Petrology, geochemistry, and metamorphic evolution of meta-sedimentary rocks in the Diancang Shan–Ailao Shan metamorphic complex, Southeastern Tibetan Plateau

Abstract

Meta-sedimentary rocks are widely distributed within the Diancang Shan–Ailao Shan metamorphic complex in the Southeastern Tibetan Plateau. Detailed geochemical analyses show that all of them have similar geochemical features. They are enriched in light rare-earth elements (LREEs) and depleted in heavy rare-earth elements (HREEs), with moderately negative Eu anomalies (Eu/Eu∗=0.55–0.75). Major and trace element compositions for the meta-sedimentary rocks suggest that the protoliths were probably claystone, siltstone, and greywacke and deposited in an active continental margin. Garnet porphyroblasts in meta-sedimentary rocks have distinct compositional zonation from core to rim. The zonation of garnet in St–Ky–Grt–Bt–Ms schist indicates an increasing P–T trend during garnet growth. In contrast, garnets from (Sil)–Grt–Bt paragneiss show diffusion zoning, implying a decreasing P–T trend. Based on mineral transformations and P–T estimates using conventional geothermobarometers and pseudosection calculations, four metamorphic stages have been determined, including an early prograde metamorphic stage (M1), a peak amphibolite–granulite facies metamorphic stage (M2), a near-isothermal decompression stage (M3), and a late amphibolites-facies retrograde stage (M4). The relic assemblage of Ms+St±Ky±Bt±Kfs+Qz preserved as inclusions in garnet porphyroblasts of the meta-sedimentary rocks belongs to prograde (M1) stage and records P–T conditions of 560–590°C and 5.5–6.3kb. Matrix mineral assemblages of Grt+Bt+Ky/Sil+Pl+Qz and Grt+Bt±Sil+Pl±Kfs+Qz formed at peak (M2) stage yield P–T conditions of 720–760°C and 8.0–9.3kb. M3 is characterized by decompression reactions, dehydration melting of assemblages that include hydrous minerals (e.g., biotite), and partial melting of felsic minerals. The retrograde assemblages is Grt+Bt+Sil+Pl+Qz formed at 650–760°C and 5.0–7.3kb. At the amphibolites-facies retrograde (M4) stage, fine-grained biotite–muscovite-bearing symplectites were formed, which occur at the grain boundaries of garnet and felsic minerals; such assemblage as Bt+Ms+Pl±Kfs±Grt+Qz were formed at 553–613°C and 4.0–5.0kb. The metamorphic history of the Diancang Shan–Ailao Shan metamorphic complex is thus defined by a clockwise P–T trajectory which includes a nearly isothermal decompression recording the subduction, collision and exhumation events between the Yangtze and Indochina blocks at Oligocene to Early Miocene, prior to the regional strong left-lateral shearing along the Ailao Shan-Red River Fault.