Contact metamorphism occurs where cooler country rocks are thermally altered by nearby intrusive bodies. The textures that develop under these low-pressure conditions typically lack strain and preserve relict features. Common textures include granoblastic polygonal textures in isotropic minerals like quartz, decussate textures in anisotropic minerals, and porphyroblasts. With increasing metamorphic grade, recrystallization becomes more prominent, grains grow larger, and evidence of strain decreases.
Komattite
Named after the Komati River in South Africa.
first described by Morris and Richard (twins) for ultramafic units in the Barberton Greenstone belt of South Africa.
Mostly of komatiite are Archean age
distributed in the Archaean shield areas.
Also a few are Proterozoic and Phanerozoic.
In all ages komatiites are highly magnesium.
Mostly a volcanic rock; occasionally intrusive.
Mafic rocks were identified as extrusive because of their volcanic textures and structures, and they seem to have been accepted as a normal component of Archean volcanic successions, Abitibi in Canada.
The ultramafic rocks were interpreted as intrusive which are founded as sills and dykes, Barberton in South Africa.
Spinifex texture-typical of Komatiites:
Komattite
Named after the Komati River in South Africa.
first described by Morris and Richard (twins) for ultramafic units in the Barberton Greenstone belt of South Africa.
Mostly of komatiite are Archean age
distributed in the Archaean shield areas.
Also a few are Proterozoic and Phanerozoic.
In all ages komatiites are highly magnesium.
Mostly a volcanic rock; occasionally intrusive.
Mafic rocks were identified as extrusive because of their volcanic textures and structures, and they seem to have been accepted as a normal component of Archean volcanic successions, Abitibi in Canada.
The ultramafic rocks were interpreted as intrusive which are founded as sills and dykes, Barberton in South Africa.
Spinifex texture-typical of Komatiites:
Information about these fluids is an invaluable aid in mineral exploration.
Conventional academic methods of analysing fluid inclusions are too slow and tedious to be of practical application in typical mineral exploration activities.
However, the academic data from numerous studies does show that CO2 is an exceptionally important indicator when exploring for most types of gold deposit.
Because the baro-acoustic decrepitation method is a rapid and reliable method to measure CO2 contents in fluids, it can be used to study a spatial array of data and it is an invaluable and practical exploration method.
Measurements of temperatures of fluid inclusions does not usually help in mineral exploration as hydrothermal minerals deposit over a wide temperature range and there is no specific temperature which is indicative of mineralisation. However, if temperatures are available on a large spatial array of samples, then temperature trends may be a useful exploration method to find the hottest part of the system, which is presumably the location of the best economic mineralisation. Baro-acoustic decrepitation is the most practical method to determine temperatures of the large numbers of samples required.
Salinities of fluid inclusions are of limited use in exploration and are difficult to measure. However, they can be used to recognise intrusion related hydrothermal systems.
Komattite
Named after the Komati River in South Africa.
first described by Morris and Richard (twins) for ultramafic units in the Barberton Greenstone belt of South Africa.
Mostly of komatiite are Archean age
distributed in the Archaean shield areas.
Also a few are Proterozoic and Phanerozoic.
In all ages komatiites are highly magnesium.
Mostly a volcanic rock; occasionally intrusive.
Mafic rocks were identified as extrusive because of their volcanic textures and structures, and they seem to have been accepted as a normal component of Archean volcanic successions, Abitibi in Canada.
The ultramafic rocks were interpreted as intrusive which are founded as sills and dykes, Barberton in South Africa.
Spinifex texture-typical of Komatiites:
The name Spinifex refer to a spiky grass in Australian.
Plate tectonics, like crustal evolution, provides a basis for understanding the distribution and origin of mineral and energy deposits. Different types of ores are characterized by distinct geological environment and tectonic settings.
GEOLOGICAL THERMOMETERS
DEFINITION AND CLASSIFICATION
Proper understanding of origin of mineral deposits and their classification requires the knowledge of formation-temperatures of these deposits. Certain minerals, present over there, give information’s with regard to temperatures of their formations and of the enclosing deposits and they are known as geological thermometers. These geological thermometers may be classed chiefly into the following groups based on their preciseness:
1. The thermometers that record fairly accurately the specific temperature condition of formation of deposits.
2. The thermometers that provide an upper or a lower temperature, above or below which the deposits do not form
3. The thermometers that provide a range of temperature within which the deposits form; and
4. The thermometers that serve as rough indications of temperatures of formation of mineral deposits.
The presence of two or more of less precise geological thermometers in a deposit narrows the range of temperature of formation for the deposits
Notes/ppt/information on texture of igneous rock geology .
