Pre-Mesozoic Iawsonite in Anglesey, northern Wales: Preservation of ancient blueschists

The existence of lawsonite blueschists in the Mona Complex of Anglesey, northern Wales has been confirmed using optical and microprobe data. The blueschists occur locally along the eastern margin of one of three belts of fine-grained mylonitic schists known as the Penmynydd Zone of Metamorphism. The Penmynydd Zone is interpreted as representing remnants of deep-seated ductile shear zones produced by the movement of sialic basement up into low-grade melange. The lawsonite schists display complex textures resulting from the overprint of blueschist conditions on greenschist facies rocks followed by a subgreenschist dynamic metamorphism. The blueschist minerals are interpreted as a late product of shear zone metamorphism under conditions of decreasing temperature and high fluid pressure. The schists are no younger than Cambrian (probably Middle Cambrian). Lawsonite of this age is exceptionally rare; older examples are reported only from Russia. Rather than invoking a change in crustal isotherms through time, the paucity of pre-Mesozoic blueschists may simply be a result of the rapid exhumation needed to expose such rocks. The appearance of a blueschist at Earth's surface will be transient unless the uplift responsible for its exposure either slows or ceases.