The Metamorphic And Deformed Characteristics Of Heilongjiang Melange In Yilan Area

Heilongjiang complex is mainly exposed in Jiayin-Luobei area in the north, inYilan-Huanan area in the center and in Mudanjiang area in the south between Jiamusi andSongnen Massifs in the eastern Heilongjiang Province. It is oriented in S-N directionalong Mudanjiang fault. The Jiamusi and Songnen Massifs are located in the eastern partof Central Asian Orogenic Belt between Siberia and North China cratons, and in the westof Pacific block. There is a growing consensus that the Heilongjiang complex is not anormal metamorphic strata unit, and is an accretionary complex, which is related to theJiamusi-Songnen collision. However, the formation age is still controversial.This paper chooses Heilongjiang mélange in Yilan area for object. Based on thecomprehensive analysis of the previous researches and the field and indoor work，weinvestigate the field distribution characteristics, material composition, the original rocktype, the age of formation, and the metamorphism and deformation of Heilongjiangmélange.The Heilongjiang mélange in Yilan area is composed of the deformed matrix andblocks. The deformed matrix includes strongly deformed felisic mylonites and mafic rocks,and the blocks consist of ultramafic rocks, metamorphic gabbro, metamorphic maficvolcanic rock marble and metamorphic silicalite. The relationship of metamorphism anddeformation between blocks and deformed matrix, and metamorphic mineral assemblageshow that most blocks are strongly deformed and occur as tectonic lens and slices in theintensively foliated felisic and mafic rocks. The undeformed blueschist blocks observed in Heilongjiang complex in Yilan are worth to notice. These blueschists are characterizedby glaucophane without directed distribution, which will provide important evidences forthe tectonic evolution of complex and metamorphism and deformation characteristics.Geochemical characteristic show that the deformed felsic rocks were formed in anactive continental margin depositional environment. The deformed mafic rocks andresidual blueschists show OIB affinity, indicating that they were formed in an intraplateenvironment.Based on petrologic and mineralogical data, the paper studied the relationshipbetween metamorphism and deformation and the type of metamorphism and pattern ofdeformation, and proposed that the Heilongjiang mélange in Yilan area has experiencedblueschist facies---glaucophane greenschist facies metamorphism. The formation ofblueschist (glaucophane) is related to subduction, and the transition from blueschist toglaucophane greenschist is related to structural uplift.Isotopic dating shows that180-154Ma is the deformation age of Heilongjiangcomplex, because the field relationship of blueschists and deformed matrix clearlyillustrates the formation age of blueschist was earlier than that of glaucophane greenschistfacies metamorphism-deformation. According to210-180Ma post-orogenic granite ofLesser Khingan and Zhangguangcai Range, emplacement age (222±5Ma) of QinshuiPluton in Yichun area and317.7Ma gneissoid adamellite of Yichun area, the paperproposed that the formation age of blueschist is317-220Ma, which is the collision ageof Jiamusi and Songnen massif.