| The eastern Harbin area is part of the East Asia Orogenic Belt,which locates in the eastern Songnen block,west of Yitong-Yilan fault zone and Jiamusi,and the southwestern of Zhangguangcai Range.Pelitic rocks in the study area comprises black,grey-black pelitic siltstones and drab fine-grained siltstones,in which clay minerals are rich and full of genetic significance.8 samples collected from measured section are systematically studied.X-ray diffraction is used to determine the whole-rock,clay mineral contents and illite genesis including illite crystallinity,polytype,and b0dimension.Chlorite composition are determined by electron microprobe.Based on clay minerals genetic information above,epigenesis-very low grade metamorphic temperatures and pressures are estimated.X-ray fluorescence spectrometer and laser ablation inductively coupled plasma-mass spectrometry are used to determine whole rock geochemistry,which is applied to elucidate the depositional setting and sedimentary environment of study area.XRD analysis results shows that:The pelitic rocks contain mainly quartz?41.2%?,albite?13.3%?,and clay minerals?45.5%?.Clay The clay minerals are illite,chlorite,mixed chlorite-smectite layers,and kaolinite.Illite appear in all samples,with an average of 89.9%.Kaolinite content average is 2.3%?n=3?.Smectite-chlorite mixed-layer content average is 5%?n=5?,in which semectite ratio is 30%.Chlorite content average is 5.7%?n=3?.Characteristics of mineral composition indicated Tangjiatun formation experienced epigenesis to very low grade metamorphism,and a metamorphic grade of low anchizone.Illite crystallinity values in the study area are 0.32–0.42,with an average of 0.37.Interpolation method suggests an average metamorphic temperature of±230°C,which indicates low anchizone and formation experienced very low grade metamorphism.1Md+2M1 mixed polytypes are apparent in all samples,indicating late diagenetic zone and a metamorphic temperature of±200°C as estimated by illite crystallinity geothermometry.Epigenesis-very low grade metamorphism temperature estimated by chlorite geothermometers are 149?271?.The results are approximate to counterpart of illite and polytype,but lower than illite crystallinity.Average of illite b0 dimenson is 8.994?,suggesting low to moderate pressures.A diagram of illite?muscovite?b dimension-temperature and pressure is introduced to semi-quantitatively estimate the pressure,suggesting relatively low pressure of epigenesis to very low grade metamorphism.Major elements were characterized by high SiO2?average 63.26%?,Al2O3?average 17.24%?content,high K/Na value?K2O/Na2O=3.35 average?,and low CaO?average 0.71%?and MgO?average 1.52%?content.Average of the compositional variability index is 0.97 indicating relatively low compositional maturity,and average chemical index of alteration is 61.66,indicating moderate chemical weathering.Trace elements were characterized by high V and Ni content and low Sr and B content.B*values<50×10-6,Sr values 51.36×10-6127.60×10-6,average81.00×10-6,B/Ga values 0.100.23,average 0.15,<1,Sr/Ba values 0.100.25,average 0.16,<1,V/Ni value 2.226.05,average 4.53,>3,V/?V+Ni?values 0.690.86,average 0.81,>0.60.These characteristics indicated that pelitic siltstone were formed in a continental and reducing environment.Rare earth elements were characterized by low fractionation degree of LREE and HREE,and Eu and Ce anomalies.?Eu ranges from 0.610.73?average 0.67?,showing strong negative anomalies,possibly suggesting felsic materials;?Ce ranges from 0.851.02?average0.92?,shows weak negative anomalies,suggesting a reduction environment.Characteristics of major,trace and rare earth elements in pelitic siltstone possibly suggested that the tectonic setting was continental island arc,and sediment source was felsic rocks of Songnen block. |
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