Sediment burial to depths beyond 60 km is documented by rare exposures and inferred from mass-balance calculations, element recycling, and numerical modeling. Although thermomechanical and geochemical models have suggested potential diapirism of subducted metasediments, natural evidence of this remains scarce, especially for large volumes. The southern Tianshan metamorphic belt (northwest China) represents a unique example of a deeply buried, ∼4–5-km-thick, coesite-bearing sequence dominated by trench infill metasediments, with kilometer-scale variations in volcanoclastic and pelitic components. We demonstrate that these rocks were subducted down to ∼80 km, as shown by evenly distributed pressure-temperature conditions of 2.5 ± 0.2 GPa and 536 ± 11 °C. Considering sedimentary budgets and geophysical constraints on subduction channel thickness (<2–5 km), this sequence likely detached as several 0.5–1.0-km-thick individual slices, and later accreted to the upper plate within >2 m.y. Results show that temperatures were buffered and cold (<550 °C), suggesting continuous refrigeration (by incoming cold material from the slab, juxtaposition to a mantle wedge “cold nose”, presence of a thick overriding plate, and/or fluid circulation along the plate interface). The absence of evidence for diapirism in this buoyant metasedimentary package, at ∼540 °C and 80 km depth, suggests that metasedimentary diapirs may occur only at greater depth and/or farther away from the plate interface.