Zircon U−Pb age and tectonic setting of the shoshonitic volcanic rocks in the Xirenbudun area, Inner Mongolia

The shoshonitic volcanic rocks in the Xirenbudun area of Inner Mongolia occur in the Meilaotewula − Diyanmiao ophiolitic melange belt of the Hegenshan suture zone. The volcanic rocks include basaltic trachyandesite and trachyandesite. The study of the formation age,petrogenesis, and tectonic setting in the volcanic rocks has important implications for the tectonic evolution of the Hegenshan suture zone. The zircon LA−ICP−MS U−Pb dating for the trachyandesite yielded a weighted mean ²⁰⁶Pb/²³⁸U age of 176.3±0.8 Ma, suggesting that its formation age is Early Jurassic. The volcanic rocks are rich in Na₂O+K₂O(5.28%~7.19%,>5%), Al₂O₃(16.13%~17.42%) and high K₂O/Na₂O ratio (0.66~1.14,>0.5), Fe₂O₃/FeO ratio (0.82~1.50,>0.5) but low in TiO₂ (0.91%~1.16%,<1.3%). The rocks are obviously enriched in Th, U, Sr and LREEs, and depleted in Nb, Ta and Ti,with remarkably negative Nb,Ta,P and Ti anomalies on primitive mantle−normalized incompatible element patterns. The contents of rare earth elements range from 168.53×10⁻⁶ to 233.03×10⁻⁶. The chondrite normalized REE distribution patterns are of right−inclined shape. The basaltic trachyandesite−trachyandesite belong to post orogenic shoshonitic rocks, were related to the subduction of the Palaeo−Asian Ocean (PAO), and formed in the subsequent subducted slab roll back−break off−post orogenic extension tectonic setting. In the end of Late Paleozoic, the fluids dehydrated from the subducted oceanic crust+sediments of the PAO metasomatized the overlying mantle wedge to form the enriched mantle of the Hegenshan suture. In the Early Mesozoic, the PAO subducted slab break off−the post orogenic extension triggered partial melting of the enriched mantle to produce the shoshonitic magma in this area.