Metamorphic Rocks

Introduction: Metamorphic rocks form as a result of significant changes in temperature, pressure, and chemical composition in regions of the Earth which we cannot directly observe. So, like igneous rocks, they provide an important 'window' to the study of internal Earth processes. Many mountain ranges (such as the Blue Ridge) are characterized by metamorphic rocks, so they also provide important information about how mountain systems form. In certain cases, metamorphism results in the formation of minerals which have important economic value.

I. Definitions

Metamorphism - The process by which rocks and minerals undergo solid state changes in response to changes in temperature, pressure, and chemical conditions.

Metamorphic Grade - The 'intensity 'of metamorphism which resulted in formation of a metamorphic rock; the maximum temperature and pressure attained during metamorphism. General rule: metamorphic grade increases with depth because temperature and pressure rise with depth (geothermal gradient - the average increase in temperature with increase in depth).

Metamorphic Facies - A set of all metamorphic rock types that formed under similar metamorphic grade conditions. Rocks within a metamorphic facies may exhibit differing mineralogical compositions due to differences in the chemical/mineralogical composition of the protolith. But, they all formed under the same conditions of pressure and temperature.

Protolith (Parent Rock) - A rock which undergoes metamorphism, resulting in formation of a new metamorphic rock. Example: The protolith of the metamorphic rock marble is the sedimentary rock limestone. The protolith of the metamorphic rock quartzite is the sedimentary rock sandstone.

II. Changes during Metamorphism

1. Change in grain size - As a general rule, grain size increases with increasing temperature and pressure

2. Increase in rock density - As a general rule, rock density increases in response to an increase in pressure

3. Development of Foliation -  (Fig. 3.29) 'Layering' in a rock which develops when micas align themselves with their flat sides perpendicular to the direction of maximum pressure (slaty cleavage, schistosity) (Fig 3.31). Also may occur when atoms recombine to form new minerals segregated into light and dark bands (gneissic banding). (Fig 3.31) (Make sure you see examples of all foliation types in lab!)

   

                     slaty cleavage                                                                        schistosity

 

gneiss

       

                   gneissic banding

4. Development of new mineral assemblages stable at the higher pressure and temperature conditions during metamorphism.

III. Types of Metamorphism

Contact Metamorphism - Metamorphism caused by heating of, and/or the addition of fluids to, cooler 'country rock' from a nearby igneous intrusion. Results in formation of a contact aureole (contact halo). Contact metamorphic rocks are commonly non-foliated.


 

Burial Metamorphism - Results from deep burial of sedimentary rocks. Burial metamorphic rocks are commonly weakly foliated, or non-foliated.

Regional Metamorphism  (Dynamothermal Metamorphism) (Fig 3.27) - Metamorphism which occurs over a large geographic region, such as along a mountain chain. Typically occurs due to convergence of the Earth's tectonic plates. Regional metamorphic rocks commonly exhibit foliation due to high pressures caused by tectonic plate convergence..



Hydrothermal Metamorphism  (Metasomatism) - Occurs when hot water and dissolved ions react with a rock to change its chemical composition and mineral content. May result in formation of hydrothermal ore deposits, such as gold, silver, copper, lead and zinc. (Fig 3.35)

 

IV. Classification of Metamorphic Rocks (Fig 3.30)

Rocks which typically form as a result of regional metamorphism: slate, phyllite, schist, gneiss, migmatite. Note: All are foliated.

Slate - A compact, fine-grained, low grade metamorphic rock with slatey cleavage that can be split into slabs and thin plates. Protolith = shale, mudstone or siltstone.





Phyllite - A fine-grained, low to medium grade metamorphic rock. Appears similar to slate, but breaks along 'wavy' surfaces. Has a glossy sheen due to larger grain size than in slate. Protolith = slate.

 

phyllite

 

Schist - A strongly foliated medium grade metamorphic rock that has well-developed parallelism of minerals such as micas. Protolith = phyllite.

Gneiss - A high grade foliated metamorphic rock with banded appearance formed by regional metamorphism. Protolith = schist, granite, or volcanic rocks.

 

Migmatite - a high grade metamorphic rock that has undergone partial melting.

 

 

Commonly non-foliated, equigranular (granoblastic) metamorphic rocks:

granoblastic texture in marble

Marble - A metamorphic rock consisting of fine- to coarse-grained recrystallized calcite. Protolith = limestone. (Fig 3.31)

Quartzite - A metamorphic rock composed mostly of quartz formed by recrystallization of sandstone. Protolith  = quartz sandstone (Fig 3.31)

Hornfels - Metamorphic rock which forms during contact metamorphism

 

hornfels

 

 

Study Questions - Metamorphic Rocks