Protracted subduction initiation of the Neo-Tethys: Insights from metamorphic sole chronology

Subduction initiation (SI), a key process in plate tectonics, remains poorly understood due to limited geological evidence. Metamorphic soles (MSs), which result from the high-temperature metamorphism of the subducted plate, hold the potential to offer crucial insights into SI. Here we reevaluate the formation time of MSs in the central Yarlung Zangbo Ophiolite (YZO) by integrating zircon, apatite, and titanite U-Pb ages with their rare earth element patterns. The results reveal that formation time of MSs is 145−115 Ma, and their protoliths were formed no later than 160 Ma. The significantly older protolith, in comparison to MSs and the majority of YZO ages (130−120 Ma), suggests that the Neo-Tethyan SI in the Yarlung Zangbo suture zone likely occurred along a transform fault or fracture zone between oceanic lithosphere with different ages or along a continental margin. The extensive age range for MSs indicates a protracted SI spanning nearly 25 m.y., which can be divided into two distinct stages: (1) the stagnant stage (145−133 Ma), marked by a slow subduction rate and a dry mantle; and (2) the unlocking stage (133−120 Ma), characterized by an accelerated subduction rate and rapid upper plate extension, leading to formation of the YZO. Additionally, we document a late-stage (post-SI stage; 120−115 Ma) of MSs that is younger than the YZO, implying that sustained subduction can produce MSs over extended periods. These observations provide new insights into the dynamics of the Neo-Tethyan SI.