Late Devonian closure of the North Qilian Ocean: evidence from detrital zircon U-Pb geochronology and Hf isotopes in the eastern North Qilian Orogenic Belt

The closure age of the North Qilian Ocean, which lay between the Alashan Terrane and Central Qilian Block during the early Palaeozoic, is intensely debated. This article presents a provenance study of detrital materials from three Upper Devonian sedimentary formations in the Pingchuan area of Gansu Province in the eastern North Qilian Orogenic Belt (NQOB), to constrain the tectonic evolution of the belt. U-Pb dating and Lu-Hf isotopic studies were conducted on detrital zircons from samples of sedimentary rocks. Four age populations of U-Pb zircon ages are defined as: 0.5-0.4Ga (peak at 472Ma); 0.8-1.3Ga (peaks at 960Ma and 1102Ma); 1.8-2.1Ga (two prominent peaks at 1963Ma and 2045Ma, and two subordinate peaks at 1816Ma and 2168Ma); and 2.4-2.5Ga (peak at 2495Ma). Zircons with U-Pb age spectra of 1.8-2.1Ga and their corresponding epsilon Hf(t) values are markedly different from those of the North China Block, Dunhuang Terrane, and Central Qilian Block, but have a strong similarity to zircons from the Alashan Terrane. Furthermore, zircons with ages ranging from 1.8 to 2.1Ga are reported for the first time in early Palaeozoic strata of the eastern NQOB. The presence of Alashan-derived clasts in the Upper Devonian strata in the NQOB indicates that the sediments were deposited after amalgamation between the Alashan and Qilian terranes. Combined with previous geochronological and geochemical data from the early Palaeozoic igneous and sedimentary rocks, we propose that the closure of the North Qilian Ocean occurred in the Late Devonian. The similar age spectra of detrital zircons from sediments of the Alashan, Cathaysia, and Australia imply that the Alashan-Qilian-Qaidam block was probably located in the periphery of Gondwana in the early Palaeozoic.