The tectonic evolution of the Garze-Litang subduction-accretionary complex in the Middle Permian Evidence from geochronology and geochemistry of the Longpan ophiolite

Longpan ophiolite is located in the southern part of the Garze-Litang subduction-accretionary complex.It was selected as a research object for detailed geochemistry and chronology studies.The petrogeochemistry study shows that it is characterized by relatively low SiO₂, TiO₂, Na₂O and K₂O, and high Al₂O₃, CaO and MgO, revealing a feature of sub-alkaline basaltic rocks.Its signature of rare earth elements shows an affinity to enriched mid-ocean ridge basalts (E-MORB) with relatively low REE content, slight enrichment of LREE, indistinctive REE fractionation ((La/Yb)_N=1.93~2.96, (La/Sm)_N=1.41~1.77), without obvious Eu negative anomaly.Basalts are depleted in Sr and Nb, enriched in Nd, while diabases are depleted in Ba and Nb, enriched in Sr.Both rocks are enriched in highly incompatible elements, which shows obvious characteristics of enriched mid-ocean ridge basalts.The LA-ICP-MS U-Pb dating of zircon from the diabase yielded an age of 262.3±1.5 Ma, which indicates that the Longpan ophiolite mélange was formed in the Middle Permian.Combined with the previous achievements, it is suggested that the Garze-Litang Ocean was still in the process of continuous expansion in the Middle Permian.The discovery can enrich the research content of the Garze-Litang ophiolite mélange belt and provide direct evidence for the tectonic evolution of the Garze-Litang in Middle Permian.