Chapter 8

What two features characterize most metamorphic rocks?

1) Metamorphic mineral assemblages- minerals uniquely produced under the temperature and pressure regimes of metamorphism 2) Metamorphic texture- grain arrangement, often involving foliation: a preferred alignment of platy grains or alternating light and dark mineral bands

Processes at work during metamorphism

1) Recrystallization- minerals change size and shape. 2) Phase change- new minerals form with the same chemical formula but different crystal structure. 3) Neocrystallization- the formation of new minerals with pressure and temp. changes. (it is the decomposition of original minerals in the protolith and the chemical reaction of the elements to form new minerals) 4) Pressure solution- mineral grains partially dissolve. 5) Plastic deformation- mineral grains soften and deform.

Differential stress

2 Types: 1) Normal- operates perpendicular to the surface. 2) Shear- operates sideways across a surface, and causes material to be smeared out.

Why does metamorphism happen at the site of meteor impacts and along mid-ocean ridges?

At a meteor impact site, pressure on minerals rises sharply at the time of impact, producing conditions favorable for new minerals that would not otherwise be present. Mid-ocean ridge settings provide ample opportunity for relatively cool water to interact with hot, recently formed rock.

How does plate tectonics explain the peculiar combination of low-temperature but high-pressure minerals found in a blueschist?

Blueschists form at the base of thick accretionary prisms, sediments scraped off of the downgoing slab at subduction zones. Because the subducting slab is relatively cool, it adds little heat to the prism, allowing for the relatively high pressures but low temperatures in which the blueschist mineral assemblage is stable.

What is metamorphic foliation, and how does it form?

Foliation is the presence of parallel planar surfaces or layers in metamorphic rock. Under sufficiently differential stress, platy or elongate grains are broken down and regrown in a preferred orientation perpendicular to maximum compressive stress.

How are metamorphic rocks different from igneous and sedimentary rocks?

Metamorphic rocks are the result of heat and stress causing an alteration of texture, mineralogy, or both within a preexisting rock, without the rock having undergone melting, weathering, or diagenesis. It is a solid-state change. Many metamorphic (but not igneous or sedimentary) rocks possess foliation.

Metamorphic environments

Metamorphism occurs in different settings via geothermal gradient, differential stress, and hydrothermal fluids, which are all governed by tectonics. Different Environments: 1) Dynamothermal (Regional) Metamorphism- formed at tectonic collision zones. Rocks caught up in mountain building are heated via the geothermal gradient and plutonic intrusions, squeezed and heated by deep burial, and smashed and sheared by differential stresses. 2) Thermal (Contact) Metamorphism- due to heat from magma invading host rock. Creates zoned bands of alteration in host rock, called a contact (or metamorphic) aureole, which is zoned from high grade (near pluton) to low grade (far from pluton). 3) Dynamic (fault zone/shear zone)- the fault location determines the type of alteration: in the shallow crust, rocks break to form fault breccia (non-metamorphic), while in the deeper crust, rocks behave in a ductile manner and minerals smear like taffy to form mylonite. 4) Burial metamorphism- must be buried below diagenetic effects. As sediments are buried in a sedimentary basin, pressure increases because of the weight of the overburden and temperature increases because of the geothermal gradient. 5) Hydrothermal (Metasomatism) Metamorphism- a dominant process near mid-ocean ridge magma (divergent plate boundary). Cold water seeps into the crust and is heated by magma, then reacts with mafic rock, and rises (as hot water) to be ejected via black smokers. 6) Shock Metamorphism- When earth is struck by a meteorite (rare), the impacts generate extremely high pressures (a shock wave), and heat that vaporizes or melts large masses of rock. These conditions generate high-pressure minerals. (ex: Coesite and Stishovite) 7) Subduction metamorphism- occurs at trenches and accretionary prisms, which have a low geothermal gradient (temp.) and high pressure. The subduction creates the unique Blueschists.

How does prograde metamorphism differ from retrograde metamorphism?

Prograde metamorphism produces higher-grade rocks. It is metamorphism via increasing T and P; common in rocks that are buried in orogenic belts. Retrograde metamorphism produces lower-grade rocks. It is metamorphism via decreasing T and P; common in rocks that are brought from depth by uplift and erosion.

