EOSC Lecture 10 Metamorphic rocks

Metamorphic rocks

change shape or form

• sedimentary rocks and igneous rocks (and metamorphic rocks) are the ‘parent rocks’ of metamorphic rocks

• with different pressure and/or temp conditions, mineral assemblages in the parent rock become unstable and recrystallize

Metamorphism

mineral (chemical) and crystalline changes in a rocks

• caused by:

  • pressure

  • temperature

  • fluids

  • time

• no melting

Country rock

the original rock around a metamorphic rock (and/or igneous intrusion)

What drives metamorphism - increased temp

average geothermal gradient is 25 degrees celsius/km depth in the upper crust - deeper is hotter

• varies depending on tectonic and geologic settings (eg. higher gradients exist close to mid-ocean ridges, close to intrusions)

what drives metamorphism - increases pressure

• lithostatic (equal in all directions) - increases with depth

• differential (directed stress, which may be compressive or shear) - usually a result of plate tectonics (sometimes meteorites)

Pressure

• pressure has implications for stability of minerals as well as texture

• rocks confined to greater pressures are typically denser and recrystallize high density minerals

• due to plate tectonics, areas in the crust are rarely under equal confining pressure, instead, pressure is directional

Foliation

produced by compressive (directed and shear) differential stress

• means layers in the rocks

• pervasive - starts at the molecular level

What drives metamorphism - fluid activity

• water and other fluids in the pore spaces of rock

• facilitates transfer of ions within rocks and minerals

• water increases the rate of reactions and metamorphism

• fluids also form “veins” - typical veins are quartz or calcite; metals are possible as trace constituents

What drives metamorphism - time

time is a very important factor in these changes

• metamorphism is a chemical reaction

• it takes time to happen

• but time alone can also lead to changes (slow reactions)

• formation of some metamorphic minerals have been estimated at 1 mm growth per 1 million years

• eg. basalt + time = greenstone

Two types of metamorphic rock

1. foliated metamorphic rocks - formed in an environment with directed or shear stress

2. non-foliated metamorphic rocks - formed close to the surface with minimal pressure

Non-foliated metamorphic rocks

formed mostly by heat, pressure not as important (eg. limestone→ marble, during metamorphism any textures or fossils are destroyed and calcite crystals become larger

Foliated metamorphic rocks

as minerals recrystallize, they do so in preferred alignment perpendicular to pressure

Metamorphic grade

the intensity of metamorphism

Metamorphic grade in foliated rocks

shale → slate → phyllite → schist → gneiss

• slate - product of low-grade metamorphism of shale, microscopic clay and mica form perpendicular to stress, minerals start to align, rock breaks into sheet ~1 mm

• phyllite - as temp and pressure increases, the product of increased metamorphism of slate. unlike slate, foliation can be wavy instead of planar. minerals grow larger, micas form larger which gives a “sheen” or shine on foliation

• schist - as temp and pressure increases, the product of increased metamorphism of phyllite. at this stage micas and metamorphic minerals are clear. individual minerals (micas, garnet) visible to the eye

• gneiss - as temp and pressure increases, no mica since at these temps mica is no longer stable. visible (big) crystals with alternating layers of mafic and felsic material

• migmatite - as temp and pressure increases, mix of both metamorphosed and igneous material, rock finally begins to melt, creates “mixed rock” with veins and patches of granite

Contact metamoprhism

magma moves in the upper curst = heat added to the rocks

• high temp/low pressure

• “cook” the rock

• results - recrystallization, new minerals, veins form, non-foliated rocks

• usually less than ~ 5km deep

Magma chambers

magma chambers in the crust are ~1000 degrees celsius in temp, and heat the surrounding rock. the zone of metamorphism around the chamber is typically very small, ~10m

Contact aureoles

zone of contact metamorphism

• the size of the contact aureole is type of country rock, the magma temp, and the size of the magma body. the larger the magma body, the more time for metamorphism to take place

Metasomatism

the magma body can heat surrounding groundwater, causing a convection system. the hot groundwater circulates for thousands of years, altering the composition of surrounding rocks. this type of metamorphism is called metasomatism, and the process of alteration this way is termed hydrothermal alteration

Regional metamorphism (mountain building)

• high temp/high pressure

• occurs over large areas of crust due to increased P and T at depths >5 km

• effects: recrystallization, new minerals form → foliated rocks

• burial of rocks to depths below 5 km is possible even if the rocks are above sea level

Regional metamorphism (divergent boundaries)

basalt and gabbro are formed at ocean ridges, sea water is drawn in and creates a convection system, heating the water up to 300 degrees, allowing for metamorphic reactions

• pyroxene in the basalt changes to chlorite/serpentine

• greenstone metamorphism is called retrograde metamorphism because the tmps at which it occurs is much lower than what it was when the basalt formed (~1200 degrees celsius)

Regional metamorphism (subduction zones)

the subducting oceanic slab is much cooler than the surrounding rock, so the metamorphism occurring is at high pressure, but low temperatures

• at low T, high P conditions, an amphibole mineral named glaucophane forms which is blue in colour. the resulting rock made is blueschist, and as it is subducted further to ~35 km (into the mantle) it turns into eclogite

Dynamic metamorphism

• low temp/high pressure

• in fault zones

• rocks grind against each other creating some friction (heat) and lots of pressure

• results: weirdness → foliated and “ground up” rocks