Model Answer
0 min readIntroduction
Andesite and dolerite are both important volcanic rocks, but they differ significantly in their composition and origin. Andesite, typically found in volcanic arcs associated with subduction zones, is an intermediate volcanic rock. Dolerite, a subvolcanic rock, is a medium-grained equivalent of basalt, commonly found in dykes and sills. Distinguishing between these rocks in thin section requires careful observation of their mineral assemblages, textures, and alteration features. This answer will detail the microscopic characteristics of each rock and highlight the key differences for identification.
Andesite: Microscopic Characteristics
Andesite is characterized by a porphyritic texture, meaning it contains larger crystals (phenocrysts) embedded in a finer-grained groundmass. The dominant minerals in andesite, visible in thin section, include:
- Plagioclase Feldspar: Typically andesine to labradorite (An50-An70), exhibiting polysynthetic twinning. These are often euhedral to subhedral phenocrysts.
- Pyroxene: Commonly augite, appearing as stubby, greenish crystals. Zoning is often present.
- Hornblende: Present as green to brown pleochroic crystals, often altered to chlorite.
- Quartz: Present in varying amounts, often as small, rounded grains.
- Groundmass: The groundmass is typically a microcrystalline aggregate of plagioclase, pyroxene, and sometimes quartz and opaque minerals.
Alteration is common in andesite, with plagioclase often showing sericitization (alteration to sericite) and kaolinization. Pyroxene and hornblende are frequently altered to chlorite and epidote.
Dolerite: Microscopic Characteristics
Dolerite exhibits a subophitic to ophitic texture, where plagioclase laths are partially enclosed by larger pyroxene crystals. Key microscopic features include:
- Plagioclase Feldspar: Typically labradorite to bytownite (An70-An90), displaying polysynthetic twinning. The plagioclase laths are generally aligned.
- Pyroxene: Predominantly augite, forming larger, well-developed crystals that partially enclose the plagioclase.
- Olivine: Often present, particularly in less-altered dolerites, appearing as rounded, yellow to brown grains. It is frequently altered to serpentine.
- Opaque Minerals: Magnetite and ilmenite are common, often forming skeletal or granular intergrowths.
- Groundmass: The groundmass is generally less prominent than in andesite, consisting of microcrystalline plagioclase and pyroxene.
Alteration in dolerite commonly involves the alteration of olivine to serpentine and the oxidation of iron-rich minerals.
Comparative Table: Andesite vs. Dolerite (Thin Section)
| Feature | Andesite | Dolerite |
|---|---|---|
| Texture | Porphyritic | Subophitic to Ophitic |
| Plagioclase Composition | Andesine to Labradorite (An50-An70) | Labradorite to Bytownite (An70-An90) |
| Pyroxene | Augite, often zoned | Augite, enclosing plagioclase |
| Quartz | Present in varying amounts | Generally absent or in very small amounts |
| Olivine | Rare | Common, often altered to serpentine |
| Hornblende | Common | Rare |
| Alteration | Sericitization, kaolinization of plagioclase; chloritization of pyroxene/hornblende | Serpentinization of olivine; oxidation of iron-rich minerals |
Conclusion
In conclusion, distinguishing andesite from dolerite in thin section relies on careful observation of their textural and mineralogical differences. Andesite is characterized by its porphyritic texture, the presence of quartz and hornblende, and a plagioclase composition towards andesine. Dolerite, conversely, exhibits an ophitic texture, typically lacks quartz, often contains olivine, and has a more calcic plagioclase composition. Recognizing these key features allows for accurate identification of these important volcanic rocks.
Answer Length
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