Whin Sill metamorphism in Teesdale

Abstract

The Whin Sill complex is briefly reviewed. Theoretical temperatures have been calculated for the metamorphic aureoles in the Teesdale, Rookhope and Ninebanks areas, for comparative purposes with the observed metamorphism. The major and trace element geochemistry of a suite of argillaceous and arenaceous sediments has been examined using R-mode factor analysis. Only one factor is interpreted as a metamorphic feature. This factor is indicative of an increase in Na in beds towards the contact but is of minor volumetric importance accounting for less than 10% of the data variance. The remaining factors can all be interpreted as sedimentary features. X-ray diffraction studies of argillaceous sediments have shown an increase in the crystallinity and a change in the polymorph, from a 1Md to a 2M variety, of illite, towards the contact. Pure limestones have been recrystallized to saccharoidal marbles, which are restricted to the Upper Teesdale region. Dark-coloured limestones are not recrystallized because the carbon present in these sediments prevents grain-boundary movement (Robinson, 1971).Calcareous sediments have been the most susceptible to the metamorphism and a varied calc-silicate mineralogy is developed up to 25 m from the Whin Sill contact. Potassic feldspar, andradite, hedenbergite, prehnite and datolite are recorded for the first time from the contact-rocks of the Whin Sill aureole. The mineralogy developed in the calcareous sediments is indicative of two or possibly three facies and is interpreted as non-equilibrium conditions. The difference in metamorphism between calcareous and non-calcareous sediments is attributed to higher reaction kinetics in the calcareous sediments.Metamorphism by the Whin Sill is virtually non-existent outside the Upper Teesdale region. This is suggested to be due to a magma source in this area, giving rise to anomalous conditions as compared to a simple intrusion into cold country-rocks. The increase of Na in certain beds within 10 m of the contact is not attributed to metasomatic introduction from the Whin Sill. Instead the Na is suggested to have been present as NaCl in hypersaline pore-waters of the sediments, at the time of the Whin Sill intrusion. It is shown that the mineralogy developed in the contact-rocks has largely been the consequence of reaction between illite and the NaCl pore-waters. Haematite has been formed by the release of Pe3+ from the illite structure.Finally it is shown that the unique geology of the Upper Teesdale area, especially the development of saccharoidal marbles, has been of major importance in the sustenance of the rare arctic-alpine flora for which the area is biologically renowned (Johnson, Robinson and Hornung, 1971).