The Geological-Geochemical Characteristics And The Structural Significance Of The Xinghuadukou Rock Group Of Paleoproterozoic In Heihe Area

The Xinghuadukou Group (Pt₁xh) is the oldest sedimentary record in the research area, which constituted the ancient crystal basement of Precambrian. It was named by Xu Lie in 1960, the era is Early Proterozoic. It was classified as the Xinghua group and the Xing'an Bridge Group in 1979. There was different idea about the group according to the 1:200,000 regional geological survey projects completed in 80-90's. It was named "Surface Crust Rock" under 1:50,000 regional geological survey and "Xinghuadukou Rock Group (Pt2xh)" under 1:250,000 regional geological survey by the mapping method of "structure - stratum". So it belonged to Mesoproterozoic Era based on its isotopic age. The book, Geology of Heilongjiang Province, published in 1993 only summarizes the 1:200,000 information of research area. The book, Cleaning Up of Rock Stratum in Heilongjiang Province, published in 1997 also didn't refer the group. It is necessary to use the latest achievements to have a new awareness of Xinghuadukou Rock Group, to determine its era, the formation setting, the correlation of formation with Songnen Plate and so on; Furthermore, we can discuss the questions of structure nature and evolution in the Precambrian. The 1: 250,000 regional geological survey and revision survey which are carrying on at present has obtained massive information of petrology, petrochemistry, geochemistry, mineral composition, and isotope and so on.. It can provide reliable data to study the Xinghuadukou Group.The study of petrology was carried out using the "structure - stratum" mapping method in exposed. Paleoproterozoic Xinghuadukou Rock Group in Heihe region. The results showed that Xinghuadukou Rock Group was composed mainly of the anorthosite-amphibolite, the gneiss, the leptynite, the schist, the marble and so on. It assumes that the inclusion are always in the shape of island and they"float"in the intrusive rock or assume the tectonic plate in the nappe structure zone. It is called"Xinghuadukou Rock Group"in contrast with Xinghuadukou Group. The original rock is a series of metamorphic basic-intermediate volcanic rocks and the metamorphic sedimentary rocks series which is made up by volcano - sedimentary formation. The stratum belongs to mesoproterozoic according to the isotopic age (Sm-Nd: 1252-1729Ma) of 1:200,000 and 1:250,000 regional geological survey from 90's. 1:250,000 Xinglong region obtains 2178Ma (Sm-Nd) the pattern age according to the latest research results, Contrasting with the series of rocks Xingdong Group(Pt₁x)in Bindong area and Baoyintu Group(Pt1b)in Inner Mongolian area, the age of the stratum was belong to Paleoproterozoic.Petrochemistry data indicated that anorthosite-amphibolite, SiO₂=44.6⁶2.84%, Na₂O>K₂O,Al₂O₃=13.53¹7.39%,MgO2=47.12⁷1.63%, Al₂O₃=12.19¹8.3%, MgO2O>K₂O,or Na₂O2O; Theschist, SiO₂=55.82 75.54%, Al₂O₃=10.92 20.96%, Na₂O > K₂O, or Na₂O < K₂O.SiO₂ is generally higher, Al₂O₃ is lower and Na₂O is higher in contrast with North American shale and Australian shale of Achaean. In marble, CaO > MgO.The result showed that the types of oriental rock include basalt, andesite, dacite, tuff, shale sandstone, and mudstone, carbonate rocks and so on. Rock composition and division series were carried through with preexistent rock is basalt rock., including alkalinity series and sub-alkalinity series.Geochemistry characteristics of volcanic rock of Xinghuadukou Rock Group are as followed:∑REE=150²00×10-6, LREE/HREE=4.57⁶.26,δEu=0.56⁰.79 is medium negative exceptionally. Distribution patterns show the character that LREE concentrated, HREE lost, Eu negative exceptionally. It is also showing a right inclining current that right side is low and the left is high and a similar distribution model. That may be the result of homologous differentiation. The trace element oceanic ridge basalts standardization diagram assumes obviously three apophyses patterns. Large ion element such as Sr, K, Rb, Ba, Th, Ta concentrate and simultaneously Ce, P, Zr, Hf, Sm also relatively concentrate. Ti, Y, Yb, Sc, Cr, and other elements are loss. The curve are similar with the