×10^-6 to 149.69×10^-6.The trace element of them are enriched in LILE,such as Rb,Sr,Ba,and depleted in HFSE such as Nb,Ta.The characteristics of A/NK?range from 1.68 to 2.00?,w?Zr+Nb+Ce+Y?(range from165.01×10^-66 to 254.16×10^-6),TFeO/MgO?range from 2.87 to 3.85?,ID?range from82.84 to 91.35?reveal that they are belong to medium-high fractionated I type granitic rocks.The ratios of Rb/Sr,Nb/Ta,La/Nb,Zr/Hf are similer to the crustal rocks.Granites are characterized by?_Hf?t?values of zircon in them range from-22.3 to-7.5 with the two-stage Hf model ages?TDM2?varying from 2325 Ma to 3643 Ma,high Sr/Y and?La/Yb?N,loss of P and Ti.Combined with the achievements of regional datas,the granitic magma is formed by the upwelling of asthenosphere intruding lower crust or ancient mafic rocks,which due to the crust accretionary resulting from continental collision relating to subduction of the Pacifific plate.Moreover,the granites in study area was formed in the early Middle Jurassic.Granites belong to I type granitic rocks and they are similar to those from the active continental margin setting.The granitic magma was derived from the partial melting of crust.Combined with the achievements of regional geological investigations,we conclude that these granites formed under collision-extension environment related to the reactivation of the late Paleozoic subduction zone in the northern margin of the North China Platform.The reactivation of the subduction zone due to the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.Medium-fine grained porphyritic monzogranite and medium grained porphyritic monzogranite have been reformed by brittle tectonic action in Late Jurassic.Their direction of structural deformation is the same as direction of Songjianghe fault and deformation extension direction of ductile shear zone.