Contact metamorphism Metamorphic rock

a contact metamorphic rock made of interlayered calcite , serpentine precambrian of canada. once thought pseudofossil called eozoön canadense. scale in mm.

contact metamorphism name given changes take place when magma injected surrounding solid rock (country rock). changes occur greatest wherever magma comes contact rock because temperatures highest @ boundary , decrease distance it. around igneous rock forms cooling magma metamorphosed zone called contact metamorphism aureole. aureoles may show degrees of metamorphism contact area unmetamorphosed (unchanged) country rock distance away. formation of important ore minerals may occur process of metasomatism @ or near contact zone.

when rock contact altered igneous intrusion becomes more indurated, , more coarsely crystalline. many altered rocks of type formerly called hornstones, , term hornfels used geologists signify fine grained, compact, non-foliated products of contact metamorphism. shale may become dark argillaceous hornfels, full of tiny plates of brownish biotite; marl or impure limestone may change grey, yellow or greenish lime-silicate-hornfels or siliceous marble, tough , splintery, abundant augite, garnet, wollastonite , other minerals in calcite important component. diabase or andesite may become diabase hornfels or andesite hornfels development of new hornblende , biotite , partial recrystallization of original feldspar. chert or flint may become finely crystalline quartz rock; sandstones lose clastic structure , converted mosaic of small close-fitting grains of quartz in metamorphic rock called quartzite.

if rock banded or foliated (as, example, laminated sandstone or foliated calc-schist) character may not obliterated, , banded hornfels product; fossils may have shapes preserved, though entirely recrystallized, , in many contact-altered lavas vesicles still visible, though contents have entered new combinations form minerals not present. minute structures, however, disappear, completely, if thermal alteration profound. small grains of quartz in shale lost or blend surrounding particles of clay, , fine ground-mass of lavas entirely reconstructed.

by recrystallization in manner peculiar rocks of distinct types produced. shales may pass cordierite rocks, or may show large crystals of andalusite (and chiastolite), staurolite, garnet, kyanite , sillimanite, derived aluminous content of original shale. considerable amount of mica (both muscovite , biotite) simultaneously formed, , resulting product has close resemblance many kinds of schist. limestones, if pure, turned coarsely crystalline marbles; if there admixture of clay or sand in original rock such minerals garnet, epidote, idocrase, wollastonite, present. sandstones when heated may change coarse quartzites composed of large clear grains of quartz. these more intense stages of alteration not commonly seen in igneous rocks, because minerals, being formed @ high temperatures, not transformed or recrystallized.

in few cases rocks fused , in dark glassy product minute crystals of spinel, sillimanite , cordierite may separate out. shales altered basalt dikes, , feldspathic sandstones may vitrified. similar changes may induced in shales burning of coal seams or ordinary furnace.

there tendency metasomatism between igneous magma , sedimentary country rock, whereby chemicals in each exchanged or introduced other. granites may absorb fragments of shale or pieces of basalt. in case, hybrid rocks called skarn arise, don t have characteristics of normal igneous or sedimentary rocks. invading granite magma permeates rocks around, filling joints , planes of bedding, etc., threads of quartz , feldspar. exceptional instances of known , may take place on large scale.