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Olivier Bruguier - One of the best experts on this subject based on the ideXlab platform.

  • Geochemistry and geochronology of Orthogneisses across a major transcurrent shear zone (East Pernambuco shear zone, Borborema Province, Northeast Brazil): Tectonic implications
    Journal of South American Earth Sciences, 2019
    Co-Authors: Rafaela H.m. França, Sérgio Neves, João P.s. Bezerra, Olivier Bruguier
    Abstract:

    The East Pernambuco shear zone (EPSZ) is a dextral shear zone bounding the Rio Capibaribe and Pernambuco-Alagoas domains of the Neoproterozoic Borborema Province (NE Brazil). This study reports new geological data from Orthogneisses across the central segment of the EPSZ. Two samples from the Bezerros Orthogneiss to the north of the EPSZ yielded 207Pb/206Pb weighted mean ages of 2111 ± 7 Ma and 2104 ± 8 Ma. The c. 2.1 Ga age is taken as the age of crystallization of the protolith. It is undistinguishable from the age of the 2.13–2.09 Ga-old Vertentes Complex, which is the main Paleoproterozoic unit of the Rio Capibaribe Domain, and of Orthogneisses that occur immediately south of the EPSZ. Like the Vertentes Complex, the Bezerros Orthogneiss has a subduction-zone geochemical signature (e.g., pronounced negative Nb-Ta anomalies), suggesting intrusion in a magmatic arc setting. South of the EPSZ, a migmatized granitic Orthogneiss and the Encruzilhada de São João Orthogneiss yielded 206Pb/238U weighted mean ages of 645 ± 3 Ma and 639 ± 5 Ma, respectively. Ages of 615 ± 17 Ma and 618 ± 12 Ma obtained at the rims of zircon grains from Encruzilhada de São João and migmatized granitic Orthogneisses, respectively, are interpreted as dating the acquisition of the low-angle gneissic fabric. The age of the EPSZ is bracketed by the 591 ± 4 Ma age of a monazite grain from the Bezerros Orthogneiss and by the of 575 ± 17 Ma age of a young zircon grain in the Encruzilhada de São João Orthogneiss. Rather than being a terrane boundary, the EPSZ more likely represents an intracontinental shear zone that nucleated at preexisting crustal heterogeneities.

  • From extension to shortening: Dating the onset of the Brasiliano Orogeny in eastern Borborema Province (NE Brazil)
    Journal of South American Earth Sciences, 2015
    Co-Authors: Sérgio P. Neves, Olivier Bruguier, José Maurício Rangel Da Silva, Gorki Mariano, Adejardo F. Da Silva Filho, Cristiane M.l. Teixeira
    Abstract:

    In pre-drift reconstructions, the central and southern parts of the Borborema Province, northeastern Brazil, belong to a large Brasiliano-Pan-African orogenic realm situated to the north of the Sao Francisco-Congo Craton. In order to better understand the timing and geodynamic setting under which this orogenic system developed, a structural, geochemical and geochronological study was conducted across the east Pernambuco shear zone (EPSZ) system, which separates the Pernambuco-Alagoas Domain (PEAL) from the Central Domain. A sample of the Pinhoes Orthogneiss (GE-1), in the Central Domain, one sample of a syenitic Orthogneiss (CA-34) wrapped by the EPSZ, and one sample of Orthogneiss named Altinho (CA-40), in the northern portion of the PEAL, were dated by LA-ICP-MS. The Pinhoes Orthogneiss yielded an age of 869 ± 9 Ma, interpreted as the emplacement age of the protolith during a late Tonian magmatic episode. Samples CA-40 and CA-34 yielded 206Pb/238U weighted mean ages of 652 ± 6 Ma and 636 ± 3 Ma, respectively, which are interpreted as dating emplacement and crystallization of the magmatic protoliths. However, it is also possible that these rocks were formed during the same magmatic event in view of the identical ages of 646 ± 13 Ma and 646 ± 11 Ma, respectively, given by the less precise upper intercept of the discordia lines. The metaluminous and magnesian nature of the Altinho Orthogneiss is akin to the calc-alkalic suite. However, some samples plot in the intraplate field in tectonic discrimination diagrams and the Nd TDM model age of 1.36 Ga is unlike that of juvenile magmas in convergent settings. The Altinho Orthogneiss is quite similar in terms of trace elements geochemistry to the syenitic Orthogneiss, which has a clearer intraplate affinity, and the dated samples have identical initial Sr isotope ratios (0.7047). Therefore, emplacement in an extensional setting is preferred over a convergent one. Two samples of paragneisses (SB-1 and BB-9) from the PEAL were also dated. The ages of the youngest zircon grains in sample BB-9 (655–642 Ma) overlap the crystallization age of the Altinho Orthogneiss, implying that sedimentation is younger than or, at best, synchronous with magmatism. The age of low Th/U grains in samples CA-34 (615 ± 8 Ma) and SB-1 (587 ± 12 Ma) are related to a subsequent metamorphic overprint, which is loosely constrained between 580 and 620 Ma. These observations, combined with evidence provided by previous studies, suggest that the change from an extensional to a contracional setting occurred at ca. 640–630 Ma. In contrast with most collisional orogens, where a long period of oceanic subduction precedes collision, the inferred tectonic evolution suggests that the Brasiliano Orogeny resulted from inversion of continental and/or proto oceanic rifts.

