桐庐杂岩体中共存的两种碱性长石研究

Study of Two Types of Alkali Feldspars Coexisting in the Tongcheng-Lujiang complex

桐庐同熔型火山—侵入杂岩中出现两种碱性长石,一种为无色透明,另一种为微红色,本文利用X射线衍射仪、电子探针、透射电镜对其结构态、成分及出溶徽结构进行了对比研究。结果表明,两者成分差别不大,而结构差别较大,无色透明的为透长石,出溶叶片发育;微红色的为正长石或低透长石,出溶结构不发育。而且随着冷却速率的变慢,无色透明碱性长石的A1/Si有序度增大,微红色碱性长石的有序度降低。分析后认为这种共存现象是岩浆混和造成的,从而为桐庐杂岩体的岩浆混合成因提供了有力的证据。

There are two types of alkali feldspars coexist in the syntexis type Tongcheng-Lujiang complex; one is colourless and transparent and the other is reddish in colour. A comparative study of structural state, composition, and exsolution microstructure was done for such alkali feldspars by using XRD, EPMA and TEM. The results indicate that the colourless and transparent one is sanidine with homogeneous exsolution microstructure and the reddish one is orthoclase or sanidine without exsolution microstructure. These two types of alkali feldspars have similar compositions. In principle, there should be a continuous range of Al and Si ordering with decreasing temperature, from completely disordered high sanidine to completely ordered low microcline. In the Tongcheng-Lujiang complex, the varying trends of Al and Si ordering of the two types of alkali feldspars are different. As the cooling rate of magmas varies from high to low, the Al and Si ordering tends to increase for the colourless and transparent alkali feldspar and decrease for the reddish one. This is obviously contradictory to previous results (Stewart et al. , 1974; Deer et al. , 1992). In the present paper, the authors discuss the genesis and petrologic significance of the coexisting alkali feldspards, which can provide evidence for the mixing of magmas in yhe Tongcheng-Lujiang complex.