Structural and geochronological evolution of an accretionary orogenic wedge during Altai orogeny: An example from the Kalasu area (Chinese Altai)

A detailed geological and structural mapping was carried out in the Kalasu area located in the southern Chinese Altai, a small segment of the giant Altai accretionary orogenic wedge, in order to understand its polycyclic evolution. The Kalasu area is predominated by felsic metavolcanic rocks in the northeastern domain and mainly by a metapelitic sequence with subordinate metavolcanic layers, intruded by gneissic granitoids cropping out over the whole area. Structural investigations reveal the presence of three deformation events with variable intensity and degree of preservation. D1 is characterized by a metamorphic S1 foliation moderately to steeply dipping to the NW or locally to the SE, preserved in the northeastern and southwestern domain. The D2 deformation is responsible mainly for the variability in dip of the originally sub-horizontal S1 fabric due to NNW-SSE F2 folding, which is not associated with penetrative metamorphic reworking. Such a structural template was subsequently reworked by orthogonal D3 deformation. This last deformation is characterized by NW-SE steeply plunging isoclinal to close F3 folds, steeply dipping S3 axial planes and S3 foliation, which is delineated by cordierite-K-feldspar leucosome in the central domain where the S1 foliation is entirely transposed. To establish the absolute age of magmatic and metamorphic events, several samples were selected for U-Pb isotope analysis on zircon using the LA-ICPMS technique