  • U-Pb ages of plutonic and metaplutonic rocks in southern Borborema Province (NE Brazil): Timing of Brasiliano deformation and magmatism
    Journal of South American Earth Sciences, 2008
    Co-Authors: Sérgio P. Neves, Olivier Bruguier, José Maurício Rangel Da Silva, Delphine Bosch, Gorki Mariano
    Abstract:

    International audienceThe Borborema Province of northeastern Brazil is divided into three main domains: northern, central, and southern. Several U-Pb zircon ages of plutons and Orthogneisses became available in the recent years in the central and northern domains, but similar results are scarce in the southern domain. This study reports U-Pb dates for single zircon grains from one Orthogneiss (Jupi Orthogneiss) and two plutons (Cachoeirinha syenitic pluton and Cabanas granite) south of the East Pernambuco shear zone system (EPSZ). The results provide geochronological constraints on the timing of deformation and magmatism in this part of the southern domain and allow correlations with the central domain. The Jupi Orthogneiss was emplaced and deformed during development of the regional flat-lying foliation. A Pb-206/U-238 weighted apparent mean age of 606 +/- 8 Ma is interpreted as the crystallization age of the protolith of the Orthogneiss and consequently the age of high-grade Brasiliano metamorphism. The NNE-trending Cachoeirinha pluton is only locally affected by strike-slip deformation, whereas the ENE-trending Cabanas granite is intensely affected by deformation related to the EPSZ. The 587 8 Ma and 573 4 Ma ages of the Cachoeirinha pluton and Cabanas granite, respectively, bracket the main period of activity of the EPSZ. Tectonomagmatic activity in the study area is similar to the age of Brasiliano events in the central domain, north of the EPSZ. In addition, xenocrystic zircons in the Jupi Orthogneiss and Cabanas granite are interpreted as inherited from Paleoproterozoic source rocks, suggesting the presence of widespread reworked old crust in the southern domain, similar to the central domain. These results support the idea that the central and southern domains belonged to the same crustal block before the onset of the Brasiliano orogeny

  • U-Pb ages of plutonic and metaplutonic rocks in southern Borborema Province (NE Brazil): Timing of Brasiliano deformation and magmatism
    Journal of South American Earth Sciences, 2008
    Co-Authors: S. P. Neves, Olivier Bruguier, Delphine Bosch, J. M. R. Silva, G. Mariano
    Abstract:

