皖南前寒武纪花岗岩类中片麻状构造的成因

本文运用岩组分析等综合研究方法,确定了该区前寒武纪花岗岩类中的片麻状构造属动力变质成因,形成于地壳较深层次(10公里左右)的准韧性剪切带内,发生于距今7.5亿年左右的地质历史时期;其变形方式及变形产物具有从准塑性向脆性转化的特点;岩石的组构类型为S-构造岩和(B+R)-复合构造岩,显示出两次变形的显微组构特征。

GENESIS OF THE GNEISSIC STRUCTURE OF THE PRECAMBRIAN GRANITOIDS IN SOUTHERN ANHUI PROVINCE

There are several explanations for the origin of the gneissic granites. In this paper the authors have discussed the genesis of gneissic structure of the precambrian granitoids in southern Anhui based on field geologic, structural, isotopic, petrographic and petrofabric evidences.These precambrian granitoids are considered to be magmatic products of the Jinning period in Proterozoic Era. Their isotopic age is 900-1000 Ma. There are about 11 intrusive bodies, which may be divided into two types on the basis of their structure feature: one is of massive granite; the other gneissic granite, to which most of granitic bodies belong.Dynamo-metamorphic phenomena of the gneissic granites are widely developed. Under the microscope quartz grains, brecciated and elongated along certain direction with X-shaped Luder’s Lines, fan-shaped extinction, "tectonic pebblets" composed of plagioclase found in; banded mylonites, some recrystallized new minerals, as well as chlorite crystals pseudomor-phosed after biotite etc. may be observed.Petrofabric analysis of gneissic granites samples shows that obvious regular orientation of optic axis of quartz was exhibited. Fabric types are the S-tectonite and (B + R)-compositional tectonite. The deformation of single crystal of quartz seems firstly to follow the "prismatic rule" and then, the "rhombohedral rule". Some tectonites are characterized by two deformed microfabric. Practice demonstrates that result of petrofabric analysis may be used to explain the deformation pattern and the mechanism of the formation of gneissic granitoids.As already discussed, the above-mentioned gneissic granites were formed by dynamo-metamorphism from the pre-existing granitoids. Thus, they are a kind of "structural rock". Under the different temperature and pressure, deformation changed from plastic state to brittle state being an outstanding feature of the deformed way and their products. The development of gneissic structure was mainly controlled by subductile shear zone, which formed at a depth of about 10 km and a time of about 750 Ma.Correct recognition of the genesis of these gneissic granitoids has important significance in retrospection of the history of regional tectonic development and understanding the post diagenesis.