Changes of provenance of Permian and Triassic sedimentary rocks from the Ailaoshan suture zone (SW China) with implications for the closure of the eastern Paleotethys

摘要

Tectonic evolution of the Ailaoshan Ocean (eastern Paleotethys), especially on the timing of opening and closure, has long been debated. Published works on the Paleotethyan evolution were mainly dedicated to structural tectonics and magmatic and metamorphic rocks, whereas studies on sediment chemistry are rare. In this study, we present new chemical data on the Permian and Middle-Upper Triassic elastic sedimentary rocks from the Ailaoshan suture zone in southwestern Yunnan (SW China). Compared with the Post-Archean Australian Shale (PAAS) and the average upper continental crust, these Ailaoshan elastic rocks are generally depleted in LILEs (e.g. K and Rb) and HFSEs (e.g. Nb and Zr), and show varying LREE/HREE enrichments ((La/Yb)(N) = 1.32-11.86), flat HREE pattern and negative Eu anomalies (Eu/Eu* = 0.3-0.9). In spite of these similarities, the Permian samples have relatively low SiO2 (avg. 71.4 wt%) contents, K2O/Na2O (avg. 0.65) and Eu/Eu* (avg. 0.65) ratios, but high Al2O3 (avg. 14.1 wt%), TiO2 (avg. 0.60 wt%), (Fe2OT 3 + MgO) (avg. 6.96 wt%) contents, Al2O3/SiO2 (avg. 0.20) and (La/Yb)(N) (avg. 6.9) ratios, similar to greywackes from typical continental arcs. In contrast, the Upper Triassic samples have relatively high SiO2 (avg. 81.9 wt%) contents, K2O/Na2O (avg. 8.48) and La-N/Yb-N (avg. 9.74) ratios, but low Al2O3 (avg. 9.98 wt%), TiO2 (avg. 0.54 wt%), (Fe2OT 3 + MgO) (avg. 5.26 wt%) contents, Al2O3/SiO2 (avg. 0.12) and Eu/Eu* (avg. 0.64) ratios, similar to greywackes from typical passive continental margin. Moreover, the Permian samples have lower CIA values (Chemical Index of Alteration: 47-74, avg. 59) but higher ICV values (Index of Compositional Variability: 0.70-1.29, avg. 1.03) than those of PAAS, indicating relatively weak chemical weathering of the source rocks with low sediment maturity. In contrast, the Upper Triassic samples have relatively high CIA (77-84, avg. 80) but low ICV values (0.52-1.12, avg. 0.79), suggesting intense chemical weathering of the source rocks with high sediment maturity. The wide range of CIA (52-84, avg. 69) and ICV (0.50-1.14, avg. 0.88) values for the Middle Triassic samples suggest multiple detrital sources with varying degree of chemical weathering and diverse sediment maturity, as also supported by their wide range of detrital contents and chemical compositions. Based on the distinct chemical contrast between the Permian and Triassic sedimentary rocks, we infer that a significant change in detrital provenance and depositional setting may have occurred across the Middle and Late Triassic boundary, which was likely caused by the closure of the Ailaoshan Ocean.