    The Borborema Province of northeastern Brazil is divided into three main domains: northern, central, and southern. Several U-Pb zircon ages of plutons and Orthogneisses became available in the recent years in the central and northern domains, but similar results are scarce in the southern domain. This study reports U-Pb dates for single zircon grains from one Orthogneiss (Jupi Orthogneiss) and two plutons (Cachoeirinha syenitic pluton and Cabanas granite) south of the East Pernambuco shear zone system (EPSZ). The results provide geochronological constraints on the timing of deformation and magmatism in this part of the southern domain and allow correlations with the central domain. The Jupi Orthogneiss was emplaced and deformed during development of the regional flat-lying foliation. A Pb-206/U-238 weighted apparent mean age of 606 +/- 8 Ma is interpreted as the crystallization age of the protolith of the Orthogneiss and consequently the age of high-grade Brasiliano metamorphism. The NNE-trending Cachoeirinha pluton is only locally affected by strike-slip deformation, whereas the ENE-trending Cabanas granite is intensely affected by deformation related to the EPSZ. The 587 8 Ma and 573 4 Ma ages of the Cachoeirinha pluton and Cabanas granite, respectively, bracket the main period of activity of the EPSZ. Tectonomagmatic activity in the study area is similar to the age of Brasiliano events in the central domain, north of the EPSZ. In addition, xenocrystic zircons in the Jupi Orthogneiss and Cabanas granite are interpreted as inherited from Paleoproterozoic source rocks, suggesting the presence of widespread reworked old crust in the southern domain, similar to the central domain. These results support the idea that the central and southern domains belonged to the same crustal block before the onset of the Brasiliano orogeny.

  • Timing of crust formation, deposition of supracrustal sequences, and Transamazonian and Brasiliano metamorphism in the East Pernambuco belt (Borborema Province, NE Brazil): Implications for western Gondwana assembly
    Precambrian Research, 2006
    Co-Authors: Sérgio Neves, Olivier Bruguier, Delphine Bosch, Alain Vauchez, José Maurício Rangel Da Silva, Gorki Mariano
    Abstract:

    The main structural feature of the central domain of Borborema Province (NE Brazil) is a network of dextral and sinistral shear zones. These shear zones rework an older, regionally developed, flat-lying foliation in Orthogneisses and supracrustal belts, which in the East Pernambuco belt was formed under amphibolite facies conditions. This study reports LA-ICP-MS U–Pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre-transcurrent tectonothermal evolution in the Eastern Pernambuco domain. Ages of 2125 ± 7 and 2044 ± 5 Ma in a mafic layer of banded Orthogneiss are interpreted as the age of the protolith of the Orthogneiss and of high-grade Transamazonian metamorphism, respectively. The latter age is consistent with the occurrence of low Th/U, metamorphic zircon xenocrysts, dated at 2041 ± 15 Ma, in the leucosome of a migmatitic paragneiss. A granitic Orthogneiss dated at 1991 ± 5 Ma reflects late to post-Transamazonian magmatic event. A similar age (1972 ± 8 Ma) was found in rounded zircon grains from a leucocratic layer of banded Orthogneiss. Ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the Archean to Neoproterozoic, with peak ages at ca. 2220, 2060–1940, 1200–1150 and 870–760 Ma. Detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 Ma. Magmatic zircons with the age of 626 ± 15 Ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the Brasiliano high-temperature metamorphism. A lower intercept age of 619 ± 36 Ma from a deformed granodiorite dated at 2097 ± 5 Ma and the crystallization age of 625 ± 24 Ma of the felsic layer of banded Orthogneiss also confirm the late Neoproterozoic metamorphism. These results show that the present fabric in basement and supracrustal rocks was produced during the Brasiliano orogeny. Paleoproterozoic ages reported in this study are similar to those found in other sectors of the Borborema Province, the Cameroon and Nigeria provinces, and the São Francisco/Congo craton. They show the importance of the Transamazonian/Eburnean event and suggest that these tectonic units may have been part of a larger, single continental landmass. Likewise, similarities in post-Transamazonian metamorphic and magmatic events in the Borborema, Nigeria and Cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the Atlantic Ocean.

Sérgio Neves - One of the best experts on this subject based on the ideXlab platform.

