Crenulation
Encyclopedia
Crenulation or Crenulation cleavage is a texture
formed in metamorphic rock
s such as phyllite
, schist
and some gneiss
by two or more stress directions resulting in superimposed foliations
.
is overprinted by a later planar fabric. Crenulations form by recrystallisation of mica
minerals during metamorphism
. Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress
field.
If a rock is subjected to two separate deformations and the second deformation is at some other angle to the original, growth of new micas on the foliation planes will create a new foliation plane perpendicular to the plane of principal stress. The angular intersection of the two foliations causes a diagnostic texture called a crenulation, which may involve folding of the earlier mica foliations by the later foliation.
Recognising a crenulation in a rock may require inspecting the rock with a hand-lens or petrographic microscope
in thin section
. Crenulations may be very cryptic, and there may be several recorded within a rock and especially, entrained within porphyroblast
s.
Crenulations may manifest as kinking of previous foliation, such that the original foliation appears to be lined or inscribed by a later foliation.
In more advanced states, the later foliation will tend to form distinct foliation planes cross-cutting the earlier foliation, resulting in breaking, warping, and micro-scale folding of the earlier foliation into the new foliation.
When the crenulation foliation begins to dominate it may totally or almost completely wipe out the original foliation. This process occurs at different rates in rocks and beds of different lithology and chemical composition so that it is usually valuable to look at a variety of outcrops to gain a better appreciation of the effect of crenulation or discover the orientation or presence of earlier foliations.
Crenulation may also be the incipient foliation plane which precipitates shearing. In this case, it s often likely that the crenulation acts as a shear plane and it may be difficult to reconstruct earlier foliations and rock units across the crenulation foliation.
.
Firstly, the crenulation must be analysed to determine the initial foliation, termed S1, and the overprinting subsequent foliation, termed S2. The intersection of these two planes forms an intersection lineation. This intersection lineation, L1-2 may approximate the plunge of F2 interference folds
.
The initial impact of a crenulation foliation may be cryptic, microscopic growth of new minerals at an angle to previous foliations. This may occur only in certain compositions of the rocks which favor growth of minerals under the P-T conditions at that time.
In more brittle conditions, especially in highly micaceous rocks, a crenulation may appear as kink bands, where S1 foliations are kinked by the S2 foliation so that the original minerals are broken or deformed. This may not result in new mineral growth.
Eventually, the crenulation foliation overprints the S1 foliation. In extreme cases, the S2 foliation will obliterate the previous foliation, especially in wet rocks which have compositions amenable to growth of minerals at that time. In this case, porphyroblast
s may be the only means in which to observe earlier foliations, assuming they have inclusion trails of the S1 foliation.
Rock microstructure
Rock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
formed in metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s such as phyllite
Phyllite
Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the...
, schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
and some gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
by two or more stress directions resulting in superimposed foliations
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
.
Formation
Crenulations form when an early planar fabricFabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
is overprinted by a later planar fabric. Crenulations form by recrystallisation of mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
minerals during metamorphism
Metamorphism
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
. Micaceous minerals form planar surfaces known as foliations perpendicular to the principal stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
field.
If a rock is subjected to two separate deformations and the second deformation is at some other angle to the original, growth of new micas on the foliation planes will create a new foliation plane perpendicular to the plane of principal stress. The angular intersection of the two foliations causes a diagnostic texture called a crenulation, which may involve folding of the earlier mica foliations by the later foliation.
Recognition
Petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...
in thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
. Crenulations may be very cryptic, and there may be several recorded within a rock and especially, entrained within porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s.
Crenulations may manifest as kinking of previous foliation, such that the original foliation appears to be lined or inscribed by a later foliation.
In more advanced states, the later foliation will tend to form distinct foliation planes cross-cutting the earlier foliation, resulting in breaking, warping, and micro-scale folding of the earlier foliation into the new foliation.
When the crenulation foliation begins to dominate it may totally or almost completely wipe out the original foliation. This process occurs at different rates in rocks and beds of different lithology and chemical composition so that it is usually valuable to look at a variety of outcrops to gain a better appreciation of the effect of crenulation or discover the orientation or presence of earlier foliations.
Crenulation may also be the incipient foliation plane which precipitates shearing. In this case, it s often likely that the crenulation acts as a shear plane and it may be difficult to reconstruct earlier foliations and rock units across the crenulation foliation.
Analysis
Crenulations, because they are the result of a second (or more) foliation, preserve important information on not only the stresses which formed the crenulation foliation, but the orientation of previous foliationsFoliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
.
Firstly, the crenulation must be analysed to determine the initial foliation, termed S1, and the overprinting subsequent foliation, termed S2. The intersection of these two planes forms an intersection lineation. This intersection lineation, L1-2 may approximate the plunge of F2 interference folds
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
.
The initial impact of a crenulation foliation may be cryptic, microscopic growth of new minerals at an angle to previous foliations. This may occur only in certain compositions of the rocks which favor growth of minerals under the P-T conditions at that time.
In more brittle conditions, especially in highly micaceous rocks, a crenulation may appear as kink bands, where S1 foliations are kinked by the S2 foliation so that the original minerals are broken or deformed. This may not result in new mineral growth.
Eventually, the crenulation foliation overprints the S1 foliation. In extreme cases, the S2 foliation will obliterate the previous foliation, especially in wet rocks which have compositions amenable to growth of minerals at that time. In this case, porphyroblast
Porphyroblast
A porphyroblast is a large mineral crystal in a metamorphic rock which has grown within the finer grained groundmass. Porphyroblasts are commonly euhedral crystals, but can also be partly to completely irregular in shape....
s may be the only means in which to observe earlier foliations, assuming they have inclusion trails of the S1 foliation.
See also
- List of rock textures
- MetamorphismMetamorphismMetamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...
- Rock microstructureRock microstructureRock microstructure includes the texture of a rock and the small scale rock structures. The words "texture" and "microstructure" are interchangeable, with the latter preferred in modern geological literature...
- Structural geologyStructural geologyStructural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover information about the history of deformation in the rocks, and...
- Shear (geology)
- Crenellation (an unrelated concept, but their similar spellings can lead to confusion)