变质作用中原岩建造的地位及意义

变质作用是地壳历史发展中的一种重要的地质作用,并随地壳的演化而表现为不同的变质作用类型。过去称为区域变质作用可根据其所处的大地构造环境而分为不同的变质作用类型,而大地构造环境的一个重要方面则是它的原岩建造的特性。变质作用开始的大地构造环境一般可认为是具有活动特点的下沉带,表现为巨厚的

The Significance of the Original Rock Sequence and its Application in Metamorphsism

Metamorphism,being a global phenomena,has beet considered as one of the most important geological processes, which is intimately linked with the development of crustal history, and varies accordingly with their tectonic setting. While in considering tectonic setting of metamorphism, a subsidence area of a crustal segment with some specific original rock sequences controlled by a definite tectonic environment in the onset of metamorphism is usually denoted. It gen- erally possesses mobile characters as exemplliled by the great subSiaence togetn- er with anomalous geothermal gradients or strong deformation effects, which may be caused by the change of thermal regimes deeply down to the mantle. Thus, it may be conceived that the tectonic setting acts as some fundamental causes in inducing different types of metamorphism. Rock types and sequences reflect both general and subtle features of tec- tonic settings. They also suggest salient aspects of dynamothermal history through time. Although they have been generally affected by the metamorphism with the elimination of original mineral associations and their fabrics, yet, their original rock types and rock sequences could be justified by careful field observations, laboratory works, and interpretatioal of petrochemical data relevant to metarocks. Original rock associations pertinent to metamorphism may be classified into several important categories, viz. the sedimentary sequences, the mixed type sedimentary sequences, the mafic and ultra.mafic sequences, and the specific types of volcanic rock series. They are largely controlled by the tecto- nic environments during the evolution of earth''s crust. In a metamorphic terrane, metamorphic rocks usually form well-defined rock sequence, named as metamorphic formation with some charateristics in its rock assemblages. The metamorphic formation is defined as a set of definite metamorphic rock sequence pevailing over metamorphic terrane, and represent the products of their original rock sequences together with the types of meta- morphism affected. Variations of metamorphic formations depend on either as the change of original rock sequence, or as the change of metamorphic types. In general, it is considered as an important generality of the relationship between the original rock sequence in a given tectonic environment and the metamorphism thus involved. Generally thoy show evolutionary trend within succession of metamorphic terranes. Several important types of metamorphic formation are also tentatively grouped under the heading of various metamor- phic grouzps.