Geochemical Evidence for the Evolution of the Deyang–Anyue Rift in the Ediacaran–Cambrian Period on the Northwestern Margin of the Yangtze Block

Abstract The evolution of a large intracratonic rift trough in the northern Sichuan Basin, China, along the northwestern margin of the Yangtze Block, is documented during the significant Ediacaran–Cambrian Transition (ECT). Three prevailing perspectives exist regarding the specific genesis of this rift: the inherited development of rift basins, erosional troughs, and tensional erosional troughs. However, the dynamic mechanisms underlying rift trough development and the precise timing of this transition remain contentious. Consequently, this paper conducts a systematic geochemical and chronological investigation for the first time, utilizing siliceous sediments from the Dengying Formation and detrital zircons from the Cambrian Canglangpu Formation. Detrital zircons are used to constrain the Neoproterozoic tectonic evolution of the northwestern Yangtze Block, and geochemical records of siliceous sediments are adopted to reconstruct the evolutionary history of the rift trough. The study’s results indicate the following: 1. Neoproterozoic zircons in the study area are dominated by continental arc signatures, with features of arc-associated orogenic compression and partially depleted mantle affinities. Crustal thickness variations calculated from zircon Eu/Eu* ratios indicate that arc-related extensional rifting occurred in the study area after ∼650 Ma. 2. The δ30Si values of chert range from −0.6‰ to 0.93‰, while those of siliceous dolostones vary between 0.66‰ and 3.26‰. These isotopic differences suggest that silica in chert originated from relatively deep-water environments with prominent biogenic sources, whereas silica in siliceous dolostones was derived from shallow marine settings through metasomatism. Trace element data reveal that both chert and siliceous dolostone experienced limited hydrothermal fluid modification during sedimentary and diagenetic stages. 3. During the ECT interval, crustal thinning drove the ascent of deep hydrothermal fluids and altered coeval seawater. Such weak hydrothermal overprint facilitated the preservation and deposition of bedded chert in deep-water conditions. Meanwhile, partial microbial dolostones underwent silicification metasomatism to form siliceous dolostones, and minor granular quartz silica filled pore spaces in microbial dolostones during late diagenesis.