Overview of metamorphism

The range of metamorphism is roughly as follows: temperature: generally 200°C to 900°C, between post-generative diagenesis and magmatism; pressure (loading pressure of rocks) from a few hundred bar to 13-14 kbar, and by depth of about 1.5-40 km. Metamorphism occurs in a complex variety of modes and processes, which include: recrystallization, metamorphic reaction and metamorphic crystallization, metamorphism, differentiation, accountability, deformation and fragmentation, etc. (Yu Zhendong, 1988, etc.). differentiation, accounting, deformation and fragmentation, etc. (You Zhendong et al., 1988). Factors affecting metamorphism are complex and diverse, including temperature, pressure, stress, fluid action, time, etc. For details, please refer to the relevant literature (You Zhendong et al. 1988; He Tongxing et al. 1988; Wang Renmin et al. 1989).

Metamorphism is characterized by diversity. According to the geological background of metamorphism, metamorphism can be classified into eight types: contact thermal metamorphism, dynamical metamorphism, regional metamorphism, mixed petrogenesis, buried metamorphism, ocean-bottom metamorphism, and impact metamorphism, among which the four types of metamorphism: regional metamorphism, mixed petrogenesis, buried metamorphism, and ocean-bottom metamorphism tend to have a certain scale and belong to regional metamorphism. The four types of metamorphism, including regional metamorphism, mixed petrogenesis, buried metamorphism and ocean floor metamorphism, tend to have a certain scale and belong to regional metamorphism.

In different geological environments, under certain physico-chemical conditions, when metamorphism is close to reaching internal chemical equilibrium, a certain chemical composition of the original rock will form a certain combination of minerals **** born. Usually a set of mineral ****-generating combinations formed simultaneously during metamorphism and the physical and chemical conditions of their formation are called metamorphic phases.F.J.Turner in 1968 had proposed a classification method of 11 phases from the sodalite-green cordite angular phase to the garnet phase, and the temperature and pressure distributions of the various phases are shown in Fig. 3-9.

It is important to note that the green-curtain sodium-long hornblende phase, hornblende hornblende phase, pyroxene hornblende phase, and diorite phase in the low-pressure part of Fig. 3-9 are commonly associated with contact metamorphism. Under regional metamorphic conditions, their metamorphic pressures are generally higher, and the most common are: green schist phase, hornblende phase, and mafic phase rocks.

Since Christian Chopin's (1984) discovery of coquartz, a high-pressure polymorphic variant of silicon oxide, in the metamorphic rocks of the Western Alpine crust, one after another mineral inclusions or outcrops that characterize the deep Earth's material have been discovered in various orogenic zones around the world, such as diamonds, potassium-rich monoclinic pyroxenes, and magnesium-aluminum garnets. Nowadays, no less than 20 places have been reported to have discovered quartz or other ultra-high-pressure minerals. The Dabie-Sulu region of China has become a typical area for international research on UHP metamorphic zones since the discovery of coquartz in garnet in the latter half of the 1980s.

Figure 3-9 P-T illustration of metamorphic phases

Traditionally, the study of metamorphism is limited to the terrestrial crust, and the metamorphic pressure does not exceed 1.4 GPa (Figure 3-10). The metamorphism with pressure above 2.5 GPa should be called ultra-high-pressure metamorphism, which means that the current research scope of metamorphism has broken through the traditional boundaries and entered the deep part of the lithosphere and even the mantle. In recent years, the study of ultrahigh-pressure metamorphic zone has shown that a huge amount of crustal material can enter the mantle, which provides a factual basis for shell-mantle interaction. Therefore, although the lighter weight of the land crust can be deep subduction into the mantle, yet to be scientifically demonstrated, but there is still reason to think that the ultra-high pressure metamorphism has the scale of the region.

Diorite is the most representative rock in the UHP metamorphic zone, so the P-T stability range of UHP diorite can reflect the P-T limit of the UHP metamorphic diorite phase, and Fig. 3-10 is the P-T diagram of the UHP metamorphic phase that has become popular in the international arena recently. The shaded area in the figure is the stabilization zone of UHP garnet, and the dashed line 5°C/km represents the lowest metamorphic temperature-pressure gradient. In addition, a more detailed delineation of the garnet (habitually referred to as high-pressure garnet) below the quartz/coquartz transition curve has been made. The two curves with steep slopes in the lower right of Figure 3-10 are melting curves for granitic rocks: one is the lowest melting curve for wet (water-bearing) granite and the other is the lowest melting curve for wet quartz diorite. They can be used to estimate the temperature-pressure conditions for partial melting of crustal rocks.