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The occurrence of continental rocks metamorphosed at high-pressure, low-temperature conditions has been interpreted as resulting from continental subduction, which occurs when continental margins are carried to depth after the subduction of dense oceanic lithosphere (Chopin, 1984;Liou et al., 2004). Exhumed subducted continental terranes occur as two endmembers, involving distinct mechanisms of subduction and exhumation (Kylander-Clark et al., 2012): (a) large, mostly coherent continental units (former downgoing plates; e.g., Western Gneiss Region,

Key Points Mylonitic metagabbros cemented by unfoliated Omp + Grt ± Lws matrices show that pristine brecciation may occur at eclogitic conditions Geochemical variations and sharp increase in fluid content of matrices points to local embrittlement prior to external fluids ingression Structural and geochemical evidence suggest that eclogite‐facies brecciation can control initial strain localization within HP shear zones