Multi-phase tectonothermal evolution in the SE Chinese Altai, central Asia: Structures, U-Pb monazite ages and tectonic implications

The tectonothermal evolution of the interior of the vast Central Asian Orogenic Belt (CAOB) is poorly understood. In this study, detailed structural observation, combined with petro-structural analysis and U-Pb monazite geochronological investigation were conducted in the southeastern Chinese Altai, aiming to decipher the Paleozoic tectonic evolution of the heartland of the CAOB. The study area is mainly composed of an Ordovician metasedimentary sequence of the Altai accretionary wedge in the north and a Devono-Carboniferous volcanoclastic sequence of the Junggar oceanic arc system in the south. An early tectonothermal event affected only the Ordovician sequence exemplified by the development of a sub-horizontal metamorphic S1 fabric, in part associated with partial melting. This fabric is characterized by the occurrence of low-­pressure/high­-temperature sillimanite-­ and/or andalusite-­bearing metamorphic assemblages in structurally lower and higher levels, respectively, interpreted as recording an extension phase of deformation. The presence of a Barrovian-type mineral residuum (e.g., staurolite) in the migmatitic S1 foliation probably reflects an earlier crustal burial and hence thickening phase. The S1 fabric was reworked by roughly E-­W-directed D2 shortening, forming a nearly N-S-oriented upright to inclined folds and linear system in the Ordovician sequence. A further NE-SW-directed D3 shortening formed heterogeneous NW-SE-trending F3 fold zones, bounded by mylonitic zones, and steep penetrative schistosity S3 in both Ordovician and Devono-Carboniferous sequences in the study area. U-Pb monazite geochronological investigations suggest that D1 extension developed around 394 Ma or earlier, D2 shortening took place soon after the extension, probably at 380–370 Ma, and D3 nearly orthogonal shortening activated during Early Permian (288–274 Ma). Combined with existing regional data, our results favor a tectonic evolution of multiple short-lived shortening-extension cycles, as a proxy of alternating retreats and advances of the subduction system, governing the Devonian tectonothermal evolution in the Chinese Altai. In contrast, the Permian tectonothermal history reflects a tectonic switching from a roughly E-W- to a N-S-orientated convergent regime in response to the terminal collision of the Chinese Altai with its southerly Junggar arc system in association with gradual closure of small residual oceanic basins in the heartland of the CAOB during the Permian.