Thermal contact metamorphism is a metamorphism that occurs at high temperature.It can be divided into two types: thermal contact metamorphism is the metamorphism that changes the mineral composition and structure of the surrounding rock of the contact zone caused by the thermal energy of magmatic intrusion, but does not change the chemical composition;Metamorphism in which the chemical composition, mineral composition and structure of surrounding rocks are changed by the replacement of late magmatic hydrothermal solution is calledcontact metasomatic metamorphism 。The former has no chemical composition change and no metasomatism, and can form hornfelsMottled slateIsothermal contact metamorphic rocks.The latter is characterized by chemical composition and lithology changes, and can form contact metasomatic metamorphic rocks such as skarn.[1]
Contact metamorphism refers to contact with magmasurrounding rockThe metamorphism occurred in.It is called thermal contact metamorphism that temperature is the dominant factor causing different recrystallization phenomena for different wall rocks.During the process, the chemical composition of the rock is generally unchanged, but only recrystallized.However, if there are impurities in the rock, new minerals will be formed (the element composition remains unchanged, but the combination form changes).
Thermal contact metamorphism is the most common kind of contact metamorphism, which usually occurs only in areas where tectonic movement is not very strong, and in adjacent wall rocks of magmatic intrusions.Temperature is the main factor that causes this metamorphism. Under the influence of the temperature of magma body, the wall rock produces endothermic reaction, and forms new mineral assemblages and fabrics through metamorphic crystallization and recrystallization. At this time, volatile matter is also of great significance to promote metamorphic crystallization and recrystallization, but they generally only act as catalysts,The chemical composition of the formed rocks is basically the same before and after metamorphism.[2]
Thermal contact metamorphic rock
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This metamorphic rock is mainly distributed near the contact zone between Indosinian Himalayan intrusive rock and wall rock, which is mostly distributed in an irregular ring with a width of several meters to hundreds of meters. In some sections, there is an incomplete thermal contact progressive metamorphic zone
1. Rock type and characteristics
Metamorphic rocks mainly include hornfels, hornfelized rocks and marbles related to contemporaneous intrusive rocks, which are distributed in the surrounding rock of the outer contact zone at the edge of the rock mass. The main rock types are as follows.
(1) Hornfelized rock: it is distributed in a ring belt around the intrusion, and some of it is distributed in the outermost zone of the thermal contact zone, which is the thermal zone in the survey areaContact metamorphic rockThe main rock types are transitional with regional normal rocks and hornfels, and various complementary structures are developed.The new minerals mainly include chlorite, epidote, actinolite, sericite, biotite, mica, quartz, albite, etc. The rock types are brecciated conglomerate, brecciated sandstoneHornfelized siltstone, brecciated silty mudstone, brecciated tuff, brecciated andesite, in addition to brecciated dacite, brecciated rhyolite, brecciated volcanic breccia and spotted slate.
(2) Hornfels: they are one of the main bodies of thermal contact metamorphic rocks in the area, and they are banded around the intrusion.It is distributed in the middle and inner zones of the thermal contact zone, with hornfels structure,Porphyroblastic texture, massive structure.In the rock, the structure is not developed. The metamorphic minerals formed by recrystallization include garnet, andalusite, cordierite, wollastonite, diopside, plagioclase, K-feldspar, biotite, chlorite, epidote, quartz, etc.The rock types include andalusite hornfels, cordierite hornfels, garnet bearing cordierite biotite hornfels, biotite felsic hornfels, diopside actinolite hornfels, garnet hornfels, felsic hornfels, albite epidote hornfels, sillimanite cordierite felsic hornfels, etc.
(3) Marble: the intrusion body is on the contact zone of carbonate rock, and the rock is in granular crystalloblastic structure,Banded structure, massive structure.The main metamorphic minerals are calciteAlmandine, diopside, tremolite, feldspar, chlorite, sericite, wollastonite, etc.The rock types include marble, wollastonite marbleDiopside marble , tremolite marble, etc.
