#16 Metamorphic Rocks

Metamorphic (Greek 'meta' = change and 'morphe' = shape) rocks - form by metamorphism which represents "solid-state" (i.e., no melting) changes in mineralogy and texture of a pre-existing rock as a result of high temperatures or high pressures or both (commonly deep within Earth); many metamorphic rocks have a foliation, banding produced during metamorphism. Metamorphic rocks can be derived from any precursor (parent) rock, e.g., sedimentary, igneous, or other metamorphic. Metamorphic environments vs. sedimentary (<~200C and <10 km depth) and igneous (>800C and variable depth) environments.

Because metamorphic environments are usually deep within Earth, this means that origin of metamorphic rocks is the most difficult to interpret because we cannot direct observe them forming like we can for sedimentary and igneous/volcanic environments. What is the evidence for metamorphism (photo #1, #2, #3, #4)?

 

 

We commonly see old metamorphic rocks at Earth's surface due to erosion of overlying rocks. Explain distribution of metamorphic rocks at surface in North America:

absence in Midcontinental USA -

presence in Appalachian and Cordilleran -

presence in central and eastern Canada -

I) Importance of studying metamorphic rocks

Natural Resources (benefit) - rocks used in construction (gneiss, slate, asbestos, quartzite, marble-sculpture); mineral resources (diamond, garnet, talc); hydrothermal ore deposits (Au, Ag, Cu, Ni, Zn, Pb); mountains (aesthetics)

Landslides (hazard) and Civil Engineering - strength of metamorphic rock is important, orientation of foliation planes with land surface affects stability





Geology - Geologic history (especially mountain-building in convergent tectonic settings), radiometric dating of time of metamorphism

II) Controls on Metamorphism

Metamorphism involves "solid-state" changes (in texture or mineral content or both) of pre-existing rock (but allows for dissolution and reprecipitation of minerals in pore water).

Why does pre-existing rock change?

 

A) Parent rock - controls composition of metamorphic rock; usually elements do not enter or leave rock, except H2O and CO2. So, limestone is converted to marble (coarse-grained, interlocking calcite) and quartz arenite is converted to quartzite (coarse-grained, interlocking quartz).

B) Temperature - increasing temperature accelerates speed of metamorphic reactions; analogy of baking bread.

C) Pressure - increasing pressure favors denser minerals and can rearrange pre-existing minerals. Types of pressure: 1) (Litho)Static = pressure applied equally on all sides, e.g., water pressure on deep submarine. Lithostatic (confining) pressure is associated with deep burial, which compresses rock into smaller, similarly shaped form, i.e., < pore space. At microscopic level, dissolution occurs at high pressure contact points and reprecipitation occurs at low pressure areas (pore spaces) that < pore space. 2) Directed pressure = involves > pressure along one preferential direction (not equally distributed); flattens objects in plane where highest pressure is applied and lengthens objects perpendicular to that plane; directed pressure is produced during tectonic plate collisions and along faults. Important effect of directed pressure = existing platy (or needle-like) minerals (e.g., mica and chlorite) change orientation and new platy minerals grow with preferential orientation (foliation) aligned perpendicular to directed pressure (figure).

D) Pore fluid (usually water) - little present during metamorphism, but it >> speed of chemical reactions; water is rapid transit mechanism for ions when they dissolve mineral somewhere and grow elsewhere; if water is absent, dissolution and recrystallization are very slow.

E) Time - need millions of years

III) Types of Metamorphism

Contact = next to hot igneous intrusions that occur ~near Earth's surface (photo).
Why not deep intrusions?

High temperature (T), low pressure (P) environment that produces zone (metamorphic aureole) of altered host rock, aureole thickness (cm - km) depends on intrusion size (adjacent to intrusion), get unfoliated metamorphic rocks (hornfels) due to low P.

Regional = large zones of metamorphism caused by high T AND high P; due to deep burial beneath Earth's crust, e.g., mountain building at convergent plate boundaries and continent-continent collision zones (regional = dynamothermal metamorphism); usually involves directed P (and foliated metamorphic rock).

