Contact metamorphism of the composition, structure and texture of rocks as a result of heating from the side of magmatic melt and post-magmatic fluids.
It appears near intrusive masses that crystallize at shallow and medium depths (up to 10-12 km), which are embedded in sedimentary or previously existing igneous rocks and introduce excess heat into the earth's crust (thermometamorphism). At great depths, contact halos merge with the fields of regionally metamorphic rocks and are not fixed.
Xenoliths captured by magmatic melt also undergo contact metamorphism. The thickness of contact aureoles is usually several tens, less often hundreds of meters, and even near large granite batholiths it does not exceed 2-3 km.
As a result of the impact of aluminosilicate melts on similar in composition, silicate or aluminosilicate sedimentary rocks ( sandstones , siltstones , mudstones , siliceous schists ), contact hornfelses are formed. From the rocks of regional metamorphism, hornfelses differ primarily in their geological position - confined to intrusive massifs. If the exposed area is good, it is possible to observe a gradual transition from contact hornfelses to their unchanged analogues - sandstones and siltstones. In addition, the transformations that the rock undergoes during contact metamorphism are mainly associated with heating, which leads to annealing; therefore, rocks of contact metamorphism are characterized by uniform massive textures, the absence of schistosity, grain idiomorphism , and the absence of intragrain dislocations.
The pressure during contact metamorphism varies between 0–3 Kbar, the temperature is 300–1200 ° C. Extremely high temperatures (900–1200 ° C) are reached only with the metamorphism of xenoliths surrounded on all sides by a magmatic melt.
A very important role is played by postmagmatic fluid. The presence of significant contact halos is characteristic of acidic intrusions, although the crystallization temperature of acidic magmas is much lower than that of the main ones. However, the main magmas are poor in fluid, and with a purely conductive transfer of heat from contact, only a narrow (up to several meters thick) zone undergoes metamorphism.
Contact halos can serve as a sign of proximity to an unopened intrusive body.
Literature
- Contact metamorphism . // Geological dictionary: in 2 volumes. - M .: Subsoil. Edited by K. N. Puffengoltz et al. 1978.