scholarly journals Deeply subducted continental fragments: I. Fracturing, dissolution-precipitation and diffusion processes recorded by garnet textures of the central Sesia Zone (Western Italian Alps)

2017 ◽  
Author(s):  
Francesco Giuntoli ◽  
Pierre Lanari ◽  
Martin Engi
Keyword(s):  
High Pressure ◽  
Diffusion Processes ◽  
Outer Part ◽  
Fluid Phase ◽  
Granulite Facies ◽  
Thermodynamic Modelling ◽  
Italian Alps ◽  
Facies Metamorphism ◽  
Eclogite Facies ◽  
And Diffusion

Abstract. Complex zoning in garnet from micaschists of the Sesia Zone (Western Italian Alps) preserves evidence of two orogenic cycles and provides detailed insights into resorption, growth and diffusion processes induced by fluid pulses at high pressure. Data on local textures and mineral chemistry are combined with data derived from thermodynamic modelling to understand and quantify these processes. Garnet shows low-Ca porphyroclastic cores that are stable at (Permian) granulite facies conditions. In one sample, the first garnet rim that surrounds the pre-Alpine granulite facies core indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. The cores show lobate edges and preserve inner fractures, which are sealed by high pressure Alpine garnet. This observation suggests that during the first stages of subduction, before hydration of these high temperature rocks, brittle failure of the garnet occurred, implying high strain rates. Several Alpine rims show different textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnets are produced by replacement reactions; (c) spatially limited resorption and enhanced transport of elements due the fluid phase is evident along brittle fractures and their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed at eclogite facies conditions. Structurally controlled samples allow these fluid-garnet interaction phenomena to be traced across a portion of the Sesia Zone, with decreasing in fluid-garnet interaction toward external areas (NW). Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100%. However, no clear relationship is visible between deformation structures and fluids that triggered eclogite facies metamorphism; suggesting disperse fluid flow.

scholarly journals Deeply subducted continental fragments – Part 1: Fracturing, dissolution–precipitation, and diffusion processes recorded by garnet textures of the central Sesia Zone (western Italian Alps)

Solid Earth ◽  
2018 ◽  
Vol 9 (1) ◽  
pp. 167-189 ◽  
Author(s):  
Francesco Giuntoli ◽  
Pierre Lanari ◽  
Martin Engi
Keyword(s):  
High Pressure ◽  
Diffusion Processes ◽  
Shear Zones ◽  
Granulite Facies ◽  
Thermodynamic Modelling ◽  
Italian Alps ◽  
Eclogite Facies ◽  
Compositional Patterns ◽  
And Diffusion ◽  
Insight Into

Abstract. Contiguous continental high-pressure terranes in orogens offer insight into deep recycling and transformation processes that occur in subduction zones. These remain poorly understood, and currently debated ideas need testing. The approach we chose is to investigate, in detail, the record in suitable rock samples that preserve textures and robust mineral assemblages that withstood overprinting during exhumation. We document complex garnet zoning in eclogitic mica schists from the Sesia Zone (western Italian Alps). These retain evidence of two orogenic cycles and provide detailed insight into resorption, growth, and diffusion processes induced by fluid pulses in high-pressure conditions. We analysed local textures and garnet compositional patterns, which turned out remarkably complex. By combining these with thermodynamic modelling, we could unravel and quantify repeated fluid–rock interaction processes. Garnet shows low-Ca porphyroclastic cores that were stable under (Permian) granulite facies conditions. The series of rims that surround these cores provide insight into the subsequent evolution: the first garnet rim that surrounds the pre-Alpine granulite facies core in one sample indicates that pre-Alpine amphibolite facies metamorphism followed the granulite facies event. In all samples documented, cores show lobate edges and preserve inner fractures, which are sealed by high-Ca garnet that reflects high-pressure Alpine conditions. These observations suggest that during early stages of subduction, before hydration of the granulites, brittle failure of garnet occurred, indicating high strain rates that may be due to seismic failure. Several Alpine rims show conspicuous textures indicative of interaction with hydrous fluid: (a) resorption-dominated textures produced lobate edges, at the expense of the outer part of the granulite core; (b) peninsulas and atoll garnet are the result of replacement reactions; and (c) spatially limited resorption and enhanced transport of elements due to the fluid phase are evident along brittle fractures and in their immediate proximity. Thermodynamic modelling shows that all of these Alpine rims formed under eclogite facies conditions. Structurally controlled samples allow these fluid–garnet interaction phenomena to be traced across a portion of the Sesia Zone, with a general decrease in fluid–garnet interaction observed towards the external, structurally lower parts of the terrane. Replacement of the Permian HT assemblages by hydrate-rich Alpine assemblages can reach nearly 100 % of the rock volume. Since we found no clear relationship between discrete deformation structures (e.g. shear zones) observed in the field and the fluid pulses that triggered the transformation to eclogite facies assemblages, we conclude that disperse fluid flow was responsible for the hydration.

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).

scholarly journals Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

2021 ◽  
Vol 2021 ◽  
pp. 1-11
Author(s):  
Xu Kong ◽  
Xueyuan Qi ◽  
Wentian Mi ◽  
Xiaoxin Dong
Keyword(s):  
Regional Metamorphism ◽  
Granulite Facies ◽  
Amphibolite Facies ◽  
Metamorphic Event ◽  
Isotopic Data ◽  
Metamorphic Condition ◽  
Eu Anomaly ◽  
Facies Metamorphism ◽  
Eclogite Facies ◽  
Plagioclase Gneiss

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Asymmetric zoning profiles in garnet from HP-HT granulite and implications for volume and grain-boundary diffusion

1999 ◽  
Vol 63 (2) ◽  
pp. 227-238 ◽  
Author(s):  
P. J. O’Brien
Keyword(s):  
High Pressure ◽  
High Temperature ◽  
Grain Boundary ◽  
Boundary Diffusion ◽  
Sharp Decrease ◽  
Fluid Phase ◽  
Granulite Facies ◽  
Grain Boundary Diffusion ◽  
Diffusion Models ◽  
High Pressure Granulite

AbstractDetailed electron-microprobe line profiles and small-area compositional maps of zoned garnets in a sample of high-pressure-high-temperature granulite show features inconsistent with commonly applied diffusion models. Larger grains of an early garnet generation have their highest Ca contents in domains away from the rim or inclusions but show a sharp fall in Ca balanced by increased Mg and Fe (and slightly higher XMg) towards inclusions and the rim. In domains with secondary biotite, the sharp decrease in Ca is accompanied by variations in XMg dependent upon proximity to biotite, thus producing one-sided, asymmetric profiles with XMg lower against biotite. As a consequence, rim compositions of the same grain are different on the sides adjacent and away from biotite and there is no relationship between grain size and rim XMg. Such a zoning pattern requires that grain-boundary diffusion is as slow as volume diffusion and implies the absence of a diffusion-enhancing grain-boundary fluid phase during the majority of the rock's high-temperature exhumation history. Diffusion models ignoring this probability could yield either cooling rates that were too fast, or extrapolated ages based on closure temperature models that were too old.A second garnet generation in the same rock, grown in a Ca-rich domain resulting from kyanite breakdown, has irregularly distributed patches, identified by compositional mapping, containing higher Ca than the first-formed garnet but at lower XMg. Use of such garnet compositions for geothermobarometrical determination of the high-pressure granulite stage would clearly lead to erroneous results. The presence of such contrasting garnet compositions in a granulite-facies rock is clearly evidence of disequilibrium, and further supports the proposition that there was a lack of an effective transport medium even at the mm scale.

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