长江源区新生代火山岩的系列及成因

长江源区的新生代火山岩系包括高钾钙碱性系列和钾玄岩系列.高钾钙碱性火山岩形成于始新世, 钾玄岩系列火山岩形成于中、上新世.总体而言, 该区火山岩富碱高钾, 富集大离子亲石元素, 稀土元素含量高且轻稀土相对富集.相对而言, 高钾钙碱性火山岩富集SiO₂、Al₂O₃, 无负Eu异常, 属于壳源岩浆系列, 其原始岩浆由加厚陆壳的榴辉岩质下地壳经部分熔融产生.钾玄岩系列火山岩富集K₂ O、TiO₂、P₂ O₅、MgO、FeO, ∑REE、HFSE、I_Sr值均较高, 弱负Eu异常, 属于幔源岩浆系列, 其原始岩浆由EMⅡ型富集地幔的部分熔融生成.2个系列的火山岩均是大陆碰撞造山后期岩浆作用的产物.始新世以来, 随着该区由碰撞、挤压作用发展到出现走滑, 应力环境由挤压转变为张性, 导致依次喷发高钾钙碱性火山岩和钾玄岩系列火山岩. 

Series of the Cenozoic Rocks in Source Region of the Yangtze River and Their Genesis

The rock series of the Cenozoic volcanic rocks include high-K calc-alkaline (HK-CA) series and shoshonite (SHO) series in the source region of the Yangtze River. The HK-CA rocks were formed in Eocene and the SHO rocks were formed in Neogene. Generally speaking, rocks with high content of alkali and K₂O are enriched in LILE and with high abundance of REE, high ratio of LREE/HREE. The HK-CA rocks have more SiO₂, Al₂O₃ and less K₂O, TiO₂, P₂O₅, MgO, FeO, ∑REE, HFSE, I_Sr than SHO, and show no Eu negative anomalies, distinguishing from SHO that show light Eu negative anomalies. Therefore, the HK-CA series belong to crust-derived magma series and are originated from melting of an eclogitic lower crust; the SHO series belong to mantle-derived magma series and are originated from melting of (EMⅡ) mantle. The Cenozoic volcanic rocks were formed in back-collision tectonic environment, and both HK-CA and SHO series magmas should be generated at the late stage of collision process, that is, when the collision changed from compression then to strike-slip movement, the SHO rocks occurred following HK-CA rocks.