长江源区新生代火山岩的系列及成因

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

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 、Al2 O3 ,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、ISr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 .

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 K_2O are enriched in LILE and with high abundance of REE,high ratio of LREE/HREE. The HK-CA rocks have more SiO_2, Al_2O_3 and less K_2O, TiO_2, P_2O_5, 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.