辽西凌源罗庄亚组白云岩成因探讨

The Origin of Dolomite of Luozhuang Subunit of Lingyuan County in the Western Liaoning Province

通过主元素、微量元素和碳、氧同位素特征分析,系统研究了燕山地区辽西凌源罗庄亚组白云岩地球化学特征及成因。碳、氧同位素和微量元素Sr分析表明,该区白云岩具有较低的成岩温度(40℃和31.7℃),结合区域地层资料、区域构造演化史推测其形成于罗庄亚组沉积时期,属同生白云岩。白云岩中Na2O质量浓度与正常海水中白云岩接近,反映其形成于正常海水盐度环境;白云岩Fe、Mn含量较低,Fe2+与Fe3+的质量浓度之比平均为0.32,表明白云石化作用发生在地表-近地表条件下的氧化环境。样品主元素因子分析提取到陆源因子和海源因子,海源因子中,SiO2与MgO、CaO的质量浓度呈负相关,可能与pH值对海源沉积作用的控制有关;而MgO与CaO的质量浓度呈正相关性,表明白云岩是沉积成因而非交代或重结晶成因。

The major elements,trace elements,carbon and oxygen isotopic compositions were analyzed to study systematically the geochemical characteristics and origin of the dolomites in the Luozhuang Subunit of Lingyuan County in the western Liaoning Province.Factors of major elements of the samples were extracted terrigenous factors(Al2O3,K2 O,TiO2,Fe2O3 and P2O5)and marine factors(MgO,CaO,SiO2,Na2 O and MnO).SiO2,MgO and CaO as negative correlations were related to pH that controlled the sedimentation of the marine resource.MgO and CaO were positive correlation which indicates dolomite being a sedimentary origin rather than metasomatism or recrystallization.The analysis carbon and oxygen isotopes and trace elements Sr showed that dolomite had a lower diagenetic temperature in the region(40℃ and31.7℃).Analysis of regional stratigraphic information and regional tectonic evolution history indicated that the dolomite formation time was equal to sedimentary stage of Luozhuang Subunit and was a syngenetic dolomite.The content of Na2 O was close to that in the dolomite formed in normal sea water,it reflected that it was created in a salt environment of normal sea water.The carbon and oxygen isotopes were used to calculate salinity,the result also indicated the dolomite was formed in the normal salinity environment of seawater.The ratio of Fe2+and Fe3+are 0.32,which shows that the dolomitization occurs in oxidation environment on the earth's surface-near surface conditions.Based on these characteristics,the results indicate that the dolomites in Luozhuang Subunit may be formed at the earth's surface-near surface oxidation condition and normal seawater salinity environment.