For more information and source of knowledge:- ·
https://egyankosh.ac.in/bitstream/123456789/66685/1/Unit-2.pdf
Information about these fluids is an invaluable aid in mineral exploration.
Conventional academic methods of analysing fluid inclusions are too slow and tedious to be of practical application in typical mineral exploration activities.
However, the academic data from numerous studies does show that CO2 is an exceptionally important indicator when exploring for most types of gold deposit.
Because the baro-acoustic decrepitation method is a rapid and reliable method to measure CO2 contents in fluids, it can be used to study a spatial array of data and it is an invaluable and practical exploration method.
Measurements of temperatures of fluid inclusions does not usually help in mineral exploration as hydrothermal minerals deposit over a wide temperature range and there is no specific temperature which is indicative of mineralisation. However, if temperatures are available on a large spatial array of samples, then temperature trends may be a useful exploration method to find the hottest part of the system, which is presumably the location of the best economic mineralisation. Baro-acoustic decrepitation is the most practical method to determine temperatures of the large numbers of samples required.
Salinities of fluid inclusions are of limited use in exploration and are difficult to measure. However, they can be used to recognise intrusion related hydrothermal systems.
Komattite
Named after the Komati River in South Africa.
first described by Morris and Richard (twins) for ultramafic units in the Barberton Greenstone belt of South Africa.
Mostly of komatiite are Archean age
distributed in the Archaean shield areas.
Also a few are Proterozoic and Phanerozoic.
In all ages komatiites are highly magnesium.
Mostly a volcanic rock; occasionally intrusive.
Mafic rocks were identified as extrusive because of their volcanic textures and structures, and they seem to have been accepted as a normal component of Archean volcanic successions, Abitibi in Canada.
The ultramafic rocks were interpreted as intrusive which are founded as sills and dykes, Barberton in South Africa.
Spinifex texture-typical of Komatiites:
The name Spinifex refer to a spiky grass in Australian.
Plate tectonics, like crustal evolution, provides a basis for understanding the distribution and origin of mineral and energy deposits. Different types of ores are characterized by distinct geological environment and tectonic settings.
GEOLOGICAL THERMOMETERS
DEFINITION AND CLASSIFICATION
Proper understanding of origin of mineral deposits and their classification requires the knowledge of formation-temperatures of these deposits. Certain minerals, present over there, give information’s with regard to temperatures of their formations and of the enclosing deposits and they are known as geological thermometers. These geological thermometers may be classed chiefly into the following groups based on their preciseness:
1. The thermometers that record fairly accurately the specific temperature condition of formation of deposits.
2. The thermometers that provide an upper or a lower temperature, above or below which the deposits do not form
3. The thermometers that provide a range of temperature within which the deposits form; and
4. The thermometers that serve as rough indications of temperatures of formation of mineral deposits.
The presence of two or more of less precise geological thermometers in a deposit narrows the range of temperature of formation for the deposits
Notes/ppt/information on texture of igneous rock geology .
For more information and source of knowledge:- ·
https://egyankosh.ac.in/bitstream/123456789/66685/1/Unit-2.pdf
Texture of Ore Minerals; Importance of Studying Textures; Individual Grains Properties; Filling of voids; Texture Types; Genetically differentiated between Texture types; Secondary textures from replacement; Hypogene Texture; Supergene Texture; Primary texture formed from Melts; Primary texture of open-space deposition; Secondary textures from cooling; Secondary textures from deformation; TEXTURES OF ECONOMIC ORE DEPOSITS; Textures of Magmatic ores; Cumulus textures; Intergranular or intercumulus textures; Exsolution textures; Textures of hydrothermal ore deposits and skarns; Replacement textures; Open space filling textures; Textures characteristic of surfacial or near surface environments and processes; Criteria for identifying replacement textures; Vein and Veining have different Nature Features
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Published classroom materials form the basis of syllabuses, drive teacher professional development, and have a potentially huge influence on learners, teachers and education systems. All teachers also create their own materials, whether a few sentences on a blackboard, a highly-structured fully-realised online course, or anything in between. Despite this, the knowledge and skills needed to create effective language learning materials are rarely part of teacher training, and are mostly learnt by trial and error.
