Contact metamorphic rocks

Figure 8.8. Crystal size distribution (CSD) plots of (a) plagioclase in igneous rock and (b) garnet porphyroblast in contact metamorphic rock [4].

Contact metamorphic rocks Groundwater from fractured bedrock Humid, arid Mo, U, Rn Drainage, mining, groundwater pumping Other metals, depending on mineralization... 

Thermal metamorphic rock, also called contact metamorphic rock, is formed not only by considerable pressure but, more importantly, by intense heat. Imagine molten rock pushing up into Earth s crust. The incredible pressure fills any empty space, every nook and cranny, with molten rock. This intense heat causes the surrounding rock to completely recrystallize. During recrystallization, the chemical composition regroups to form a new rock. An example of this type of thermal metamorphic rock is marble, which is actually limestone whose calcite has recrystallized. Sandstone made mostly of quartz fragments recrystallizes into quartzite. Thermal metamorphic rocks are not as common or plentiful as regional metamorphic rocks. Sometimes a metamorphic rock can become metamorphosed. This is known as polymetamorphism. 

Pyroxenes Inosilicates Igneous and contact metamorphic rocks R Source of Fe, Ga, Mg, and Mn unstable in highly leached soil... 

Soil water flow is decidedly episodic. During dry times the water solutions in the soil are probably fairly concentrated and not very reactive. Time-averaged reaction rates should be roughly proportional to the fraction of time reacting minerals are in contact with thermodynamically imdersaturated (and reactive) water. In a study of the relationship between denudation rate and runoff for rivers draining igneous and metamorphic rock in Kenya, Dunne (1978) obtains the relationship of (denudation rate in tons/km per year) = 0.28 (runoff in mm/ year)°. ... 

Host rocks in the Hitachi area suffered regional metamorphism, contact metamorphism by Cretaceous granitic rocks and hydrothermal alteration associated with sulfide mineralization. 

Host rocks in deeper parts of the Besshi mine suffered contact metamorphism probably due to Cretaceous granitic intrusion. Kase (1972, 1977, 1988) suggested the contact metamorphism in the Besshi mine from the occurrence of large amount of pyrrhotite and As, Zn, Ag and Sn vein mineralization in deeper levels. 

In the other areas, generally, the host rocks weakly suffered regional and contact metamorphisms but suffered ocean-floor hydrothermal alteration. For example, hydrothermal alteration mineral assemblages in the Minamidani mine district in the Maizuru range from prehnite-pumpellyite facies to a transition state from green schist to amphibolite... 

In contact-metamorphic zones, in association with magnetite and Fe-rich silicates (feldspathoid-bearing igneous rocks)... 

Diopside-hedembergite (Ca,Mg,Fe)2Si206 Cr-rich ultramafic rocks contact metamorphism (cipollines, comubianites)... 

The rocks at Morila record an intimate interplay between magmatism, contact metamorphism (and metasomatism), deformation, and mineralization. The spatial, temporal, and textural associations between contaminated post-D2 intrusions, contact metamorphism, and polymetallic post-D2 mineralization strongly suggests an intrusion-related origin for Morila. Other controls on Au mineralization include ... 

In regional metamorphic rocks and contact metamorphic (metasomatic) rocks, new mineral crystals grow in solid rocks in which there was a change in conditions. However, the process is not the same as straightforward solid state growth or recrystallization. Since volatile components such as H O and CO, which were originally present in the rock, are involved, it is better to assume a... 

Many authors have studied anthraxolite and related substances since Vanuxem s first account in 1842 (39). It has been reported as cavity fillings, veins, and dikes and as laminations in rocks of all geologic ages and types on all continents (e.g., I, 15, 20, 31, 32, 37, 38). Except for obvious cases of contact metamorphism near igneous intrusions, earlier workers have generally recognized a fluid origin and low temperature history for these substances. 

Another proof of the importance of temperature is the fact that there is often a strict relationship between the run of isovols and the run of isotherms in deep profiles, both being influenced no doubt by the varying thermal conductivity of the different rocks. The strong influence of temperature on the rank of coal is obvious in the case of contact-metamorphic coals, whose rank increases distinctly when approaching the intrusive body. Apart from these geological observations, all experiments on artificial coalification have shown that temperature is the decisive factor in the coalification process. Thermodynamic and reaction kinetic considerations (9) also support this opinion. 

Scapolite is found in the metamoiphic locks, particularly those, rich in calcium also in contact metamorphic deposits in limestones. It has been found in basic igneous rocks, probably as a secondary mineral. Notable localities are Lake Baikal, Siberia Arendal. Norway and Madagascar. In the United States, it is found in Massachusetts, New York, and New Jersey. Greenville, in the Province of Quebec, Canada is an important locality. Superb transparent yellow gem crystals have recently been found in Brazil and Tanzania. Wernente (scapolite) was named in honor of A.O. Werner, a famous German mineralogist (1749-1817). 

Contact metamorphism Localized metamorphism of rocks involving heat, fluids, and minor deformations resulting from the intrusion of a magma (compare with regional and cataclastic metamorphism). 

Skarn A calcite- or dolomite-rich rock produced by contact metamorphism (compare with marble). 

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