Metamorphism
Metamorphism can be defined as the solid state recrystallisation of pre-existing
rocks due to changes in heat and/or pressure and/or introduction of fluids i.e without melting. there will be
mineralogical, chemical and
crystallographic changes.
Metamorphism produced with increasing pressure and temperature conditions is known as
prograde metamorphism. Conversely, decreasing temperatures and pressure characterize
retrograde metamorphism.
The temperature lower limit of metamorphism is considered to be between 100 - 150°C, to exclude
diagenetic changes, due to compaction, which result in
sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic, but some types of metamorphism can occur at extremely low pressures (see below).
The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The temperature interval is between 700 - 900°C, with pressures that depend on the composition of the rock.
Migmatites are rocks formed on this borderline. They present both melting and solid-state features.
Regional metamorphism
Regional or Barrovian metamorphism covers large areas of
continental crust typically associated with mountain ranges, particularly subduction zones or the roots of previously
eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an
orogenic event. The collision of two
continental plates or
island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism. These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the
mantle in a
subduction zone also produce regional metamorphic effects. The techniques of
structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into
metamorphic facies or zones of temperature/pressure conditions throughout the orogenic terrane.
Metamorphic facies Metamorphic facies are recognizable terranes or zones with an
equilibrium assemblage of key minerals that were in equilibrium under specific range of temperature and pressure during a metamorphic event. The facies are named after the metamorphic rock formed under those facies conditions from
basalt. Facies relationships were first described by Eskola (1920).
Facies:
*Low T - low P :
Zeolite*Mod - high T - low P :
Prehnite-Pumpellyite*High-P low T :
Blueschist*Mod P - Mod to high T:
Greenschist -
Amphibolite -
Granulite*High P - Mod - high T :
EclogiteMetamorphic grades
Metamorphic grades are also classified by mineral assemblage based on the appearance of key minerals:
Low grade --- High grade:Greenschist - Amphibolite --- Granulite:
Slate - >>>melt:
Chlorite zone::::
Biotite zone:::::::
Garnet zone::::::::::
Staurolite zone:::::::::::::
Kyanite zone::::::::::::::::
Sillimanite zone
Contact metamorphism
Contact metamorphism occurs typically around
intrusive igneous rocks as a result of the temperature increase caused intrusion of
magma into cooler
country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the
metamorphic aureole. Contact metamorphic rocks are usually known as
hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained.
Contact metamorphism is greater adjacent to the intrusive and dissipates with distance from the contact. The size of the aureole depends on the heat of the intrusive, its size, and the temperature difference with the wall rocks.
Dykes generally have small aureoles with minimal metamorphism whereas large
ultramafic intrusions can have significantly thick and well-developed contact metamorphism.
The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of
pelitic or alumonisilicate rocks and the minerals they form. The metamorphic grades of aureoles are andalusite hornfels, sillimanite hornfels, pyroxene hornfels.
Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into
metasomatism. If the intruded rock is rich in
carbonate the result is a
skarn.
Fluorine-rich magmatic waters which leave a cooling granite may often form
greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic
ore minerals and thus are of economic interest.
Hydrothermal metamorphism
Hydrothermal metamorphism is the result of the interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and
metasomatic reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating
groundwater, or ocean water. Convective circulation of water in the ocean floor
basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.
Impact metamorphism
This kind of metamorphism occurs when either an extraterrestrial object (a
meteorite for instance) collides with the Earth's surface or during an extremely violent
volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO
2 polymorphs
coesite and
stishovite) and textures are characteristic of these conditions.
Dynamic metamorphism
Dynamic metamorphism is associated with major
fault planes. Metamorphism is localised adjacent to the fault plane and is caused by frictional heat generated by the fault movement.
Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.
The textures of dynamic metamorphic zones are dependant on the depth at which they were formed, as the confining pressure determines the deformation mechanisms which predominate. Within depths less than 5km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of
breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a
mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.
Within the depth range of 5-10km pseudotachylyte is formed, as the confining pressure is enough to prevent brecciation and milling and thus energy is focused into discrete fault planes. The frictional heating in this case may melt the rock to form pseudotachylyte glass or mylonite, and adjacent to these zones, result in growth of new mineral assemblages.
Within the depth range of 10-20km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout
shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms
mylonite, with dynamothermal metamorphism observed rarely as the growth of
porphyroblasts in mylonite zones.
Overthrusting may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages in this case are diagnostic of the depth and temperature and the throw of the fault and can also be
dated to give an age of the thrusting.
*
Metamorphic rock*
Metasomatism*
RecrystallizationEskola P. 1920.
The mineral facies of rocks. Norsk. Geol. Tidsskr.,
6, 143-194.
Winter J.D., 2001.
An introduction to Igneous and Metamorphic Petrology. Prentice-Hall Inc. , 695 pages. ISBN 0-13-240342-0.
*
James Madison University: Metamorphism*
BARROVIAN METAMORPHISM: Brock Univ.*
Metamorphism of Carbonate Rocks: University of Wisconsin - Green Bay
