Structural evolution of zonal metamorphic sequences in the southern Chinese Altai and relationships to Permian transpressional tectonics in the Central Asian Orogenic Belt

Abstract

The Chinese Altai in the Central Asian Orogenic Belt is characterized by the occurrence of kyanite- and andalusite-type metamorphic zones. However, the tectonic setting associated with these metamorphic zones is still debated. Here we present structural observations from the southern Chinese Altai (Aletai area), where a relict kyanite zone occurs within an andalusite-type metamorphic zonal sequence of sillimanite, andalusite-sillimanite, andalusite-staurolite, staurolite-garnet and biotite. We document two major phases of folding in this area. The earlier structures (D1) developed synchronously with kyanite growth and are represented by an axial planar fabric of tight folds. D1 fabric is mostly obliterated by later deformation, but is inferred to be penetrative as indicated by a profound intersection lineation (L21). The second generation of deformation (D2) is associated with superimposed F2 folds and a dominant axial planar foliation (S2) with ~NW–SE orientation. Microstructural observations show that D2 structures are associated with the andalusite-type metamorphic series, which is characterized by an early-D2 occurrence of sillimanite, staurolite-garnet and biotite zonal sequence overprinted by the late-D2 growth of andalusite. Combining with previous metamorphic data, we consider that these two folding events (D1 and D2) correspond with the development of kyanite- and andalusite-type metamorphic zones, respectively, which records the burial and exhumation history of metamorphic rocks possibly in response to Devonian-Carboniferous accretion/collision and Permian transpressional tectonics. The latter was associated with oblique convergence between the Chinese Altai and the intra-oceanic arc system of the East/West Junggar.