Metamorphic Textures

Metamorphic Texture: Unveiling Nature's Transformations

Texture or fabric = small-scale features that are penetrative (occurs in virtually all of the rock body at the microscopic level).

Structure:

larger-scale features; found in hand-sample, outcrop, or regional scale.

Texture: 

Is a term that describes the size, shape and orientation of the grains constituting a rock, as well as the relationship between these grains.

1- Crystal size:

<0.1 mm very fine-grained

0.1-1 mm fine-grained

1-5 mm medium-grained

5-10 mm coarse-grained

> 10 mm very coarse-grained

2-Shape:

Idioblastic: If the mineral grain is euhedral

Subidioblastic: If the grain is subhedral

Xenoblastic: If the grain is anhedral

Metamorphic textures can be grouped into three main groups:

A- Relict textures (palimpsest textures): 

Relict textures are textures inherited from the original rock type, and which have survived metamorphism.

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 lowgrade rocks.

Examples of such textures include:

• blasto-porphyritic • blasto-ophitic • blasto-intergranular • blasto-amygdaloidal • blasto-pisolitic • blasto-oolitic

B- Typomorphic textures: 

textures characteristic of metamorphism.

Textures based on thermal effect can be :

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.

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:

1. Fish-net or skeletal texture: rapid crystallization
2. Sieve texture
3. 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.
4. 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.
5. e- Helicitic: Where the poikiloblasts overgrow the pre-existing foliation. This texture therefore indicates post-tectonic crystallization of the poikiloblasts.

C- Superimposed textures: 

textures characteristic of a postmetamorphic 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.

C- 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, multi-crystal

D- 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.