Abstract:

Moerdaoga district is situated in the northern Eerguna block. There occur a lot of Precambrian intrusive and metamorphic rocks, including Fengshuishan complex, intermediate and mafic complex, macrophenocryst syenogranite, adamelite, etc. The authors collected a series of intrusive rock thought to form in Neoprotozoic period, and separated the zircons from the rocs for U-Pb dating by LA-ICP-MS method. The obtained ages suggested that the majority of rocks, which were thought to form in Neoprotozoic period, emplaced in Early Mesozoic period, except the migmatic syenogranite formed in Neoprozoic. It can be conclude that there took place intensive tectonic and magmatic activities in Moerdaoga and adjacent areas during the Early Mesozoic period. According their ages, the Early Mesozoic intrusive and metamorphic complexes in Eerguna and northern Daxinganling Mountain could be divided into such three groups as 243-246 Ma, 210-229 Ma, 200-205 Ma, and the 200-205 Ma group is the most extensive. The 243-246 Ma group is dominated by the intermediate and mafic intrusive complex, maybe related to the extension occurred in after the Late Permian orogensis of Xing’an-Mongol collision. The rocks formed from 200 Ma  to 229 Ma are dominated by granite and metamorphic complexes which have such characteristics formed in continental collision setting as calc-alkaline, per aluminous, S-type granite, migmatites, biotite, muscovite and horbenlende as well as lacking associated volcanic rocks. The geology and geochemistry of the Early Mesozoic granites and metamorphic complex indicate that the middle part of Mongol-Okhotsk Ocean (Transbaikal) might be closed at the end of Middle Triassic to the beginning of Late Triassic period when the collisional orogeny might start. The peak collision might took place at the end of Triassic period (200 Ma±). The confirmation of the Early Mesozoic granite and its geodynamic setting in Eerguna and northern Daxing’anling is also important to reconsider the geological evolution of the area.

Key words: Daxing’anling, Moerdaoga, Early-Mesozoic;granite, LA-ICP-MS dating, Mongol-Okhotsk orogenic belt, continental collision