metamorphic rocks

What is metamorphism? It is a re-crystalization that alters the mineral composition and texture igneous or sedimentary rocks. The protolith dictates characteristics of resulting metamorphic rock. Alterations proceed until equlibrium with new conditions is reached.

Metamorphism three principal agents: Heat, Pressure, fluids

- They occur in the solid state (no melting occurs)

- metamorphic rocks often exhibit complex folding (protoliths must have become ductile)

Amphibolite can be produced from basalt. Different mineralogical composition and texture:

Basalt: mafic mineral comprised of plagioclase feldspar, pyroxene with low silica content

Amphobolite: comprised mostly of amphibole and plagioclase feldspar with little to no quartz

Marble is the metamorphic equivalent of limestone. Similar mineral and chemical composition. Has distinct difference in texture:

- fossils present in limestone protolith

- Marcle comprised of fine-grained calcite

- schist produced from metamorphism of shale, has a similar chemical composition but is different and has their own texture in comparison:

Shale is fine-grained and exhibits bedding, while Schist's texture is dominated by large mica crystals

Metamorphism is driven by four principal factors:

1. Temperature: increases at a rate of 30c per km depth

2. Pressure: Increases at a rate of 0.23 kbar per km depth

3. Fluid chemistry: Introduces dissolved minerals

4. Time: Reactions involving silicate minerals are very slow

These all typically occur between 200 and 800 C. Diagenesis occurs at <200C, while melting occurs at >800C. These temperatures are typically encountered at depths of 10-30km within the crust

The role of heat:

It changes composition, mineralogy and texture due to heat are associated with breaking of chemical bonds, changing of crystal structure. Recrystalization and stability of minerals are temperature dependent:

- Atoms combine differently at different temperatures

- Increasing temperature speeds up chemical reactions

Geologists can use mineral compositon of metamorphic rocks as a geothermometer.

Role of pressure: Pressure can also alter chemical composition, mineralogy and texture of rocks, they can control which minerals form and which are stable. Minerals formed at a higher pressure exhibit higher density.

Two basic kinds of pressure involved in metamorphism:

1. Confining pressure (lilthostatic pressure): Force is applied equally in all directions and increases with depth in the crust. Recrystallized minerals become reoriented and tightly locked

2. Directed pressure (differential stress): extered on rocks at convergent plate boundaries. Forced applied in a perticular direction and usually concenntrated within discrete zones. Recrystallized minerals exhibit parallel alignment of textural and structural features. Ductile rocks can be severely distorted.

The role of Fluids: Water and carbon dioxide present in varying amounts = along grain boundaries and in pore spaces. They accererate chemical reactions where ions are transported rapidly from one place to another. Different sources of fluids:

- trapped in sedimentary rocks

-released from magmas

- breakdown of hydrated minerals

Recrystallization: hydrothermal fluids can also transport heat and promote recrystallization.

Types of Metamorphism: composition and texture can provide information about metamorphic conditions: temperature, pressure, protolith. Linking metamorphism to geologic setting requires understanding of:

- regional metamorphism

- contact metamorphism

- Hydrothermal (seafloor) metamorphism

Other types of metamorphism:

Burial: low grade metaphorhism from increasing pressure, typically begins at depths of 6-10 km, "basement" of oil and gas development

High pressure and ultra-high pressure: 8-12kbar (HP) and >28kbar (UHP), rocks form at depths of >30km in subduction zones, can produce rocks called eclogites.

Shock metamorphosm: energy derived from meteorite impacts, localized metamorphism, planar fracturing and deformation

Metamorphic Grades: rocks evolve through a sequence of metamorphic grades, index minerals provide an indication of metamorphic grade

Metamorphism: some silicate minerals ae found only in metamorphic rocks, texture and mineral composition are used to study metamorphic processes and conditions

Metamorphic textures are determined by properties of consituent minerals:

- size, shape arrangement, mineral.

These are seperated into three general classes:

- foliated rocks

- Non-foliated rocks (granoblastic) rocks

- Prophyroblasts

Foliation: Flat or wavy parallel planes produced by directed pressure. More pronounced with increaseing metamporhic grade. Produced bby prederred orientation of minerals with platy crystal habit.

Their texture is produced bgy directed stress related to regional metamorphism: slate (fractures along parallel planes due to aligned micas) > phyllite (parallel or wavy fabric surface sheen due to micas) > Schist (sub-parallel alignment of visible mica crystals) > Gneiss (minerals segregated into bands; feldspar and amphibole)

Foliated rocks are igneous or sedimentary protoliths and are classified according to four principal criteria:

1. Metamorphic grade

2. Grain (crystal) size

3. Type of foliation

4. Banding

Systematic relationship between foliation texture and metamorphoc grade, higher grade produced more pronounced foliation

Non-foliated rocks contain crystals with equi-dimensional shapes and form due to confining pressure; directed pressire produces foliation. Often associated with contact metamorphism.

Hornfels: high-temperature contact metamorphism. granunlar texture, little deformation, commonly contains pyroxene

Greenstone: seafloor metamorphosm of mafic volcanic rocks, abundance of chlorite produces green colour

Amphibolites: medium to high-grade metamorphism, contain amphibole and plagioclase felspar, foliated textures also produced.

Granilite: high-grade metamorphism, equant crystals showing faint foliation, formed from shale, impire sandstone and many igneous rocks

Prophyroblasts: metamorphic minerals can grow to large crystals surrounded by much finer-grained matrix. Crystal growth due to recrystallization of rock matrix at high temperature and pressure.

Metamorphics Facies: relationship between mineral assemblage and metamorphic grade