Metamorphism and Metamorphic Rocks
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The study area is located in southwest of Qayen and in Northern part of Sistan suture zone in east of Iran and it is in real a ophiolitic complex with approximate complete sequence and with a special metamorphic zone including metabasite rocks such as greenschist, epidote amphibolite, amphibolite and granulite in its eastern part. The metabasite rocks are associated to ophiolite, have been affected by low to high grade dynamothermal regional metamorphism that has out cropped as a special metamorphic zone in eastern part of ophiolite. On the base of geochemical diagrams most of these metamorphic rocks are result of dynamothrmal regional metamorphism of primary pillow basaltic lavas and magmatic series of metabasite protoliths, is tholeiitic to high Fe tholeiitic. This composition is showing their relationship with MORB origin magma. The thermobarometric investigations has done on these rocks with varieties of methods, are showing that these metamorphic rocks have been affected by metamorphism in maximum pressure of 3.9 – 5.3 Kbar and maximum temperature of 725 ± 25 degrees of centigrade and in most extensive metamorphic position, changing of metamorphic pressure has had an important effect in parageneses changes.
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Greenschist
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Ciletuh Complex is considered to be one among other localities (Luk Ulo, Bantimala, Meratus
Complexes) in Indonesia in which Cretaceous subduction fossil was occurred. The objective of this study
is determining the variation of metamorphic rocks in Ciletuh Complex by petrographical analysis
through their texture and mineral assemblages.
Metamorphic rocks in Ciletuh Complex were collected in Gunung Badak and Tegal Pamidangan areas.
Based on petrographical analysis, metamorphic rocks in Gunung Badak area consist of Grt-Ms-Qz
schist, Ms phyllite, quartzite and serpentinite. Meanwhile in Tegal Pamidangan area, consist of Ms-Qz
phyllite and slate. The metamorphic rocks indicate low-grade metamorphism in the greenschist-facies.
The protolith of metamorphic rocks are suggested from pelitic, ultramafic, and quartz-rich rocks.
Present study did not recognize the blueschist or eclogites-facies rocks which indicates high-pressure
and low-temperature metamorphism in the subduction system. The present of serpentinite among the
low-grade metamorphic rocks indicates that metamorphic environment still correlate with oceanic crust
environment or mantle. Low-grade metamorphic rocks might be developed on the near surface of the
subduction system.
Keywords : petrography analysis, mineral assemblages, metamorphic facies, protolith
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Greenschist
Phyllite
Blueschist
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Ultramafic rock
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Metamorphism is the sum of all changes that take place in a rock as a result of changes in the rock's environment; that is, changes in temperature, pressure (directed as well as lithostatic), and composition of fluids. The changes in the rock may be textural, mineralogical, chemical, or isotopic. These changes proceed at varying rates, so time is an important factor in metamorphism. Any kind of rock can be metamorphosed; the starting rock is called the protolith. Common protoliths include the spectrum of igneous rocks from ultramafic to felsic, as well as sedimentary rocks such as sandstones, alumina-rich shale (pelite), and carbonate rocks (limestone and dolostone).
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Ultramafic rock
Felsic
Dolostone
Pelite
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Bixiling and Shuanghe are two typical outcrop areas in the Dabie ultrahigh-pressure (UHP) metamorphic terrane. Based on field outcrop observations and petrographic and petrochemical studies, the authors find that the rock association consists predominantly of eclogite, amphibolite, gneiss and foliated garnet-bearing granite. Eclogite has the characteristics of the crustal source and is considered to form by UHP metamorphism crustal rocks in the deep interior of the mantle; amphibolite is the product of retrogressive metamorphism of eclogites during exhumation of the plate; gneiss was produced by anatexis and progressive evolution of amphibolite in an ideal environment during exhumation; and foliated garnet-bearing granite is the result of partial melting of gneiss. In conclusion, there is a reworked in-situ relationship between eclogite and gneiss (and/or foliated garnet-bearing granite).
Anatexis
Outcrop
Migmatite
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Micro-inclusions and the U-Pb age of zircons were investigated from the banded gneiss in the northern Dabie complex, central China Mineral inclusions in zircon from the gneiss,combined with cathodoluminescene images and isotopic dating, can be divided into three generations, i. e. primary magma assemblage of feldspar + biotite + quartz + apatite, ultrahigh-pressure ( UHP) metamorphic assemblage of diamond + garnet + rutile, and granulite-facies retrograde assemblage such as diopside. Diamond and garnet occurred as inclusion in diopside, which is representative retrograde mineral of granulite-facies in the northern Dabie complex. The zircons from the studied gneiss, analyzed by SHRIMP U-Pb, yielded peak metamorphic age of 218±3Ma and granulite-facies retrograde age of 199±10Ma. The evidences for UHP metamorphism were firstly discovered on the gneiss from the northern Dabie complex, indicating the banded gneiss of northern Dabie complex suffered Triassic UHP metamorphism as well as the enclosed eclogites.
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Most metamorphic rocks have mineral assemblages that can be shown to have crystallized at elevated pressures and temperatures. With certain assemblages, these conditions can be specified quite closely using thermodynamic, kinetic, and experimental data. The important question that we must now address is: to what point in the history of a metamorphic rock do these conditions relate? The protoliths of many metamorphic rocks have their origins on the surface of Earth as sediment or lava. With burial, increasing pressure and temperature lead to the development of metamorphic minerals, which undergo successive changes as pressure and temperature continue to change. Then, through erosion and exhumation, the rocks return to Earth's surface. The rocks consequently undergo changes in pressure and temperature, which are both functions of time. The question, then, is: what part of this pressure–temperature–time (P–T–t) path is recorded in the final metamorphic rock?
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