Textures and Structures of Metamorphic Rocks | Geology Optional for UPSC PDF Download

Textures in Metamorphic Rocks

Metamorphic Textures Overview:

• Definition: Metamorphic textures refer to the varied size, shape, orientation, and spatial arrangement of crystals in rocks, influenced by different pressure and temperature conditions during metamorphism.
• Diversity: These textures exhibit a wide range of characteristics due to the changing P-T conditions.

Categories of Metamorphic Textures:

• Palimpsest Texture: Shows evidence of earlier stages of metamorphism that have been partially or wholly overprinted by subsequent changes.
• Typomorphic Texture: Characteristic textures of a particular mineral or group of minerals, aiding in their identification.
• Reaction Textures: Form as a result of reactions between minerals due to changes in pressure and temperature during metamorphism.
• Corona Texture: A texture where a mineral is surrounded by concentric layers of other minerals due to reactions at the mineral's rim.
• Intergrowth Texture: Involves the interlocking growth of different minerals, creating intricate patterns within the rock.

Textures and Structures of Metamorphic Rocks

Palimpsest Texture

• The primary texture of the original rock persists through metamorphism, showcasing the inheritance of protolith rock textures.
• Also referred to as relict texture because it survives metamorphism, especially in low-grade metamorphic rocks where deformation is limited, allowing for preservation.

Examples:

• Blasto-ophitic texture: Plagioclase laths suspended in a pyroxene matrix.
• Blasto-intergranular texture: Igneous relict textures in metamorphic rocks with ferromagnesium minerals occupying interstices between plagioclase crystals.
• Blasto-porphyritic texture: Larger grains or crystals surrounded by a fine-grained matrix or glassy groundmass.

Metamorphic Petrology

Typical Textures in Metamorphic Rocks

• Blasto-intergranular texture: Coarse-grained appearance with angular interstices filled with pyroxene minerals, commonly seen in rocks derived from basalt.
• Blasto-porphyritic texture: Larger crystals of plagioclase, biotite, and clinopyroxene surrounded by a fine-grained groundmass.
• Blasto-cumulate texture: Results from the settling of early-formed, high-density crystals, preserved as a relict texture.

Typomorphic Texture

Porphyroclastic texture

• Occurs due to deformation within the rock, resulting in the crushing of softer minerals to form a groundmass while more resistant minerals appear as larger fragments (porphyroclasts).
• Shows two distinct grain sizes of the same mineral.

Mylonitic Texture

Characterized by a fine-grained, streaky appearance due to intense shearing forces.

Types of mylonitic textures

• Protomylonitic: Initial stage of foliation mylonitic texture.
• Orthomylonitic: Well-defined foliation with quartz grains oriented in a ribbon-like manner.
• Ultramylonitic: Highly strained crystals recrystallizing into smaller ones, forming a granoblastic polygonal texture.

Textures Based on Thermal Effect

• Nodular Texture: Oval-shaped porphyroblasts of minerals like cordierite or scapolite grow alongside randomly distributed quartz and other minerals.
• Granoblastic Texture: Found in non-foliated metamorphic rocks like marble or quartzite, where grains are equigranular to nearly equigranular, creating a mosaic of recrystallized mineral grains without porphyroblastic grains.

Characteristics of Granoblastic Texture

• Variability: Ranges from equidimensional grains with straight grain boundaries and well-developed crystal faces to irregular grain boundaries.

Types:

• Polygonal Granoblastic Texture: Seen in monomineralic quartz in quartzite.
• Interlobate Granoblastic Texture: Irregular grain boundaries.
• Amoeboid Granoblastic Texture: Features irregularly shaped mineral grains.
• Decussate Granoblastic Texture: Mineral grains are interlocked, randomly oriented, with common triple junctions.

Crystallisation Textures

• Porphyroblastic Texture: Large mineral grains within a fine-grained ground mass.
• Example: Large garnets embedded in a matrix of fine-grained muscovite or biotite.
• Poikiloblastic Texture: Fine-grained inclusions within porphyroblast grains, randomly oriented or exhibiting helictic patterns.

Reaction Textures in Metamorphic Petrology

• Reaction-Rim Texture: A new mineral forms by replacing an older mineral along the rim.
• Kelyphitic Texture: Replacement of a mineral through the intergrowth of two or more minerals, forming a rim around the replaced mineral.
• Corona Texture: Develops in both prograde and retrograde phases of metamorphism, forming concentric rims.

Intergrowth Texture

• Symplectite texture: Worm-like appearance of minerals along boundaries of older reacting minerals.
• Myrmekitic texture: Vermicular or wormy quartz intergrowing with plagioclase.

Recrystallization

• Crystals change size and shape without melting due to heat and pressure.
• Example: Limestone turning into marble.

Other Textures

• Palimpsest Texture: Original texture partially preserved.
• Example: Fossil shells preserved in a metamorphic rock.
• Porphyroblastic Texture: Large crystals under specific P-T conditions.
• Example: Garnet porphyroblasts in a schist matrix.
• Kelyphitic Texture: Rims of new minerals forming around pre-existing grains.

Structures in Metamorphic Rocks

Foliation and Lineation

• Foliation: Layering within metamorphic rocks, characterized by parallel alignment of mineral grains.

• Lineation: Parallel or nearly parallel lines forming a linear fabric element.

Classification

1. Spaced Foliations: Includes compositional, disjunctive, and crenulation foliations.
2. Continuous Foliations: Fine-grained, with aligned flaky minerals.

Common Structures

• Schistosity: Strongly foliated texture from mineral growth.
• Found in mica schist and chlorite schist.
• Gneissose Structure: Alternating bands of felsic and mafic minerals, forming striped patterns.
• Cataclastic Structure: Results from intense shearing along fault zones.
• Pseudotachylite: Dark, fine-grained rock from frictional melting.