Knowledge and skills frameworks, generally called competency frameworks, for ELT teachers, trainers and managers have existed for a few years now. However, until I created one for my MA dissertation, there wasn’t one drawing together what we need to know and do to be able to effectively produce language learning materials.
This webinar will introduce you to my framework, highlighting the key competencies I identified from my research. It will also show how anybody involved in language teaching (any language, not just English!), teacher training, managing schools or developing language learning materials can benefit from using the framework.
2. Deffinition of Contact Metamorphism
•Contact metamorphism occurs in aureoles around intrusive bodies
and is a response of cooler country rocks to the thermal metasomatic
effects of the intrusion.
Parameters of contact metamorphism that influence the textures
•The textures of contact metamorphism are typically developed at low
pressures and thus under conditions of low deviatoric stress.
•The thermal maximum in a contact aureole should also occur much
later than any stress imparted by forceful intrusion. Crystallization
(including recrystallization) may therefore occur in a near-static
environment, and contact metamorphism is typically characterized by
a lack of significant preferred mineral orientation. Many minerals are
equidimensional, rather than elongated, and the elongated minerals
that do form are orientated randomly.
•Relict textures are also common in contact metamorphic rocks
because there is little accompanying shear to destroy them.
3. •Static recrystallization occurs after deformation ceases at elevated temperatures
or when a thermal disturbance occurs in a low-stress environment.
•The process acts to reduce both lattice strain energy and overall surface energy.
anisotropic & isotropic mineral
•The textures that result typically depend on the minerals involved. In
“structurally anisotropic” minerals, the surface energy of a grain boundary
depends strongly on the lattice orientation of the boundary in question, whereas in
“structurally isotropic” minerals the grain boundary energy is about the same for
any surface. If the orientation dependence is low (as in quartz or calcite), no
particular faces are preferentially developed.
4. TEXURES OF CONTACT METAMORPHISM
Texture of Structurally isotopic minerals
Granoblastic polygonalTexture:
•Monomineralic aggregates of structurally isotropic minerals (quartzites or marbles),
grain boundary area reduction leads to an equilibrium texture in which grains meet
along straight boundaries (resulting in low surface area for each grain). The texture is
called granoblastic polygonal (or polygonal mosaic).
•grains appear in two-dimensional thin sections as equidimensional polygons with
grain boundaries that meet in triple junctions with approximately 120° between them
.
•The size of the polygonal grains depends mainly on temperature and the presence of
fluids (higher temperatures and aqueous fluids promote larger crystals).
5. TexturesofStructurally anisotropic minerals
Decussate texture:
•Structurally anisotropic minerals, such as micas and amphiboles, have some
crystallographic surfaces with much lower energy than others.
•This affects the shape of the grains, and thus the equilibrium recrystallization
texture.
•High surface energy boundaries grow faster, so that low- energy surfaces
become larger.
•To understand this, imagine a cube on a table. If the four vertical faces are of
higher energy (less stable) and grow more quickly than the top and bottom
faces, the cube will expand laterally faster than it does vertically, and it
gradually becomes more plate-like. Low-energy surfaces in such minerals
(e.g., the top and bottom of the plate) predominate in the final static
recrystallization texture, even in monomineralic rocks, so that simple regular
polygons are no longer abundant.The result is called decussate texture.
6. polygonal textures of polymineralic aggregates :
•In polymineralic rocks, the grain boundary energy also depends on whether the
boundary is between like minerals or unlike minerals. In general, same-mineral (A-
A) grain boundaries have higher energy than different-mineral (A-B) boundaries, so
that the final static equilibrium texture will tend to minimize the total area of A-A
boundaries and increase the area of A-B boundaries (thereby resulting in lower
total energy).
•This brings us back to the concept of dihedral angles.
•If the dihedral angle, θ, becomes smaller, the total area of A-B grain boundaries
increases with respect to A-A boundaries.
•Polymineralic rocks thus develop a modified type of granoblastic texture in which
120° angles occur only at A-A-A or B-B-B triple junctions, and the other junctions
depend on the relative grain-boundary energies of the different minerals involved .