  • Geochemistry and geochronology of Orthogneisses across a major transcurrent shear zone (East Pernambuco shear zone, Borborema Province, Northeast Brazil): Tectonic implications
    Journal of South American Earth Sciences, 2019
    Co-Authors: Rafaela H.m. França, Sérgio Neves, João P.s. Bezerra, Olivier Bruguier
    Abstract:

    The East Pernambuco shear zone (EPSZ) is a dextral shear zone bounding the Rio Capibaribe and Pernambuco-Alagoas domains of the Neoproterozoic Borborema Province (NE Brazil). This study reports new geological data from Orthogneisses across the central segment of the EPSZ. Two samples from the Bezerros Orthogneiss to the north of the EPSZ yielded 207Pb/206Pb weighted mean ages of 2111 ± 7 Ma and 2104 ± 8 Ma. The c. 2.1 Ga age is taken as the age of crystallization of the protolith. It is undistinguishable from the age of the 2.13–2.09 Ga-old Vertentes Complex, which is the main Paleoproterozoic unit of the Rio Capibaribe Domain, and of Orthogneisses that occur immediately south of the EPSZ. Like the Vertentes Complex, the Bezerros Orthogneiss has a subduction-zone geochemical signature (e.g., pronounced negative Nb-Ta anomalies), suggesting intrusion in a magmatic arc setting. South of the EPSZ, a migmatized granitic Orthogneiss and the Encruzilhada de São João Orthogneiss yielded 206Pb/238U weighted mean ages of 645 ± 3 Ma and 639 ± 5 Ma, respectively. Ages of 615 ± 17 Ma and 618 ± 12 Ma obtained at the rims of zircon grains from Encruzilhada de São João and migmatized granitic Orthogneisses, respectively, are interpreted as dating the acquisition of the low-angle gneissic fabric. The age of the EPSZ is bracketed by the 591 ± 4 Ma age of a monazite grain from the Bezerros Orthogneiss and by the of 575 ± 17 Ma age of a young zircon grain in the Encruzilhada de São João Orthogneiss. Rather than being a terrane boundary, the EPSZ more likely represents an intracontinental shear zone that nucleated at preexisting crustal heterogeneities.

  • Timing of crust formation, deposition of supracrustal sequences, and Transamazonian and Brasiliano metamorphism in the East Pernambuco belt (Borborema Province, NE Brazil): Implications for western Gondwana assembly
    Precambrian Research, 2006
    Co-Authors: Sérgio Neves, Olivier Bruguier, Delphine Bosch, Alain Vauchez, José Maurício Rangel Da Silva, Gorki Mariano
    Abstract:

    The main structural feature of the central domain of Borborema Province (NE Brazil) is a network of dextral and sinistral shear zones. These shear zones rework an older, regionally developed, flat-lying foliation in Orthogneisses and supracrustal belts, which in the East Pernambuco belt was formed under amphibolite facies conditions. This study reports LA-ICP-MS U–Pb zircon ages of metaigneous and metasedimentary rocks aiming to constraint the pre-transcurrent tectonothermal evolution in the Eastern Pernambuco domain. Ages of 2125 ± 7 and 2044 ± 5 Ma in a mafic layer of banded Orthogneiss are interpreted as the age of the protolith of the Orthogneiss and of high-grade Transamazonian metamorphism, respectively. The latter age is consistent with the occurrence of low Th/U, metamorphic zircon xenocrysts, dated at 2041 ± 15 Ma, in the leucosome of a migmatitic paragneiss. A granitic Orthogneiss dated at 1991 ± 5 Ma reflects late to post-Transamazonian magmatic event. A similar age (1972 ± 8 Ma) was found in rounded zircon grains from a leucocratic layer of banded Orthogneiss. Ages of detrital zircons in a paragneiss sample indicate derivation from sources with ages varying from the Archean to Neoproterozoic, with peak ages at ca. 2220, 2060–1940, 1200–1150 and 870–760 Ma. Detrital zircons constrain the deposition of the supracrustal sequence to be younger than 665 Ma. Magmatic zircons with the age of 626 ± 15 Ma are found in the leucosome of a migmatitic paragneiss and constrain the age of the Brasiliano high-temperature metamorphism. A lower intercept age of 619 ± 36 Ma from a deformed granodiorite dated at 2097 ± 5 Ma and the crystallization age of 625 ± 24 Ma of the felsic layer of banded Orthogneiss also confirm the late Neoproterozoic metamorphism. These results show that the present fabric in basement and supracrustal rocks was produced during the Brasiliano orogeny. Paleoproterozoic ages reported in this study are similar to those found in other sectors of the Borborema Province, the Cameroon and Nigeria provinces, and the São Francisco/Congo craton. They show the importance of the Transamazonian/Eburnean event and suggest that these tectonic units may have been part of a larger, single continental landmass. Likewise, similarities in post-Transamazonian metamorphic and magmatic events in the Borborema, Nigeria and Cameroon provinces suggest that they shared a common evolution and remained in close proximity until the opening of the Atlantic Ocean.

