松辽盆地北部断陷区营城组火山岩锆石SHRIMP年代学、地球化学及其意义

Geochronology of Zircon SHRIMP, Geochemistry and Its Implication of the Volcanic Rocks from Yingcheng Formation in Depression Area, North of Songliao Basin

松辽盆地北部徐家围子断陷区营城组广泛发育有一套以酸性喷发岩为主的中酸性火山岩,包括玄武粗安岩、安山岩、英安岩、流纹岩及流纹质火山碎屑岩等,属钙碱性岩类,亚碱性系列。锆石SHRIMPU-Pb年代学研究揭示研究区营城组火山岩形成于113Ma至111Ma之间,属早白垩世的Aptian期至Albian早期。岩石地球化学表明,流纹岩类均表现为富集Rb、Th、Zr等元素,亏损Sr、Ba、P、Ti、Eu等元素,安山岩类则亏损不明显,英安岩类介于两者之间,结合Nd同位素测试结果,认为营城组中-酸性火山岩是同源岩浆演化的结果。通过构造环境图解判别表明营城组火山岩形成于板内拉张构造背景,是中国东部早白垩世大规模岩石圈减薄和岩浆作用的产物。

A suite of intermediate or acidic volcanic rocks distributed widely in the Yingcheng Formation of the Xujiaweizi depression, north of the Songliao Basin, consist of basaltic trachyte andesite, andesite, dasite, rhyolite and rhyolitic volcaniclastic rocks, etc., which mostly belong to subalkaline and parapelite. Geochronological study through zircon SHRIMP UPb of the volcanic rocks shows that the volcanic eruption concentrates between 113Ma and 111Ma, which experienced from the early Aptian to the Albian of the early Cretaceous. Geochemical study and Nd isotopic data shows that the volcanic rocks of the Yingcheng Formation in the depression were from the same magma source, for the rhyolites are rich in such elements as Rb, Th, Zr, etc., and have obvious negative anomalies of Sr, Ba, P, Ti, Eu, etc., while the andesites have a notable loss of those elements, and the characteristics of dacites are between them. The tectonic discrimination diagrams of granite for the volcanic rocks indicate that they were formed in the extensional tectonic environment of intraplate and is the product of a largescale lithospheric thinning and magmatism in eastern China during the Early Cretaceous.