Metamorphism and Metamorphic Rocks

Greek Etymology:
- Meta– means "Change"
- Morpho– means "Shape"
Definition of Metamorphism:
- Metamorphism is the process by which rocks beneath the Earth's surface change due to new conditions, resulting in rocks that exhibit gained or new properties.
- The rocks transformed from parent rock, formed under new conditions, are called metamorphic rocks.
- Source Rocks:
- Metamorphic rocks can form from:
  - Igneous rocks
  - Sedimentary rocks
  - Other metamorphic rocks

Role of Heat:
- Increases the rate of chemical reactions that produce different minerals.
Sources of Heat:
- Intrusive magma: Heat from intrusive igneous bodies can bake surrounding rocks.
- Deep burial along subduction zones: Earth's internal heat is manifested when rocks are subject to great depths of pressure.

Types of Pressure:
- Lithostatic Pressure:
- Applied equally in all directions, resulting from the weight of overlying rocks.
- Pressure increases with depth (P increases with D).
- Causes mineral grains to pack more closely together.
- Recrystallization may occur, producing smaller and denser minerals.
- Differential Pressure:
- Results from unequal forces applied to the rock (e.g., during mountain building).
- Differential Stress:
  - Creates a metamorphic texture called foliation.
  - Texture refers to the size, shape, and arrangement of grains within a rock.
  - Foliation Definition:
  - Any planar arrangement of mineral grains or structural features within a rock.

Role of Fluids:
- Water and carbon dioxide are almost always present in metamorphic environments and enhance metamorphism by increasing the rate of chemical reactions.
Fluid Sources:
- Water trapped in the pore spaces of sedimentary rocks.
- Volatile magmatic fluids.
- Dehydration of water-bearing minerals (such as clays, micas, and amphiboles) subject to heat and pressure.

Definition:
- Occurs when a body of magma alters the surrounding country rock.
Factors Affecting Contact Metamorphism:
- Initial temperature and size of the intrusion.
- Presence and chemistry of fluids.
Metamorphic Aureoles:
- Zones of mineral assemblages surrounding the intrusion, created due to a decrease in temperature with distance from the intrusion.

Definition:
- Results from high differential pressures, often found near faults.
- Resulted rock types are typically mylonites, which are hard, dense, and fine-grained and are restricted to narrow zones along the faults.

Definition:
- A consequence of tectonic forces that build mountains.
- Characterized by pronounced differential stresses and extensive mechanical deformation, accompanied by chemical recrystallization.
Characteristics:
- Most significantly observed along convergent plate margins but also occurs in divergent areas.
- Results from extreme heat and pressure due to tectonic processes at convergent plate boundaries.
- Produces the majority of metamorphic rocks, forming the geological shields.
- Covers large geographic areas, usually showing gradation of deformation in relation to the areas of intense heat and pressure.

As temperature and pressure rise, one mineral assemblage transforms into another.
Each assemblage is characteristic of a specific rock composition.

Definition:
- Heat and differential pressure cause minerals to arrange in a parallel fashion known as foliation.
- Texture:
- Determined by size and shape of crystals; ranges from fine to coarse with increasing grain size.
Common Examples:
- Slate
- Phyllite
- Schist
- Gneiss
Note: Metamorphic grade increases with the degree of foliation.

Definition:
- In these rocks, minerals do not exhibit a clear preferred orientation.
Common Examples:
- Marble (mostly composed of calcite)
- Quartzite (primarily quartz)
- Very fine-grained textures like those in greenstone or hornfels.

Index Minerals:
- In the Upper Peninsula of Michigan, certain minerals indicate different grades of metamorphism; the appearance of index minerals allows for zonation.
Metamorphic Facies:
- A group of metamorphic rocks formed under the same broad conditions of temperature and pressure, characterized by specific minerals reflecting these conditions.

A pressure-temperature diagram illustrates where various metamorphic facies occur.
Facies:
- Defined as groups of rocks characterized by mineral assemblages formed under similar temperature and pressure conditions.

Convergent Boundaries:
- Here, temperature and pressure increase due to tectonic collisions.
- Several facies are recognized in this model based on the conditions of temperature and pressure.
Note: Metamorphism can occur along all types of plate boundaries but is most common along convergent ones.
Example:
- The Franciscan Complex in California is a notable example, exhibiting the blueschist facies, which indicates a former subduction zone characterized by low-temperature, high-pressure conditions.

Geological Significance:
- Exposed metamorphic rocks make up extensive parts of continents.
Mineral Relevance:
- Certain minerals present in metamorphic rocks provide insights into the conditions that existed during their formation.
Utilitarian Value:
- Used widely as building materials and in various manufacturing industries.
Location Reference:
- Mines along Greenstone Belts in Manitoba are examples of economically significant locations connected to metamorphic activity.