Comments to “high-pressure eclogite facies metamorphism and decompression melting recorded in Paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)”

2020 ◽  
Vol 99 ◽  
pp. 102495 ◽  
Author(s):  
Hans-Joachim Massonne
Keyword(s):  
High Pressure ◽  
Accretionary Wedge ◽  
Facies Metamorphism ◽  
Eclogite Facies

High-pressure eclogite facies metamorphism and decompression melting recorded in paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)

2019 ◽  
Vol 94 ◽  
pp. 102226 ◽  
Author(s):  
Alexandre de Oliveira Chaves ◽  
Luiz Emanoel Alexandre Goulart ◽  
Raphael Martins Coelho ◽  
Daniel Andrade Miranda ◽  
Ramon de Oliveira Aranda ◽  
...  
Keyword(s):  
High Pressure ◽  
Accretionary Wedge ◽  
Facies Metamorphism ◽  
Eclogite Facies

scholarly journals Eclogites and related high-pressure rocks from North-East Greenland

1994 ◽  
Vol 162 ◽  
pp. 77-90
Author(s):  
J.A Gilotti
Keyword(s):  
High Pressure ◽  
Northern Limit ◽  
Medium Temperature ◽  
East Greenland ◽  
High Pressure Metamorphism ◽  
North East ◽  
Gneiss Complex ◽  
Facies Metamorphism ◽  
Eclogite Facies

Eclogite, gamet clinopyroxenite, gamet websterite and websterite bodies were discovered within the Skærfjorden gneiss complex during recent mapping in North-East Greenland. These eclogitic pods extend from Danmarkshavn (c. 76° 40'N) to the northern limit of the area mapped (78°N), and attest to widespread high-pressure metamorphism. Eclogites with the assemblage omphacite + garnet ± quartz ± futile are common. The protoliths of some of the eclogites were xenoliths within the precursor batholiths to the gneisses. Field relations, regional correlations and preliminary geochronology indicate that the eclogite facies metamorphism is Caledonian. The eclogites formed at minimum pressures between 10-15 kilobars and temperatures between 600–900°C, and hence are the medium-temperature type typically formed in over-thickened crust during continent collision.

Mechanism of formation of atoll garnet during high-pressure metamorphism

2010 ◽  
Vol 74 (1) ◽  
pp. 111-126 ◽  
Author(s):  
S. W. Faryad ◽  
H. Klápová ◽  
L. Nosál
Keyword(s):  
High Pressure ◽  
Short Distance ◽  
Mechanism Of Formation ◽  
Medium Temperature ◽  
High Pressure Metamorphism ◽  
The Core ◽  
Fluid Infiltration ◽  
Garnet Core ◽  
Facies Metamorphism ◽  
Eclogite Facies

AbstractAtoll garnet has been found in metabasites and quartz- and mica-rich rocks that have experienced low- to medium-temperature, high-pressure eclogite facies metamorphism in the Krušné Hory (Erzgebirge). They occur in several localities but are restricted to thin, texturally distinct zones, even on a thin-section scale. The mechanism of atoll garnet formation is documented by a series of micrographs and compositional maps and profiles of atoll garnet in combination with textural relations to other phases in the rocks. The core of full garnet or its relics in the atoll garnet have larger Ca and Fe, but smaller Mg contents, compared with the thin rim (ring). In addition to quartz, Na-Ca amphibole and phengite, the atoll cores are filled by a new garnet that has a composition similar to the outer rim. Formation of the atoll garnet is interpreted as resulting from fluid infiltration and element exchange between the garnet core and matrix, a process facilitated by a temperature increase during eclogite facies metamorphism. In addition to fluid access, the primary textures, mainly grain size, were also effective for the atoll garnet formation. Small grain fractions with thin rims were easily infiltrated by fluid, which used the short distance for element exchange between core and matrix. The core garnet was gradually dissolved and replaced by new garnet having the same crystallographic orientation as the rim or relics in the core.

scholarly journals High pressure metamorphism in the peridotitic cumulate of the Marun-Keu complex, Polar Urals

2019 ◽  
Vol 27 (2) ◽  
pp. 138-160
Author(s):  
Y. Y. Liu ◽  
A. L. Perchuk ◽  
A. A. Ariskin
Keyword(s):  
High Pressure ◽  
Crystallization Process ◽  
Ultramafic Rocks ◽  
Magmatic Chamber ◽  
Polar Urals ◽  
Tectonic Model ◽  
Facies Metamorphism ◽  
Formation Conditions ◽  
Eclogite Facies ◽  
Cumulate Texture

The Marun-Keu Complex of high-pressure rocks comprises granitoids, gneisses, schists, gabbroids and peridotites, which are unevenly and variably metamorphosed to the eclogite facies. A representative sample of garnet–amphibole lherzolite from the Mount Slyudyanaya area shows a cumulate texture and well preserved magmatic mafic minerals (olivine and pyroxenes) but practically no preserved plagioclase. The eclogite-facies metamorphism produced corona textures of newly formed minerals: amphibole, garnet, orthopyroxene and spinel. The metamorphic parameters of the garnet–amphibole lherzolite were estimated by geothermobarometry and by modeling phase equilibria at Р ~ 2.1 GPa and T ~ 640–740°C and are well consistent with our earlier estimate of the formation conditions of eclogites in the area. Computer simulation of the crystallization process of the gabbroic melt with the COMAGMAT program package, using literature data on the composition of the least altered plagioclase peridotites and gabbroids from the Marun-Keu Complex, shows that the mafic and ultramafic rocks are genetically interrelated: they crystallized in a single magmatic chamber. According to the modeling, the origin of the cumulate texture in the lherzolite was controlled by the peritectic reaction Ol + melt → Opx at a pressure of 0.7–0.8 GPa and a temperature of 1255–1268°C. Differences between thermodynamic parameters in the eclogites and garnet peridotites are discussed within the framework of a tectonic model for subduction and subsequent exhumation of the Baltica paleocontinent.

