北祁连西段熬油沟蛇绿岩的矿物学研究

MINERALOGICAL STUDIES ON THE AOYOUGOU OPHIOLITE IN THE WESTERN PART OF NORTHERN QILIAN MOUNTAINS

本文对北祁连熬油沟蛇绿岩中的辉石、长石、铬铁矿和铁钛氧化物的成分进行了研究 ,结果表明 ,铬铁矿具有高的 Al、Mg含量和低 Fe3 + 及 Cr含量 (Cr* 值为 0 .4 8～ 0 .4 9)的特点 ,反映了其形成于大洋中脊下的低熔地幔。辉石、铬铁矿和铁钛氧化物中 Fe3 + 的含量均较低 ,表明这些矿物的分离结晶作用是在低氧逸度条件下进行的 ,因而熬油沟蛇绿岩中的玄武质岩石均不是初始岩浆 ,而是分离结晶以后形成的残余岩浆 。

The mineralogy of pyroxene, feldspar,chromite and ilmenite from the Aoyougou ophiolite shows that the chromite is characterized by high contents of Al and Mg and low contents of Fe 3+ and Cr (Cr =Cr/Cr+Al=0.48～0.49),which indicates that it was formed by low degree of mantle melting under the mid ocean ridge. The low contents of Fe 3+ in pyroxene, chromite and ilmenite suggest that the fractional crystallization of these minerals in the magma chamber occurred under low oxygen fugacity, and all of the basaltic rocks were not formed by the primary magma melted directly from the mantle, but by the evolved magmas. The primary magma may be a more Mg and Cr rich picritic magma.