Petrological and geochronological evidences of tectonic exhumation of Heilongjiang complex in the eastern part of Heilongjiang Province,China

Heilongjiang complex,along the Mudanjiang fault in the western part of the Jiamusi massif,was due to the collision between the Jiamusi and Songnen massifs.The complex is composed predominately of strongly deformed mylonites with a large number of masses or slices of ultrabasite,blueschist,metasilicatite and marble in varied sizes and deformation.Although most masses are strongly transformed to be mylonites marked by new metamorphic mineral assemblage of crosstie,stilpnomelane,phengite and albite,some undeformed glaucophane schist masses with assemblage of glaucophane,albite and epidote are locally preserved in Yilan area in west of Jiamusi massif.The relationships of metamorphism and deformation between undeformed masses and deformed mylonites indicate the Heilongjiang complex was at least subjected to two phase metamorophism of early high-pressure metamorphism and late lower P/T metamorphism associated by ductile deformation.The muscovites associated with crossite in Huanan area give the 40Ar/39Ar ages of 176.5±1.9Ma and 184.5±2.1Ma respectively.This event is prevalently recorded in Heilongjiang complex in Jiayin,Yilan and Mudanjiang areas,indicating that the entire Heilongjiang complex experienced a major tectonic exhumation in the Early-Middle Jurassic.This event not only had exhumated the early blueschists to the surface,but also controlled the formation and evolution of the Late Mesozoic basins in the Northeast China.