Decompression time scales of mantle fragments constrained by secondary chemical zoning of garnet from the Gföhl Unit, Moldanubian Zone

Mafic–ultramafic lenses embedded in felsic granulites of the Gföhl Unit, Moldanubian Zone, are considered to be mantle fragments incorporated into mid-crustal levels of the Variscan orogenic crust. We investigated a several 100 m sized mafic lens mainly formed by garnet pyroxenite. The primary mineral assemblage comprises calcium-rich garnet (X_Grs = 0.4), kyanite, and sodium-rich clinopyroxene (X_{Na_M2} = 0.29) (± quartz), which indicates pressures above 1.8 GPa and temperatures around 1000 °C. Towards the margins of the mafic lens, the garnet pyroxenites were increasingly overprinted at lower pressures leading to the destabilization of kyanite, Na-rich clinopyroxene, and garnet. A first decompression phase is represented by garnet-hosted sapphirine–spinel–plagioclase symplectites supposedly replacing kyanite and clinopyroxene. A second stage is evident from the partial resorption of garnet by plagioclase and clinopyroxene in the form of a peculiar corrosion tubes penetrating the garnet in a worm-like fashion. Finally, the third stage decompression assemblage is represented by plagioclase–orthopyroxene–spinel symplectites partially replacing garnet. In all cases, garnet shows pronounced secondary compositional zoning towards the decompression products. The secondary zoning is qualitatively similar for the sapphirine–spinel–plagioclase symplectites and the plagioclase–clinopyroxene corrosion tubes and is characterized by a strong decrease of the Grs content accompanied by an increase of the Alm and Prp contents towards the decompression products. For the sapphirine–spinel–plagioclase symplectite, the garnet composition changes from Alm₁₄Prp₄₂Grs₄₄ in the pristine garnet to Alm₂₂Prp₆₃Grs₁₅ at the interface to the symplectite. The compositional change towards the corrosion tubes is from Alm₁₉Prp₄₀Grs₄₁ to Alm₃₀Prp₅₄Grs₁₆. The secondary zoning towards the plagioclase–orthopyroxene–spinel symplectites is characterized by an increase of X_Alm from 0.19 to 0.27 and a concomitant decrease of X_Prp from 0.55 to 0.49 at constant X_Grs of 0.25. In all cases, the compositional changes are gradual suggesting diffusion-mediated re-equilibration of the garnet at decreasing pressures. Time scales for the duration of decompression were estimated by fitting a multicomponent diffusion model to the observed compositional patterns. Depending on the choice of the diffusion coefficients, the time scales vary from several hundreds to hundred thousands of years, whereby the earliest decompression features yield time scales that are five times longer than those obtained from the corrosion tubes and about ten times longer than those obtained from the plagioclase–orthopyroxene–spinel symplectites. These timescales reflect the duration from the onset of the different decompression-induced mineral reactions to the time when the rocks cooled below about 700 °C and the composition patterns of the garnet were effectively frozen. The longest timescales obtained from the early decompression reactions are on the order of 100,000 years and the shortest timescales obtained from the late-stage symplectites are on the order of 1,000 years. Considering the regional metamorphic setting of the Moldanubian Zone, such timescales are remarkably short and suggest rapid transport of the mafic–ultramafic lithologies from mantle depths to the mid-crustal level. Concomitant incorporation into a dominantly felsic environment led to immediate cooling.