Barrovian and Buchan metamorphic series in the Chinese Altai: P−T−t−D evolution and tectonic implications

Abstract

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