松辽盆地北部上二叠统林西组地球化学特征及地质意义

Geochemical Characteristics and Geological Significance of Upper Permian Linxi Formation in Northern Songliao Basin

为探讨松辽盆地北部林西组地球化学特征、沉积环境及构造背景,对该区黑富地1井所钻遇的上二叠统林西组暗色泥质板岩进行主量元素、微量元素和稀土元素测试分析,结果表明:样品主量元素平均质量分数SiO2为65.05%、Al2O3为17.23%、CaO为2.58%、Na2O为2.84%、K2O为3.29%,K2O/Na2O值为0.20~3.48,Al2O3/(CaO+Na2O)值为0.41~8.01;稀土元素w(∑REE)值介于80.68×10^-6~215.59×10^-6之间,平均值为173.41×10^-6,δEu值介于0.61~0.88之间,平均值为0.70,表现为Eu的负异常,δCe值介于0.91×10^-6~1.01×10^-6之间,为微弱的Ce负异常,LREE富集,HREE亏损;微量元素以亏损Nb、Ta、Sr,相对富集Rb、Ba、La、Ce、Pb、Nd、Sm为特征。样品元素分析和物源w(TiO2)-w(Ni)、La/Th-w(Hf)等判别图解表明,林西组沉积物源具有多样性,主要来源于上地壳长英质物源,还有少量来自中、基性岩火成岩物源区。样品微量元素标准化蛛网图,稀土元素球粒陨石标准化配分模式图,K2O/Na2O-w(SiO2)、Th-Co-Zr/10、F2-F1构造背景判别图及地球化学参数对比结果表明,松辽盆地北部林西组物源构造背景主要为活动大陆边缘和大陆岛弧,可能形成于靠近大陆岛弧的活动大陆边缘构造背景。林西组应形成于海陆过渡相到陆相环境,水体由半咸水逐渐转变为淡水。

Experimental data of the major elements, trace elements and rare earth elements of the dark argillaceous slate from the Upper Permian Linxi Formation in Well Heifudi 1 of the northern Songliao basin were examined so as to discuss the geochemical characteristics, sedimentary environment and tectonic background of the Linxi Formation. The average content of major elements is SiO2=65.05%, Al2O3=17.23%, CaO=2.58%, Na2O=2.84%, and K2O=3.29%. K2O/Na2O is 0.20-3.48, and A12O3/(CaO+Na2O) is 0.41-8.01. The ΣREE is between 80.68×10^-6 and 215.59×10^-6(average 173.41×10^-6), the δEu negative anomaly is between 0.61 and 0.88(average 0.70), and the Ce negative anomaly is between 0.91×10^-6 and 1.01×10^-6, relatively weak. The clastic rocks are characterized by LREE enrichment and HREE depletion. The trace elements are characterized by a loss of Nb, Ta, and Sr, and are relatively rich in Rb, Ba, La, Ce, Pb, Nd, and Sm. The elemental analysis of core samples and the results of w(TiO2)-w(Ni)and La/Th-w(Hf)indicate that the sediment source of Linxi Formation is diverse, mainly from felsic rock of the upper crust with a minority of intermediate-basic igneous provenance. The results of trace element standardized spider web map, rare earth element chondrite standardized partition pattern,diagrams of K2O/Na2O-w(SiO2), Th-Co-Zr/10, F2-F1 structural background discrimination,and geochemical parameters comparison indicate that the provenance of Linxi Formation in Well Heifudi 1 is mainly from active continental margin and continental island arc,which would like be formed in the marine-terrestrial transitional to terrestrial environment.