藏北赤布张错地区埃达克岩的厘定及其意义

Identification of the adakitic rock association in Chibzhang Co area, northern Tibet, and its significance

常量、微量及Sr、Nd同位素研究表明,藏北赤布张错地区分布的一套始新世(35～4 0Ma)高钾钙碱性火山岩具有C型埃达克岩的地球化学特征。火山岩SiO2 含量为5 8%～6 9% ,K2 O、Al2 O3 、Sr的含量相对偏高,而Y、Yb的含量和Mg# 值相对较低,Nb明显亏损,Zr、Ti、P弱亏损,LREE富集,无明显Eu负异常,ISr(0 .70 72 3～0 .70 777)值偏高,而INd(0 .5 12 32 0～0 .5 12 35 2 )值偏低,指示其为一套壳源中酸性火山岩系,源自加厚陆壳下部的一个榴辉岩质源区的部分熔融。这套新生代埃达克质火山岩的厘定,指示藏北羌塘地区或更大范围的大陆地壳在4 0Ma左右已经加厚到超过4 0km ,且青藏高原的隆升开始于4 0Ma以前。

Major and trace elements and Sr-Nd isotope data of adakitic high-K calc-alkaline volcanic rocks in Chibzhang Co area are presented, which yield ages of 35～40 Ma. All of the rocks exhibit characteristics of adakites formed by partial melting of an eclogitic lower crust: SiO-2=58%～69%,high K-2O, Al-2O-3 and Sr contents,low Y, Yb contents and Mg~# values, strong depletion of Nb and Zr, weak depletion of Ti and P,LREE enrichment with no remarkable Eu negative anomaly,I-(Sr)=0.707?23～0.707?77 and I-(Nd)=0.512?320～0.512?352. The adakitic geochemical features and crustal anatectic isotopic characteristics suggest a primitive magmatic generation process responsible for the partial melting of an eclogitic lower crust of northern Tibetan Plateau. It is also implied that the crust was thickened to over 40 km and an eclogitic lower crust was formed in northern Tibet area during Eocene. The adakitic volcanic rocks of 40 Ma indicate that the continental crust of Qiangtang area in Tibetan Plateau has been thickened to over 40 km,and the plateau probably started its uplifting 40 Ma ago.