A greenschist protolith for blueschist in Anglesey, U.K.

New exposures of Monian schists in SE Anglesey reveal a transition from weakly foliated, coarse, porphyroblastic actinolite greenschist to intensely foliated, fine-grained blueschist dominated by epidote and sodic amphibole. Textures are interpreted as recording an initial, near-static greenschist recrystallisation of a basic plutonic igneous rock, followed by deformation during continuing metamorphism at greenschist, then blueschist, grades. The sodic amphibole ranges in composition from glaucophane to ferro-glaucophane and crossite. A balanced reaction expressing the greenschist to blue-schist transition is: 0.52 actinolite+1.67 albite+0.73 magnetite+0.15 chlorite+0.09 H2O=1.0 crossite+0.4 epidote+0.44 hematite+0.32 quartz.During this reaction actinolite initially becomes increasingly barroisitic by glaucophane, Al-tschermakitic and edenitic coupled substitutions. Probe analyses indicate complete miscibility between actinolite and barroisite which then converts to sodic amphibole across a compositional gap spanning Ca/(Ca + Na)M4 values of 0.3-0.44. Epidotes in both greenschist and blueschist show Fe-depleted rims. Whereas blueschist epidotes show very high Fe content (79.5-92 mole% Al2FeEp), those in the greenschist are lower and span an apparent compositional gap of 60-72 mole% Al2FeEp. Low NaM4 values in unaltered actinolitic cores are similar to those in actinolites formed during sea-floor metamorphism. This suggests initially low-P metamorphic conditions for the green-schist, followed by deformation under rapidly increasing P which deflects the PT-time path away from the amphibolitic field into the blueschist facies. A preferred explanation for this unusual PT evolution is to invoke the subduction of young, hot oceanic crust previously recrystallised under low P during ocean-floor metamorphism.