Lab 7 Metamorphic Rocks

What is a Metamorphic Rock?

-rock changed by heat, pressure or fluids

Parent Rock (Protolith)

-The original rock type before it underwent metamorphism

Two types of Metamorphism:

1) Contact Metamorphism-

- rocks that are close to an igneous intrusion, like a batholith, undergo metamorphism from the heat and fluids given off by the intrusion.

2) Regional Metamorphism-

- large scale metamorphism of rocks within the crust, like underneath mountain belts, produces regional metamorphism. Rates of deformation are very slow, rocks are folded and metamorphosed over a long period of time.

Metamorphic Rock Textures:

Foliated vs. Non-Foliated

-Foliated means a parallel alignment of platy minerals like mica. The rock appears layered, with “stripes” of crystals

· Foliated Metamorphic Rock Textures-Increasing met. grade

Slaty Rock Cleavage-

-Microscopic faults within the rock produce a sheet like cleavage. Slate is a good example of a metamorphic rock with slaty cleavage.

Phyllite Texture-

-Rock has a shiny luster (sheen) resulting from the growth of platy micas. The surface of the rock may be wrinkled and wavy. Intermediate grade.

Schistosity-

-Mica minerals are visible to the naked eye, rock appears to have scales and is very reflective. Intermediate to High grade.

Gneissic Banding-

-Alternating layers, or lenses, of light and dark minerals. Mafic minerals usually form the dark bands, and felsic minerals usually form the light colors.

· Non-Foliated Metamorphic Rock Textures-

-Crystalline Texture

-Med to coarse grained aggregate of intergrown xtals

-Microcrystalline Texture

- very fine grained aggregate of minerals

-Sandy Texture- (not sedimentary)

- A sandstone that has undergone metamorphism would have a sandy texture, the grains would be recrystalized and interlocking. Ex- quartzite

Textures that occur in any Metamorphic Rock-

-Stretched or Sheared Grains-

-deformed grains (pebble, fossil etc) are stretched out like putty

-Folds

-Initially linear features can be bent and buckled into a folded geometry

Many more on Page 140

· Classification of Metamorphic Rocks:

-Metamorphic rocks are named based on their texture and mineralogical compositon…

ex- A rock that has large, visible grains of muscovite mica roughly aligned in a foliation with small garnets would be called a-

Garnet Schist

Name then Texture

If more than one accessory mineral list in order from least to most abundant.

Ie. If the above described rock were to have a small amount of biotite then you could call it a Biotite Garnet Schist