Ocean-plate stratigraphy (central Tibet) reveals Early Mesozoic double-subduction tectonics in the northern Meso-Tethys margin: A central piece of the Cimmerian geopuzzle

The timing and nature of initial Meso-Tethys subduction beneath the Cimmerian South Qiangtang (SQT) are highly disputed and impact global plate reconstructions. To address this issue, the internal structure and ocean-plate stratigraphy (OPS) of an accretionary complex at the SQT margin were investigated herein. Our integrated studies of field mapping, stratigraphy, zircon U-Pb dating, and geochemistry document an OPS sequence of Late Triassic-early Jurassic trench-fill turbidites intercalated with arc-affiliated high-Nb basalts (HNB; ∼219 Ma; SiO₂ = 45–48 wt%, Nb = 19.8–27.8 ppm, Nb/U = 32–48), Nb-enriched basalts (NEB; ∼217–214 Ma; 48–54 wt%, Nb = 5.2–10.46 ppm) and andesitic rocks (∼192 Ma). The results indicate simultaneous operations of two north-dipping subduction zones, which initiated at ∼ 220 Ma, in the Meso-Tethyan SQT margin. The Southern Subduction Zone produced a short-lived intra-oceanic arc, represented by the HNB and NEB, fringing the SQT. The Northern Subduction Zone facilitated the fringing arc to collide with the SQT at ∼ 210 Ma, and terminated at ∼ 190 Ma when the microcontinents (Amdo and Jiayuqiao) collided with the SQT. The Southern Subduction Zone migrated northward and brought about an Early Jurassic Andean-type arc, which delivered pyroclastic deposits with abundant high-Mg^# andesitic clasts (SiO₂ = 53–58 wt%, Mg^# = 52–54) to the trench. Together with regional information, it is envisaged that the Late Triassic-Early Jurassic double-subduction system was active along the entire southern Cimmerian margin: the Northern Subduction Zone bounded the main Cimmerian ribbon continent in the south and a series of microcontinents (e.g., Amdo, Jiayuqiao and Tengchong) was scattered in-between the two subduction zones. Proceeding of the double subduction until termination of the northern one at ∼ 190 Ma likely induced the Early Jurassic, synchronous occurrences of continental-arc magmatism, exhumation of high-pressure metamorphic rocks, emplacement of ophiolite and amalgamation of microcontinents, along this margin.