我国前寒武纪变质岩的构造特征

我国前寒武纪变质岩系的主要构造变形特征是:线型韧性剪切带及逆冲推覆构造发育,一般都遭受了多期变形、变质和强烈的构造置换作用,显生宙再造作用强烈,发育各种类型的穹隆构造。这些特征反映了前寒武纪地壳结构的不均一性和水平运动为主的构造体制.据构造置换、构造组合样式等划分出五种变质岩系构造类型,并对它们的制图原则和研究方法,作了相应的讨论。

TECTONIC FEATURES OF PRECAMBRIAN METAMORPHIC ROCKS IN CHINA

The bulk of the earth's continental lithosphere was produced in the Precambrian which constitutes about seven-eighths of geological time. Thus detaied study of the precambrian geology is vital to understanding of the origin and evolution of the continental crust, in which the key problem is how to identify and analyse the structural properties of the precambrian metamorphic rocks theoretically and practically.Archaeozoic and Proterozoic metamorphic rocks in China are essentially restricted to the North China Platform,the Jiangnan Uplift of the Yangtze Platform and the core of younger orogenic belts. Detailed structural and petrological studies of selected areas as well as regional reconnaissance work suggest that many of these rocks show specific deformation patterns. Their tectonic features are as follows:1. Linear shear belts, which can be grouped into EW-, NE-and NW trending systems, are particularly developed. The continuous belts of layered rocks in these belts are the products of strong ductile deformation in zones of high strain,which lie between displaced rhomb or lensshaped blocks containing distinctive rock associations, metamorphism and structural style and orientation on regional scale. The ductile belts have a long history, probably beginning in the late Archaean. It is marked by mylonites, blastomylonites and straight gneisses etc. Also in the shear zones there are small tight-recumbent folds and many of these have a characteristic sheath-fold shape.2. Major nappe and overthrust tectonics may have been very common in the Archaean and proterozoic terrains. It is capable of considerably shortening and thickening continental crust.3. The metamorphic series are mainly complexes composed of rocks with different lithological features and have undergone multiple deformation, metamorphism and intense structural transposition. It is the result from long-standing evolution of original layered sequence due to folding, shearing, flowing and dislocation. Consequently what we see on plan or on the steep mountain walls is not a stratigraphic succssion, but a tectonic pile or stack.4. Phanerozoic reworking or reactivation has deeply affected the basement rocks within an ancient platform and orogenic belt. The reworking refers to partial melting of the basement itself, intense metamorphic and structural changes, so that it is often very difficult to determine the nature of the early structures which have produced a new tectonic layering.5. The gneiss domes are one of the most striking structural features in the Precambrian crust. However, their origin and evolution models are various. For example, the granitic gneiss domes were formed by solid -state diapirism, while "false" domes were produced by complex overfolding. Some of other domes are comparable to the classic "mantled gneiss domes" of Eskola ( 1949)Although the expression of the above features in various regions is discrepant to some extent, overall, they reflect the bulk of horizontal tectonic regimes and textural anisotropy in the Precambrian crust. The recognition of these features are necessitated to make a revaluation of some previously detemined conclusions about the evolution of continental crust of China.The structural features of the Precambrian metamorphic rocks in China are classified into five types based on transposition, metamorphism, together with large structural associations and others. Methods of mapping and structural work for each of these types have been discussed respectively. It shows that in spite of their structural complexity, the deformation history of the Precambrian metamorphic rocks including the rocks at middle to deep levels is interpretable by structural analysis.