Early Neoproterozoic assembly of the Yangtze Block decoded from metasedimentary rocks of the Miaowan Complex

Abstract The present Yangtze Block was proposed to be formed as a unified block in the Mesoproterozoic-Neoproterozoic transition, but the detailed timing and process are poorly constrained. Here we report integrated in situ U–Pb and Hf–O isotopic analyses of detrital zircons from the Xiaoxikou metamorphosed sedimentary rocks of the Miaowan Complex in the Yangtze Block, as well as U–Th–Pb and Nd–O isotopic analyses of magmatic monazites from an undeformed and unmetamorphosed leucocratic vein intruding these metasedimentary rocks. U–Pb dating results for detrital zircons from the metasedimentary rocks display two main age peaks at ca 970 and 900 Ma. The 970 Ma zircons are dominantly characterized by mantle-like δ18Ozir values (4.5–5.8‰) and highly depleted eHf(t) values (+5 to +14). The 900 Ma zircons mostly have O and Hf isotopic features similar to the 970 Ma zircons, except relative small proportion of grains with high δ18O values (6.0–9.2‰) and variable eHf(t) values (–33 to +7). Monazites from the vein are dated at ca 850 Ma and have uniform δ18O values (5.4–6.6‰), but significantly enriched eNd(t) values (–14.5 to –12.6). Based on the new isotopic results and immature lithology, the Xiaoxikou metasedimentary rocks are interpreted to be deposited between 900 Ma and 850 Ma in a fore-arc basin, and clastic materials derived from 970–900 Ma arc rocks were related to the northward subduction (present-day coordinates) of the Miaowan oceanic plate. The 850 Ma leucocratic vein shows geological and isotopic characteristics consistent with local 860–840 Ma post-tectonic granitic plutons, and may represent a fractionated product of local coeval granitic magmas. We propose that the closure of the Miaowan oceanic basin and formation of the current unified Yangtze Block occurred between 900 Ma and 860 Ma, i.e., later than the deposition of the fore-arc metasedimentary rocks while earlier than the emplacement of the post-tectonic plutons.