Geochemistry, geochronology and geological implication of amphibolites in Ailao Shan-Day Nui Con Voi metamorphic complex belt, southeastern Tibetan Plateau.

The Ailao Shan-Day Nui Con Voi metamorphic complex belt (ADB), located on the southeastern margin of Tibetan Plateau, is a vital NW trend orogenic belt in southeastern Tibetan Plateau. The ADB mainly consists of various paragneisses, schists, quartzites, marbles, amphibolites, which have experienced extensive mylonitization. In this paper, we discovered the Middle Permian-Early Triassic amphibolites in the Yuanjiang, Jinping, Lao Cai and Pho Rang areas within the ADB. The amphibolites can be subdivided into four groups by the integrated studies of the geochemistry and zircon U-Pb geochronology. Group 1 is distributed in Yuanjiang area. Its REE and trace element patterns are similar to those of E-MORB. LA-ICP-MS zircon U-Pb dating of these rocks yields an age of 272. 5 +/- 1. 7Ma. Group 2 is collected from Mengqiao and Maandi. Compared with Yuanjiang amphibolites (Group 1), it shows higher ratios of LREE/HREE, and the REE and trace element patterns are similar to the enriched mid-ocean ridge basalts (E-MORB). The signatures of trace element ratios (e. g. Nb/Yb, Th/Yb, etc.) show the trend from MORB to OIB. The Mengqiao and Maandi amphibolites are similar to Emeishan Low-Ti basalts and High-Ti basalts, respectively, based on their Ti contents and Ti/Y ratios. The zircon U-Pb dating results yield weighted mean ages of 265. 2 +/- 1.0 0Ma similar to 266. 2 +/- 1. 0Ma and 250.4 +/- 1. 5Ma similar to 248. 7 +/- 1. 6Ma for Mengqiao and Maandi amphibolites, in which the late rocks contain 261.2 +/- 1. 5Ma similar to 257.9 +/- 1. 6Ma of inherited zircon. These ages of protoliths and inherited zircon are similar to Emeishan basalt (similar to 260Ma). All of these evidences indicate that the Mengqiao-Maandi mafic rocks are mainly derived from the mixing of mantle source components (E-MORB-like) and Emeishan plume material (OIB-like). Group 3 is distributed in Lao Cai and Pho Rang in Day Nui Con Voi, and has OIB-like characteristics of rare earth and trace elements. The contents and radio of trace elements (TiO2 = 3. 28% similar to 4. 31%, Nb/La = 0. 84 similar to 1. 01, Ti/Y >500, and so on) are similar to high-Ti (HT) Emeishan basalt, indicating that the Emeishan basalt is widely distributed in the ADB, and these rocks may be one of the end-member components of the Mengqiao-Maandi amphibolites (Group 2). Group 4 is distributed in Pho Rang with geochemical features similar to the Yuanjiang amphibolites (Group 1). Further research suggests that the amphibolites (E-MORB-like) in ADB are characterized by varying degrees of negative Nb, Ta anomalies and positive Rb, Ba anomalies. The ratios of incompatible trace elements (Nb/Yb (0.72 similar to 5. 29), Th/Yb (0. 11 similar to 0. 87), La/Nb (0.91 similar to 8. 83), and so on) are similar to island arc basalts. Such geochemical signatures indicate that these rocks were derived from partial melting of mantle wedge in the subduction zone. In combination with previous studies on the time-space distribution between the ADB, Ailao Shan-Song Ma suture and Yangtze block, we proposed that the East Paleo-Tethys Branch Ocean (Ailao Shan-Song Ma Ocean) subducted eastward beneath the Yangtze Block during the Middle Permian-Early Triassic (272 similar to 248 Ma), and accompanied formation of E-MORB-like magmas in the ADB.