The Material Composition And Tectonic Evolution Of Majiajie Group In Jiamusi Block

Majiajie group which exposed in central Huanan region, Jiamusi block, is mainly a set of metamorphic rocks with contact metamorphic characteristics. Previous researchers considered it as an integral part of Jiamusi block, one of the three main metamorphic sets exposed in the block (Mashan Group, Heilongjiang group and Majiajie group). However, the distribution of Majiajie group is limited, and the metamorphism type is significantly different from the Mashan granulite Group and the Heilongjiang blue schist group. Therefore, the latter two have been the focus of research and the former one was studied on low level.In recent years, studies on the composition and evolution of the Jiamusi block have been made in great progress, mainly in three aspects.①propose the so-called Heilongjiang Group is not a normal metamorphic formation, but a set of the structure mélange with ophiolite remaining. (Xing-Zhou Zhang, 1992; Jin-Yi Li, 1999; Caoxi, 1992).②zircon date testing results show that metamorphic times of Mashan Group in granulite face is not Archean or early Proterozoic, but early Paleozoic, 520 Ma(Wilde et.at.2001).③it is traditionally considered that granite rocks in Jiamusi block were formed in Precambrian, but new research shows most of which are formed in late Proterozoic (Sun and Wu have yuan, 2001).The above progresses have put forward a crucial issue that Jiamusi block may not be a consolidation unit in Precambrian, as it was still in strong tectionic activities in Phanerozoic.In this view, the topic of this paper is Majiajie group with the field geological investigation and indoor analysis, so as to understand the metamorphic characteristics and the formation times of Majiajie group, and reveal its geological significance by comprehensive comparison with Mashan group and Heilongjiang group, and provide further evidences for understanding the tectonic properties and evolution of Jiamusi block in-depth.In this paper, aimed at the above problems and based on collection and analysis of previous results, the field work on Huanan and the surrounding areas was engaged from July to September in 2007. Systematically inspected the rock assemblages and occurrences of the Majiajie group and the surrounding rocks, collected rock samples for analysis and testing, and studied the above problems by methods of structural geology, petrology, isotopic chronology and genetic mineralogy separately.By the field geological work and thin section examination indoor, the Majiajie Group contact metamorphic rocks can be divided into the four metamorphic mineral zones, zircon ages is obtained by zircon date testing on the underlying granite gneiss and the western margin of Meizuo intrusive rock, separately 504 Ma, 260Ma, which confine the original rock formation age of Majiajie group. It is identified by rock geochemistry analysis that the original rock in Majiajie group may be the re-sediments resulted from weathering and erosion of the silt rocks on the top the Mashan group, the source region has the same characteristics with the young continental crust in post-Archean, source district type is mainly the mainland island arc, with characteristics of stable continental margin. According to the fact that Mashan group metamorphism times is 500 Ma and contact metamorphism times is 260 Ma, with the comparability of Majiajie Group and the Late Paleozoic sediments, it is determined that the original rock times is lower late Paleozoic.