The fundamental cause of rock metamorphism is the change of geological environment, and it should be said that the fundamental factors that control metamorphism are geological factors, such as: geotectonic location (island arc, ocean trench, mid-ocean ridge, etc.), tectonic process (subsidence, uplift, etc.), magmatism, etc. However, from the perspective of physical chemistry, although there are various geological factors that control metamorphism, physical and chemical factors such as temperature (T), pressure (P), fluid composition (x), and time (t) can be abstracted, which is also the starting point for introducing physical chemistry into petrology. The most important change in the metamorphic process is the change in mineral composition. Metamorphic rocks are formed under P-T conditions at a certain depth underground, and their mineral assemblages are adapted to certain P-T conditions. When the P-T conditions change, they become unstable, and a chemical reaction (metamorphic reaction) occurs to form a new mineral assemblage that is stable under the new P-T conditions. This shows the importance of T and P as metamorphic factors.
(1) Temperature (T)
The minimum temperature of metamorphism is recorded by the transition from diagenesis to metamorphism (Fig. 14-3), which is usually 150~200°C. The high temperature limit of metamorphism is limited by the transformation of metamorphism and magmatism. As shown in Figure 14-3, there is a wide range of P-T transition zones between metamorphism and magmatic conditions. Due to the average composition of the earth's crust felsic, this transition zone is generally considered to be between the excess water solidus (EHGS) and dry solidus (DGS) of the simplified granite system.
The increase in temperature is conducive to endothermic reaction (such as the above-mentioned calcite + quartz = wollastonite + CO2↑ reaction), and the increase in temperature can greatly accelerate the metamorphic reaction rate and crystal growth, which is the decisive factor for metamorphic crystallization. The increase of temperature can also change the deformation behavior of the rock, from brittle deformation to plastic deformation. The increase in temperature will also affect the metamorphism through the dehydration reaction and decarbonization reaction to form a metamorphic hydrothermal liquid as a catalyst, transport agent and heat medium. In addition, the increase in temperature can lead to partial melting and mixed lithification. Because of the dominant role of temperature in metamorphism, metamorphism is usually divided into four metamorphic grades according to temperature: very low (VLM), low (LM), intermediate (MM), and high (HM). Metamorphic rocks of different metamorphic grades have different basic characteristics such as composition, structure, and structure.
(2) Pressure (P)
There are two main types of pressure in underground metamorphic environment: lithostatic pressure and di-rected pressure. The load pressure Pl is mainly derived from the overlying rock column and is a uniform stress that is equal in all directions. Directional pressure comes from tectonic movements (e.g., extrusion, shear) and is a differential stress. The rocks formed by crystallization under the two stress states have different appearances even if they are of the same composition. Crystallized rocks under uniform stress have minerals randomly distributed and have non-directional structures such as massive structures, such as granite (Fig. 14-4a), and rocks crystallized under differential stress, with mineral orientation and gneiss
Isodirectional structures, such as granitic gneiss (Figs. 14-4b). Most of the intrusive rocks are formed under uniform stress, and most of the rocks have non-directional structures. On the contrary, the metamorphic rocks are formed in the state of differential stress, and the rocks have various directional structures, except for a few cases where the contact metamorphic rocks produced in the contact zone of the intrusion are close to the formation under uniform stress and the rocks have no directional structure. This is the main difference between metamorphic and igneous rocks.
From the surface downwards, the pressure (load pressure) increases with depth at a rate of approximately 0.029 GPa/km. The average stable continental crust is 35km thick, and its bottom pressure is about 0.1 GPa. The maximum thickness of the continental crust observed in the modern and Cenozoic orogenic belts is about 70 km, and its bottom pressure is about 2.0 GPa. According to the determination of geological pressure gauges, most of the metamorphic rocks exposed on the surface are crystallized in the range of pressure 0.1~1.0GPa and depth of about 3~35km. At shallower depths, the temperature is usually too low to cause crystallization. The metamorphism at greater depths must be extensive, but it is difficult for the formed metamorphic rocks to be able to lift the outcropping surface.
Temperature and pressure are both important factors in metamorphism, but they are not isolated. From the surface down, as the depth increases, the pressure increases, and the temperature increases. The rate of change of temperature to pressure (depth), i.e., the geothermal gradient, reflects the combined effect of temperature and pressure. The "geothermal gradient" recorded in the rock-mineral assemblage of the actual metamorphic body is called the apparent geothermal gradient, which reflects the characteristics of metamorphism in an area.