Types Of Metamorphic Textures

 

STRUCTURES OF METAMORPHIC ROCKS (MACROTEXTURES):

(i)                Slaty cleavage: A pervasive, parallel foliation (layering) of fine-grained platy minerals (chlorite) in a direction perpendicular to the direction of maximum stress. It produces the rocks slate and phyllite.

(ii)          Schistose: Schist has a lepidoblastic foliation if this foliation is defined by oriented mica, and a nematoblastic foliation if such a foliation is defined by the orientation of prismatic minerals as amphiboles and pyroxenes.

(iii)      Gneissic:  A complex banded texture made of schistose layers or bands alternating with bands commonly characterized by a granoblastic texture.

(iv)          Granoblastic: Granular, interlocking equi-dimensional grains of subequal size; no preferred orientation or cleavage.

(v)             Hornfelsic: Fine-grained, granular interlocking grains, possibly of variable shapes and sizes. No preferred orientation.

(vi)            Cataclastic:

• produced under stress and in absence of high temperature, whereby rocks are subjected to shearing and fragmentation.

• Only the durable mineral partly survive the crushing force and the less durable ones are powdered.

• Thus, when resistant minerals and rock fragments stand out in a pseudo porphyritic manner in the finer materials, it is known as 'porphyroclastic structure. Phenocrysts are called 'porphyroclasts.

 • Argillaceous rocks develop slaty cleavage, harder rocks may be shattered and crushed forming crush breccia and crush conglomerate.

• When the rocks are highly crushed into fine grained rocks, they are known as mylonites. Since these structures are formed due to cataclasis, they are, as a whole, known as cataclastic structure

 

Types Of Metamorphic Textures And Mineralmineral Relations

Metamorphic textures can be grouped into three main groups:

A-  Relict textures (palimpsest textures): are textures inherited from the original rock type, and which have survived metamorphism.

B-    Typomorphic textures: textures characteristic of metamorphism.

C-   Superimposed textures: textures characteristic of a post metamorphic event, e.g. alteration, weathering etc. Other smaller groups as “reaction textures”, “polydeformational textures” etc. may also be typomorphic or replacement, but are grouped separately because they have some genetic connotation.

 

Position and meaning of Blasto- and –blastic

 • Suffix -blast or -blastic indicates that a feature is of metamorphic origin.

 • Prefix blasto- (meaning that a feature is not of metamorphic origin but is inherited from the parent rock)

 • Porphyroblastic means a porphyritic-like texture (coarse grains in a finer matrix) that is of metamorphic origin.

 • Blastoporphyritic indicates an igneous porphyritic texture that survived metamorphism to the extent that it can still be recognized.

 • Sedimentary/ Igneous protolith that have undergone very low grade metamorphism, Also termed as relict textures. Other examples: ophitic, intergranular, amygdaloidal, spherulitic, pisolitic, oolitic

 

RELICT TEXTURES

 There are several types of relict textures. Relict textures in metamorphic rocks are indicated by applying the prefix "blasto" to the original textural name. Relict textures are best preserved in low grade rocks.

Examples of such textures include: blasto-porphyritic, blasto-ophitic, blasto-intergranular, blasto-amygdaloidal, blasto-pisolitic, blasto-oolitic.

Textures Characteristic Of Thermal/ Contact Metamorphism:

When thermal metamorphism is not associated with any deformation, the mineral grains are randomly oriented, resulting in either granoblastic or hornfelsic textures. Note that the granoblastic texture can also develop in regionally metamorphosed rocks. The following are some of the types of granoblastic textures:

1-    Granoblastic polygonal: where the equidimensional grains may have well developed crystal faces resulting in straight grain boundaries, and where triple junctions are common.

2-    Granoblastic interlobate: where the grain boundaries are somewhat irregular.

3-  Granoblastic amoeboid: where all the grains have irregular outlines, and all the minerals are anhedral.

4- Granoblastic decussate: where the interlocking randomly oriented crystals are somewhat elongate, prismatic or subidioblastic. Usually applied to rocks with one or two mineral species only. Triple junctions are common.

5-    Nodular: results from the growth of oval - shaped porphyroblasts of such minerals as cordierite or scapolite in association with other randomly oriented minerals as Quartz, etc.

Figure. Typical textures of thermal metamorphism.

TEXTURES OF CONTACT METAMORPHISM

• Typically shallow pluton aureoles (low-P)

• Crystallization/recrystallization is near-static

▪ Monomineralic with low D surface energy →granoblastic polygonal

▪ Larger D S.E. → decussate

• Isotropic textures (hornfels, granofels)

• Relict textures are common

Textures of Regional Metamorphism

– Dynamo-thermal (crystallization under dynamic conditions)

– Orogeny- long-term mountain-building

 - May comprise several Tectonic Events

–May have several Deformational Phases

– May have an accompanying Metamorphic Cycles with one or more Reaction Events.

Tectonite- a deformed rock with a texture that records the deformation

 -Fabric- the complete spatial and geometric configuration of textural elements

• Foliation- planar textural element

• Lineation- linear textural element

• Lattice Preferred Orientation (LPO)

• Dimensional Preferred Orientation (DPO)

TEXTURES OF DYNAMIC METAMORPHISM:

1- Porphyroclastic: A texture produced by the crushing or fragmentation of large grains, resulting in two distinct grain size distributions of the same mineral: coarser grained porphyroclasts and finer grained fragments.

2- Mortar: similar to porphyroclastic but in which the smaller fragments are further crushed to finer and finer sizes (close to becoming powders), while some porphyroclasts still persist.

3- Protomylonitic: A more advanced stage of cataclasis, where some minerals begin to deform in a ductile manner, giving rise to an incipient foliation or preferred orientation.

