Detrital zircon provenance of metasedimentary rocks in the Proterozoic Caiziyuan‐Tongan accretionary complex: Constraints on crustal and tectonic evolution of the Yangtze Block, South China

Abstract

Detrital zircon U–Pb ages and Lu–Hf isotopes of three metasandstone samples from the recently identified Caiziyuan‐Tongan accretionary complex (CTAC) in the southwestern Yangtze Block are presented. The detrital zircons are characterized by high Th/U ratios, oscillatory zoning, and nonabrasive crystal shapes, implying that these metasandstones in the CTAC were dominantly derived from proximal magmatic sources. The detrital zircon age spectrum shows three prominent intervals of 2.8–2.6 Ga, 2.5–2.2 Ga, and 2.1–1.7 Ga with minor dispersions between 3.8 and 2.9 Ga, matching well with those of the exposed magmatic rocks in the Yangtze Block. Two samples show consistent youngest age peaks at ~1.72 Ga, but the remaining sample has the youngest age peak at ~1.56 Ga, indicative of their differences in depositional ages. The Eoarchean zircons have negative εHf(t) values with Hadean depleted mantle two‐stage (TDM2) model ages, indicating the earliest crustal growth of the Yangtze Block in the Hadean. The Mesoarchean to Neoarchean zircons show both negative and positive εHf(t) values, suggesting a prolonged and complicated crustal reworking and growth process. By contrast, the Paleoproterozoic zircons have dominantly negative εHf(t) values, suggestive of extensive crustal reworking in response to the Nuna supercontinent assembly. Available detrital zircon TDM2 model ages from the southwestern Yangtze Block produce three prominent peaks at ~3.06 Ga, ~2.95 Ga, and ~2.84 Ga, suggesting significant crustal growth pulses in the Mesoarchean. Given that the ~1.38 Ga Caiziyuan SSZ‐type ophiolitic mélange and the complex tectonothermal evolution process, we deduce that the CTAC belt may represent a relict suture zone between micro‐blocks in the Yangtze Block and may have witnessed the Nuna breakup and subsequent Rodinia assembly.