A new age, provenance and tectono-sedimentary setting of the middle Dengying Formation of the terminal Ediacaran in the western Yangtze Block, South China

Abstract

The widespread terminal Ediacaran Dengying Formation in the western Yangtze Block, South China, is dominated by thick marine dolomite successions deposited in shallow-water platform environments. However, a distinctive succession of mixed clastic and carbonate rocks occurs widely in the middle Dengying Formation, representing sedimentary environment variations. The absolute age of the mixed sedimentary succession is poorly constrained, and tectonic and sedimentary settings remain controversial. In this study, we report a new zircon U–Pb age of 547.3 ± 4.8 Ma from a volcanic ash bed intercalated within a mudstone succession (ca. 40–100 cm thick) in the middle Dengying Formation in the western Yangtze Block. This age is consistent with previously published ages of the middle Dengying Formation or its equivalents in the Yangtze Block and in other blocks worldwide. Field investigations show that the mudstone succession overlies above an unconformity resulted from the lower Dengying Formation weathering and shows a deepening upwards depositional succession representing a period of transgression. Mineralogical analyses show that the mudstones are composed of clay minerals (ca. 68.7%, illite and illite/smectite mixed layers), quartz (24.2%) and minor K-feldspar, indicating a high compositional maturity. Redox sensitive element results (e.g., Mo and U) indicate an increasing anoxic setting upwards. Both elemental geochemical and Lu-Hf-O isotopic analyses suggest that the siliciclastic sediments were moderately weathered (average CIA = 71) and were derived from felsic-intermediate rocks, such as granites and granodiorites (or geochemically-equivalent rocks). We suggest that the mixed sedimentary succession represents as a short-lived, regionally-correlated sea level change event, rather than a tectonic uplift event. Our geochemical data reveal that the mudstone succession accumulated most likely in a passive margin setting, supporting the proposition that the Yangtze Block was located on the periphery of the Gondwana supercontinent during the Ediacaran Period.