Prograde metamorphic history of UHP granulites from the Moldanubian zone (Bohemian Massif) revealed by major element and Y + REE zoning in garnets
Abstrakt
Major and trace element distribution in garnet crystals from felsic granulites of the Kutna Hora Complex provides evidence of two metamorphic events, the first related to continental subduction at ultrahigh-pressure (UHP) conditions and the second to a granulite-facies overprint that occurred at mid- to lower crustal levels. To reconstruct P-T paths of both metamorphic events, pseudosection modeling was combined with major and trace element zoning in garnet.
The granulites contain lenses and boudins of mantle-derived garnet peridotite and eclogite. UHP conditions are confirmed by the presence of inclusions of micro-diamond and coesite in garnet and zircon in the granulites.
Compositional zoning in garnet provides evidence for a pre-granulite-facies high-pressure (HP)-UHP metamorphic history of the rocks. The two distinct metamorphic events are documented by high Y + HREE concentrations in the garnet cores and annuli in the mantle part of the garnet grains.
The cores of large garnets with relatively high Ca contents and bell-shaped Mn profiles suggest formation during a prograde low- to medium-temperature metamorphic event that was coeval with HP-UHP metamorphism. Decompression and cooling during exhumation of the rocks led to partial resorption of garnet and release of trace elements into the matrix.
The new garnet with high Y + HREE in the annuli was formed during the granulite-facies event at crustal levels.