Nonfoliated metamorphic rocks

Quartzite- formed by metamorphism of quartz sandstone, when sand grains in the protolith recrystallize and fuse. It is almost pure quartz in composition, and is hard, glassy, and resistant like quartz. Marble- forms from a limestone or dolostone protolith, it is coarsely crystalline calcite or dolomite. Recrystallization completely changes the rock and it exhibits a variety of colors.

How is a slate different from a phyllite? How does phyllite differ from a schist? How does schist differ from a gneiss?

Slate and its characteristic slaty cleavage arise from the preferred orientation of clay minerals resulting from the relatively low-temperature and low-pressure metamorphism of a body of shale. Phyllite arises when significantly higher temperatures and pressures cause clay grains within slate to be recrystallized to form mica grains, which retain a preferred orientation. Unlike slate, which is rather dull, mica gives phyllite a silky luster. Schist differs from phyllite in that, as a result of greater heat and pressure, its mica grains are large, visible discrete plates, unlike the smooth sheen of tiny mica grains within phyllite. Gneiss is compositionally banded, with alternating bands or swirls of light- and dark-colored minerals, including additional minerals besides mica (quartz, feldspar, amphibole).

Foliated rocks

Slate- a fine clay, low grade metamorphic shale that has a distinct foliation called slaty cleavage. It develops by parallel alignment of platy clay minerals. It can break along this foliation to create flat sheets. Phyllite- a fine mica-rich rock that was formed by a low-medium grade alteration of slate. The clay minerals neocrystallize into tiny micas. It has slaty cleavage, like slate. Schist- a medium-coarse grained rock with larger micas that was formed from phyllite at higher temp. and pressure. Has a distinct foliation called schistosity, which is a parallel alignment of large mica crystals, and often has other minerals due to neocrystallization. Gneiss- has a distinct banded foliation, with light bands of felsic minerals (quartz and feldspar), and dark bands of mafic minerals (biotite and amphibole).

Why do some environments yield both foliated and nonfoliated rocks, whereas others produce only nonfoliated rocks?

The pressure applied to the reforming rock causes the differences in the way the rock looks once recrystallized and determines whether it will be foliated or nonfoliated. Foliated metamorphic rock: has a going-through planar fabric (a layered or banded appearance). It was formed because it was subjected to differential stress, and pressure is applied to the recrystallizing rock unequally. It has a significant amount of platy (or elongate) minerals and is classified by composition, grain size, and foliation type. Nonfoliated metamorphic rock: has no planar fabric evident. It was formed because it was not subjected to differential stress, and the pressure applied to the recrystallizing rock is equal all over. It has equant minerals only and is classified by mineral composition.

Describe the geologic settings where thermal, dynamic, and dynamothermal (regional) metamorphism take place.

Thermal metamorphism takes place in a zone of country rock surrounding a pluton, where the country rock's mineral assemblage becomes recrystallized. Dynamic metamorphism occurs in fault zones, where shearing force recrystallizes minerals at depth. Dynamothermal metamorphism occurs within the cores of mountain ranges, induced by increased heat and pressure associated with crustal thickening and the shear that arises in the development of fold-and-thrust belts.

Where would you go if you wanted to find exposed metamorphic rocks? How did such rocks return to the surface of the Earth after being at depth in the crust?

You would go look for the site of an ancient, greatly eroded mountain range, such as the modern Appalachians. The bases of mountain ranges produce large volumes of metamorphic rock. As overlying layers of sediment and rock are weathered and eroded away, isostatic pressure causes the basement to be buoyed upward until these rocks are finally exposed at the surface.

Metamorphic grade

a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form. Also a measure of metamorphic intensity. Low grade (slightly intense); High grade (very intense). High grade rocks are formed at high temperatures and pressures. PROGRADE metamorphism- produces higher-grade rocks. It is metamorphism via increasing T and P; common in rocks that are buried in orogenic belts. ex: Shale --> Gneiss Low grade rocks are produced at lower temperatures and pressures. RETROGRADE metamorphism- produces lower-grade rocks. It is metamorphism via decreasing T and P; common in rocks that are brought from depth by uplift and erosion.

Metamorphic rocks form

from pre-existing rock or "protolith" at high temperature and pressure. It is a solid-state change, and metamorphic rocks often look totally unlike protoliths. Due to changes in physical and chemical conditions: burial, tectonic stresses, heating by magma, or fluid alteration. Agents of metamorphism are heat (temp.), pressure, differential stress, and hydrothermal fluids.