Geochemical zonation across a Neoproterozoic orogenic belt: Isotopic evidence from granitoids and metasedimentary rocks of the Jiangnan orogen, China

摘要

In order to understand how Late Mesoproterozoic to Early Neoproterozoic orogenic belts evolved during the assembly and rifting of the supercontinent Rodinia, we have carried out detailed studies on the geochemical compositions of Early Neoproterozoic crust across the Jiangnan orogen (JO) which connects the Yangtze and Cathaysia blocks on the northwestern margin of Rodinia. A gradual geochemical variation is recognized from east to west, based on comparisons of whole-rock Nd isotopes, U–Pb age spectra of detrital zircons, and Hf isotopes in magmatic zircons and detrital zircons from the Neoproterozoic granitoids and metasedimentary basement sequences in the JO. LA-ICP-MS U–Pb dating of detrital zircons from the sediments and magmatic zircons from interlayered volcanic rocks suggests that the folded basement sequences formed within the span 860–825 Ma, implying the final amalgamation of the Yangtze and Cathaysia blocks occurred no older than ca 825 Ma. The 950–820 Ma detrital zircons strongly dominate in the eastern basement sequences, and most of them show moderately to highly positive ε_(Hf)(t). In contrast, many of the Early Neoproterozoic detrital zircons in the western JO have moderately negative ε_(Hf)(t) and more older (>1.0 Ga) detrital zircons were found in this area. We suggest that the metasedimentary basement sequences in the JO were deposited in retro-arc foreland basins which originally evolved from back-arc basins, and the change of provenance controlled the variation in crustal geochemistry of crust across the JO. Sediments in the eastern basement sequences have been sourced mainly from the juvenile subduction-related igneous rocks to the east, with a few from the central Yangtze Block, whereas those in the western JO may have been located far from arc terranes to the east and thus received more older recycled detritus from the southern part of the South China Block. Moreover, the Hf isotopes of the detrital zircons imply episodic crustal growth in the provenance of the JO metasedimentary basement sequences, with age peaks at 1.0–0.8 Ga, 1.75–1.50 Ga and 2.60–2.45 Ga. The early crust in South China may have been formed mainly at around 3.8 Ga ago, and contains some Hadean components (ca 4.1 Ga).