Geology Exam 2 Lecture 11

What tectonic setting is low temperature - high pressure metamorphism characteristic of?

-Low-temperature =low grade High-temperature=high grade -As grades increase, recrystallization and neocrystallization tend to produce coarser grains and new mineral assemblages that are stable at higher temperature and pressure. -Metamorphism that occurs while temperature and pressure progressively increase is called prograde metamorphism. As grades increase, metamorphic reactions release water, so high grade rocks tend to be "drier" than low-grade rocks. - Metamorphism that takes place when temperature and pressure progessiely decrease is known as retrograde metamorphism.

What is the progression of rocks derived from shale towards higher metamorphic grades?

-Under low-grade metamorphic conditions and differential stress, shale transforms into slate. In slate, the clay flakes are a bit bigger, are better formed, and align parallel to cleavage. As metamorphic grade increase a bit more, the clay flakes decompose and new crystals o fine-grained white mica as well as new crystals of chlorite and quartz form, transforming the rock into phyllite.

Hormfels

A fine-grained nonfoliated rock that contains a variety of metamorphic minerals. The specific minerals assemblage in a hornfels depends on the composition of the protolith and on the temperature and pressure of metamorphism.

Quartzite

A metamorphic rock composed of quartz and transformed from a protlith of quartz sandstone.

Serpentinite

A metamorphic rock formed by regional metamorphism of deep-sea rocks from the oceanic mantle

Foliation

Assemblage of parallel planar surfaces and/or layers in a metamorphic rock.

What are the parent rocks of the main metamorphic rocks covered in class? (Protolith)

Basalt, greenstone, sandstone, metasandstone, quartzite, granite, gneiss

Metasomatism

Chemical alterations of a rock by hydrothermal or other fluids.

Gneiss

Compositionally layered metamorphic rock, typically composed of alternating dark-colored and light colored layers that range in thickness from millimeters to meters. This compositional layering gives gneiss a stripped appearance. They are formed at very high temperatures, however, does not contain mica, because at high temperatures, mica reacts to form other minerals.

Eclogite

Eclogite is a rare but geologically significant rock type. It is common in the upper mantle, especially in regions occupied by subducted oceanic plates. Eclogite is unusually dense for a silicate rock which suggests that very high pressure was involved in the formation of this rock type

Phyllite

Fine-grained metamorphic rock with a foliation caused by the preferred orientation of very fine-grained whit mica. They form by the metamorphism of slate at a high temperature high enough to cause neocrystalization of white mica. The formation of foliation in phyllite is due to differential stress during metamorphism.

How can you distinguish granite from gneiss and basalt from amphibolite?

Granite vs gneiss: - Granite is an igneous rock, whereas gneiss is formed after metamorphosis of an existing igneous rock. -The mineral composition of both granite and gneiss is same but alteration of granite because of very high pressure and temperature leads to the formation of gneiss. -Minerals are seen arranged in bands, in gneiss. Basalt vs Amphibolite: Amphibolite is a rock of convergent plate boundaries where heat and pressure cause regional metamorphism. It can be produced through the metamorphism of mafic igneous rocks such as basalt.

Regional metamorphism

If metamorphism affects a large region.

Schist

Medium to coarse-grained metamorphic rock that possesses a type of foliation defined by the preferred orientation of large mica flakes. It forms at a higher temperature than phyllite and it typically contains other minerals. Can form from shale but also from a variety of other protoliths.

How are metamorphic rocks formed? At what range of temperature? What happens at the atomic scale - how are new minerals made?

Metamorphic rocks are rocks that have "morphed" into another kind of rock usually between 250 degrees c and 850 degrees. These rocks were once igneous or sedimentary rocks. Metamorphic rocks are formed due to heating, pressure, changes in pressure and temperature together, and compression. The formation of metamorphic minerals take place very slowly and involves many processes such as, RECRYSTALIZATION (changes the shape and grains without changing the identify of the minerals making up the grains), PHASE CHANGE (transforms one minerals into another with the same compostion), METAMOPRHIC REACTION (the growth of new mineral crystals that differ from those of protolith), PRESSURE SOLUTION (wet rock is squeezed more strongly in one direction than others), PLASTIC DEFORMATION (rock is squeezed or sheared at elevated temperatures and pressure).

Index mineral

Minerals that serve as good indicators of metamorphic rocks. It indicates the approximate metamorphic grade of the rock

Slate

The finest-grained foliated metamorphic rock formed by metamorphism of shale of mudstone under relatively low pressure and temperatures. They contain foliation called slaty cleavage, which allows it to split into thin sheets that make excellent roofing shingles.

Contact metamorphism

The local metamorphism caused by igneous rocks is called this to emphasize that it develops adjacent to the contact of an intrustion with its wall rocks.

Marble

The metamorphism of limestone yields marble. During the formation of marble, calicite composing the protolith recrystallizes, so fossil shells, pore space, and the distinction between grains and cement disappear. Marble typically consists of a fairly uniform mass of interlocking calcite crystals.

Protolith

The original rock which a metamorphic rock formed.

Metamorphism

The process at which one kind of rock transforms into a different type of rock

Polymorph

Two minerals that have the same chemical composition but a different crustal lattice structure

What role do fluids play in metamorphism?

Water permenates the crust • - Metasomatism - Loss or gain of elements in rock - Water, CO2 • Pluton contact - Increased T drives off H2O, CO2; magma brings H2O • Hydrothermal alteration - Seafloor metamorphism - Pyroxene and olivine à serpentine, chlorite, talc (forming serpentinite)

What two textures are characteristic of metamorphic rocks?

• Directed/differential stress à foliation - Minerals have preferred orientation • Alignment of platy minerals • Alternating compositional layers - Increased grain size at times w/metamorphism Nonfoliation: contain minerals that crystallized during metamorphism. They have an absence of compression and shear.