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 (<6 kbar). Microtextures indicate garnet 2, staurolite, and sillimanite crystallisation in the extension-related foliation. Monazite in garnet 1 is Carboniferous ( ca. 350–340 Ma) while monazite related to garnet 2, staurolite and sillimanite is Permian ( ca. 290–285 Ma). Migmatite contains Devonian zircon ( ca. 375 Ma) and Permian monazite ( ca. 285 Ma). Anhedral garnet core in a garnet-kyanite-sillimanite migmatite documents an undated early subsolidus metamorphism. Resorbed kyanite, euhedral sillimanite, and garnet rim document a suprasolidus isothermal decompression (9 to 4–7 kbar at ~750 °C). Melt crystallisation indicates subsequent cooling under the solidus (<700 °C). Formerly shallow-dipping migmatitic foliation is related to the decompression. Monazite crystals associated with either kyanite or sillimanite systematically indicate a Permian age, related to the decompression. Consequently, lower crustal rocks decompression was coeval with magmatism, shearing, and heating higher in the crust. The shear zone accommodated the exhumation of a migmatite dome and focused an intense magmatism, inducing the isobaric heating and related Barrovian metamorphism in the hangingwall micaschists. As such, the envelope of extensional domes appears a favourable site for the development of the Barrovian metamorphic sequence.
Marien et al. (Mon,) studied this question.