Regional metamorphic rocks

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... 

Figure 13.5. White mica in a regional metamorphic rock using the decoration method [5]. 

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. 

Regional metamorphic rocks containing orthopyroxene and K-feldspar i.e., charnockites, must have crystallized in H20-poor environment, or have a significant Fe content. If sufficient H2O were present, such rocks would become either quartz-biotite schists or silicate melts. The low H2O activity can be accoxmted for either by the intrinsic composition of the rocks (metavolcanics) or by the introduction of CO2 into the system by virtue of decarbonation reactions. The fact that many charnockites are fo md in marblebearing terrains is in accord with the requirement for low H2O activity. 

Chlorite is a common accessory mineral in low-to-medium grade regional metamorphic rocks and may be the most abundant mineral in metamorphic rocks of the chlorite zone. It is an occasional constituent of igneous rocks, in most cases probably forming secondarily by deuteric or hydrothermal alteration of primary ferromagnesian minerals, such as mica, pyroxene, amphibole, garnet, and olivine. Chlorite is found in pegmatites and fissure vein... 

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

The geological cycle sediments from deep regions of the Earth, which are subject to high pressure and high temperature, are brought back to the Earth s surface by geological dynamics. The metamorphic rock is more labile than the starting material and can be leached more easily, so that a cycle is set up in which new clay species are formed. 

Heinrich CA (1986) Eclogite facies regional metamorphism of hydrous mafic rocks in the Central alpine Adula nappe. J Petrol 27 123-154... 

In thermal metamorphism, biotite occurs in the clorite-sericite facies as a dispersed phase in the argillitic matrix and is stable up to low-grade cornubianites. In regional metamorphism, biotite is typical of argillitic and pelitic rocks up to the staurolite-garnet facies (biotite, biotite-sericite, biotite-chlorite, and albite-biotite schists, and garnet-staurolite micaschists). 

In metamorphic rocks, muscovite occurs in low-grade terrains of the regional metamorphism (albite-chlorite-sericite schists). It must be noted here that the term sericite identifies fine-grained white micas (muscovite, paragonite). 

Feldspars are the most abundant minerals of igneous rocks, where their ubiquity and abundance of their components influence normative classifications. They are also abundant in gneisses, and may be observed in several facies of thermal and regional metamorphic regimes. Notwithstanding their alterability, they are ubiquitously present in sedimentary rocks, as authigenic and/or detritic phases. Only in carbonaceous sediments is their presence subordinate. 

Harrison T. M. and McDougall I. (1981). Excess " °Ar in metamorphic rocks from Broken Hill, New South Wales Implications for " °Ar/ Ar age spectra and the thermal history of the region. Earth Planet. Sci Letters, 55 123-149. 

The Pebble deposit is located in the Kahiltna terrane, near the boundary between two lithologic packages Jurassic and older magmatic-metamorphic rocks to the southeast, and an assemblage of Mesozoic volcaniclastic and sedimentary rocks overlain by Tertiary volcanic rocks, to the northwest. An extended discussion of the regional geology is given in Kelley etal. (this volume). 

The rocks in the study have undergone zeolite facies regional metamorphism but locally intense penetrative fabrics are recognized in areas adjacent major faults. 

Regional metamorphism proceeds in a wide area, and characteristic mineral paragenesis appears in zones depending on the temperature and pressure conditions that were confronted by the original rock. These are called metamorphic... 

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