Closure time of the Ailaoshan Paleo-Tethys Ocean: Constraints from the zircon U-Pb dating and geochemistry of the Late Permian granitoids

The Jinshajiang-Ailaoshan Paleo-Tethys suture zone is an important tectonic boundary developed along the western margin of the South China block. The closure time and related magmatism in the south segment, commonly known as the Ailaoshan, were less constrained for lacking available geochronological and geochemical data. This paper reports zircon U-Pb dating, Hf isotopic compositions and trace element chemistry of the granite porphyry and quartz porphyry from the Laowangzhai area in the northern Ailaoshan. Two granite porphyry samples record Pb-206/U-238 weighted mean age of 247.7 +/- 1.8Ma and 255.0 similar to 251.7Ma, and one quartz porphyry sample yields age of 255.1 +/- 3.4Ma, indicating that these porphyries were emplaced in Late Permian. Whole rock trace elements result shows distinctive enrichment in the RE, U, Th, and Pb, and depletion in Ba, Sr, Ti, and Nb, and strong fractionated REE (LREE/HREE = 7.2 similar to 18.2) and negative Eu anomalies (0.22 similar to 0.24). In combination with the peraluminous and high-K calc-alkaline properties revealed by previous major elements analysis, the granite porphyry was classified as the S-type granite which was produced under a thickened crust setting. The zircon epsilon(Hf)(t) of the granite porphyry are mainly of slightly positive values and clustered in 0.8 similar to 3.3, with several elevated positive (7.2) and negative values (-5.1 and -2.9). It was explained that the primary magma was derived from a mixture from mantle component and ancient crust materials (sediments source-dominant). The zircon epsilon(Hf)(t) of the quartz porphyry sample are characterized by clustered negative values (-5.5 similar to -2.3) and it was interpreted by partial melting of crustal materials, which is most likely the arc magmatic rocks in lower crust formed during the Ailaoshan oceanic slab subduction. The more fractionated HREE and higher temperature calculated by the zircon Ti thermometry in the granite porphyry than those in the quartz porphyry are consistent with the explanation that the granite porphyry has more mantle contribution as melting. The mantle melting probably induced by mantle upwelling under a thickened crust condition may denote a post-collision extensional setting after the closure of Ailaoshan Paleo-Tethys Ocean. It was summarized that the transition from subduction-related to collision-related magmatisms along the Ailaoshan suture was at similar to 260Ma. This time is earlier than that of its north extension, Jinshajiang suture (similar to 245Ma) and that of the paralleled Changning-Menglian suture (similar to 235Ma) to the west.