B. Petri - One of the best experts on this subject based on the ideXlab platform.

  • Microstructural and metamorphic evolution of a high-pressure granitic Orthogneiss during continental subduction (Orlica-Snieznik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: F. Chopin, Pavel Pitra, Karel Schulmann, Jean-emmanuel Martelat, P. ŠtÍpskÁ, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from

  • microstructural and metamorphic evolution of a high pressure granitic Orthogneiss during continental subduction orlica śnieznik dome bohemian massif
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

  • Microstructural and metamorphic evolution of a high‐pressure granitic Orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

Karel Schulmann - One of the best experts on this subject based on the ideXlab platform.

  • Petrological evolution of a high-P migmatitic Orthogneiss in Orlica–Śnieżnik Dome (NE Bohemian Massif)
    2018
    Co-Authors: Carmen Aguilar, Francis Chopin, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, Pavel Pitra
    Abstract:

    428 Petrological evolution of a high-P migmatitic Orthogneiss in Orlica–Śnieżnik Dome (NE Bohemian Massif) Carmen Aguilar ∗1, Francis Chopin 2, Pavla štípská 1,2, Karel Schulmann 1,2, Jean-Emmanuel Martelat 3, Pavel Pitra 4 1 Centre for Lithospheric Research, Czech Geological Survey (cz) – République tchèque 2 IPG UMR 7516, Université de Strasbourg, ESPE – France 3 Université Claude Bernard et ENS Lyon, LGL–CNRS UMR5276 – France 4 Université Rennes 1, CNRS UMR 6118 – Université Rennes – France Petrological study and pseudosection modelling have been carried out in a high-grade Orthogneisses of the southern domain of the Orlica–Snieznik Dome (NE Bohemian Massif). The studied samples are from an outcrop dominated by vertical foliation with gradual transition from mylonitic augen Orthogneiss (Type I) to banded Orthogneiss (Type II) with elongated cm-scale quartz and feldspar monomineral layers, passing to schlieren Orthogneiss (Type III) with preserved felsic monomineral aggregates, and to nebulitic orthogniess (Type IV) with faint foliation marked by micas. The field and microstructural observations reveal a first subhorizontal foliation vertically folded and to various degrees reworked by a vertical foliation during E–W lateral shortening. The mineral assemblage of all types consists of biotite, phengite, garnet, quartz, K-feldspar and plagioclase, and accessory apatite, ilmenite, zircon and monazite. The transition from the Type I to IV is characterized by increasing nucleation of interstitial phases along like-like grain boundaries, by a decrease of grain size of all phases and by progressive disintegration of recrystallized K-feldspar grains by embayments of fine-grained myrmekite. In the mineral equilibria modelling, the core of garnet (alm0.58, py0.02-0.03, grs0.34, sps0.05) and phengite (Si = 3.38–3.20 p.f.u) is consistent with a P–T peak at 10–13 kbar and 720–750°C in the dominant the grt-bt-ph-rt-qtz-pl-kfs mineral assemblage. The garnet (alm0.68, py0.02-0.03, grs0.11, sps0.21) and white mica (Si = 3.10 p.f.u) rims together with unzoned biotite (XFe = 0.76–0.78) compositions match the modelled isopleths in the middle-P part of the grt-bt-ph-ilm-qtz-pl-kfs field to reach the solidus at 7–8 kbar and 630–650°C. In addition, the absence of prograde garnet zoning in the Type I to III suggests that the garnet was completely re-equilibrated during the retrograde history, whereas in the Type IV the HP garnet chemistry was preserved. This is discussed in frame of melt presence in different migmatite types along their P–T path. Based on the mineral equilibria modelling it is argued for fluid/melt-fluxed melting at HP conditions and on exhumation. The migmatite textural types are a result of grain-scale melt migration process and not of a localized melt transport in dykes as known from metasediments.

