皖南姚村花岗岩体环斑长石和暗色包体的成因机制

GENESIS OF RAPAKIVI FELDSPAR AND DARK-COLOURED ENCLAVES OF THE YAOCUN GRANITE BODY IN SOUTHERN ANHUI

本文研究表明,姚村花岗岩体中的暗色包体是少量中基性岩浆与花岗质岩浆不完全混合的结果。包体中的环斑长石特别是其钾长石核是从花岗质岩浆中结晶后,机械地迁入包体中的。混合作用使花岗质岩浆微区成分不均一,一些微区钙的含量增高,导致钾长石停止生长,而斜长石围绕钾长石得以生长成为环边。尔后这种粥状岩浆进一步侵位,并快速结晶形成相对细粒的基质。

The Yaocun granite body is characterized by a lot of dark-coloured enclaves and rapakivi feldspars. It was discovered that there is close relationship between them. there are no rapakivi feldspars in the granite unless the appearanee of dark-coloured enclaves. In dark-coloured enclaves there are often rapakivi ieldspar whose characters are the same as those of rapakivi feldspar in host rocks; for example, their cores (perthite) have Carlsbad twins and resorbed outlines, the contents and changing trend of the major elements and barium are similar, The dark enclaves are fine-grained quartz-hornblende monzonite with typical igneous textures such as girdles and resorption of hornblende and plagioclase and poikilitic texture. This suggests a magmatic genesis for the enclaves, Further research came to the conclusion that dark enclaves were formed by incomplete mixing of intermediate-basic magma with granitic magma. The chief evidence is as follows: fluidal structure and fine-grained rims along the contact of enclaves with granite, existence of acicular apatite in enclaves, and presence of broken rapakivi feldspar whose cracks are infilled with minerals of host rocks. This implies that K-feldspar in enclaves crystallized in granitic magma and migrated into intermediate-basic magmatic balls trapped by the granitic magma, thus forming rapakivi feldspar, As granite magma was contaminated by intermediate-basic magmas, the Ca content increased locally and K-feldspar ceased to grow. whereas plagioclase crystal- lized by means of selecting K-feldspar with lattice similar to that of plagioclase as the growth base, thus forming rims of plagioclase. Afterwards such mush magma was further emplaced to a higher level and crystallized rapidly, thus forming relatively fine-grained matrix.