metamorphism

Metamorphism involves a solid-state recrystallization of a pre-existing rock – igneous, sedimentary, or metamorphic rocks – under the agency of :
● high temperature
● high temperature and pressure (prograde)
● reduced temperature and pressure (retrograde)
● high pressure
● percolating chemical solutions (metasomatism)

Metamorphic changes in rock are caused by:
● regional or Barrovian metamorphism
● contact metamorphism
● hydrothermal metamorphism
● impact metamorphism
● dynamic metamorphism
● thermal metamorphism

Metamorphic grade reflects systematic sequence of mineralogic and textural changes:

Grade: Low------------Medium----------High

For a shale parent, which is a rock fine-grained sedimentary rock whose original constituents were clays or muds, the sequence is:
-------slate ------phyllite ------ schist -------------gneiss -----------> melt (magma)

For a mafic parent rock such as basalt or gabbro, rock facies follow the series:
greenschist------- ---- amphibolite ------------- granulite facies

A metamorphic facies is defined as "a set of mineral assemblages repeatedly associated in time and space, such that there is a constant and therefore predictable relation between mineral composition and (bulk rock) chemical composition."*

Within metamorphic facies, minerals change so as to accommodate changes in temperature and pressure:
-----chlorites-------------
--------------epidote---------
------no Al-----------------amphiboles----------------Al-
---------------------------------------garnets-----------
-------------------------------------------------pyroxenes--

Another index of metamorphic intensity (beginning with a clay-rich parent rock) depends upon zones in which index minerals appear in sequence:

Zone: chlorites → biotite → garnets → staurolite → kyanite → sillimanite index minerals

Facies: greenschist------------- amphibolite -------------- granulite
Grade: ----- Low-----------------Medium--- - ---------High
------slate ---phyllite -------- schist ------------------------gneiss

Zone: index minerals are bold and large:
----------chlorites------
-----------------------quartz/feldspars----------------------------------
--------------------------muscovite---------------
--------------------------biotite--------------------
----------------------------------- garnets ------
-------------------------------------- --staurolite--
-------------------------------------------kyanite---
-----------------------------------------------------------sillimanite--

Metamorphic facies, grades, and zones are linked. The graded trajectory of mineralogic changes through which a rock passes when subjected to varying degrees of textural change in response to heat/pressure/metasomatism defines the metamorphic facies, and the volume of rock that contains altered index minerals is the metamorphic zone.