Late Neoproterozoic development of the Nanhua Basin and implications for assembly of the South China Block

Abstract

Whole rock geochemistry, detrital zircon U-Pb geochronology, and Lu-Hf isotopic data from late Neoproterozoic strata within the Nanhua Basin, South China, provide crucial insights into the amalgamation history of the Yangtze and Cathaysia blocks. Strata from the central part of the basin (Xinyu and Ji’an regions) overlie the western margin of the Cathaysia Block and consist of, from bottom to top, the Shenshan, Kuli, Shangshi, Xiafang, Bali, and Laohutang formations. Detrital zircon age spectra of the samples from the Shenshan Formation to the Xiafang Formation show a unimodal distribution around 770–700 Ma. In contrast, samples from the overlying Xiafang and Bali formations are dominated by zircons with ages in the range 1100–900 Ma, along with some aged 2600–2400 Ma, 1950–1750 Ma, 1500–1300 Ma, and ca. 688 Ma. Based on the ages of the youngest detrital zircons and regional stratigraphic correlations with the well-established glacial and interglacial strata in the Jinping-Congjiang region, the maximum depositional ages of the units are 717 Ma for the Shenshan Formation, 717–690 Ma for the Kuli to Shangshi formations, between 690 Ma and 635 Ma for the Xiafang Formation, between 635 Ma and 550 Ma for the Bali Formation, and post-550 Ma for the Laohutang Formation. U-Pb ages, Lu-Hf isotopic compositions, and whole-rock geochemistry data indicate that detrital zircons with ages from 770 Ma to 700 Ma were sourced from the coeval arc-related magmatic rocks along the northern margin of the Yangtze Block. The transport of sediments across the Yangtze and Cathaysia blocks suggests that the Nanhua Basin is a late Tonian to Cryogenian intracontinental basin, and the amalgamation of the Yangtze and Cathaysia blocks occurred before the late Tonian. Older zircons aged 1100–900 Ma, 1500–1300 Ma, 1950–1750 Ma, and 2600–2400 Ma came from the Gondwana continent. Synchronous sediment input into the Nanhua Basin from both the northern margin of the Yangtze Block volcanic arc and Gondwana sources during the late Tonian to Cryogenian, suggesting a back-arc tectonic setting developed along the northern margin of East Gondwana. Integration with previous geochronological data, the two episodic propagation processes of Gondwana-derived sediments toward the northeast of the Nanhua Basin in the late Cryogenian and Cambrian are likely related to pulses of crustal thickening and topographic uplift during the assembly of Gondwana.