Textures and Structures of Metamorphic Rocks | Geology Optional Notes 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.

Clear all your UPSC doubts with EduRev Join for Free

Join for Free

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.

Download the notes Textures and Structures of Metamorphic Rocks

Download as PDF

Download as PDF

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.