The Evidence Of Petrology And Geochronology On Tectonic Evolution Of Heilongjiang Complex In Eastern Heilongjiang Province, China

Heilongjiang complex, bounded by the Mudanjiang fault on the west, is distrubuted between the Jiamusi and Songnen massifs. The complex as a melange zone resulted from rupture and collision of Jiamusi and Songnen massifs. The complex is mainly exposed in three regions of Mudanjiang - Muling, Huanan - Yilang and Luobei– Jiayin areas in the eastern part of Heilongjiang Province, and is composed predominately of strongly deformed mylonites with a large number of blocks or slices of ultrabasite, blueschist, metasilicatite and marble in varied sizes and deformation, constituting a typical tectonic mélange with matrix and blocks.The relationship of metamorphism and deformation between blocks and mylonites ,and metamorphic mineral assemblage features show that, although most metabasalt blocks are strongly deformed and transformed to be mylonites marked by new metamorphic mineral assemblage of crosstie, stilpnomelane, phengite and albite, some undeformed glaucophane schist blocks with assemblage of glaucophane, albite and epidote are locally preserved in mylonites.The relationships of metamorphism and deformation between undeformed masses and deformed mylonites indicate the Heilongjiang complex was at least subjected to two phase metamorphism of early high-pressure metamorphism and late lower P/T metamorphism associated by ductile deformation.In order to further insight into the tectonic setting of Heilongjiang complex and its relation with the Jiamusi and Songliao massifs, we present geochemical and LA-ICP-MS U-Pb Zircon data for undeformed blueshist facies metabasalts and deformed blueshist facies metabasalts in Heilongjiang Complex.The results show that the protolith of the blueschists is subalkali-basalt with OIB features,instead of ocean rift. The zircon datings futher surport the result above. The zircons from metabasalts are complex. LA-ICP-MS U-Pb dating from 96 zircon analyses gives five age groups of 210~330Ma with a peak age of 240Ma, 380~420Ma with a 400Ma, 480~540Ma with a 503 Ma, 1600~1900Ma with a 1824Ma and 2400~2500Ma with a 2512Ma. These imformation from zircons in the metabaslts also prove that the protolith of the metabaslts is not formed in ocean rifting.The zircons with age group of 380~420Ma are predominent in dated zircons. The crystal forms and composition of the zircon indicate they are originated from baslt magma.At present, this age group has not been reported in Jiamusi and Songnen massifs. Consequently, it is suggested that Jiamusi and Songnen massifs was an united microcontinent block before 380~420Ma. The block not only has a Precambrian crystaline basement, but also subjected to the strongly metamorphism and granitic magmatism in 503Ma. Therefore we favour the view that the basalt most likely formed in a rift environment within the Jiamusi-Songnen microcontinent similar to the current Red Sea.The age group of 210~330Ma with a peak age of 240Ma is the youngest zircon ages with metamorphogenic features. According to the fact that the basalt had been metamorphosed to be glaucophane schist,we suggest this age group from zircons records the form time of bluschist metamorphism in Heilongjiang complex. Combined with postorogenic granites with the ages of 210-180Ma widely developed in XiaoXinganLing and Zhangguangcailing regions to west of Heilongjiang complex, it is reasonable that this age group of 210~330Ma represents the time of blueschist metamorphism and the collision of Jiamusi with Songnen massifs.In order to reveal the metamorphic and deform age of predominent mylonites in Heilongjiang Complex,we present the results of geochemical analysis and muscovite Ar-Ar dating from mylonites.The results indicate that the protolith of mylonites for the most part formed in continental margin envirorment. The muscovites coexisting with crossite in mylonites in Huanan area give the (40)~Ar/(39)~Ar 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 form the general tectonic framwork in NE direction in the northeast China.