Model Answer
0 min readIntroduction
Metamorphism, the transformation of existing rocks by heat, pressure, or chemically active fluids, results in significant changes in mineralogy and texture. Basalt, a common extrusive igneous rock, undergoes predictable metamorphic changes as it is subjected to increasing metamorphic grade. The transition from greenschist to amphibolite facies represents a crucial shift in metamorphic conditions, marked by increasing temperature and pressure. This transition leads to distinct mineralogical and mineral-chemical alterations in basaltic rocks, reflecting the stability fields of different minerals. Understanding these changes is fundamental to deciphering the metamorphic history of a region.
Initial Mineralogy of Basalt
Fresh basalt typically consists of plagioclase feldspar (typically labradorite or bytownite – (Ca,Na)(Al,Si)4O8), pyroxene (augite – (Ca,Mg,Fe)2Si2O6), and olivine ((Mg,Fe)2SiO4). Accessory minerals may include magnetite (Fe3O4) and ilmenite (FeTiO3). The initial mineralogy dictates the starting point for metamorphic reactions.
Greenschist Facies – Initial Changes
As basalt is subjected to greenschist facies conditions (typically 300-500°C and 1-4 kbar), the first changes involve hydration and alteration of primary minerals. Key reactions include:
- Olivine Breakdown: Olivine reacts with water to form serpentine ((Mg,Fe)3Si2O5(OH)4) and magnetite.
3(Mg,Fe)2SiO4 + 4H2O → 2(Mg,Fe)3Si2O5(OH)4 + Mg3SiO4 (forsterite, which may also react further) - Pyroxene Alteration: Augite alters to form chlorite ((Mg,Fe)5Al(AlSi3O10)(OH)8) and actinolite (Ca2(Mg,Fe)5Si8O22(OH)2).
Augite + H2O → Chlorite + Actinolite - Plagioclase Alteration: Plagioclase may begin to alter to form epidote (Ca2(Al,Fe)3(SiO4)3(OH)).
Plagioclase + H2O + Fe3+ → Epidote
The resulting mineral assemblage in the greenschist facies typically includes chlorite, actinolite, epidote, albite (NaAlSi3O8), and magnetite. The rock develops a characteristic greenish hue due to the presence of chlorite and epidote.
Greenschist-Amphibolite Facies Transition – Key Reactions
The transition to amphibolite facies (typically 500-700°C and 4-8 kbar) is marked by dehydration reactions and the formation of amphibole. Crucial reactions include:
- Serpentine Breakdown: Serpentine breaks down to form olivine (or pseudomorphs), and water is released.
2(Mg,Fe)3Si2O5(OH)4 → 3(Mg,Fe)2SiO4 + 4H2O - Chlorite Breakdown: Chlorite reacts to form amphibole (hornblende – (Ca,Na)2-5(Mg,Fe,Al)5(Si,Al)8O22(OH,F)2) and plagioclase.
Chlorite + Plagioclase → Hornblende + H2O - Epidote Breakdown: Epidote reacts to form plagioclase, amphibole, and quartz (SiO2).
Epidote + Plagioclase → Hornblende + Quartz + H2O - Albite + Anorthite → Plagioclase: A shift in plagioclase composition towards more calcium-rich varieties occurs.
NaAlSi3O8 + CaAl2Si2O8 → (Ca,Na)(Al,Si)4O8
Mineralogical Changes and Assemblages in Amphibolite Facies
The amphibolite facies assemblage typically consists of hornblende, plagioclase (andesine to bytownite), garnet (Ca3Al2(SiO4)3), and sometimes quartz. The presence of garnet indicates higher temperatures and pressures. The rock loses its greenish color and becomes darker due to the increased proportion of hornblende. The water released during these reactions plays a crucial role in facilitating further metamorphic processes.
Mineral-Chemical Changes
Beyond the formation of new minerals, significant mineral-chemical changes occur. For example, the Ca/Na ratio in plagioclase increases with increasing metamorphic grade. Iron content in hornblende also varies depending on the bulk composition of the rock and the prevailing P-T conditions. The stability of different polymorphs of silica (quartz, tridymite, cristobalite) is also dependent on temperature and pressure.
| Facies | Typical Mineral Assemblage | Key Reactions |
|---|---|---|
| Greenschist | Chlorite, Actinolite, Epidote, Albite, Magnetite | Hydration of Olivine & Pyroxene |
| Greenschist-Amphibolite Transition | Hornblende starting to appear, Serpentine breakdown | Dehydration reactions, Chlorite & Epidote breakdown |
| Amphibolite | Hornblende, Plagioclase, Garnet, Quartz | Formation of Amphibole, Garnet stability |
Conclusion
The metamorphic transition of basalt from greenschist to amphibolite facies involves a series of complex mineralogical and mineral-chemical changes driven by increasing temperature and pressure. These changes are characterized by dehydration reactions, the breakdown of hydrous minerals, and the formation of new, higher-grade minerals like hornblende and garnet. Understanding these reactions is crucial for interpreting the metamorphic history of basaltic rocks and the tectonic processes that have affected them. Further research into fluid-rock interactions and the role of trace elements can provide even more detailed insights into these metamorphic transformations.
Answer Length
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