Petrological and Sedimentological Fingerprinting of Late Paleozoic Tianshan Accretionary Orogenesis from the Kangkelin Formation (Wushi, South Tianshan)

Abstract The South Tianshan Orogenic Belt (STOB) is located between the Central Tianshan Block and the Tarim Craton, and its tectonic evolution was closely associated with the Central Asian Orogenic Belt. However, the timing of the Paleozoic accretionary orogenesis in the STOB remains controversial, especially due to the absence of the comprehensive regional comparative investigations. The Paleozoic sedimentary and magmatic rocks within the STOB can provide a crucial window to evaluate the final closure processes of the South Tianshan Ocean. We carried out zircon U–Pb dating and whole-rock geochemical analysis of the Wushibei basaltic andesites and investigated the sedimentology and paleontology of the related Kangkelin Formation in the Wushi area of the STOB. Zircons from the basaltic andesites yield crystallization ages of 286 ± 2.5 Ma to 288.4 ± 2 Ma. The Wushibei basaltic andesites have continental magmatism-like geochemical affinities and are slightly enriched in light rare earth elements with high (La/Yb)N ratios (4.46, 5.45), which indicate the magma is production of the enriched mantle. The enrichment of incompatible elements (Cs, Rb, Th), along with negative Nb, Ta, and positive Pb anomalies, indicates that Wushibei magmatism resulted from decompression melting of upwelling asthenospheric mantle and substantial melting of the mantle wedge. The Kangkelin Formation is dominated by shallow-marine carbonate rocks deposited in the Wushi sag, which are intercalated with clastic rocks. The fossils Plicochonetes paeckelmanni, Sphaeroschwagerina moelleri Rauser, and Schwagerina sp. in the Kangkelin Formation constrain its age to the early Permian (i.e. no older than the Zisongian or Asselian stages). We speculate that the Wushibei basaltic andesites formed in a post-collisional magmatism tectonic setting during the early Permian. The Kangkelin Formation carbonates were deposited on tidal flats and a carbonate platform in the Wushi sag. Our results provide new insights into the tectonic evolution of the STOB.