Intermediate and Acidic Magmatic Rocks: Composition, Structures, and Occurrence
Intermediate Rocks
Intermediate rocks have a SiO2 content of 52-63%. They can contain up to 40% modal quartz.
Composition:
- Mafic Minerals: A few mafic minerals, such as MgO and FeO.
- Plagioclase: Sodium-rich plagioclase (Na).
- Potassium Feldspar and Biotite: Due to the presence of K2O.
- Biotite and Hornblende: Due to the presence of H2O.
Diorite
Diorite is characterized by An40 plagioclase, clinopyroxene (Cpx) ± orthopyroxene (Opx), hornblende (HBL), biotite, quartz (Qz), and interstitial potassium feldspar (FeldK). Accessory minerals include apatite (Ap), zircon (Zr), spinel, magnetite, and ilmenite. The color index (CI) ranges from 25-50. It differs from gabbro in the anorthite content (An) of the plagioclase, which is typically less than 50% in diorite, and the presence of hornblende.
If quartz content is less than 5%, the rock is classified as hornblende diorite, with augite and hornblende. If quartz content is between 5-20% (with An35 plagioclase), it is classified as quartz diorite.
Tonalite
Tonalite contains more than 20% quartz, An30 plagioclase, biotite, and hornblende. Plagiogranite and trondhjemites are related rock types.
Structures in Diorite
Diorite can exhibit igneous lamination, defined by the parallel arrangement of clinopyroxene and plagioclase, formed by deposition in the magma chamber under weak currents. Xenoliths and autoliths may be present. Diorite can also display gneissic banding, characterized by light and dark bands, particularly at the edges due to intrusive contamination. Schlieren-like structures, which are mixtures of mafic minerals or crushed and partially digested xenoliths, can be found closer to the contact with the surrounding rocks. Pegmatite and miarolitic cavities may also occur.
Occurrence Modes of Diorite
Diorite occurs in two primary modes:
- Dioritic Massifs: These are commonly found in quartz diorites, often originating from the differentiation of basic magma in calc-alkaline subduction zones.
- Differentiated Stocks at Invasive Edges: These are smaller bodies found continuously around different intrusive bodies. Migmatites may be observed in these settings.
Monzonite
Monzonite is a relatively rare rock type found between diorite and syenite. It exhibits zoning in plagioclase with An68-35. Quartz monzonite (Mz Qz) can contain biotite and hornblende, or clinopyroxene and olivine. Monzonite can evolve into syenite.
Syenite
Syenite is an evolved rock type that can further evolve into trachyte.
Andesites
Andesites are the extrusive equivalent of diorite and monzodiorite, with a SiO2 content typically ranging from 57-63%, although it can be as low as 52-57%. They are characterized by strongly zoned plagioclase (An80-50, both normal and reverse zoning), orthopyroxene, clinopyroxene, and Fe oxides. The texture often indicates magma mixing, with the presence of sieve textures and brown (neutral) glass. The color index is typically around 35. Andesites are primarily found in subduction zones but can rarely occur in other environments (in such cases, they are poor in Al2O3 and aphanitic). Andesite lava flows can be up to 10 km long, 1 km wide, and 30 m thick.
Structures in Andesite Lava Flows
Andesite lava flows exhibit various structures:
- Block Lava Flows: These flows are characterized by a blocky surface.
- Aa Lava Flows: These flows can contain levees, formed by the layering of water percolation and oxidation along fractures, and pressure ridges, which are wrinkles on the surface of the flow.
| Lava | Phyllosilicate Mantle |
|---|---|
Conglomerate, tuffaceous ash breccia (autobreccia), cubic to spherical vesicles, massive texture (few vesicles), prismatic jointing, zone with clinopyroxene, aphanitic zone with some platy jointing, lenticular vesicles, and gap-matrix conglomerate with groundmass, generally oxidized with carbonized material. | Conglomerate, vitreous to aphanitic texture, vesicular-fluidal texture, vesicular with clinopyroxene, massive with clinopyroxene, regular prismatic jointing with clinopyroxene, vesicular and vesicular-fluidal texture, vitreous to aphanitic texture, sandstone (symmetric). |
Olivine, pyroxene, hornblende, and biotite can be present.
Acidic Lavas
Acidic lavas include dacite and rhyolite. These lavas are viscous and tend to form volcanic domes. Explosive eruptions can produce lateral flows or coulées. Phenocrysts are common. Domes often have a flat surface and steep fronts. Low domes are referred to as “cake” domes.
Volcanic Domes
Volcanic domes can range in thickness from tens to hundreds of meters. They are typically short, have a small volume, and form in the last stage of volcanic activity, often as resurgent domes (tholoids). Pelean domes, characterized by a needle shape, can generate lateral blasts. The most frequent dome types are rhyolitic and dacitic, with dacite often associated with explosive events.
Internal Structure of Volcanic Domes
From the ceiling to the base, the internal structure of a volcanic dome typically consists of:
- Gap zone with blocks of obsidian and pumice.
- Finely vesicular pumiceous lava.
- Obsidian layer.
- Spherulitic obsidian layer.
- Central part composed of brecciated rhyolite.
Towards the base, the sequence is reversed, but the obsidian is less dense. Coarse-grained pumice and pyroclastic deposits from previous eruptions can be found in the soil. Pumiceous and obsidian layers may exhibit flow structures. Alternating bands of different lithology, grain size, and composition can be observed. Phenocryst alignment, which can be folded (fluxion structure, with visible bands of differential alteration), is also common.