Middle Permian-Late Triassic magmatism in the Deqen-Weixi area of the Sanjiang Orogenic Belt: Implications for Paleo-tethyan evolution

The Middle Permia-Late Triassic magmatic rocks are widespread in the Deqen-Weixi area of the Sanjiang Orogenic Belt. However, the relation between their petrogenesis and the two branches of the Paleo-Tethys Ocean remains the subject of intensive debate, which constrains our understanding of the Paleo-Tethyan evolution. Based on field investigation, the petrological study, zircon U-Pb isotopic dating, and major and trace element analyses are carried out upon the basalts and rhyolites near the Derong County, and andesites near the Lanping County and sedimentary volcanic breccia near the Deqen County to investigate their petrogenesis and tectonic settings, and further provide insights into the Paleo-tethyan evolution. The LA-ICP-MS zircon U-Pb analyses yield Late Triassic eruption ages for the basalts (similar to 236Ma), the andesites (similar to 233Ma) and the sedimentary volcanic breccia (similar to 232Ma), and a Middle Permian age of similar to 261Ma for the rhyolites. The Derong basalts are tholeiitic, with variable TiO2 and MgO contents, and low LREE/HREE ratios, and are characterized by varying degrees of negative Nb, Ta and Ti anomalies. They are inferred to be derived from high degree partial melting of a relatively fertile lithospheric mantle in the spinet stability field within a continental arc, with some fractional crystallization and crustal assimilation. The Lanping andesite has geochemical features of high-Mg andesites (high MgO content and Mg-# value) with trace elements similar to continental crustal material and may have been generated by interaction between sediment derived melts and a mantle wedge. The Derong rhyolite is characterized by high SiO2 and low TiO2 contents, and high Zr saturation temperatures, with geochemical features similar to fractionated A-type granites, and it was produced by partial melting of crustal material at high temperatures and low pressures, corresponding to an extensional setting with mantle upwelling. These data, in combination with previous studies on these Middle Permian-Late Triassic magmatic rocks and palaeogeographic reconstruction of Paleotethyan realm, allow us to discuss the tectonic evolution of the Paleo-Tethys Ocean described as belows: (1) During the Middle Permian-Middle Triassic, the eastward or northwestward subduction of the Changning-Menglian Paleo-Tethys Ocean (CMPTO) opened the Jinshajiang back-arc basin between the South China Block (SCB) and the Changdu-Lanping-Simao Block (CLSB); (2) During the Late Triassic, the continent-continent collision between the SCB and the CLSB following the closure of the CMPTO resulted in the westward or southwestward subduction of the Jinshajiang back-arc basin.