Igneous intrusion refers to a body of igneous rock that has made its way into pre-existing rock (known as country rock). Igneous intrusions are emplaced as magma, which is less dense than solid rock and therefore tends to move upwards. It can then force its way through cracks in the rocks and can wedge them apart or, if it is hot enough, it can melt and replace them.
Igneous intrusions can be of a variety of shapes and sizes, ranging from huge batholiths to bodies only one or two meters across. The general term 'pluton' can be applied to any of these. Intrusions may cut across the bedding of the country rock. They are then termed 'discordant' or 'transgressive'; dykes and laccoliths are examples. Intrusions that follow the bedding of the country rock, such as sills, are termed 'concordant'.
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Metamorphic rock is rock that has been altered by intense heat, pressure, or both. It may originally have been sedimentary or igneous rock, or even different metamorphic rock. The changes that take place during metamorphism can affect both the structure of the rock and its composition. Very often the rock is re-crystallized. Contact metamorphism is a localized form of metamorphism that is produced by the heat of an igneous intrusion. Limestone may then be altered into marble, and clay into a hard, tough rock (a hornfels). The zone affected in this way is called a metamorphic aureole. Hydrothermal metamorphism, or metasomatism, is produced by hot aqueous fluids emanating from igneous intrusions. China clay is produced in this way from granite. In dynamic metamorphism, or cataclasis, rocks are broken down mechanically by shearing and crushing; mylonite, a fine-grained banded rock, is a typical product. Regional metamorphism takes place on a large scale.
The rocks are subjected to heat, deformation, and the action of hot fluids that may affect their chemical composition. In the lowest grades of regional metamorphism, slates and phyllites (the latter with better- developed crystals than slate) are formed. More intense regional metamorphism results in the development of schists, rocks with a characteristic wavy foliation. At the highest grades, gneisses are formed: coarsely crystalline rocks with alternate light and dark bands. The normal sequence, from lower to higher metamorphic grade, is known as prograde metamorphism. The process can be reversed if, for example, rocks of a high grade are subsequently maintained for a long time at a lower temperature than was reached during the first metamorphism. Alteration from a higher to a lower grade is termed retrograde metamorphism.
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Scapolite has the formulae (Na,Ca,K)4A13(Al,Si)3Si6O24(Cl,SO4,CO3) and a relative hardness of 6. It shows fluorescence. Occurs in the crystalline schists, gneisses, and often is probably derived from the alteration of plagioclase feldspars. Also occurs in crystalline limestones formed by metamorphic contact or igneous intrusion. Associated with diopside, amphibole, garnet, apatite, and zircon.
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