High-pressure polymetamorphic garnet growth in eclogites from the Mariánske Lázně Complex (Bohemian Massif)
Abstrakt
The Mariánské-Lázně Complex is a Cambro-Ordovician terrane with metabasites of oceanic-crust affinity and tectonically emplaced between the Saxothuringian Zone and the Teplá-Barrandian Unit (western part of the Bohemian Massif). It is formed by amphibolite (retrogressed after eclogite), serpentinized peridotite, coronitic metagabbro with paragneiss, and felsic orthogneiss.
Available geochronological data support Variscan ages for the eclogite-facies metamorphism and subsequent amphibolite-facies reequilibration. Garnet crystals with multi-stage growth features in eclogite were studied to analyse the relation of their chemical zoning with pressure-temperature (P-T) changes and/or reactions among coexisting phases during metamorphism.
The core garnet has highly contrasted compositions with high Mg and low Ca compared to the surrounding host-garnet crystal. Compositional zoning and crystallographic orientation of the garnet showed that the cores and rims were formed during two different metamorphic events.
In addition to conventional geothermobarometry, pseudosection and garnet isopleths were used to estimate PT conditions for both metamorphic events. The older core (garnet I) indicates high-pressure amphibolite facies and the younger rim (garnet II) indicates eclogite-facies conditions.
Garnet (II) contains inclusions of Na-Ca amphibole and omphacite, suggesting a prograde metamorphism from blueschist- to eclogite-facies conditions. Full multicomponent-diffusion modelling of compositional zoning at the interface of amphibolite-/eclogite-facies garnets in conjunction with the retrieved P-T paths were used to evaluate the average heating/cooling rates during the eclogite-facies event.
These two metamorphic events deciphered from different garnet generations are consistent with available geochronological data and bring new insight into the subduction history of the Saxothoringian and/or Rehic oceanic basins during the Variscan orogeny in the Bohemian Massif.