Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan, NW China: implications for the tectonic evolution and Phanerozoic continental growth

International audienceSituated in the southwest of the Central AsianOrogenic Belt (CAOB), the South Tian Shan (STS) Blockis a key area for understanding the final accretion of theCAOB. A suite of volcanic rocks interbedded with continentalsediments from the Xiaotikanlike Formation liesalong the southwestern edge of the Tian Shan orogen.Laser-ablation-inductively coupled plasma-mass spectrometerU–Pb dating provided a crystallization age of295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an EarlyPermian magmatic event. The volcanic rocks show a variablecomposition, with dominant rhyolites and dacites,subordinate basaltic andesites and few basalts. The felsicrocks are enriched in K and exhibit remarkably negativeanomalies in Ba, Sr, Eu, P and Ti. These anomalies associatedwith their high negative !Nd(t) values and old Ndmodel ages suggest that they are most likely sourced fromancient lower crustal rocks. The mafic rocks are characterizedby high Mg#, Cr, Ni contents, negative Nb, Taanomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophileelements. The mafic rocks are thus inferred to derivefrom enriched subcontinental lithospheric mantle. Thepetrographic and geochemical characteristics of the XiaotikanlikeFormation volcanic rocks indicate that they weregenerated under a post-collisional regime. Therefore, thefinal collision between the Tarim Craton and the Kazakhstan–Yili terrane took place before Early Permian, mostprobably at Late Carboniferous. Differing from othertectonic units of the CAOB, the recycling of ancient lithosphericcrust played a significant role in the continentalgrowth of the STS Block