scholarly journals Eclogites overprinted in the granulite facies from the Ďumbier Crystalline Complex (Low Tatra Mountains, Western Carpathians)

2009 ◽  
Vol 60 (3) ◽  
pp. 193-204 ◽  
Author(s):  
Marian Janák ◽  
Tomáš Mikuš ◽  
Pavel Pitoňák ◽  
Ján Spišiak
Keyword(s):  
High Pressure ◽  
Phase Equilibrium ◽  
Granulite Facies ◽  
Western Carpathians ◽  
Tatra Mountains ◽  
Crystalline Complex ◽  
High Pressure Granulite ◽  
Facies Metamorphism ◽  
Eclogite Facies ◽  
Thermal Overprint

Eclogites overprinted in the granulite facies from the Ďumbier Crystalline Complex (Low Tatra Mountains, Western Carpathians)Metabasites with evidence for breakdown of former eclogites and recrystallization under granulite facies conditions occur in the Ďumbier Crystalline Complex of the Low Tatra Mountains, Central Western Carpathains. Textural relationships, phase equilibrium modelling and thermobarometry have been used to determine theP-Tevolution of these rocks. Omphacite diagnostic for the eclogites facies stage is absent but its former presence is inferred from the symplectitic intergrowths of clinopyroxene + plagioclase. The re-equilibration in high-pressure granulite facies conditions is demonstrated by the assemblage garnet + clinopyroxene (< 10 % Jd) + plagioclase + quartz. The phase equilibrium modelling using THERIAK-DOMINO program and conventional geothermobarometry suggest theP-Tconditions of 750-760 °C and 1.1-1.5 GPa for the high-pressure granulite stage. Orthopyroxene formed in the clinopyroxene + plagioclase symplectites and kelyphites and coronas around garnet atP-Tconditions of ca. 0.7-1.0 GPa and 650-700 °C.P-Tevolution of granulitized eclogites is interpreted as the result of two metamorphic events; early Variscan eclogite facies metamorphism was followed by granulite facies thermal overprint in the Carboniferous time. The second metamorphic event was crucial for breakdown of eclogites, these are only seldom preserved in the pre-Alpine basement of the Western Carpathians.

Transitional granulite-eclogite facies metamorphism of basic supracrustal rocks in a shear zone complex in the Precambrian shield of south India

1994 ◽  
Vol 58 (390) ◽  
pp. 97-118 ◽  
Author(s):  
Biswajit Mukhopadhyay ◽  
Mihir K. Bose
Keyword(s):  
High Pressure ◽  
South India ◽  
Bulk Composition ◽  
Spatial Association ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Exchange Reactions ◽  
Intermediate Pressure ◽  
Facies Metamorphism ◽  
Eclogite Facies

AbstractThe phase petrology of two bodies of basic granulites occurring in association with supracrustal sequences within a dextral oblique-slip shear complex in the high-grade terrain of south India, have been studied. Some metabasites are characterized by garnet + clinopyroxene assemblages such as Grt + Cpx + PI ± Hbl ± Opx ± Qtz + Fe-Ti oxide(s) which are high-pressure granulites (garnet-clinopyroxene subfacies) transitional between the intermediate-pressure granulites (orthopyroxene-plagioclase subfacies) and eclogite. Mineralogical characteristics suggest that metamorphic conditions did not reach the eclogite facies but certainly approached that level. Close spatial association of the garnetiferous varieties with garnet-free assemblages (Opx + Hbl + Pl ± Cpx) and intimate juxtaposition of anhydrous and hydrous assemblages indicate a complex interplay of variable μH2O and bulk composition, especially αSiO2. Due to variable and low closure temperature of Fe-Mg exchange reactions between coexisting pyroxene and garnet, reliable estimation of peak temperatures during this near eclogite-facies metamorphism is problematic. A combination of a garnet-clinopyroxene barometer and various temperature estimates suggests P-T conditions of 875 ± 25°C and 8 ± 1 kbar for the Mettuppalaiyam samples and 900 ± 50°C and 14 ± 2 kbar for the Kanjamalai samples. Bulk rock chemical differences in terms of SiO2-content can account for the development of similar mineral assemblages along a near adiabatic metamorphic gradient between these two localities. These assemblages represent some of the highest grade granulite facies rocks in south India. The high-pressure granulites are not in situ samples of ancient lower continental crust, but low-pressure protoliths buried at depth. Textural features suggest that the high-pressure garnet-clinopyroxene subfacies rocks evolved from an amphibolitic assemblage by formation of garnet at the expense of hornblende and plagioclase and this high-pressure assemblage underwent an isothermal decompression indicating a clockwise P-T-t path resulting from crustal thickening and exhumation. A later isobaric cooling, possibly accompanying shearing and high H2O influx (aH2O≈0.78) gave rise to the coexisting orthopyroxeneplagioclase subfacies rocks (intermediate pressure granulites).

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