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

Lawsonite-bearing metamorphic/metasomatic rocks form at high-pressure-ultrahigh pressure (H P-UH P) and low-temperature (L T) conditions, commonly in Pacific-type subduction zones. The P-T stability 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 ( d P/ d T) 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 in P-T space. The oldest occurrences of lawsonite-bearing blueschists are latest Neoproterozoic, suggesting that subduction-zone thermal structures evolved towards the necessary L T conditions 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 H P serpentinite 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. [ABSTRACT FROM AUTHOR]