Vein Rocks

in petrology, magmatic rocks that occur in the earth’s crust in the form of veins or dikes. Vein rocks may be associated with intrusive masses that have developed in the region (clearly derived from them or territorially related), or they may be completely independent. In chemical and mineral composition they partly correspond to the igneous rocks making up large masses, and at the same time they differ significantly from them. Vein rocks that are similar in composition to the intrusive rocks are called aschistic in contrast to diaschistic rocks; among the diaschistic rocks a distinction is made between those rich in dark minerals (melanocratic or lamprophyric) and those with a low percentage of dark minerals (leucocratic, or aplitic). Vein rocks of coarse grain are called pegmatite veins. Almost every type of intrusive rock has a corresponding type of vein rock, either leucocratic or melanocratic. Vein rocks usually have a porphyritic structure and a crystalline or glassy groundmass.