辽河盆地东部凹陷早第三纪火山岩地球化学及形成环境

Geochemistry and the Tectonic Setting of Early Tertiary Volcanic Rocks in the East Depression,Liaohe Basin

中新生代频繁的岩浆活动是辽河盆地重要特征之一,东部凹陷作为各时期岩浆活动的中心,下第三系火山岩基本覆盖了整个凹陷。受构造运动差异影响,东部凹陷各地区不同时期岩浆活动强度不同,房身泡组火山岩分布最广,从沙三段到东营组沉积时期岩浆活动具有自中部向南、北转移的特征。岩石学和地球化学研究表明,主要火山熔岩类型为碱性玄武岩、粗面岩和辉绿玢岩;岩石化学组成上具有高碱、高铝和镁,较富集轻稀土元素,Eu异常不明显(δEu为0.70～1.05),弱亏损Ti、P、Sr、Ta元素的特征。岩石εNd(t)和(N(87Sr)/N(86Sr))i组成分别为-2.8～2.8和0.7046～0.7067。综合研究表明,玄武质岩浆来源于岩石圈地幔,而粗面岩和辉绿玢岩可能源于幔源玄武质岩浆的分异,但辉绿岩受地壳物质污染较重;该火山岩形成于大陆裂谷环境,反映出早第三纪辽河盆地处于拉张构造背景。

Frequent magmatic activity is one of the important characteristics for the Liaohe basin. The east depression in the Liaohe basin was covered with lower Tertiary volcanic rocks. The volcanic rocks are composed of basalt, dolerite and trachyte. The analysis of major and trace elements show that the volcanic rocks are characterized by high ω（K2O＋Na2O）, high ω（Al2O3 ） and ω（MgO）, enrichment of Light rare earth elements, no pronounced Eu anomaly （δEu= 0. 70～ 1. 05） and weak negative Ti, P, Sr and Ta anomalies. These geochemical characteristics are similar to those of continental alkali basalt. Isotopic geochemistries show that values of εNd（t） and （N（^87Sr）/ N（^86Sr））i of the volcanic range from -2.8 to 2.8 and from 0. 704 6 to 0. 706 7, respectively. These geochemistries show that the basaltic magma was formed in partial melting of asthenosphere mantle. Based on above mentioned characteristics, combined with the regional researching results, we suggest that the east depression was very likely an extensional rift environment during the early Tertiary.