Longlin-Ruili subduction-accretionary complex belt in the southeastern Gaoligong orogen, and its relationship with the evolution of the Meso-Tethyan Ocean

The tectonic setting of the Longlin-Ruili m lange belt is a key to identify the Gaoligong tectonic zone as the suture zone between Tengchong and Baoshan blocks, and the southward extension of the Bangonghu-Nujiang suture. Here we present new geological, petrochemical, mineral chemical, geochronological and isotopic data for the Longlin-Ruili m lange belt in the southeastern Gaoligong orogen. The slices inside this m lange belt include serpentinization peridotite, basalt/gabbro, chert, carbonite and pelagic sedimentary rock with radiolarian bearing layered siliceous rocks and manganese nodules distributed in the turbidites, showing typical rock assemblage of a subduction-accretionary complex. The peridotite consists of the harzburgite and a small amount of dunite. It is characterized by the slightly enriched LREE, high values of Mg# (88 92), high values of Fo (90 similar to 95) for olivine, and Cr-# and Mg-# values for chrome-spinel are 60 70 and 20 26, respectively. The Mg# (spinel) vs. Cr-# (spinel) and Mg-# (olivine) vs. Cr-# (spinel) diagrams show the peridotite is the residual phases after high degree partial melting (> 30%) and extraction of melt in suprasubduction environment. The basalt and gabbro are oceanic island or seamount type whose mafic magmas are derived from a metasomatized mantle, with features of the enriched TiO2 (>2. 26%) and high values of Mg# (49 similar to 57), and their REE patterns and trace element spider lines are consistent with that of OIB, with low ENd (t) (+ 2. 2 to + 5. 1). Furthermore, a small amount of Ti enriched biotite and hornblende occurred in mafic rocks. The U-Pb dating of the zircons from pyroxenites within harzburgite yielded weighted mean Pb-206/U-238 ages of 183 similar to 185 Ma. The U-Pb dating of the detrital zircons from greywackes in turbidite yielded the o6youngest group Pb-206/U-238 ages of 212 similar to 241Ma. There are forearc and inter-arc sedimentary rocks with Early Cretaceous rhyolite or tuff in m lange belt. The Upper Cretaceous continental sedimentary rocks unconformably onlapped on the m lange zone. All these features suggest that this m lange belt was formed in a supra-subduction zone during Late Triassic to Early Cretaceous. The tectonic setting and age of the Longlin-Ruili m lange belt are consistent with that of the Bangonghu-Nujiang and Myitkyina ophiolitic m lange belts which are related with the subduction of the Meso-Tethyan Ocean. As a result, the Longlin-Ruili m lange belt should be connected with Bangonghu-Nujiang suture zone in the north, while the Myitkyina ophiolitic m lange belts in the southeast during Mesozoic. After that, the Myitkyina ophiolitic m lange belt was displaced to present position by Sagaing dextral slip -shearing in Cenozoic.