Figure b & c also illustrate modified polygonal textures of polymineralic aggregates
of quartz–mica and quartz–pyroxene, respectively.
7. Fig:
(a) Dihedral angle between two mineral types . When the A-A grain
boundary energy is greater than for A-B, the angle θ will decrease.
(b) so as to increase the relative area of A-B boundaries.
8. • In highly structurally anisotropic minerals in which the energy of one face
is unusually low, such as in mica, that face will predominate and will not be
altered {001} much by the interfaces of more isotropic minerals such as
quartz (Figure 3).
•As a result, quartz–quartz interfaces usually intersect faces of mica at right
angles or join to the edges of mica flakes (Figure 5).
• In situations where micas are foliated and closely spaced, this tendency
for right-angle intersections may cause the quartz grains to be elongate
(Figure 6, lower half), contrary to their usual tendency.
•This dimensional elongation is not caused by deformation of the quartz
crystals but is an artifact of quartz conforming to the mica foliation.
•Another common feature of polymineralic rocks is that the occurrence of a
minor phase, such as graphite, may retard grain boundary migration of a
more abundant phase and “pin” (or anchor) the boundaries, resulting in a
finer grain size than might otherwise have occurred (Figure 6).
9. porphyroblasts
•Although not restricted to contact metamorphic rocks, porphyroblasts are
very common. In examples of contact metamorphism, the rocks are typically
charac - terized by large porphyroblasts of biotite, andalusite, or cordierite.
•Poikiloblasts are porphyroblasts that incorporate numerous inclusions .
•This texture is common in garnet, staurolite, cordierite, and hornblende.
• Poikiloblastic texture is a high-energy texture because it represents a high
surface area situation.
•It probably occurs as a result of poor nucleation and rapid porphyroblast
growth, which envelops neighboring grains.
•In some cases, a high grain boundary energy between inclusion minerals may
be reduced by the formation of a layer of another (poikiloblast) mineral
between them. The surface energy of an included mineral is also lowered by
rounding the edges, which is common among poikiloblast inclusions.
10.
11. •Skeletal texture (also called web, or spongy) is an extreme example of poikiloblast
formation , in which the inclusions form the bulk of the rock and the enclosing mineral
phase occurs almost as an intergranular, crystallographically continuous, network.
•It may develop at the margins of some porphyroblasts where they are growing into
the matrix, but the occurrence is more pervasive in some rocks.
•Skeletal texture may result from rapid poikiloblast growth or from the introduction of
poikiloblast-building or reactive components via an intergranular fluid.
Porphyroblasts typically form in minerals for which nucleation is impeded.
They are larger than the groundmass minerals and are separated by greater distances.
Crystallizing groundmass minerals typically grow among preexisting grains and thus have
little nucleation problem, whereas porphyroblasts must nucleate from scratch. Porphyroblast
growth requires that components diffuse farther in order to add onto the growing
porphyroblast surface.
12. Nodular
•Cordierite, biotite, and some other minerals commonly form ovoid porphyroblasts in
contact aureoles, particularly when the matrix is very fine grained. The texture is called
nodular .
•A field term for rocks that in hand specimen contain small porphyroblasts (usually
ovoid, but not necessarily so) in a fine matrix is spotted. If the matrix is non-foliated the
rock is
•commonly called a spotted hornfels.
•Contact metamorphism overprinting regional metamorphism is common
(reflecting either post-orogenic magmatism or the time required for magmas to
rise to shallow regions following an orogenic metamorphic- plutonic event).
The result for lowgrade regional metamorphic rocks is spotted slates or spotted
phyllites.
13. Overprint of contact metamorphism on regional.
(a) Nodular texture of cordierite porphyroblasts developed during a thermal
overprinting of regional metamorphism .
(b) Spotted phyllite in which small porphyroblasts of cordierite (and one larger
pyrite) develop in a preexisting phyllite.
(a) (b)
14. Conclusion
As metamorphic grade (mostly temperature) increases in contact
aureoles recrystallization becomes more dominant. Thus we might
summarize the following as the most pronounced textural effects of
increasing metamorphic grade in contact aureoles:
1. Fewer relict textures and a more fully recrystallized
metamorphic texture
2. Increased grain size
3. Straighter grain boundaries and less evidence of strain