辽河坳陷大民屯凹陷古近系沙河街组三段地球化学特征及其地质意义

对辽河坳陷大民屯凹陷沈检5井古近系沙河街组三段(简称沙三段,Es3)的地球化学特征进行分析,明确大民屯凹陷沙三段的母岩类型、恢复该区的古环境并对构造背景进行判别和讨论,从而确定最终物源。结果显示,大民屯凹陷沙三段的母岩以大陆上地壳的长英质火山岩和太古宙的古老岩石为主,并受到来自幔源的大陆岛弧中基性岩浆影响。A-CN-K、SiO₂-Al₂O₃、Th-Th/U判别图显示源区的化学风化程度较弱。本文根据聚类分析选取出Sr/Cu、Sr/Ba、V/(V+Ni)值等10种元素组合,并根据其纵向分布特征对研究区的古环境进行了恢复。结果显示,Es₃⁴Ⅱ至Es₃³Ⅲ沉积时期,大民屯凹陷的古气候以干热为主并整体呈增强趋势,该区古湖泊的水体盐度低,以弱还原环境为主。研究表明,古气候是控制沙三段元素分配的主因,这与研究区的沉积环境和较为封闭的构造环境有关。稀土元素参数和主微量元素判别图表明,大民屯凹陷同时具有大陆岛弧和被动大陆边缘构造背景。本文认为,研究区复杂的物源和构造背景与晚侏罗世—晚始新世阶段太平洋板块向欧亚板块俯冲的大背景有关。大民屯凹陷沙三段沉积岩主要来自太古宇变质岩基底中的混合花岗岩和混合片麻岩,以及中生代多期次构造运动过程中生成的中酸性火成岩,同时受中生代晚期的幔源物质影响。

Geochemical characteristics and its implications of the third Member of Paleogene Shahejie Formation from the Damintun sag,Liaohe depression

Based on the geochemical data from the third Member of Shahejie Formation(Es3),from the well Shenjian 5 in the Damintun sag, the sedimentary environment, geological structural background, parent rocks and provenance are studied in detail. Results show that the parent rocks of Es3 are felsic volcanic rocks from the upper continental crust and the Archaean rocks. The A-CN-K, SiO₂-Al₂O₃, and the Th-Th/U discrimination diagrams display the low degree of chemical weathering in the Damintun sag. Based on cluster analysis, the vertical distribution characteristics of 10 element ratios such as Sr/Cu, Sr/Ba and V/(V+Ni) values are selected to reconstruct the paleoenvironment in the study area. Paleoclimate of the sag is mainly dry-heat from Es₃⁴ Ⅱ to Es₃³ Ⅲ, with low salinity and weak restore environment in the ancient lake. It shows that the significant influence of paleoclimate on the elemental distribution in the study area is mainly related to the depositional environment and closed tectonic environment in Es₃. The Damintun sag was situated in a tectonic background characterized by both continental island arc and passive continental margin, under the general background of subduction of the Pacific plate beneath the Eurasian plate in the late Jurassic-late Eocene stage. It is speculated that the sediments of Es3 mainly came from the Archean migmatite and migmatite gneiss in the Archean metamorphic basement in this area and intermediate acid igneous rocks formed in the Mesozoic. The parent rocks were also affected by the volcanic activities in the late Eocene.