西藏措勤-赛利普地区新生代火山岩地球化学特征

Geochemical characteristics of Cenozoic volcanic rocks in the Cuoqin-Sailipu area,Tibet,China

青藏高原拉萨地块是印度板块与欧亚板块碰撞的重要地区之一,其中广泛发育的碰撞—后碰撞岩浆岩记录了新生代以来印度大陆岩石圈向北俯冲的全过程以及拉萨地块在碰撞-后碰撞之后的岩浆作用类型。基于对措勤—赛利普地区新生代火山岩地球化学及Sr、Nd同位素资料的分析,表明火山岩中以明显富碱和高钾为特征;Sr同位素呈递增而Nd同位素呈递减趋势,其中87Sr/86Sr=0.710 352～0.732 268,εNd=-3.41～-15.82,其地球化学类型明显不同于高原北部地球化学省,即本区87Sr/86Sr比值更高,εNd更低,揭示了新生代以来,印度大陆岩石圈向北俯冲过程中,拉萨地块比青藏高原北部融入了更多的印度板块地壳端元的成分;在中新世中早期印度大陆板块已经向北A型俯冲到拉萨地块基底西段并发生岩石圈不同层次的部分熔融、岩石圈断离等作用。

The Lhasa block of the Tibetan plateau is one of the main areas collided by the Indian plate and Eurasian plate.The extensively developed magmatic rocks in this area record the whole process which the Cenozoic India continental lithosphere subducted to north and the type of magmatism of Lhasa block after collision and collision-after.The geochemical analysis of the Cenozoic volcanic rocks and Sr,Nd isotopic data from the Cuoqin-Sailipu area indicate that the volcanic rocks transiste from calcium rock to alkaline-high potassium calcium rock and then to alkaline-shoshonitic rock and are characterized by rich alkali and high potassium.Sr isotope increases while Nd isotope decreases,and 87Sr/86Sr=0.710352~0.732268,εNd=-3.41^-15.82.The geochemical type is obviously different from that in northern Tibetan plateau of geochemical provinces,namely,the 87Sr/86Sr ratio in this area is higher while the εNd is lower.This shows that the Lhasa block absorbs more components of the Himalayan continent curst end than the north of Qinghai-Tibet plateau in the northward subduction process of the Cenozoic India continental lithosphere;In the early and middle Miocene,Indian continent already subducted with A type northward to Lhasa block western basement fault and affected the lithosphere partial melting and lithosphere dialysis on different levels of rocks.