Provenance and tectonic setting transition as recorded in the Neoproterozoic strata, western Jiangnan Orogen: Implications for South China within Rodinia

摘要

The Neoproterozoic Tonian strata (ca. 870–725?Ma) in the western Jiangnan Orogen archive the records of sedimentary provenance and tectonic setting which can be used to understand the geological evolution of the South China Continent. These strata are separated into the basement and cover sequences by a regional angular unconformity. The basement sequence can be subdivided into the lower and the upper parts by the widespread interbedded ca. 840 Ma basalt with pillow structure. In the present work, 234 concordant detrital zircon analyses are obtained from three Tonian sandstone samples in the Fanjingshan district, Guizhou Province. Combined with previous results, a total of 1736 analyses of detrital zircon U-Pb ages derived from 12 formations of Tonian strata in the western Jiangnan Orogen are used to decipher the integrated sedimentary and tectonic histories. The zircons from the lowermost part of the basement sequence (the Yujiagou Formation) show oval morphology and display two Paleoproterozoic age peaks at 2325?Ma and 1845?Ma which are similar with the detrital zircon age peaks from the Late Paleoproterozoic to Early Mesoproterozoic Dongchuan/Dahongshan/Hekou groups, suggesting a passive margin basin in which the sediments were mainly sourced from the southwestern Yangtze Block. However, the zircon age population of the lower part of the basement sequence (the Xiaojiahe, Huixiangping formations and their equivalents) indicates the sedimentary derivation from bidirectional sources (the ca. 870 Ma arc materials in the south and the old detritus from the southwestern Yangtze Block) which is consistent with a back arc setting for the deposition of the sediments. Zircons from the upper part of the basement sequence (the Duyantang Formation and its equivalent) show euhedral and subangular morphology and display a unimodal age peak at ca. 835 Ma. This sequence was possibly deposited in a convergent setting and the detritus were came from the locally distributed syn-collisional igneous rocks. The lower part of the cover sequence (the Xinzhai and Wuye formations and their equivalents) shows a distinct zircon age peak at 815–809?Ma and two subordinate peaks at 2485?Ma and 2018?Ma, suggesting that the basin had gradually transformed into a continental rift basin and received the detritus from the ca. 815 Ma post-collisional magmatic rocks as well as from different Paleoproterozoic source rocks in the northern Yangtze Block. We propose a tectonic evolution model that envisages eruption of ca. 840 Ma basalt in a back arc basin that existed during ca. 870–835?Ma, an angular unconformity was formed during amalgamation of the Yangtze Block and the Cathaysia Block at ca. 835–820?Ma and the rifting of the South China Continent was initiated at ca. 800 Ma. Our study concludes that the South China Continent was formed on the periphery of the Rodinia supercontinent.