  • Microstructural and metamorphic evolution of a high-pressure granitic Orthogneiss during continental subduction (Orlica-Snieznik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: F. Chopin, Pavel Pitra, Karel Schulmann, Jean-emmanuel Martelat, P. ŠtÍpskÁ, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from

  • microstructural and metamorphic evolution of a high pressure granitic Orthogneiss during continental subduction orlica śnieznik dome bohemian massif
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

  • Microstructural and metamorphic evolution of a high‐pressure granitic Orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

  • origin of migmatites by deformation enhanced melt infiltration of Orthogneiss a new model based on quantitative microstructural analysis
    Journal of Metamorphic Geology, 2008
    Co-Authors: Pavlina Hasalova, Pavla Štípská, Karel Schulmann, O. Lexa, Frantisek Hrouda, Stanislav Ulrich, Jakub Haloda, Patricie Týcova
    Abstract:

    A detailed field study reveals a gradual transition from high-grade solid-state banded Orthogneiss via stromatic migmatite and schlieren migmatite to irregular, foliation-parallel bodies of nebulitic migmatite within the eastern part of the Gfohl Unit (Moldanubian domain, Bohemian Massif). The Orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well-equilibrated banded microstructure by crystallization of new interstitial phases (Kfs, Pl and Qtz) along feldspar boundaries and by resorption of relict feldspar and biotite. The grain size of all felsic phases decreases continuously, whereas the population density of new phases increases. The new phases preferentially nucleate along high-energy like–like boundaries causing the development of a regular distribution of individual phases. This evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases. To explain these data, a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded Orthogneiss during deformation. In this model, infiltrating melt passes pervasively along grain boundaries through the whole-rock volume and changes completely its macro- and microscopic appearance. It is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation. The melt topology mimicked by feldspar in banded Orthogneiss forms elongate pockets oriented at a high angle to the compositional banding, indicating that the melt distribution was controlled by the deformation of the solid framework. The microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms. The migmatite microstructures developed by granular flow accompanied by melt-enhanced diffusion and/or melt flow. However, an AMS study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains.

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  • Petrological evolution of a high-P migmatitic Orthogneiss in Orlica–Śnieżnik Dome (NE Bohemian Massif)
    2018
    Co-Authors: Carmen Aguilar, Francis Chopin, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, Pavel Pitra
    Abstract:

    428 Petrological evolution of a high-P migmatitic Orthogneiss in Orlica–Śnieżnik Dome (NE Bohemian Massif) Carmen Aguilar ∗1, Francis Chopin 2, Pavla štípská 1,2, Karel Schulmann 1,2, Jean-Emmanuel Martelat 3, Pavel Pitra 4 1 Centre for Lithospheric Research, Czech Geological Survey (cz) – République tchèque 2 IPG UMR 7516, Université de Strasbourg, ESPE – France 3 Université Claude Bernard et ENS Lyon, LGL–CNRS UMR5276 – France 4 Université Rennes 1, CNRS UMR 6118 – Université Rennes – France Petrological study and pseudosection modelling have been carried out in a high-grade Orthogneisses of the southern domain of the Orlica–Snieznik Dome (NE Bohemian Massif). The studied samples are from an outcrop dominated by vertical foliation with gradual transition from mylonitic augen Orthogneiss (Type I) to banded Orthogneiss (Type II) with elongated cm-scale quartz and feldspar monomineral layers, passing to schlieren Orthogneiss (Type III) with preserved felsic monomineral aggregates, and to nebulitic orthogniess (Type IV) with faint foliation marked by micas. The field and microstructural observations reveal a first subhorizontal foliation vertically folded and to various degrees reworked by a vertical foliation during E–W lateral shortening. The mineral assemblage of all types consists of biotite, phengite, garnet, quartz, K-feldspar and plagioclase, and accessory apatite, ilmenite, zircon and monazite. The transition from the Type I to IV is characterized by increasing nucleation of interstitial phases along like-like grain boundaries, by a decrease of grain size of all phases and by progressive disintegration of recrystallized K-feldspar grains by embayments of fine-grained myrmekite. In the mineral equilibria modelling, the core of garnet (alm0.58, py0.02-0.03, grs0.34, sps0.05) and phengite (Si = 3.38–3.20 p.f.u) is consistent with a P–T peak at 10–13 kbar and 720–750°C in the dominant the grt-bt-ph-rt-qtz-pl-kfs mineral assemblage. The garnet (alm0.68, py0.02-0.03, grs0.11, sps0.21) and white mica (Si = 3.10 p.f.u) rims together with unzoned biotite (XFe = 0.76–0.78) compositions match the modelled isopleths in the middle-P part of the grt-bt-ph-ilm-qtz-pl-kfs field to reach the solidus at 7–8 kbar and 630–650°C. In addition, the absence of prograde garnet zoning in the Type I to III suggests that the garnet was completely re-equilibrated during the retrograde history, whereas in the Type IV the HP garnet chemistry was preserved. This is discussed in frame of melt presence in different migmatite types along their P–T path. Based on the mineral equilibria modelling it is argued for fluid/melt-fluxed melting at HP conditions and on exhumation. The migmatite textural types are a result of grain-scale melt migration process and not of a localized melt transport in dykes as known from metasediments.

