Model Answer
0 min readIntroduction
Granite, a ubiquitous and economically important igneous rock, is a coarse-grained, felsic rock primarily composed of quartz, alkali feldspar, and plagioclase feldspar. Its formation is intimately linked to the evolution of continental crust. Calc-alkaline peraluminous granites represent a significant subset of granites, commonly found in subduction zone settings and characterized by a high alumina content relative to alkali and calcium. Understanding their petrogenesis provides insights into the processes driving continental crust formation and differentiation. This answer will define granite and then delve into the complex petrogenetic pathways leading to the formation of calc-alkaline peraluminous granite.
Defining Granite
Granite is an intrusive igneous rock, meaning it cools slowly beneath the Earth’s surface. Its defining characteristics include:
- Mineral Composition: Typically >20% quartz, 65% feldspar (alkali feldspar – orthoclase, microcline; and plagioclase – albite, oligoclase), and minor amounts of mica (biotite, muscovite), amphibole (hornblende), and accessory minerals like zircon, apatite, and magnetite.
- Texture: Phaneritic – meaning the individual mineral grains are large enough to be visible to the naked eye.
- Color: Varies depending on mineral composition, ranging from pink to gray to white.
- Density: Approximately 2.6 – 2.7 g/cm3.
Petrogenesis of Calc-Alkaline Peraluminous Granite
Calc-alkaline peraluminous granites are formed through a complex interplay of processes. Their petrogenesis can be broadly divided into the following stages:
1. Source Rock and Melting
The source rocks for these granites are typically metasedimentary rocks (e.g., pelitic schists, gneisses) derived from the partial melting of subducted oceanic crust and sediments. These rocks are rich in aluminum, hence the ‘peraluminous’ designation (Al2O3 > Na2O + K2O). The addition of water from the subducting slab lowers the solidus temperature of the mantle wedge and overlying crust, initiating partial melting.
2. Magma Generation and Ascent
Partial melting of the source rocks generates a magma that is initially relatively mafic. This magma is less dense than the surrounding solid rock and begins to ascend through the crust. The ascent is facilitated by buoyancy and the opening of fractures and pathways within the crust.
3. Magma Evolution: Fractional Crystallization and Assimilation
As the magma ascends, it undergoes significant evolution through two key processes:
- Fractional Crystallization: Minerals with higher melting points (e.g., olivine, pyroxene, amphibole, plagioclase) crystallize out of the magma as it cools. This changes the composition of the remaining liquid, increasing its silica and alumina content.
- Assimilation: The magma interacts with and melts the surrounding crustal rocks. This process incorporates crustal material into the magma, further enriching it in silica, alumina, and incompatible elements. Assimilation is particularly important in generating peraluminous granites.
The combination of fractional crystallization and assimilation leads to a progressive change in magma composition towards a more felsic, peraluminous, and calc-alkaline character.
4. Role of Magma Mixing
Magma mixing can also play a role in the petrogenesis of these granites. Mixing of more mafic magmas (derived from the mantle wedge) with the felsic melts generated from crustal sources can create hybrid magmas with intermediate compositions. This process can contribute to the diversity of granite types observed in subduction zone settings.
5. Crystallization and Textural Features
As the magma reaches shallower levels in the crust, it cools more slowly, allowing for the growth of large, well-formed crystals. The characteristic texture of granite – phaneritic – is a result of this slow cooling. The presence of specific minerals, such as muscovite (a hallmark of peraluminous granites), and the textural relationships between minerals can provide clues about the magma’s evolution history.
Table: Comparison of Granite Types
| Granite Type | Source Rock | Alumina Saturation | Tectonic Setting |
|---|---|---|---|
| S-type (Peraluminous) | Sedimentary Rocks | Peraluminous (Al2O3 > Na2O + K2O) | Subduction Zones, Continental Collision |
| I-type (Metaluminous) | Igneous Rocks | Metaluminous (Al2O3 < Na2O + K2O) | Within continental crust, often associated with mantle plumes |
| A-type (Alkaline) | Variable | Peraluminous or Metaluminous | Rifting environments, Anorogenic settings |
Conclusion
Calc-alkaline peraluminous granites are complex products of magmatic processes operating in subduction zone settings. Their formation involves partial melting of crustal sources, magma ascent, fractional crystallization, assimilation, and potentially magma mixing. Understanding these processes is crucial for deciphering the evolution of continental crust and the dynamics of plate tectonics. Further research utilizing geochemical and isotopic analyses continues to refine our understanding of these fascinating igneous rocks and their role in Earth’s geological history.
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.