The Alashan Terrane was not part of North China by the Late Devonian: Evidence from detrital zircon U–Pb geochronology and Hf isotopes

Abstract

The relationship between the Alashan Terrane and North China is a contentious issue given the discovery of allochthonous detrital zircons in Middle Ordovician flysch sandstones from the southwestern Ordos Margin and the large differences in palaeolatitudes between the North China and Tarim cratons. We have collected a suite of Middle to Late Devonian sedimentary rocks from the Niushoushan Mountains at the southeastern margin of the Alashan Terrane, adjacent to the western margin of the Ordos Basin of the North China Craton (NCC). U–Pb dating and Lu–Hf isotopic studies were carried out on detrital zircons from these rocks. The zircon U–Pb ages define five age populations: 0.4–0.7Ga (peak at 488Ma), 1.0–1.3Ga (peaks at 1001 and 1152Ma), 1.5–1.8Ga, 2.4–2.8Ga (prominent peak at 2506Ma and secondary peaks at 2668 and 2796Ma) and >3.0Ga (peak at 3332Ma). One detrital zircon yielded a Hadean age of 4022±17Ma. Zircons with U–Pb age spectra of 2.4–2.7 and >3.0Ga and their corresponding εHf(t) values are significantly different from those in the NCC, indicating that these detrital zircons are not from the NCC, which implies that the Alashan Terrane was not part of North China until the Middle to Late Devonian. U–Pb age spectra of zircons dated at 1.0–1.3Ga, 2.4–2.7Ga, and >3.0Ga, and their corresponding Hf isotope data, have a strong similarity with zircons from East Gondwana and the South China Craton.