Other types of metamorphism = hydrothermal alteration (by hot, flowing waters, e.g., serpentinite at oceanic ridge), fault-zone metamorphism (grinding of rock along fault produces directed P, frictional heat and fine-grained rock = mylonite and large, recrystallized grains = augen), shock metamorphism (e.g., from meteorite impact), and from lightning strikes (ultra-high T).

IV) Metamorphic Rock Classification - subdivided as foliated or unfoliated, depends on nature of parent rock.

A) Foliated - most from shale/mudstone precursor, contains abundant platy clay minerals. Why are foliated metamorphic rocks more common than unfoliated metamorphic rocks? Four rocks below form with > metamorphic grade, i.e., > T and P.

Slate (photo) = breaks in thin flat sheets (foliation = slaty cleavage), contains microscopic, oriented micas; uses = chalkboard, roof shingle, and pool tables. Colors = red (hematite-rich), green (chlorite-rich), and black (organic carbon-rich).

Phyllite (photo) = silky luster due to barely visible mica, straight or wavy (crenulated) cleavage.

Schist (photo) = coarse-grained, visible mica with parallel orientation; > mica size; foliation = schistosity.

Gneiss (photo) = bands of light and dark layers (different minerals); foliation = gneissic (compositional) banding. Schist and gneiss have most abundant mineral names before rock name (e.g., quartz, garnet schist and quartz, feldspar, biotite gneiss).

B) Non-foliated = no platy minerals; equant grains (sides with equal lengths) or form under low P (contact metamorphic) conditions.

Quartzite (photo) = from quartz arenite; very hard; interlocking quartz grains; quartz arenite breaks within cement, quartzite breaks through quartz grains.

Marble (photo) = from limestone or dolostone (dolomitic marble); coarse, interlocking calcite (dolomite) grains; sugary; lose sedimentary features (e.g., fossils, bedding).

Hornfels = from any parent rock by contact metamorphism (high T and low P), no directed P so non-foliated even if platy minerals present; often dark, dense, and hard (easiest to identify in field).

V) Metamorphic Facies (Grade) - With > metamorphism usually little change in chemical composition (< H2O and CO2), but can be big changes in mineral content, especially for shale parent rock. Fig. 7.21 (bottom) shows key indicator minerals for low-grade (T = 200 - 400C, P = 1 - 6 kbar or depth = 3 - 18 km; chlorite, biotite and muscovite), medium grade, and high-grade (T = 500 - 1,000C, P = 12 - 40 kbar, or depth = 36 -120 km; staurolite and sillimanite) metamorphism. Metamorphic grade in rocks from eastern USA due to Paleozoic convergent plate tectonics (> grade to east).

Certain metamorphic minerals (index minerals) form under narrow ranges in T and P (e.g., Al2SiO5 polymorphs - kyanite, andalusite and sillimanite). Metamorphic index minerals not always present (need correct chemical composition); metamorphic facies = assemblage of minerals in rock which indicates metamorphic T and P. Fig. 7.23 = major metamorphic facies (assuming basalt parent rock) over T and P. Different T - P pathways for different types of metamorphism - Middle path = regional metamorphism (normal geothermal gradient, high T and P, includes zeolite, greenschist, amphibolite, granulite, and eclogite facies), upper path = contact metamorphism (high geothermal gradient, high T, low P, includes hornfels facies), lower path = subduction-zone metamorphism (low geothermal gradient - high P, low T, includes blueschist facies).

VI) Metamorphism and Plate Tectonics - plate tectonics explains distribution of metamorphic facies for regionally metamorphosed rocks. See paired (foliated) metamorphic facies in mountain belts (high P, low T = blueschist facies plus high T and P = eclogite/amphibolite/greenschist facies). Explanation = regional metamorphism at convergent plate boundaries (subduction zones). Get foliation from directed P due to convergent plate motion. High P/low T (blueschist) facies forms in rocks of accretionary prism due to cold, ~rapidly descending oceanic lithosphere. High T and P facies form in overriding tectonic plate (continental or oceanic lithosphere) due to upwelling magma that provides heat source. See greenschist (and other facies) metamorphism in rocks of magmatic arc. Ancient paired metamorphic facies in rock record indicates fossil convergent plate boundary.