Whin Sill contact metamorphism in the Cow Green reservoir boreholes, Northern England: evidence for an Upper Teesdale source for the Whin Sill magma

Contact metamorphism of Carboniferous rocks by the Whin Sill in Upper Teesdale is documented utilizing boreholes drilled in the 1960s ground investigation for the Cow Green reservoir. Hedenbergite, prehnite and datolite are recorded for the first time, and the first analyses for seven contact minerals are presented. Pure limestones are recrystallized into saccharoidal marbles with average calcite grain sizes increasing from 20 m from the contact, up to 0.5 mm within 5 m from the contact. Grossular is the most abundant mineral, and along with epidote is recorded over 20 m from the sill contact; all other minerals are restricted to < c. 10 m from the contact. This substantial contact metamorphism around Cow Green is unique in comparison with other boreholes across the Alston Block that have penetrated even thicker leaves of the sill, and for which no contact metamorphic mineral development has been reported. The currently favoured model has the magma sourced from dykes on the southern and northern borders of the Alston Block, but the evidence in support is circumstantial. The contrast in the metamorphic development is definitive evidence that the heat flow linked to the Whin Sill intrusion in the Upper Teesdale area lasted much longer than other areas across the Block, giving strong support to the magma being sourced in this area, rather than from the dykes.