Tectonometamorphic evolution of the Atbashi high-P units (Kyrgyz CAOB, Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction-exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high-P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic melange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100km long) high-P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P-T conditions of 23-25kbar and 560-570 degrees C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319Ma at regional scale. These ages are interpreted as (re-) crystallization ages of phengite during T-max conditions at a pressure range of 20-25kbar. Thermobarometry on samples from the HP tectonic melange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P-T conditions of 5-6kbar and 290-340 degrees C, highlighting a 17-20kbar pressure gap between the HPMU-tectonic melange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340Ma), HP metamorphic peak (330Ma), the T-max (328-319Ma) and the final exhumation of the HPMU (303-295Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70-75km up to 60km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental-derived character from the North Tien Shan suggest this unit corresponds to the hyper-extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N-dipping subduction below the Kazakh margin of Middle Tien Shan until 390-340Ma and (b) S-dipping subduction of remaining Turkestan marginal basins between 340 and 320Ma.