  • Microstructural and metamorphic evolution of a high-pressure granitic Orthogneiss during continental subduction (Orlica-Snieznik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: F. Chopin, Pavel Pitra, Karel Schulmann, Jean-emmanuel Martelat, P. ŠtÍpskÁ, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from

  • microstructural and metamorphic evolution of a high pressure granitic Orthogneiss during continental subduction orlica śnieznik dome bohemian massif
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

  • Microstructural and metamorphic evolution of a high‐pressure granitic Orthogneiss during continental subduction (Orlica–Śnieżnik dome, Bohemian Massif)
    Journal of Metamorphic Geology, 2012
    Co-Authors: Francis Chopin, Pavel Pitra, Pavla Štípská, Karel Schulmann, Jean-emmanuel Martelat, O. Lexa, B. Petri
    Abstract:

    A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic Orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic Orthogneisses develops three distinct microstructural types, as follows: type I augen Orthogneiss, type II banded Orthogneiss and type III mylonitic Orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I Orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II Orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III Orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III Orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II Orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III Orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III Orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

  • Early Permian extensional shearing of an Ordovician granite: The Saint-Eutrope “C/S-like” Orthogneiss (Montagne Noire, French Massif Central)
    Comptes Rendus Geoscience, 2012
    Co-Authors: Pavel Pitra, Marc Poujol, Jean Van Den Driessche, Jean-charles Poilvet, Jean-louis Paquette
    Abstract:

    International audienceDating the magmatic events in the Montagne Noire gneiss dome is a key point to arbitrate between the different interpretations of the Late Carboniferous-Early Permian tectonics in this southern part of the Variscan belt. The Saint-Eutrope Orthogneiss crops out along the northern flank of the dome. We show that the protolith of this Orthogneiss is an Ordovician granite dated at 455 ± 2 Ma (LA-ICP-MS U-Pb dating on zircon). This age is identical to that previously obtained on the augen Orthogneiss of the southern flank, strongly suggesting that both Orthogneiss occurrences have the same Ordovician protolith. The Saint-Eutrope Orthogneiss experienced intense shearing along the Espinouse extensional detachment at ca. 295 Ma (LA-ICP-MS U-Pb-Th on monazite), an age close to that determined previously on mica by the 39Ar-40Ar method and contemporaneous with the emplacement age of the syntectonic Montalet granite farther to the west. This normal sense shearing reworked previous fabrics related to Variscan thrusting that can be still observed in the augen Orthogneiss of the southern flank, and is responsible for the spectacular "C/S-like" pattern of the Saint-Eutrope Orthogneiss. This work also shows that care is needed when dealing with C/S-type structures, since they can develop not only in syntectonic intrusions, but also in Orthogneisses affected by an intense secondary deformation, at decreasing temperature