Carboniferous Barrovian to Permian Buchan-type metamorphic cycles in the Mongolian Altai Zone: Implication for pressure cycles in accretionary orogens

Abstract

In the Altai Accretionary Wedge, several periods of Barrovian- and Buchan-type metamorphic cycles were dated from Ordovician to Permian. However, the timing and link between these cycles are not clear, and their causes are debated. In order to contribute to the understanding of Barrovian- to Buchan-type evolution of the accretionary wedges, we studied an area composed of three parallel belts in the easternmost extremity of the Hovd domain located in Mongolian Altai Zone: garnet gneiss in the north, garnet–staurolite–kyanite schist overprinted by ±sillimanite±cordierite±andalusite-bearing assemblages in the centre and garnet–sillimanite gneiss in the south. Petrography, garnet zoning and thermodynamic modelling indicate that the garnet gneiss from the northern belt records burial from ~510°C and ~3–4 kbar to ~600°C and ~5 kbar, followed by heating to ~660°C and decompression to ~4.5 kbar. The garnet–staurolite–kyanite schist from the central belt records burial from ~550°C and ~3–4.5 kbar to ~640–680°C and ~7 kbar, followed by decompression to the sillimanite stability field at ~650°C and ~6 kbar. Crystallization of cordierite, andalusite, late muscovite and chlorite in some samples indicates cooling on decompression to ~540°C and ~3.5 kbar. In the southern gneiss belt, the garnet–sillimanite gneiss with almost unzoned garnet suggests re-equilibration at ~6 kbar and ~710°C. In situ U–Pb monazite and xenotime dating carried out inclusions in porphyroblasts and matrix grains revealed Carboniferous and Permian ages. The monazite and xenotime from gneisses of the northern and southern belts record Carboniferous and Permian ages, which are interpreted as Carboniferous crystallization at c. 347 Ma associated with metamorphic peak, followed by Permian (re)crystallization at c. 300 and 283 Ma. In the central belt, rare Carboniferous xenotime grains in a garnet–staurolite–kyanite–andalusite–muscovite schist indicate a possible Carboniferous age of the prograde metamorphism. Predominant ages between c. 280 and 260 Ma recorded by monazite are interpreted as a result of complete recrystallization during an LP metamorphic overprint. The Carboniferous ages from the gneisses can be interpreted as constraining the timing of the exhumation of deep crustal rocks to shallow crustal levels. This event corresponds to the formation of crustal-scale migmatite-magmatite domes in the Mongolian Altai Zone. The prograde Barrovian assemblages in the central schist belt are interpreted as having formed contemporaneously during burial in a synform between the migmatite-magmatite domes. The Permian ages reflect LP–HT metamorphism, best recorded by the Buchan-type assemblages in the central schist belt, and are related to massive heat flux from tectonically mobile deep partially molten crust. Correlation with similar Barrovian- and Buchan-type episodes from the Chinese Altai Zone indicates multiple compressional and extensional events in the upper plate accretionary wedge, probably related to retreating and advancing modes of the subduction zone.