| Zhalantun area of Inner Mongolia is located in the eastern central Asian orogenic belt.Its geotectonic units are composed of the Ergun,Xing'an and Songnen block.The lower Triassic Laolongtou Formation studied is dominated by a series of pelitic including pelite,silty pelite,pelitic siltstone,which contains abundant clay minerals.Sedimentary environment of the lower Triassic Laolongtou Formation were studied by measuring the major elements,trace elements and rare elements content using XRF and LA-ICP-MS respectively,and the clay minerals genesis were discussed based on mineral assemblages,relative content,illite crystallinity,illite polytypism,illite b0 dimension and chemical composition of chlorite by means of XRD and EPMA.The XRF spectrum results showed major elements in study area is characterized by high Si?Al and low Mg?Ca?P and Mn.CIA index value is in the range of 62.28~71.28,with an average of 66.62;ICV index value is in the range of 0.67~1.04,with an average 0.86;ratio of SiO2/Al2O3 is in the range of 4.31~5.73,with an average of 5.08 while K2O/Na2 O is in the range of 1.84-4.80,with an average of 3.53.Trace elemnt results showed that B value changes from 0.53×10-6 to 1.77×10-6 averaged at 1.05×10-6,equivalent B value changes from 0.48×10-6 to 1.89×10-6,averaged at 1.08×10-6,Sr changes from 76.43 ×10-6 to 209×10-6,averaged at 120.05×10-6,Sr/Ba changes from 0.05 to 0.46,averaged at 0.18,B/Ga changes from 0.030-0.090,averaged at 0.058;Rb/K changes from 23.92 to 33.51,averaged at 28.19;V/(V+Ni)changes from 0.78 to 0.88;averaged at 0.83.In La/Th-Hf discrimination diagram,most samples falled into felsic provenance area,while in La-Th-Sc tectonic discrimination diagram,most samples falled into the continental island arc area.Chondrite-normalized REE patterns showed that light rare elements were enriched but heavy rare elements were deficited,and Eu showed a negative abnormal.La/Yb ranges from 9.63 to 15.77,with an average of 11.96,indicating the fractionation degree of LREE and HREE was high;Ce/Ce* changes from 2.02 to 2.35,with an average of 2.14.Characteristics of major,trace and rare elements suggest that study area experienced medium weathering,the materil source were mainly felsic and the sedimentary environment is continental,reducing environment.The XRD analytical results indicated that the pelitic rocks of the Laolongtou Formation are characterized by illite + chlorite ±mixed-layer illite / smectite and detrital quartz + potassium feldspar+ albite.Content of quartz varies from 46.2 to 62.6%,averaged at 54%;albite varies from 7.4 to 16.2%,averaged at 9.8%;potassium feldspar varies from 1.2~4.9%,averaged at 3.6%.Total clay minerals content varies from 23 to 42.4%,averaged at 32.7%.In clay minerals,illite varies from 11 to 23%,averaged at 16.5%,content of mixed-layer illite / smectite varies from 76 to 88%,averaged at 82.3%,while chlorite varies from 0 to 5%,averaged at 1.9%.No mixed-layer chlorite / smectite occurred.These indicate the strata in study area experieced epigenesis,the epigenesis temperature was lower than 200?.Illite in the samples has a crystallinity of 0.718~0.85(average=0.809)and comprises 2M1 polytype,indicating that the pelitic rocks in study area have undergone epignensis with the temperature of 165?±?Main chemical composition of chlorite are as follows:content of SiO2 varies from 26.843~33.672%,with an average of 31.781%;Al2O3 varies from 15.816~22.084%,with an average of 18.333%;FeO varies from 25.557~35.941%,with an average of 29.240%;MgO varies from 0.827~2.283%,with an average of 1.625% According to the relationship between chemical composition and d001 proposed by Rausell-Colom(1991)and Nieto(1997)and geothermometer based on d001 and temperatures put forward by stefano(1999),the estimated temperature varies from 127~164?,with an average of 144??Illite b0 dimension varies from 8.994? to 9.004?,with an average of 8.995,indicating the formation experienced low–medium pressureThe clay mineral genesis reveals that the study area experienced epigenesis,the epigenesis temperature is about 150?,low–medium pressures conditions.The strata belongs to senior diagenesis belt. |
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