Geochemical and zircon U-Pb-Hf isotopic study of metasedimentary rocks from the Huangyuan Group of the Central Qilian block (NW China): Implications for paleogeographic reconstruction of Rodinia

Abstract

We present a systematic study of micaschists and felsic gneisses from the Huangyuan Group of the Central Qilian block in NW China, with aims to unravel the connection with the Rodinia supercontinent. The micaschists have detrital zircon ages of 2895–928 Ma that peaking at 1.80–1.40 Ga. They show strongly increasing zircon εHf(t) values of −8.1 to +12.1 from 1.6 Ga to 1.4 Ga. Detrital zircon ages from the felsic gneisses are dominantly 960–913 Ma with εHf(t) values of −0.1 to −10.7. The micaschists have a wide range of whole-rock major element compositions, and the felsic gneisses have higher SiO2 contents, combined with lower other major element contents than those of the micaschists. All samples have trace element compositions consistent with upper continental crustal origin. The protoliths of the micaschists are dominantly shales and minor wackes with maximum depositional ages from ca. 1317 to 928 Ma. The protoliths of the felsic gneisses are mostly wackes with a maximum depositional age of ca. 927 Ma. The source materials for these metasedimentary rocks originated from intermediate to felsic igneous rocks. The variable maximum depositional ages of the metasedimentary rocks in the Huangyuan Group indicate that their protoliths constituted a sedimentary series with a long history of deposition starting at ca. 1317 Ma in an oceanic island arc-related basin that developed through a transitional continental arc-related basin into an active continental marginal basin at ca. 927 Ma. It is inferred that the 1795–1321 Ma detritus was sourced from juvenile arc crust at the margin of the Indian or the Western Australian craton. The source rocks for 1317–913 Ma detritus were arc magmatic rocks formed during assembly of Rodinia. A sequence of initial intra-oceanic subduction (ca. 1317–967 Ma) and continuous oceanic crust-continent subduction with formation of a mature continental arc (ca. 967–896 Ma) at the margin of Rodinia during the formation of the Central Qilian block is suggested.