The origin, evolution and provenance of the Northern Belt (Ordovician) of the Southern Uplands Terrane, Scotland: a heavy mineral perspective

The discovery of lawsonite, as part of a ferroglaucophane-glaucophane blueschist assemblage in the Portpatrick Formation, is important yet enigmatic. Ordovician crosstie-winchite-ferroglaucophane-phengite assemblages are known throughout the northwestern Appalachian/Caledonian belt and may have provided the source for the Portpatrick blueschist-facies detritus. However, a possible source was from the Cadomian blueschist belt of Anglesey, then on the southern margin of the Iapetus Ocean. This is supported by garnets with sodic/calcic amphibole inclusions, unknown in the Dalradian. Perhaps Cadomian 'terrane fragments' were rifted from the southern margin of Iapetus to drift towards and collide with Laurentia to shed detritus into the Laurentian margin trench.