青藏高原东北缘伯阳地区第三系流纹岩地球化学及岩石成因

Geochemistry and petrogenesis of the Tertiary rhyolite from the Boyang area,northeastern margin of the Qinghai-Tibet plateau

伯阳第三系流纹岩出露于青藏高原东北缘特殊的构造部位，位于青藏、华北和扬子三大构造域的交接转换区域。岩石的w（SiO2）介于68％～76％，w（K2O）〉w（Na2O），w（K2O）／w（Na2O）平均值1．25，为一套典型的壳源流纹岩岩石系列。岩石微量及稀土元素具有典型的板内火山岩特征，K、Th、Rb等元素呈较明显的富集状态，而岩石显著的低Sr特征（（19～120）×10^-6）表明其并非源自加厚的下地壳，而是起源于斜长石稳定的正常下地壳。正是由于新生代期间青藏高原东北缘强烈的造山环境，加之渭河断裂的发育为下地壳物质提供了减压熔融的有利条件，从而诱发下地壳的局部熔融，形成伯阳酸性火山岩的原始岩浆。该岩浆体系沿区域断裂构造体系上升，并经历了较强的结晶分异和演化，最终形成伯阳第三系流纹岩系列。

Tertiary rhyolites from the Boyang area, northeastern margin of the Qinghai-Tibet plateau, are located at the convergent juncture of the Qinghai-Tibet plateau, the North China plate and the South China plate. These rocks exhibit SiO2 between 68% and 76%, K2O〉Na2O,and K2O/Na2O with a mean value of 1.25,and represent a typical crust-derived calc-alkaline rhyolite. Their trace and rare earth elements show geochemical characteristics of intra-plate volcanism. Their K, Th and Rb values are obviously enriched and the Sr is depleted （ （19-120） ）× 10^-6 ）, indicating that the primary magma was not generated from a thickened deep crust, but from a normal deep crust where plagioclase is stable. The collisional orogenic setting of the Plateau during the Cenozoic era, and the development of the Weihe fault system induced partial melting of the deep crust that generated the primary magma of the Boyang rhyolite.