METAMORPHIC ROCKS

Metamorphism refers to a set of processes 
that result in changes in mineralogy and 
texture accompanying changes in 
temperature and pressure.



As noted previously, the boundary between 
diagenesis (sedimentary) and the onset of 
metamorphism is truly fuzzy.


Metamorphic Types	



Contact Metamorphism - nearby heat source - 
an intrusive igneous body is injected into a 
colder, older rock - heat flows from the 
intrusive body into the country rock



Regional Metamorphism - no obvious heat 
source - increasing depth of burial plus 
deformation results in an increase in 
temperature and pressure



Dynamic Metamorphism - variable pressure 
at relatively low temperatures - often 
associated with fault zones
n High Pressure - Low Temperature 
Metamorphism - associated with subduction 
zones 


Effects of Metamorphism



Increasing Grain Size



Increasing Temperature and Pressure may 
aid in the RECRYSTALLIZATION of minerals 
in the rock



Small grains become larger - oriented with 
respect to direction of applied pressure(s) - 
stress



Clay minerals are often enlarged with 
increasing metamorphism


Growth of New Minerals



New minerals may grow during 
metamorphism



CaCO3 + SiO2 = CaSiO3 + CO2



The presence of wollastonite can be used as 
an indicator of the Degree of Metamorphism



ISOGRAD - a line on a map connecting 
points of equal degree of metamorphism

Classification



Is the rock banded? - each band is often a 
single mineral - GNEISS



Does the rock exhibit foliation - parallelism 
of the cleavage of micas?

	Schist - coarse


	Phyllite - fine - barely visible


	Slate - very fine

If the rock is neither foliated nor banded it is 
called a granofels if it is coarse grained or a 
hornfels if it is fine grained



Marble - a metamorphosed limestone


Quartzite - a metamorphsed quartz arenite



Degree of Metamorphism	



A function of the composition of the parent 
rock - the PROTOLITH


		Marble - had a parent rich in carbonate


		Quartzite - quartz sandstone parent


		Slate, Schist, Gneiss - clay mineral rich 
	parent



Metamorphic Facies - attempts to deduce 
degree of metamorphism


Plate Tectonics and Metamorphism



Divergent Boundaries - contact 
metamorphism : basaltic lavas in contact 
with sediments



Convergent Boundaries : Subduction Zones : 
high pressure/low temperature 
metamorphism



Convergent Boundaries : 
Continent/Continent collision - regional 
metamorpism