curve of Chile island arc calcium alkalinity basalt.In the samples whose preexistent sedimentary ,∑REE=126¹90×10-6,that is similar with the Russian platform mudstone and Guangdong Luoding sillimanite schist .LREE/HREE=4.75 to 8.06,δEu=0.64-0.84 is for the weak Eu negative abnormal. Distribution patterns show LREE enrichment, HREE loss, the feature of Eu negative anomaly; it also shows low left high right - right tilt, and the original rock and the distribution model are similar with the volcanic rocks. In this area, Gd/Yb ratio generally is more than 2.0 which shows the mother rock of source area oldest rock in the region. The trace element oceanic ridge basalts standardized graphical displays that large ion elements Sr, K, Rb, Ba, Th, concentrate, Nb, Ce, P, Zr, Hf, Sm also relatively concentrate simultaneously. Ti, Y, Yb, Sc, Cr, and other elements are losses. The curve was similar with which of the volcanic rock.Thus the material origin was connected with the volcanic rock..Mudstone can be classifed garnet zone, staurolite zone, basic rocks can be classifed plagioclase-hornblende belt according to the characteristic of metamorphic mineral combination. Plagioclase is andesine according to electron microprobe analysis of the chemical constituents; biotite is lepdomelane, it is belong to the amphibolite facies; amphibole is hornblende in calcium amphibole group of magnesium;The more aluminum the 4 coordinates Hornblende contains, the higher the temperature reflects; Garnet is manganese aluminum garnet of almandine garnet, which explained metamorphic temperature is located the low temperature part contrast to hornblende. The chemical composition diagram falls the kyanite belt and the amphibolite facies. According to the mineral ingredient estimate metamorphic the temperature and pressure conditions: T=450⁶50℃, P=0.2⁰.6GPa. It is the region power heat flow metamorphism product.The level of metamorphism is amphibolite facies which belongs to middle P/T types.According to the content of trace elements, such as B, Cr, Cu, Ni, V, and the ratio of Sr / Ba and Mg/Al, it reflects land-sea transition and sea water sedimentary environment. According to the ratio of V/Cr, Ni/Co, the U/Th, it shows the neritic environment which it forms are possibly rich of oxygen. According to the ratio of Th/Co, Th/Cr, the Cr/Zr, the source rocks are felsic rock and basic rock.The study of the ratio of trace elements and major elements showed that the basalt of the Xinghuadukou Rock Group reflects complex tectonic setting which includes the ocean floor of basalt, mainland volcanic rock, island arc of calcium alkali basalt and also potassium basalt rock. They are close paragenesis in space, that shows that it was ocean basin environment in Heihe region in the Proterozoic Era. But the island basalt and potassium basalt are volcanic rocks above the eruption to the ocean floor basalt or island arc. It relative to the magma caused by mantle plume(hot spot).However the active continental margin or the mainland island arc area of structural environment which sedimetary rock categories reflect has explained the ingredient characteristics: the composition set of metasedimentary rocks between the mainland island margin argillaceous sandstone should belong to the transitional types of clastic sedimentary rocks.The study of petrology and geochemistry of the Xinghuadukou Rock Group showed that in Proterozoic, Heihe area is related to a land-based source - formation of island-arc deposition, the rock combination which is with the mantle plume (hot spot) activities. It is also a combination of magma ocean floor basalt, volcanic island arc basalt, and different tectonic setting. The primitive environment could possibly be ocean basin environment with multi-islands. It indicated something by development in various combinations between the rock and the existence of the internal faults, as well as the characteristics which happened after the rock experiences the function of amphibolite facies. The rocks are formed under several different tectonic setting are joined together in the same place by the multi-islands ocean basin which is reduced with long complex structure process of oceanic crust and the arc–continenta collision.