TY - JOUR
KW - Eclogite-facies metamorphism
KW - exhumation
KW - Lu-Hf and Sm-Nd garnet geochronology
KW - Phase-equilibria modelling
KW - Variscan orogeny
AU - Stephen Collett
AU - Pavla Štípská
AU - Karel Schulmann
AU - Vit Peřestý
AU - Jérémie Soldner
AU - Robert Anczkiewicz
AU - Ondrej Lexa
AU - Andrew Kylander-Clark
AB - Abstract Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P–T–t–d evolution of eclogites from the Mariánské Lázně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P–T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33–36) and garnet core (Alm33–38Prp38–42Grs22–25Sps1) compositions are  25 kbar at 650–750 °C. A \HT\ overprint occurred at  14–18 kbar and >800 °C based on coexisting clinopyroxene (Jd18–24), plagioclase (An18–35), and amphibole (Na(B) <0.20; Al(C) = 0.60–1.15) in symplectite after original omphacite and phase-equilibria modelling of garnet mantle compositions. Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of \HP\ metamorphism, while c. 15 Ma younger ages were obtained by the Sm-Nd method. As temperatures for the \HT\ overprint exceed the empirically and experimentally determined closure temperature of the Sm-Nd system the Sm-Nd ages are interpreted to date cooling following the \HT\ overprint. Combined together, contrasting eclogite and amphibolite-facies migmatite fabrics, the mineral textures, calculated P–T conditions, and distinct Lu-Hf and Sm-Nd ages, provide a complete P–T–t–D path characterised by rapid ( 15 Ma) transition from \HP\ subduction, crustal thickening to extensional \HT\ shearing. This unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the Münchberg and ŻEV\ Devonian \HP\ units to the west. In contrast, the post-peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the Iberian subduction system both in time scales and sequence of tectonic events.
BT - Lithos
DO - https://doi.org/10.1016/j.lithos.2018.02.007
N2 - Abstract Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P–T–t–d evolution of eclogites from the Mariánské Lázně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P–T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33–36) and garnet core (Alm33–38Prp38–42Grs22–25Sps1) compositions are  25 kbar at 650–750 °C. A \HT\ overprint occurred at  14–18 kbar and >800 °C based on coexisting clinopyroxene (Jd18–24), plagioclase (An18–35), and amphibole (Na(B) <0.20; Al(C) = 0.60–1.15) in symplectite after original omphacite and phase-equilibria modelling of garnet mantle compositions. Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of \HP\ metamorphism, while c. 15 Ma younger ages were obtained by the Sm-Nd method. As temperatures for the \HT\ overprint exceed the empirically and experimentally determined closure temperature of the Sm-Nd system the Sm-Nd ages are interpreted to date cooling following the \HT\ overprint. Combined together, contrasting eclogite and amphibolite-facies migmatite fabrics, the mineral textures, calculated P–T conditions, and distinct Lu-Hf and Sm-Nd ages, provide a complete P–T–t–D path characterised by rapid ( 15 Ma) transition from \HP\ subduction, crustal thickening to extensional \HT\ shearing. This unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the Münchberg and ŻEV\ Devonian \HP\ units to the west. In contrast, the post-peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the Iberian subduction system both in time scales and sequence of tectonic events.
PY - 2018
SP - 74
EP - 94
T2 - Lithos
TI - Combined Lu-Hf and Sm-Nd geochronology of the Mariánské Lázně Complex: New constraints on the timing of eclogite- and granulite-facies metamorphism
UR - https://www.sciencedirect.com/science/article/pii/S0024493718300495
VL - 304–307
SN - 0024-4937
ER -