Lawsonite blueschists and lawsonite eclogites as proxies for palaeo‐subduction zone processes: a review

Abstract Lawsonite‐bearing metamorphic/metasomatic rocks form at high‐pressure–ultrahigh pressure (HP–UHP) and low‐temperature (LT) conditions, commonly in Pacific‐type subduction zones. TheP–Tstability fields of lawsonite blueschists and lawsonite eclogites represent subfacies of the blueschist and eclogite facies, respectively. Although the lawsonite–epidote transition boundary has a positive Clapeyron (dP/dT) slope, the blueschist‐to‐eclogite transformation within the lawsonite stability field in metabasaltic rocks is gradual and cannot be defined by a specific discontinuous reaction inP–Tspace. The oldest occurrences of lawsonite‐bearing blueschists are latest Neoproterozoic, suggesting that subduction‐zone thermal structures evolved towards the necessary LTconditions for lawsonite formation only by the late Neoproterozoic. A clear difference in frequency between Phanerozoic lawsonite and epidote blueschists does not exist, but our new compilation found a global lawsonite hiatus in the Permian that is a robust indication of relatively warm subduction‐zone thermal regimes. Lawsonite eclogites have been confirmed from at least 19 localities; they are classified as L‐ (lawsonite only), E‐ (lawsonite + epidote), and U‐type (lawsonite + coesite). Complete preservation of L‐type lawsonite eclogites attending their return to the surface is uncommon. Rare evidence of progressive eclogitization within the lawsonite stability field is preserved in some zoned garnets, as growth isolates a significant volume of precursor phases and textures during incipient eclogitization. Brittle fracturing and fluid infiltration are common during prograde eclogite facies metamorphism. Certain lawsonite‐bearing metasomatic rocks record multiple fluid‐infiltration events. Significant cooling and continuous H2O supply from the dehydrating oceanic plate to exhuming HPserpentinite mélange may cause lawsonite blueschist facies overprinting and prevent breakdown of lawsonite during decompression. The subduction records of lawsonite blueschists and eclogites agree with numerical modelling of subduction zones.