Age,metamorphism And Petrogenesis Of The Amphibolites From Heilongjiang Complex,yilan Area

The Heilongjiang Complex is a set of tectonic mélange formed by the collision between the Jiamusi and the Songnen-Zhangguangcai Range massifs.At present,some research achievements have been made in the field of the Heilongjiang Complex.However,there are still some problems to be solved:the metamorphism of the Heilongjiang Complex,except for blueschist,is poorly studied;the formation age and evolution mechanism of the Heilongjiang Complex remain controversial.In order to solve these problems,this thesis carried out petrographical,chronological,geochemical,mineral chemical,and EBSD fabric analyses on the amphibolite from the Heilongjiang Complex and granodioritic mylonite from the Jiamusi Massif in the Yilan area.The following five findings have been obtained.Zircon U-Pb dating results show that the protolith ages of the amphibolite are 261.3±3.0 Ma and 261.8±3.3 Ma,while that of the granodioritic mylonite is 207.8±2.2 Ma.Combined with macro-and microscopic structures and geochemical characteristics,the protolith of the amphibolite is inferred to be gabbro,formed in an active continental margin environment,and derived from partial melting of the lithospheric mantle metasomatized by subduction fluids.The protolith of the granodioritic mylonite is granodiorite,I-type granite,formed in a post-orogenic extensional environment and originated from partial melting of the lower crust.Through petrographical and mineral chemical studies,combined with traditional geothermobarometer,it can be concluded that the amphibolites experienced two phases of metamorphism and deformation.During the first phase metamorphism,the amphibolites experienced three metamorphic stages:prograde metamorphic stage,peak metamorphic stage,and retrograde metamorphic stage,which records a clockwise P-T path.The mineral assemblage of the prograde metamorphic stage may be Chl+Zo+Q,which reached low greenschist facies(300?450?/0.40?0.50 GPa).The mineral assemblage of the peak metamorphic stage is Grt+Amp_core+Pl₂,and the metamorphic degree of this stage is low amphibolite facies(622?664?/0.64?0.90 GPa).The mineral assemblage of the retrograde metamorphic stage is Amp_rim+Bt+Pl₂,and the metamorphic degree of this stage is high greenschist facies(518?579?).The first period of metamorphism is not accompanied by deformation.The mineral assemblage of the second period of metamorphism is Act+Chl+Zo+Ms,and the metamorphic degree of this period is low greenschist facies(310?410?/0.34?0.46 GPa).The second phase of metamorphism is accompanied by deformation,which is characterized by mylonitic texture:amphiboles appear as porphyroclasts,part of which are elongated to be the lenticular shape and oriented;the matrix consists of plagioclase,zoisite,muscovite,chlorite,and actinolite,with very fine grain size.According to petrographical and quartz EBSD fabric characteristics,it is concluded that the granodioritic mylonite underwent two phases of metamorphism and deformation.The mineral assemblage of the first phase metamorphism is Grt+Pl+Q₁.The deformation of this phase is marked by the grain boundary migration recrystallization and subgrain rotation recrystallization of the quartz grains.The deformation temperature of the first period of deformation is 550?650?.The mineral assemblage of the second phase of metamorphism is Zo+Ms+Q₂.During this phase,the deformation was characterized by bulging recrystallization of the quartz grains.The deformation temperature of the second phase of deformation is 350?450?.The metamorphic degree of the two periods of metamorphism experienced by the amphibolite and granodioritic mylonite is the same,which reflects that they have undergone the same tectonic evolution process.The first period of metamorphism and deformation is possibly related to the amalgamation of the Jiamusi and Songnen-Zhangguangcai Range massifs.The second period of metamorphism and deformation may be associated with the tectonic exhumation of the Heilongjiang Complex.