Barrovian and Buchan metamorphic series in the Chinese Altai: <i>P–T–t–</i> D evolution and tectonic implications

The relations between Barrovian- and Buchan-type metamorphic series in the Chinese Altai remain obscure, hence a representative region of central part was investigated to address this issue, using combined microstructural, petrological geochronological methods. In region, Barrovian-type garnet, staurolite, kyanite sillimanite zones are locally overprinted by andalusite- cordierite-bearing domains. Microstructural analysis shows that Barrovian staurolite grew synchronously with earliest regional foliation S1B. A sillimanite-bearing assemblage zone parallel S1B, assigned as S1M. originally subhorizontal S1B-M foliation, mineral isograds were folded F2 upright folds, leading their inclination juxtaposition upper crustal levels. Subsequent D3 deformation affected heterogeneously all previous structures producing vertical high-strain around low-strain vicinity Permian pegmatites associated metamorphism characterized syn-D3 growth andalusite cordierite. Phase equilibria modelling staurolite/kyanite-bearing assemblages suggests prograde P–T path an apparent thermal gradient ~23°C/km S1B fabric. partial re-equilibration occurred stability at ~670°C, corresponding ~34°C/km S1M Garnet subsequently exhumed without during D2 event. interpreted pressure–temperature (P–T) evolution on basis thermobarometry phase significant heating processes, 41°C/km or more. situ U–Pb dating monazite inclusions revealed predominantly 280–260 Ma ages minor older scattering 350 290 Ma, important recrystallization Monazite cordierite yielded only these minerals. Lu–Hf garnet–whole rock isochron garnet–cordierite–chlorite schist gave age ca. 262 overlapping time Combined available data, results suggest cycle reflects continued burial followed decompression, probably connected Devonian suprasubduction tectonic switching shortening extension events. contrast, documents heat input NE–SW shortening, related Early collision southerly Junggar arc system.