4- Orthomylonitic (mylonitic): Where the rocks develop a well - defined foliation. In quartz rich rocks, an orthomylonitic fabric is often indicated by quartz crystals elongated like ribbons or flames (ribbon quartz).

5- Polygonized/ recrystallized/ annealed (ultramylonitic):

The most advanced stages of cataclastic metamorphism result in the recrystallization of the highly strained crystals into smaller ones developing a granoblastic polygonal texture. At the same time, a foliation defined by micaceous or prismatic minerals persists

 

CRYSTALLIZATION TEXTURES:

1. Porphyroblastic: Where coarse-grained metamorphic minerals (porphyroblasts) occur in a matrix of finer grained crystals.

2. Poikiloblastic: Where coarse-grained metamorphic minerals contain numerous inclusions of finer-grained crystals of other minerals. It is of different types:

a-    A   Fishnet or skeletal texture: rapid crystallization

b-  B   Sieve texture

c-   C    Rotational texture

 where the inclusions are oriented at an angle that suggests that the poikiloblast may have rotated during its growth, thus indicating syndeformational or syntectonic growth. An alternative interpretation of such texture is the rotation of the foliation during the growth of the poikiloblast, which still makes the growth syndeformational.

  d- Snowball: Similar to rotational texture, but where the inclusions define a spiral shaped trail, which may have developed from the "rolling over" of the poikiloblasts.

  e- Helicitic: Where the poikiloblasts overgrow the pre-existing foliation. This texture therefore indicates post-tectonic crystallization of the poikiloblasts.

REPLACEMENT TEXTURES (SUPERIMPOSED IN PART)

1. Mesh texture: develops in serpentinites, where the needle shaped serpentine minerals occur in aggregates interwoven like a mesh.

2. Hour-glass texture: Also in serpentinites, where the serpentine minerals replace the granular olivine crystals giving rise to hour-glass like appearances.

3. Bastite texture: A third texture that occurs in serpentinites, where Opx crystals were completely replaced by aggregates of serpentine minerals retaining the prismatic shape of the original Opx.

4. Pseudomorphic replacement textures:

(i) single-crystal

(ii) multi-crystal

(iii) multi-phase

REACTION TEXTURES

1. Epitaxial overgrowth: Epitaxial overgrowth is characterized by optical continuity between the mineral and its overgrowth. Both the mineral and the overgrowth must belong to the same structural group, and may possibly be the same mineral. This type of overgrowth is controlled fully by the the matrix mineral.

2. Topotactic replacement: One mineral overgrows another of a similar structure (e.g. Actinolite rims on glaucophane). Orientation of overgrowing mineral is

controlled by that of the overgrown one.

3. Kelyphitic texture (also a replacement texture): A kelyphitic texture is a replacement of one mineral along its rim by an intergrowth of two or more minerals, in a way that the new minerals almost completely surround the mineral being replaced. The term is most commonly used when the replacing minerals form during retrogression.

Examples include kelyphitic rims of chlorite + Fe-oxides after garnet.

4. Reaction-rim texture: when one mineral replaces another along its rims, suggesting a reaction between both phases. The contacts between both phases are irregular

5. Corona texture: several concentric layers of one or more minerals completely encircling an older phase. The layers (which range from one to five in number) represent a sequence of reactions that have taken place (none to completion) to replace the mineral in the core or center of the corona. Coronas form during both prograde or retrograde metamorphism. Monomineralic coronas are also known as moats.

6. Atoll texture: where the core of a mineral is dissolved or replaced leaving behind a surviving rim. Such textures usually form due to an original compositional zoning within the mineral with the replaced core.

INTERGROWTH TEXTURE

1. Symplectites (also a reaction texture): Are irregular fine grained mineral intergrowths that form as a result of a certain reaction that did not go to completion. These intergrowths are often recognized by their wormy appearance and often occur along the boundaries of reacting minerals (or ones not in equilibrium).

Examples of commonly intergrown mineral pairs: Qz- Feldspar/ Amph-Spinel/ Plag-Mgt/ Gt-Qz/ Plag-Cpx/ Bt- Qz/ Ep-Qz/ Amph-Plag.

 Note that a common type of symplectitic intergrowth is the myrmekitic texture

commonly observed in granites, where wormy quartz occurs in plagioclase crystals in contact with biotite. Symplectitic intergrowths are more common in high temperature rocks.

POLYDEFORMATIONAL / POLYMETAMORPHIC TEXTURES

1. Crenulated cleavage/schistosity: Results from the folding of a foliation.

2. S-C fabric: A more advanced stage of crenulation, where one or more minerals are orientated along the crenulated surfaces to define a new foliation (S2) at an angle to the older one (S1). This commonly involves some form of "recrystallization".

SPECIAL TEXTURES AND FEATURES

1. Pressure shadows: are ellipsoidal regions adjacent to a rigid crystal where minerals grow developing textures that differ from those defined by the same minerals in the rest of the sample. Growth in a pressure shadow is therefore influenced by the crystal faces of the rigid mineral which seem to "protect" the minerals in its immediate vicinity from the deformation affecting the same minerals in other parts of the sample. Accordingly, the foliation wraps around the rigid crystal and its shadow.

2. Mica fish: Are lenticular porphyroblasts of mica which commonly develop in a shear stress environment and can be used to indicate the sense of shear.

3. Kink bands (deformational bands): Are bends and twists within some minerals as a result of their deformation. Kink bands develop in pre-tectonic minerals.

4. Zoning: Compositional change of a crystal, often accompanied by a change in some of its optical properties.

5. Twinning: Some twinning may be induced by deformation.

6. Exsolution texture: results from the incomplete miscibility between two components (endmembers) of a solid solution series. A decrease in temperature may result in the separation of these two phases, one in the other commonly along cleavage planes. Common in high grade rocks that cooled slowly.