Model Answer
0 min readIntroduction
Igneous rocks, formed from the cooling and solidification of magma or lava, exhibit a wide range of textures that provide valuable insights into their cooling history and petrogenesis. Gabbro, a coarse-grained, mafic igneous rock, commonly displays distinct textures like porphyritic and ophitic. These textures arise from the sequential crystallization of minerals under varying conditions of temperature and pressure. Understanding the formation of these textures requires an understanding of phase diagrams, which graphically represent the stability of minerals under different conditions. This answer will illustrate the formation of porphyritic and ophitic textures in gabbroic rocks, supported by sketches and a relevant binary phase diagram.
Porphyritic Texture
Porphyritic texture is characterized by large, well-formed crystals (phenocrysts) embedded in a fine-grained groundmass. This texture indicates a two-stage cooling history. Initially, the magma cools slowly at depth, allowing for the formation of large crystals. Subsequently, the magma is rapidly cooled, often due to volcanic eruption or shallow intrusion, resulting in the formation of a fine-grained groundmass.
Sketch of Porphyritic Texture
(Image depicts large, visible crystals of plagioclase and pyroxene surrounded by a fine-grained matrix.)
Ophitic Texture
Ophitic texture is common in gabbroic rocks and is characterized by lath-shaped plagioclase crystals enclosed within pyroxene crystals. This texture forms when plagioclase begins to crystallize first, forming laths. As the magma cools further, pyroxene crystallizes and grows around the plagioclase laths, effectively enclosing them. This indicates relatively slow cooling, but not as slow as that required for a purely equigranular texture.
Sketch of Ophitic Texture
(Image depicts lath-shaped plagioclase crystals completely enclosed within larger pyroxene crystals.)
Binary Phase Diagram: Pyroxene – Plagioclase
The formation of porphyritic and ophitic textures can be understood using a binary phase diagram of pyroxene and plagioclase feldspar. The diagram shows the stability fields of each mineral and the coexisting phases at different temperatures and compositions.
Binary Phase Diagram Sketch
(Image depicts a binary phase diagram with temperature on the y-axis and composition (pyroxene-plagioclase ratio) on the x-axis. Solidus and liquidus lines are clearly marked. Areas representing pure pyroxene, pure plagioclase, and a mixed phase region are labeled.)
Explanation of Texture Formation using the Phase Diagram:
- Porphyritic Texture: If the magma starts within the mixed phase region (lower temperature), plagioclase will begin to crystallize as phenocrysts. A rapid change in conditions (e.g., ascent to the surface) will then cause the remaining liquid to cool quickly, forming the fine-grained pyroxene-rich groundmass.
- Ophitic Texture: If the magma cools slowly within the mixed phase region, plagioclase will crystallize first as laths. As the temperature decreases further, pyroxene will crystallize and grow around these laths, resulting in the ophitic texture. The slow cooling allows for the pyroxene to fully enclose the plagioclase.
Factors Influencing Texture Development
- Cooling Rate: The primary control on texture. Slow cooling favors larger crystals and ophitic textures, while rapid cooling leads to fine-grained groundmasses and porphyritic textures.
- Magma Composition: The relative proportions of pyroxene and plagioclase influence the crystallization sequence and resulting texture.
- Pressure: Higher pressure can affect mineral stability and crystallization rates.
- Presence of Volatiles: Water and other volatiles can lower the melting point of the magma and influence crystallization.
Conclusion
In conclusion, the textures of gabbroic rocks, specifically porphyritic and ophitic, are direct consequences of their cooling history and magma composition. Porphyritic textures indicate a two-stage cooling process, while ophitic textures suggest relatively slow cooling with sequential crystallization of plagioclase followed by pyroxene. Binary phase diagrams provide a powerful tool for understanding these crystallization processes and predicting the textures that will form under different conditions. Analyzing these textures is crucial for deciphering the petrogenetic history of igneous rocks and understanding the processes occurring within the Earth’s mantle and crust.
Answer Length
This is a comprehensive model answer for learning purposes and may exceed the word limit. In the exam, always adhere to the prescribed word count.