Lecture 6 -- Metamorphic Rocks

Useful Web page -- UBC provides an excellent homepage for Metamorphic Rocks
Want to know Earth history -- history of rocks
How hot? What pressures? How deep?
Where were collisions? What were mountain-building events like?
To answer these types of questions we need to understand metamorphic rocks.

What factors cause metamorphism?
What are the various kinds of metamorphism?
What are the chief types of metamorphic rocks?

Why do rocks metamorphose?

At high P and T minerals are more able to react and form the most thermodynamically stable mineral assemblage.
At higher P, minerals want to be more tightly packed, hence different mineral assemblage.

Metamorphism -- all changes to texture and mineral assemblage of a rock that occur within the Earth's crust that occur because of changes in P and T. Mostly isochemical process. Composition usually does not change in these closed systems.

Metasomatism -- Change in composition (except H2O) due to infiltration of fluids through the rock. Minerals may change, though texture may stay largly the same.
-These fluids often originate from nearby igneous intrusion.

Low-Grade metamorphism refers to relatively low pressures and temperatures - 0-6 kb; 0-600 degrees
High-Grade metamorphism refers to relatively high pressures and temperatures.
- higher Ps and Ts

Geotherm (from the Latin Earth-Heat) -- The change in temperature in the Earth with depth

Wet partial melting refers to the wet solidus for basaltic crustal rocks.
-If metamorphic rocks melt, they form granitic rocks.
-Metamorphic rocks with little bits of granite melt are called migmatites.

What do Metamorphic Rocks Look like?

Texture

Foliation refers to any planar fabric in a mm rock. It is usually formed by the alignment of micas (a common metamorphic mineral) or other flat minerals. Foliation falls into two main types:
Slaty Clavage - low grage, microscopic mica grains, causes rocks to break along these planes. Caused during folding, almost exclusively in meta-pelites
Schistosity - higher grade, macroscopic mica grains, grain size is most important.
Compositional Banding -- forms a texture or rock referred to as a gneiss (from the German gneisto, to sparkle)

Textural names of rocks -- most predominant in the meta-pelites

slate -- clays are texture-defining mineral, contains slaty cleavage
phyllite -- microscopic micas
schist -- macroscopic micas
gneiss -- compositional layering

Mineral Assemblages

Mineral assemblages depend on:

Metamorphic grade
Protolith --- what the original rock was -- i.e., the parent material

Protolith refers to the pre-metamorphic rock type. The principle protoliths we will worry about are

Clastic edimentary rocks rich in Al, referred to as pelitic; these are usually shales, mudstones and sandstones
Basaltic rocks rich in Mg, Fe, and Ca.

In meta-pelites, many of the metamorphic minerals are rich in Al.
In meta-basaltic rocks, many of the metamorphic minerals are rich in Mg, Fe, and Ca.

Metamorphic Facies

Refer to mineral assemblages generated during specific temperatures and pressures [fig 5.16].
The mineral assemblages don't stay the same, but the Ps and Ts do.
Were developed based on meta-basalts
Are named, in part, after the minerals found in meta-basalts.

Mineral assemblages for various protoliths follow (see table 5.1)

Basalt
greenschist -- epidote and chlorite (two green minerals [diagnostic]) + calcte, plag
amphibolite -- amphiboles, plagioclase
granulite -- pyroxenes, plag, the last thing before melting, dry,
blueschist -- glaucophane -- Na-amphibole (blue mineral), epidote, phengite
eclogite --jaditic pyroxene, garnet, kyanite
hornfels --pyroxene, plagioclase

Al-Rich Sedimentary Rocks
greenschist --chlotie muscovite, plagioclase, quartz
amphibolite --garnet, biotite, muscovite, sillimanite
granulite --biotite, k-feldspar, quartz, sillimanite
blueschist --chlorite, muscovite, plagioclase, quartz
eclogite --not yet observed
hornfels --andalusite, biotite, K-feldspar, quartz

Other Sediments
Marble -- recrystallized calcite
Quartzite -- recrystalized qtz

The Plate Tectonic Connection

Convergent plate margins often have paired metamophic belts. Closest to the trench is a blueschist belt, while close to the volcanic arc is a greenschist/amphibolite belt. Close to the trench, high pressures are accomponied by low temperatures, whereas closer to the volcanic arc, high temperatures are attained at moderate pressures.