minpetexam

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gabbro

a dark, medium- to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and minor olivine, but no quartz. It is the intrusive equivalent of a basalt mafic intrusive plagioclase, pyroxene, trace amts of olivine, biotite phaneritic

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diabase

Diabase or dolerite or microgabbro is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro mafic, intrusive plagioclase, pyroxene, opaques phaneritic

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types of metamorphism

contact metamorphism

Contact metamorphism occurs adjacent to igneous intrusions and results from high temperatures associated with the igneous intrusion. Since only a small area surrounding the intrusion is heated by the magma, metamorphism is restricted to the zone surrounding the intrusion, called a metamorphic or contact aureole. Outside of the contact aureole, the rocks are not affected by the intrusive event. The grade of metamorphism increases in all directions toward the intrusion. Because the temperature contrast between the surrounding rock and the intruded magma is larger at shallow levels in the crust where pressure is low, contact metamorphism is often referred to as high temperature, low pressure metamorphism. The rock produced is often a fine-grained rock that shows no foliation, called a hornfels.

regional metamorphism

Regional metamorphism occurs over large areas and generally does not show any relationship to igneous bodies. Most regional metamorphism is accompanied by deformation under non-hydrostatic or differential stress conditions. Thus, regional metamorphism usually results in forming metamorphic rocks that are strongly foliated, such as slates, schists, and gniesses. The differential stress usually results from tectonic forces that produce compressional stresses in the rocks, such as when two continental masses collide. Thus, regionally metamorphosed rocks occur in the cores of fold/thrust mountain belts or in eroded mountain ranges. Compressive stresses result in folding of rock and thickening of the crust, which tends to push rocks to deeper levels where they are subjected to higher temperatures and pressures.

cataclastic metamorphism

Cataclastic metamorphism occurs as a result of mechanical deformation, like when two bodies of rock slide past one another along a fault zone. Heat is generated by the friction of sliding along such a shear zone, and the rocks tend to be mechanically deformed, being crushed and pulverized, due to the shearing. Cataclastic metamorphism is not very common and is restricted to a narrow zone along which the shearing occurred.

hydrothermal metamorphism

Rocks that are altered at high temperatures and moderate pressures by hydrothermal fluids are hydrothermally metamorphosed. This is common in basaltic rocks that generally lack hydrous minerals. The hydrothermal metamorphism results in alteration to such Mg-Fe rich hydrous minerals as talc, chlorite, serpentine, actinolite, tremolite, zeolites, and clay minerals. Rich ore deposits are often formed as a result of hydrothermal metamorphism.

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grade of metamorphism

how-grade metamorphism

takes place at temperatures between about 200 to 320oC, and relatively low pressure. Low grade metamorphic rocks are generally characterized by an abundance of hydrous minerals.With increasing grade of metamorphism, the hydrous minerals begin to react with other minerals and/or break down to less hydrous minerals

high-grade metamorphism

takes place at temperatures greater than 320oC and relatively high pressure. As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O, and non-hydrous minerals become more common

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amygdaloidal basalt

basalt is full of olivine and pyroxenes because it is mafic plagioclase, quartz, opaques, olivine, epidote mafic extrusive

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ultra-mafic

relating to or denoting igneous rocks composed chiefly of mafic minerals

mafic

relating to, denoting, or containing a group of dark-colored, mainly ferromagnesian minerals such as pyroxene and olivine

intermediate

roughly even mixtures of felsic minerals (mainly plagioclase) and mafic minerals (mainly hornblende, pyroxene, and/or biotite). There is little or no quartz. Felsic rocks are mostly feldspar (especially K-feldspar), at least 10% quartz, and less than 15% mafic minerals (biotite, hornblende).

felsic

relating to or denoting a group of light-colored minerals including feldspar, feldspathoids, quartz, and muscovite

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hornblende gabbro

plagioclase, hornblende, pyroxene, biotite mafic intrusive Gabbro refers to a large group of dark, often phaneritic, mafic intrusive igneous rocks chemically equivalent to basalt, being its coarse-grained analogue

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norite

norite may be very similar to gabbro but contains orthopyroxene, not clinopyroxenemafic intrusive plagioclase, pyroxene, biotite phaneritic

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anorthosite

mostly plagioclase feldspar, with a minimal mafic component pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present mafic intrusive

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peridotite

minerals: olivine, pyroxene ultra mafic / intrusive / igneous rock, magnesium rich Peridotite is the dominant rock of the upper part of the Earth's mantle.