1,721,027 research outputs found
P-T evolution of Punta Orvili metabasite, NE Sardinia, Italy: some constraints from pseudosection modelling
No full textRe-equilibration history of retrogressed eclogites hosted in the medium- grade metamorphic Complex from north-central Sardinia
No full textFormation of clinopyroxene + spinel and amphibole + spinel symplectites in coronitic gabbros from the Sierra de San Luis (Argentina): a key to post-magmatic evolution
No full textSEM/TEM, EPMA and μ-RAMAN Spectroscopy applied to the characterization of Al-and Ni- Rich Chrysotile
No full textMetamorphic evolution of non-equilibrated granulitized eclogite from Punta de li Tulchi (Variscan Sardinia) determined through texturally controlled thermodynamic modelling
No full textThe metamorphic evolution of a granulitized eclogite from Punta de li Tulchi NE Sardinia, Italy,
reconstructed utilizing a combined microstructural (symplectitic, coronitic and kelyphytic features) and
thermodynamic approach, involved a complex metamorphic history with equilibrium attained only at a
domainal scale. Microstructural analysis and mineral zoning allow recognition of reactants and products
involved in successive balanced mineral reactions. The P–T conditions at which each microstructure was
formed are further constrained by calculating isochemical phase diagrams (pseudosections) for the
composition of effectively reacting domains. A pre-symplectite stage developed during prograde
metamorphism under conditions ranging from 660-680°C, 1.6-1.8 GPa to 660-700°C at 1.7-2.1 GPa.
Pseudosections calculated for subsequent clinopyroxene + plagioclase and orthopyroxene +
plagioclase symplectitic coronae using the composition of effectively reacting microdomains temperature in excess of 800°C and pressures of 1.0-1.3 GPa. Modelling the development of later plagioclase + amphibole coronae around garnet during decompression yields conditions of 730-830°C and 0.8-1.1 GPa. H2O (wt%) isomodes indicate that the granulitized eclogites were H2O-undersaturated
at peak-P conditions and during most of the subsequent heating and decompression. This allowed the
preservation of prograde garnet zoning in spite of the strong granulite facies overprint. The P–T
evolution of Punta de li Tulchi granulitized eclogite is very similar in shape to that registered by other
NE Sardinia retrogressed eclogites thus suggesting a common tectonic scenario for their evolution
Micro-Raman spectroscopy for a quick and reliable identification of serpentine minerals from ultramafics
No full textP-T evolution of the metabasite with eclogite facies relics from Punta de Li Tulchi (NE Sardinia, Italy) as revealed by the petrological approach of pseudosections
No full textClockwise exhumation path of granulitized eclogites from the Ama Drime range (Eastern Himalayas)
No full textThe metamorphic evolution of a granulitized eclogite from the Phung Chu Valley (Eastern Himalaya) was reconstructed combining microstructural observations, conventional thermobarometry and quantitative pseudosection analysis. The granulitized eclogite consists of clinopyroxene, plagioclase, garnet, brown amphibole, and minor orthopyroxene, biotite, ilmenite and quartz. On the basis of microstructural observations and mineral relationships, four metamorphic stages and related mineral assemblages have been recognized: (i) M1 eclogite-facies assemblage, consisting of garnet, omphacite (now replaced by a clinopyroxene + plagioclase symplectite) and phengite (replaced by biotite +plagioclase symplectite); (ii) M2 granulite-facies assemblage, represented by clinopyroxene, orthopyroxene, garnet, plagioclase and accessory ilmenite; (iii) M3 plagioclase + orthopyroxene corona developed around garnet, and (iv) M4 brown amphibole + plagioclase assemblage in the rock matrix. Because of the nearly complete lack of eclogitic mineral relics, M1 conditions can be only loosely constrained at > 1.5 GPa and > 580 degrees C. In contrast, assemblage M2 tightly constrains the peak granulitic stage at 0.8-1.0 GPa and > 750 degrees C. The second granulitic assemblage M3, represented by the plagioclase + orthopyroxene corona, formed at lower pressures (similar to 0.4 GPa and similar to 750 degrees C). During the subsequent exhumation, the granulitized eclogite experienced significant cooling to nearly 700 degrees C, marked by the appearance of brown amphibole and plagioclase (M4) in the rock matrix. U-Pb SHRIMP analyses on low-U rims of zircon from an eclogite of the same locality suggest an age of 13-14 Ma for the M3 stage. The resulting decompressional clockwise P-T path of the Ama Drime eclogite is characterized by nearly isothermal decompression from > 1.5 GPa to similar to 0.4 GPa, followed by nearly isobaric cooling from similar to 775 degrees C to similar to 710 degrees C. Modelling of phase equilibria by a calculated petrogenetic grid and conventional thermobarometry on a biotite-garnet-sillimanite metapelite hosted in the country rock granitic orthogneiss extends the inferred P-T trajectory down to similar to 630 degrees C and similar to 0.3 GPa
P-T evolution of eclogite-facies metabasite from NE Sardinia, Italy: insights into the prograde evolution of Variscan eclogites
Full text linkA petrological study of the Punta Orvili metabasite in NE Sardinia has been carried out, integrating
quantitative pseudosection modelling with reaction balancing, with the aim of reconstructing the
metamorphic evolution and P–T path.
The Punta Orvili metabasite preservesmicrostructural evidence of: (i) a pre-symplectite polyphase and prograde
stage (M1) indicated by the occurrence of amphibole inclusions in garnet, by Na-rich diopside and by
compositional zoning in garnet, clinopyroxene and amphibole; (ii) a symplectite stage (M2) represented by the
occurrence of Cpx2+Pl1 symplectite; (iii) a corona stage (M3) documented by the formation ofmicrometre-thick
Pl2±Am3±Ilm coronas around garnet; and (iv) a late stage (M4), documented by the growth of epidote and
albite and by the replacement of biotite and clinopyroxene by chlorite. The M1 pre-symplectite stage has been
modelled by P–T pseudosections calculated in the NCKFMASTH model system at a(H2O)=1 (for a(H2O)=0.5
temperature values are ~50 °C lower). Mg and Ca zoning in garnet and Na zoning in clinopyroxene testify to a
progressive increase in temperature and pressure during garnet and clinopyroxene growth from610bTb630 °C,
1.7bP b1.8 GPa up to 620bTb650 °C, 1.9bPb2.1 GPa, allowing to reconstruct a prograde segment of the P–T path.
Peak pressure conditionswere reachedinthe amphibole–eclogite-facies field.Destabilization of clinopyroxene led
to the formation of Cpx2+Pl1 symplectite (M2 stage) at P–T conditions of 760bTb800 °C, 0.9bPb1.0 GPa.
The corona stage (M3) was modelled in the NCFMASTH model system using the bulk composition of the
effectively reacting microdomain, calculated from mineral compositions and stoichiometric coefficients of the
corona-forming reaction. T=610–670 °C and P~0.7 GPa have been determined for this stage. Presumed P–T
conditions of the latest re-equilibration stage (M4 stage) are around 300–400 °C and 0.2–0.3 GPa.
The prograde evolution of the Punta Orvili metabasite took place under a geothermal gradient of ~10 °C/km,
compatible with a relatively hot subduction of a small, youngmarginal basin. Subsequent increase of the geothermal
gradient up to 20–30 °C/km suggests that subduction was followed by the Variscan continental collision.
The P–T path of the Punta Orvili metabasite has significant analogies with that of the retrogressed eclogite of Golfo
Aranci, NE Sardinia, and with other eclogites from the Migmatite Complex of NE Sardinia
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