Magmatic enclaves and magma mingling/mixing
Magmatic enclaves are inclusions of igneous rocks, generally with mafic to intermediate composition, within igneous rocks of more evolved composition that are thought to result from the mechanical mingling and chemical mixing of two molten magmas in a magma chamber.
Magma mingling and mixing
Magmas with different composition commonly interact in magma chambers, producing a wide range of hybrid magmas derived the mixing of two (or more) parent magmas. In most cases, the process results in the complete mixing of the starting magmas, leaving behind only geochemical and petrographic clues, like a hybrid geochemical signature or the presence of reverse or oscillatory zoning in phenocrystals, indicating the arrival of multiple magma pulses in a magma chamber. This is because magma mixing commonly occurs between magmas with fairly similar compositions or at depth in the crust during magma ascent, allowing most molten magmas to mix and homogenize. However, there are several, documented cases in which magma mingling/mixing is incomplete, resulting in the formation of magmatic enclaves that are visible in the field.
Features of magmatic enclaves
Magmatic enclaves, also known as mafic enclaves or mafic microgranular enclaves (MME), occur as round or lobate dark-colored and fine-grained igneous blobs within igneous rocks with different composition. Differently from xenoliths – which are fragments of solid rocks incorporated in the magma during its ascent and emplacement – magmatic enclaves derive from the mechanical mingling and incomplete chemical mixing of two magmas with different composition. Magmatic enclaves vary in size from a few millimeters to several meters and, in general, they are commonly found as groups of objects with variable sizes. Magmatic enclaves almost universally show a mafic or intermediate composition and are found within more evolved igneous bodies, like granitic and granodioritic intrusions or rhyolitic/dacitic lava flows. This happens because the interaction of a hot basaltic/andesitic magma with a colder and more viscous granitic magma causes the rapid cooling and crystallization of the mafic magma, allowing the preservation of structures that indicate the incomplete mixing of the two melts. When magmatic enclaves occur in volcanic rocks, they are commonly considered to indicate mingling/mixing at depth before the eruption took place. In many such cases, the arrival and mingling of new magma in a magma chamber has also been addressed as the possible cause of eruption.
Several structures associated with magmatic enclaves, indicating mixing of two magmas at the liquid state, can be observed in the field and in thin section:
- Enclaves are commonly mafic (dark-colored) and occur in granitic/rhyolitic rocks.
- Enclaves show a spheroidal to irregular shape with a rounded outline. This shape is typical of the interaction of two liquids with different viscosity (think of droplets of oil in water).
- Enclaves deflect/fold around solid objects in the host, like phenocrystals, that were already present when the two magmas mingled.
- The magmatic enclave may contain xenocrystals derived from the surrounding magma, which may be corroded by the mafic magma.
- The mafic magma in the enclave undergoes rapid crystallization/cooling because it is surrounded by much colder, felsic magma. As a result mafic enclaves appear fine- to very fine-grained. By contrast, the surrounding felsic rock tends to be coarse-grained (in the case of enclaves in intrusive rocks). Even thin injection veins of granite into the enclave do not show a decrease in grain size.
- Large enclaves are commonly surrounded by smaller enclaves, sometimes organized in trails of oval bodies. This is an indication of the dispersal of mafic magma into the surrounding acid melt by mingling.
- Enclaves may be involved in the flow of the surrounding magma and hence be sheared and ductively deformed parallel to a magmatic foliation. Ductile deformation localized within the enclaves and absent in the surrounding rock indicates that they were solid or nearly solid while the surrounding magma was still flowing.
- A rim or selvage with hybrid composition may develop at the contact between the mafic enclave and the surrounding felsic rocks. This selvage is commonly of tonalitic (biotite-plagioclase) composition and indicates partial (chemical) mixing between the two magmas.
- Additional microstructures that may develop during magma mingling are oscillatory or reverse zoning (for example in plagioclase phenocrystals) and resorption of some grains that have become in chemical disequilibrium with the surrounding magma after mixing.
Interaction between solid objects in magma and mafic enclaves
Above: the black mafic enclave to the right encountered an alkali feldspar megacrystal during mingling with the surrounding monzogranite. The megacrystal ‘poked’ the surface of the enclave, unable to break through due to surface tension. Monte Capanne monzogranite, Elba, Italy.
Above: xenocrysts of alkali feldspar originated in the surrounding monzogranite that have been partially corroded by the mafic magma of the enclave (note the anhedral habit). From the same outcrop of the first slider.
Examples of magmatic (mafic) enclaves in intrusive rocks
Examples of enclaves in volcanic rocks
Mingling between synplutonic dykes and intrusive rocks
Most granitic and granodioritic intrusions remain partially molten in the crust for thousands to millions of years as crystal mushes, ‘hot soups’ characterized by solid crystals and very low melt fraction. Because of this elevated solid content, a crystal mush is extremely viscous and can be crosscut by igneous dykes filled by magma. However, since some melt is still present in the surrounding intrusive, the dykes may be slowly deformed after their emplacement and be in turn crosscut by granitic magma coming from the surrounding intrusives. Mafic dykes in particular are significantly hotter than granitic rocks and they can release heat to the surrounding rocks, increasing the local presence of granitic melt. Evidence of mutually crosscutting relationships between dykes and the host intrusives and structures like lobate/rounded boundaries and injection veinlets at the contact indicate that both magmas were partially molten and that the dyke can be classified as synplutonic.
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Magmas and Magmatic Rocks: An Introduction to Igneous Petrology
Petrology: Principles and Practice