Geology 111G/Lecture 11

Classification of Metamorphic Rocks

Foliated Rocks

Non-foliated Rocks

Pressure-Temperature Domains

Metamorphic Facies

Metamorphism and Plate Tectonics

I. Foliated Rocks: progressive metamorphism of a fine-grained rock, for example, a mudrock results in development of increasing grain size and more readily apparent foliation. The classification of these rocks is based on that grain size and also the protolith or starting rock type:

A. Shale protolith. Progressive metamorphism of sedimentary rock.

1. Slate. Fine grained rock with a definite foliation. Minerals are microscopic micas with parallel alignment.

2. Phyllite. Somewhat larger micaceous minerals gives rocks a silky or glossy sheen. Still to small to see with unaided eye. May contain small knots of non-platy minerals.

3. Schist. Rock with visible platy minerals, sometimes with large knots of non-platy minerals such as garnet. The well developed foliation of this rock type is called schistosity.

4. Gneiss. Strongly layered rock with alternating bands of platy (micaceous) minerals and non-platy (granular) minerals, usually quartz and feldspar. These mineral segregations result in dark-light banding.

B. Basalt protolith. Progressive metamorphism of oceanic crust.

1. Blueschist. Blue foliated rock formed at low T but high P.

2. Greenschist. Green foliated rock formed at moderate T and P.

3. Amphbolite. Black foliated rock containing abundant amphibole (hydrous). Forms at high T and P.

4. Granulite. Foliated gnessic rock formed at very high T and P. Anhydrous, formed near base of continental crust.

II. Non-foliated rocks. These form in contact metamorphic settings or the protolith composition is not conducive to formation of platy minerals.

A. Hornfels. Nondescript, compact rock found in contact-metamorphic aureoles.

B. Quartzite. Metamorphosed quartz-rich sandstone.

C. Marble. Metamorphosed limestone or dolostone. Large crystals of CaCO3 are equant and tend not to form foliation. Tends to be banded in different colors of gray because of variations in organic carbon (graphite) content.

III. Pressure-Temperature Domains. In the case of regional metamorphism, variations in the PT conditions result in corresponding changes in metamorphic grade. These variations recur so that we recognize them and group them.

A. Metamorphic facies. Groupings of rocks of various mineral composition s formed under different PT conditions. These are named after the rocks formed from a basalt protolith.

IV. Metamorphism and Plate Tectonics. Different types of metamorphic faces occur at different plate boundaries, because different P/T conditions are inherent to those boundaries.

A. Metamorphism at Convergent boundaries.

1. Subduction zone. Introduces cold oceanic crust rapidly to high P conditions, resulting in blueschist facies.

2. Magmatic arc. Rise of magma transfers heat to mid and shallow crustal levels, tends to result in greenschist facies, but could be hornfels too.

3. Continental collision. Thickening of crust results in both amphibolite and granulite facies.

B. Metamorphism at Divergent boundaries. These tend to be high temperature, but moderate pressure.

1. Mid ocean ridge. Hydrothermal metamorphism tends to create greenschist facies rocks. Abundant elemental exchange between sea water and rock.