Evidence for metamorphic remagnetisation of upper Precambrian tillite in the Dalradian Supergroup of Scotland

Abstract

ABSTRACTA conglomerate test using 36 clast cores (76 specimens) from the upper Precambrian Port Askaid Tillite of the Dalradian Supergroup in the Garvellach–Islay region of Scotland shows that the tillite was remagnetised during orogenic activity in Lower Ordovician time. After 550°C thermal demagnetisation, the clast remanence directions give a mean of 155°, 14° (α95—12°). This is similar to the mean from four tillite matrix sites of 162°, 4° (α95—9°) and the mean ‘primary component direction’ of 166°, −12° (α95—6°) previously determined from the interbedded siltstones. This indicates that the matrix and siltstones were also remagnetised. These mean directions give virtual geomagnetic poles after tilt correction that fall within the cluster of British Ordovician poles from rocks in the Caledonian orogen, and thereby indicate a prefolding Ordovician age for the remagnetisation. Thermal demagnetisation studies and observations from polished and thin sections of tillite clast and matrix specimens show that secondary hematite derived from partially to completely altered magnetite is the dominant carrier of the metamorphic remagnetisation. These results show that remanence data from the Port Askaid Tillite cannot be used to support the hypothesis that a worldwide glaciation occurred during the Late Precambrian.