Petrological evolution of a high-P migmatitic orthogneiss in Orlica–Śnieżnik Dome (NE Bohemian Massif)
Résumé
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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.