SEVERAL PROBLEMS ON THE DIVISION OF THE LITHODEMIC UNITS OF GRANITOIDS

According to the concept of stratigraphy sensu lato, granitoids should also belong to stratigraphy. Like stratified geologic bodies such as sedimentary rocks, this kind of bounded masssive geologic body is recognized and delineated by means of lithologic features and calls for a hierarchical system and is expressed with hierarchical terms. But the fundamental difference between both lies in that: the stratified geologic body generally follows the law of sequence and generally lithostratigraphic units are established, while the massive geologic body generally does not follow the law of sequence but may be ascribed to a new kind of stratigraphic units, i. e. lithodermic units. For stratified sedimentary rocks, stratified extrusive volcanic rocks and metamorphic bodies distinctly of sedimentary or volcanic origin, the hierarchical system is easy to understand and popularize; whereas for plutons, the hierarchical system may be considered to be a special type of stratigraphic units. For the twenty-odd years, geologists have gained new understanding of plutons as the study of regional granitoid petrology and regional geological mapping have been carried out. In 1972 W. S. Pitcher put forward a new concept about the definition of plutons: it is a rock body composed of several kinds of magmas and consanguineous magmas, and these consanguineous magmas are intruded largely simultaneously and occur within a single contact surface. Plutons with different association types are primary magmatotectonic units with distinctive features. Their classification has not yet been definitely made, but according to foreign data coupled with the actual case of granitoids of China, they may be classified into the following three kinds of plutons with different association types. Simple pluton It is formed by a single pulsatory intrusion, i. e. it is a simple intrusion. It is the most essential tectonic unit in respect to the geology of granitoids. Multiple pluton It results from multiple pulsatory intrusions and shows distinct zoning. It is composed of a series of concentric shells, each of which is actually an intrusion with its own system and represents a separate magmatic intrusion. The contact relationships between intrusions may be either pulsatory and notably abrupt, or hidden and indistinct emergence. The inner part of a multiple pluton has compositional or textural zoning which is formed by repeatedly appearing magma flows. Composite pluton It is composed of various plutons （including simple and multiple ones） of different natures. The contact relationships between various plutons are distinct emergence. To disintegrate the above-mentioned plutons with different association types, merge and establish lithologic units of different ranks is the basic core of the lithodemic units; while to understand and reconstruct the tectonic unit of primitive magma is the important basis for establishing units and merg them into a super-unit. Generally speaking, the composite pluton formed by a particular tectono-magmatic cycle is the basis for establishing a super-unit association. Within each super-unit association or a separate magmatic activity, magmatism has its own specific emplacement phase and spatial distribution. The multiple pluton resulting from a comagmatic evolutionary sequence occurrig in a melting event is the basis for establishing a super-unit, while each intrusion within a superunit is the basis for establishina a unit.