Provenance and tectonic setting of the Ordovician sedimentary succession at the southeastern margin of the Yangtze Block, South China: Implications for paleotopographic evolution of northestern Gondwana

摘要

Ordovician sedimentary strata at the southeastern margin of the Yangtze Block consist of, from bottom to top, fine-grained siliciclastic rocks, cherts with interlayers of siliceous/carbonaceous slate, and coarse-grained siliciclastic rocks, which are important for understanding the tectono-sedimentary evolution in South China. A combined analysis of petrography, whole-rock geochemistry, and zircon U-Pb dating was carried out on these strata. The cherts with radiolarian fossils have high Al/(Al + Fe + Mn) values (0.38-0.94) and no obvious Ce anomalies (1.09 +/- 0.05), suggesting a biogenic origin on a continental margin. The geochemical features of the siliciclastic rocks, including ratios of elements, discriminant functions, and ternary diagram positions, suggest a passive margin setting that received detritus mostly derived from intermediate to felsic igneous rocks and recycled quartzose sedimentary rocks. The source areas of the Ordovician siliciclastic rocks existed in semiarid-arid paleoclimate and moderate weathering conditions. The sandstone modal grain compositions and detrital zircon cumulative curves indicate a collisional setting of the source areas. Zircon age populations have been time-varying during the Early Paleozoic. Based on the above-mentioned evidences, the following basin filling and paleotopographic evolution process has been proposed in research area. In the Cambrian, a bilateral filling pattern reflected a flat and broad basin. In the Early Ordovician, longer transportation of SE-sourced sediments reflected the uplift of Gondwana. In the Late Ordovician, uplift of the Wuyi-Yunkai Orogen blocked some Gondwanan detritus (500-700 Ma) and released recycled (900-1100 Ma) and syn-orogenic detritus. Meanwhile, synchronous uplift of the Jiangnan Orogen led to a steeper and narrower basin, and sediments could reach farther distances in a shorter time. In the Silurian, a unidirectional filling pattern with overwhelming SE-sourced detritus indicated intense uplift of Gondwana and the Wuyi-Yunkai Orogen.