Model Answer
0 min readIntroduction
Igneous rocks, formed from the cooling and solidification of magma or lava, exhibit a wide range of textures reflecting their cooling history and composition. These textures are crucial for understanding the petrogenesis of the rock. ‘Intergranular’ and ‘sub-ophitic’ are two common textures observed in mafic igneous rocks like basalt and gabbro. Intergranular texture signifies a relatively slow cooling allowing for the development of well-formed, equigranular crystals, while sub-ophitic texture indicates a slightly faster cooling rate where plagioclase laths partially enclose pyroxene crystals. Understanding these textures provides insights into the conditions under which the rock formed.
Intergranular Texture
Intergranular texture is characterized by equidimensional, well-formed crystals of different minerals that are interlocked with each other, resembling grains fitting together. This texture typically forms during slow cooling of magma at depth, allowing sufficient time for all minerals to nucleate and grow to a comparable size. The boundaries between crystals are relatively straight and well-defined.
Key Features:
- Equidimensional crystals
- Well-defined crystal boundaries
- Slow cooling rate
- Common in coarse-grained rocks like gabbro.
Sub-Ophitic Texture
Sub-ophitic texture is a variant of ophitic texture, commonly found in mafic rocks. It is characterized by lath-shaped plagioclase crystals partially enclosing rounded pyroxene crystals. The term 'sub' indicates that the enclosure is not complete; pyroxene crystals protrude through the plagioclase matrix. This texture suggests a cooling rate faster than that producing intergranular texture, but slower than that resulting in a glassy texture.
Key Features:
- Lath-shaped plagioclase crystals
- Rounded pyroxene crystals
- Partial enclosure of pyroxene by plagioclase
- Intermediate cooling rate
- Common in basalts and diabases.
Coexistence of Intergranular and Sub-Ophitic Textures in Mafic Rocks
The presence of both intergranular and sub-ophitic textures within the same mafic rock indicates a complex cooling history. This is often observed in layered intrusions or differentiated magma chambers. The following explains this coexistence:
- Differential Cooling: Different parts of the magma chamber may have cooled at different rates. Areas closer to the chamber walls or in contact with cooler country rock would cool faster, promoting sub-ophitic texture. The central portions, insulated by the surrounding magma, would cool slower, resulting in intergranular texture.
- Magma Mixing: Mixing of magmas with different compositions and temperatures can also lead to textural variations. A more slowly cooled, intergranular portion might be intruded by a faster-cooling, sub-ophitic magma.
- Crystallization Sequence: Pyroxene and plagioclase have different crystallization temperatures. Initially, pyroxene may crystallize as rounded grains. As the temperature drops, plagioclase starts to grow as laths, partially enclosing the already formed pyroxene, leading to sub-ophitic texture. If cooling is slow enough, further growth of both minerals can result in an overall intergranular appearance, but with remnants of the sub-ophitic relationship.
- Zoning: The rock may exhibit compositional zoning, where different zones have different mineral compositions and cooling rates, leading to different textures.
Therefore, the coexistence of these textures is a testament to the dynamic processes occurring during the formation and cooling of mafic magmas.
Conclusion
In conclusion, intergranular and sub-ophitic textures are valuable indicators of the cooling history and crystallization conditions of mafic igneous rocks. The presence of both textures within a single rock suggests a complex magmatic environment characterized by differential cooling, magma mixing, or a changing crystallization sequence. Analyzing these textures, alongside mineral compositions, provides crucial insights into the petrogenesis of these rocks and the geological processes that shaped them.
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