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Ophiolites commonly sample the uppermost parts (15-20 km) of fossil oceanic lithosphere. However, in recent years, the documentation of diamonds, super-reducing (e.g., SiC), and other "unusual" minerals from several ophiolitic peridotites and chromitites (e.g., Tibet and the Polar Urals) has caused debate concerning their origin (i.e., deep vs. shallower upper mantle). Here we report on symplectite-bearing lherzolites from the Purang ophiolite in Tibet, which preserve the first compelling evidence of garnet-facies protoliths. These lherzolites were previously formed and stabilized at a depth of similar to 85-100 km, which is much deeper than generally suggested and approaches the depth (similar to 120 km) required for stabilizing the diamonds. Combining with other key observations, we suggest the Purang garnet-bearing peridotites may represent mixtures of oceanic lithosphere domains with diverse origins; they were rapidly exhumed at a variety of mantle depths within a subduction channel associated with oceanic slab retreat.