Earth Materials Lecture 17: Introduction to Metamorphic Petrology Flashcards
Define Metamorphism
Transformation of pre-existing rock as a result of Temperature, Pressure and chemical conditions.
Involves recrystallisation
Does not involve melt = sub-solidus
The pre-existing rock is known as the protolith
Protoliths can be igneous, sedimentary or metamorphic
Describe where the word Metamorphism comes from
Transformation of pre-existing rock as a result of Temperature, Pressure and chemical conditions
From Greek - Meta = changed, altered
- Morphos = form, shape
Describe Limitations for metamorphism
Low-temperature limit grades into diagenesis
Processes are indistinguishable
Metamorphism begins in the range of 100-150oC for the more unstable types of protolith
Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
High-temperature limit grades into melting
Different Ts depending on protolith and fluid
Migmatites are the metamorphic product of partial melting and the extremity
Describe necessary components for metamorphism to occur
Temperature
Pressure
Protolith
Heat source moves towards protolith (e.g. granite intrusion near protolith)
Describe pressure in the context of metamorphosis
CONFINING PRESSURE i.e. the amount of overburden
Varies with depth in crust
-Burial in sedimentary basin
-Subduction
-Thickening of crust within a mountain belt (orogen)
An important variable, not quite as important as temperature
Traditionally recorded in KILOBARS (kbar) – 1 atmosphere = 1 bar
1 kbar approximates to 3.3 km
SI unit is PASCALS (Pa) – 1 Pa = 0.00001 bar
1 GIGAPASCAL (GPa) = 10000 bar or 10 kbar
Describe temperature in a metamorphosis context
Temperature CAN increase with P, but it is not linear
In a stable environment, such as a craton interior, the GEOTHERMAL GRADIENT or GEOTHERM is higher due to radioactive decay (of K, Th, U chiefly)
In thinner basaltic oceanic crust heat produced much less and lost more rapidly
Give the three types of metamorphism
Contact
Regional
Impact (rare, not discussed further)
Describe contact metamorphism
A type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma.
Localised metamorphic affect
Typically formed around plutons in the upper parts of the crust
Heat source is transient
Typically associated with the later stages of orogenesis
Tends to affect rocks having already undergone regional metamorphism = polymetamorphism
Rocks regionally metamorphosed have already been deformed and contain a fabric (slaty cleavage)
Describe Regional Metamorphism
Metamorphism that occurs over a wide area of Earth (contrast with local affect of contact metamorphism)
metamorphism affecting rocks over an extensive area as a result of the large-scale action of heat and pressure.
Orogenic
Subduction zone
Define orogenesis
The process of Mountain building
Describe subduction metamorphism
Clearly part of the process of orogenesis
Distinct in that restricted to the downgoing slab of ocean floor
Different Pressure and Temperature regime as the downgoing slab refrigerates the surrounding rocks and creates a cooler geothermal gradient, promoting high-pressure low-temperature metamorphism
Define a foliation
FOLIATION – any planar feature that occurs penetratively in a body of rock
Could refer to rhythmic bedding in a sedimentary rock, to compositional layering in igneous rocks, or to cleavage, schistosity and gneissosity in metamorphic rocks
Can be defined by spatial variation in mineral composition or grain size
Describe the types of foliation
Compositional layering
Preferred orientation of platy minerals
Shape of deformed grains
Grain size variation
Preferred orientation of platy minerals in a matrix without preferred orientation
Preferred orientation of lenticular mineral aggregates
Preferred orientation of fractures
Define cleavage
Traditionally: the property of a rock to split along a regular set of sub-parallel, closely-spaced planes
A more general concept adopted by some geologists is to consider cleavage to be any type of foliation in which the aligned platy phyllosilicates are too fine grained to see individually with the unaided eye
Describe Slate and Phyllite
Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull
Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface.
Define Schistosity
A preferred orientation of inequant mineral grains or grain aggregates produced by metamorphic processes
Aligned minerals are coarse grained enough to see with the unaided eye
The orientation is generally planar, but linear orientations are not excluded
Define a Gneissose structure/Gneiss structure
Either a poorly-developed schistosity or segregated into layers by metamorphic processes
Gneissose rocks are generally coarse grained
Gneissose rocks and Gneissosicity are poorly developed schistosity
Describe non foliated metamorphic rocks
Simpler than for foliated rocks
Again, this discussion and classification applies only to rocks that are not produced by high-strain metamorphism
Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation)
Hornfels is a type of granofels that is typically very fine-grained and compact, and occurs in contact aureoles. Hornfelses are tough, and tend to splinter when broken.
Describe the way in which Metamorphic rocks are classified
Two basic criteria:
Modal compositions (actual percentage of minerals)
Structures
Naming consists of a root name and prefix
Root name is the structural name (or other specific name), meaning Schistosity or Gneissosity
Prefix refers to the mineral composition
e.g. Quartz Schist
or
Garnet Gneiss
Describe;
Serpentinite
Blueschist
Eclogite
Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.
Blueschist: a blue amphibole-bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that it is even applied to non-schistose rocks.
Eclogite: a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). The protolith is typically basaltic.
Describe:
Granulites
Migmatites
Granulite: a high grade rock of pelitic, mafic, or quartzo-feldspathic parentage that is predominantly composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common.
Migmatite: a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.
Define Porpyhroblastic
Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast
Define what the prefixes ortho and para
Ortho- a prefix indicating an igneous parent, and
Para- a prefix indicating a sedimentary parent
