西藏边坝地区早白垩世火山岩年代学、地球化学及其大地构造意义

白垩纪是班公湖-怒江洋演化的重要构造转折期,该时期火山岩对于研究古大洋的构造演化具有重要意义。本文对西藏边坝地区早白垩世火山岩进行了岩相学、年代学和地球化学研究,结果表明,该套火山岩由底到顶,主要为玄武安山岩、安山岩、英安岩和流纹岩,锆石U-Pb年龄为116~120 Ma。中基性火山岩SiO₂为54.07%~62.63%,Mg^#为48~67,SiO₂-Mg^#显示良好线性关系;酸性火山岩SiO₂为72.64%~75.41%,Mg^#为40~50。火山岩样品相对富集轻稀土元素、亏损重稀土元素,富集K、Rb、Th等大离子亲石元素,相对亏损Nb、Ta、P、Ti等高场强元素,CaO-δEu和SiO₂-Na₂O没有明显线性关系。因此,西藏边坝地区火山岩应形成于班公湖-怒江洋盆南向俯冲活动大陆边缘环境,中基性火山岩与酸性火山岩岩浆源区不同。在班公湖-怒江洋萎缩关闭早期,板块俯冲流体交代古老岩石圈地幔发生部分熔融,镁铁质矿物分离结晶而产生中基性火山岩;晚期则发生地壳部分熔融形成酸性火山岩。

Geochronology,Geochemistry and Tectonic Significance of the Early Cretaceous Volcanic Rocks in the Bianba Area,Tibet

An important tectonic switch of the Bangong-Nujiang Ocean took place in the Cretaceous,thus,the volcanic rocks of this period are critical to decode the tectonic evolution of the Bangong-Nujiang Ocean.Petrographical,geochronological and geochemical investigations were conducted on the Cretaceous volcanic rocks in the Bianba area,Tibet.From the bottom to the top,the volcanic rocks are mainly composed of basaltic andesite,andesite,dacitic and rhyolite.U-Pb dating of zircon grains from these rocks yielded ages of 116 Ma to 120 Ma.Geochemical results show that the intermediate-basic volcanic rocks have SiO₂ contents of 54.07%–62.63%and Mg^#values of 48–67,with a linear relationship between SiO₂ and Mg^#.The acidic volcanic rocks have SiO₂ contents of 72.64%–75.41%and Mg^#values of 40–50.The overall volcanic rock samples show enrichment of LREE and large-ion lithophile elements such as K,Rb,Th and depletion of HREE as well as some high field strength elements such as Nb,Ta,P,Ti.There are no significant linear relationships between CaO andδEu,and SiO₂ and Na₂O.Thus,these intermediate-basic and acidic volcanic rocks from the Bianba area may have different magma sources.The intermediate-basic volcanic rocks are likely results of partial melting of the ancient lithospheric mantle that metasomatized by subduction-derived fluid and underwent fractional crystallization of mafic minerals in the early stage of the closure of the Bangong-Nujiang Ocean.In contrast,the acid volcanic rocks were likely derived from partial melting of the curst in the late stage.