北京云蒙山浅色花岗岩脉及韧性剪切变形的地球化学特征

对云蒙山地区的花岗岩、糜棱岩化花岗岩及周围太古宙花岗片麻岩中的浅色花岗岩脉的主要及稀土元素地球化学研究结果表明：与未糜棱岩化花岗岩相比，浅色花岗岩脉具有较低的LREE和P2O5含量及（La／Gd）N比值，Sm／Nd比值较高，而剪切应变岩石的LREE相对富集；糜棱岩化花岗岩具有近平行于未糜棱岩化花岗岩的稀土元素配分模式；浅色花岗岩脉BH-2-5和BH-2-6具有和未糜棱岩化花岗岩相似的重稀土元素配分模式；浅色花岗岩脉BH-2-3的稀土元素配分模式和所有分析的样品都不一样，推断BH-2-3有可能是古老变质岩部分熔融的产物。在部分熔融过程中，诸如独居石这样的富含轻稀土元素的副矿物以残留体的形式出现，不参与部分熔融，导致BH-2-3具有很低的LREE和P2O5含量及高达0．4122的Sm／Nd比值。而其他两条浅色花岗岩脉有可能是云蒙山花岗岩后期岩浆分异的产物，随分异程度的增强和富含轻稀土元素的副矿物的分离结晶作用，导致最后岩浆的SiO2增高、LREE含量减少及Sm／Nd比值发生变化。

Geochemical characteristics of leucogranitic dikes and sheared granites in the Yunmeng Mountains, northeastern Beijing

Based on a detailed structural analysis and a preliminary study of major and rare earth elements in a suite of undeformed granites, mylonitized granites and leucogranitic dikes within the Yunmeng Mountains of northeastern Beijing, the authors have reached some conclusions: with increasing shear strain, SiO-2 decreases whereas Al_2O_3,TiO-2,H-2O,and LREE increase in mylonitized granites; compared with the undeformed granite, the leucogranitic dikes have lower LREE and P-2O-5 contents and (La/Gd)-N ratios, but elevated Sm/Nd ratios; and the REE distribution patterns of the mylonitized granites are parallel to the pattern of the undeformed granite, whereas the HREE distribution pattern of leucogranitic dike BH-2-5 and BH-2-6 is similar to that of the undeformed granite; the REE distribution pattern of BH-2-3 differs however from the patterns of all the other samples. The major and rare earth element geochemistry of BH-2-3 suggests that it was derived from partial melting of ancient metamorphic rocks. In such partial melting reactions, accessory phases that concentrated LREE like monazite occurred as residue phases, which resulted in extremely low LREE and P-2O-5 contents and elevated Sm/Nd ratios in the melts. The other two leucogranitic dikes may represent the latest crystallization differentiation product of the Mesozoic Yunmeng Mountain granites. The increasingly intense crystallization differentiation of a granitic magma resulted in the elevated SiO-2 and lowered LREE contents as well as the variation of Sm/Ndratios in the final silica saturated products .