Permian Syn-Extensional Barrovian Metamorphism in the Envelope of a Migmatite Dome of the Mongolian Altai – Petrostructural and Geochronological Arguments

The Paleozoic Altai orogenic belt results from a suprasubduction polycyclic metamorphism of a sedimentary accretionary wedge. In the Mongolian-Altai, a Barrovian-type metamorphic sequence marks the transition between exhumed lower- and upper-crustal domains. It is studied by field and petrological observations, phase equilibria modeling, U-Pb zircon LA-ICP-MS, and in-context monazite LASS geochronology. Prograde micaschist and retrogressed migmatite are juxtaposed by an extensional shear zone hosting variably deformed Permian dikes ( ca. 295–290 Ma, U-Pb zircon) and a sharp metamorphic gradient (< 5 km from biotite to sillimanite zone). In the hangingwall micaschist, garnet and staurolite size increase in the neck zone of boudinaged quartz veins suggests syntectonic growth during the activity of the shear zone. Sequential growth of garnet 1-chlorite (M1), garnet 2-staurolite or garnet 2-staurolite-kyanite (M2) and sillimanite (M3) indicate M1 P–T increase (525–550 °C, 3–4 kbar to 550–575 °C, 5–6 kbar); M2 isobaric heating (~7 kbar, from 550–575 °C to 625–680 °C) and M3 decompression (