The Structural Features And Tectonic Setting Of Erguna Fault

The Erguna fault is located in Erguna massif which is an important metallogenesis,transmitting and hosting structure of Derbugan metallogenic belt. After a multitude ofnonferrous and precious metal deposits were found for the past few years, the Ergunafault has drawed geologists the great attention. However, there are still some infers andcontroversies in fault’s structural features, forming ages and geological settings.According to the field features, we divide the Erguna fault into North section（Shiwei-Heishantou town） and south section（Manzhouli-New Barag Right Banner）. Inthe north section, the fault displays as a long and narrow ductile shear zone and cutsthrough Cambrian Erguna river Formation, Ordovican Wubinaobao Formation, LowerCarboniferous Hongshuiquan Formation, late Permian and early Jurassic granite. Theductile shear zone is about120km long and0.5-2km wide. In order to study themacroscopic and microcosmic structure of the ductile shear zone, the author choosesthree quarries that located NNE of the Shiwei town, Shiwei gas station and Qika village.The σ feldspar porphyroid, S-C fabric and domino structures indicate that the shear zoneis a dextral fault, and indicate top to the NE motions. In Qika village, the foliation andlineation of mylonite is developed. The preferred occurrence of foliation is270°∠24°.The occurrence of lineation is also very steady, preferred occurrence is299°∠20°. Thesefeatures illustrate that Erguna ductile shear zone is a low angle dip-slip fault.We providing new LA-ICP-MS U-Pb zircons ages from mylonite, and indicate thatthe mylonite was formed from256Ma to252Ma. According to the ductile elongationand necking of feldspar, and the development of GBM of quartz, we conclude that thedeformation temperature is between⁴⁰0℃and600℃. The biotite⁴⁰Ar/³⁹Ar agesacquired from the mylonites are106.16±0.79Ma,111.55±0.67Ma and121.81±0.98Ma,which are interpreted as the cooling ages of the ductile shear zone. This period is thefastigium of large-scale NW–SE extensional tectonic event in the northeast Asian region. In Transbaikalia and northern Mongolia, dozens of late Mesozoic NE-SW trendingmetamorphic core complexes （MCCs） and small size rift basins arranged alternately. Bycomparing the cooling ages with MCCs in Transbaikalia and northern Mongolia, wefound that Erguna fault is slightly late, we consider it may relate to the inhomogeneous ofcrustal arching or a relatively low uplift/denudation rate. However, the mechanism forthe lithosphere extension and thinning remain controversial. The author disscused thedistribute characteristics, forming time, and tectonic settings of magmatic rocks, MCCsand rift basins in an attempt to investigate the geodynamic background of Erguna fault. Itwas believed that Erguna ductile shear zone was formed after the closer of theMongol-Okhotsk Ocean which induced upwelling of asthenospheric mantle material andlead to lithosphere delamination and thinning.Derbugan metallogenic belt is well-known for considerable nonferrous and preciousmetal deposits. For the past few years, a batch of large or small gold deposits and goldmineralizations were found. These deposits are differ from common epithermal golddeposits, scholars named them as ductile shear deposits. In addition, the research groupfound that the pyromeride in the Shangkuli Formation contains a large amount of asphaltand represents a form of deepvolcanic oil reservoir directly exposed to the surface alongthe south section of the Erguna fault, with excellent opportunities for studying oil and gasreservoirspaces. It is obviously different from sedimentary rocks as the main reservoir inthe Hailaer Basin and is the new and important volcanic reservoir type of the basin. Italso is the important research direction of reservoir in the Hailaer Basin in the future.Hence, the study of the structural features, deformation ages and structure attribute ofErguna fault has an important theoretical and practical significance to determine therelation between the fault and late Mesozoic volcanic, the mineralization and regularityof precious metal, nonferrous metal, uranium, and oil-gas reservoir in volcanic.