Enclaves
Usually, the term enclave is associated with a territory with political and cultural distinctions whose geographic borders are completely within the limits of another territory. In Europe and in terms of entire nations included in the territory of another country, two enclaves are known: the Republic of San Marino and the Vatican City. Both enclaved within Italy.
The word enclave is French and first appeared in the mid-15th century as a derivative of the verb enclaver (1283), from the colloquial Latin inclavare (to close with a key). Originally, it was a term of property law that denoted the situation of a land or parcel of land surrounded by land owned by a different owner, and that could not be reached for its exploitation in a practical and sufficient manner without crossing the surrounding land.
In geology, an enclave is an aggregate of minerals or rock observed inside a larger rock body. Usually, it refers to such situations in plutonic rocks. Micro-granular enclaves in felsic plutons result from the introduction of mafic magma into the magma chamber and its subsequent cooling following incomplete mixing. Roughly it can be divided in four types of enclaves: xenoliths, microgranular enclaves, magma mixture and micaceous enclaves (restites).
Xenoliths
They are fragments of rock that the magma has included during its rise or emplacement. They represent solid material completely unrelated to magma whose origin is usually the bedrock. Its nature can be very varied: metamorphic, plutonic, volcanic or sedimentary.
The image on the left shows a metric-sized xenolith of metamorphic rock, and probably metasedimentary (fine-grained). Although the xenolith is an angular fragment, some edges show symptoms of some digestion (recrystallization and granitization or partial melting and dissolution). It is also observed how micro-venules of granitic melt permeate and wedge, splintering the enclave, in the form of small magmatic mining or stopping phenomena. When the xenolithic material corresponds to a single crystal, the term xenocrystal is used.
The image on the right shows a xenolith embedded in a granite rock. The characteristics of this enclave are different from the previous one. It is mafic, being composed mainly of biotite, although it preserves felsic minerals, and it has foliation, so it must be an enclave of metamorphic origin. The mafic character may be due to having partially melted in contact with the magma or having been gored.
Microgranular enclaves
These enclaves are characterized by having normally rounded or ellipsoidal shapes, having a texture between porphyritic and equigranular with a tendency to fine grain and a high content of mafic minerals. They are typical of granites, and their origin is also igneous (basic magma), which is why they can be considered proof of magma mixing (without reaching intermediate terms). They usually correspond to rocks of the dioritic, tonalitic or granodioritic type. The fact that the contacts are rounded or sinuous implies that they were totally or partially molten when they were included in the granitic magma. Two examples of microgranular enclaves included in monzogranites can be seen in the upper images. The porphyritic texture can be appreciated in both cases, with feldspar phenocrysts. The contacts are quite clear, although in the left image a felsic, feldspathic halo can be sensed, perhaps of reactional origin.
The microgranular enclaves can be mixed to a lesser or greater degree with the granitic melt. When they do, their contacts are transitional and their form more irregular. The left image shows a more "digested" enclave with irregular contact, larger grain size and less mafic. The right image shows a rounded shape, net contact and a greater abundance of biotite on the edges, developing a quartz-feldspathic halo. This shape can be caused by the viscous flow of the two melts, or also by deformation of the intrusion.
Magma mixture
The coincidence in time and space of two contrasting magmatic baths (fine-grained equigranular tonalite and porphyritic granite) allows us to observe various structures of igneous mixture in the images. The general contact between the two is very sinuous, lobed and scalloped. It is possible to see very fine grained (chilled) edges of the tonalite against the granite. Fragmentation and formation of mafic microgranulate enclaves are seen. The microgranular enclaves of the previous images are also examples of mixing, however, the greater the compositional contrast of the magmas, the more difficult it is to obtain an authentic mixture.
Micaceous enclaves
They are normally small enclaves, with a high content of mafic minerals, within which biotite and foliation normally abound. The presence of foliation indicates that they are fragments of metamorphic rocks, foreign to magma. The abundance of mafic phases, some of them representative of medium-high pressures and temperatures (garnet, sillimanite, cordierite), has led to interpreting these enclaves as fragments of a restitic metamorphic source area, that is, partially molten, which have been dragged by the melt during their ascent from deep levels. The images show mafic enclaves, composed mainly of biotite and with a fine grain size. Observe the main differences with the microgranular enclaves: presence of a metamorphic fabric, elongated or rounded shapes, and planar, absence of porphyritic textures and scarcity of quartz and feldspars.
ref: wikipedia
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