The Moorby Microgranite: a deformed high level intrusion of Ordovician age in the concealed Caledonian basement of Lincolnshire

SUMMARY A granophyric microgranite has been proved in the sub-Carboniferous basement at Moorby, near Horncastle in south Lincolnshire. The microgranite is interpreted as a high level intrusion which has suffered strong alteration and deformation subsequent to emplacement, including the development of a spaced pressure-solution foliation and brecciation. The microgranite has yielded a U-Pb zircon age of 457 ± 20 Ma (late Ordovician) interpreted as the age of emplacement. A Rb-Sr whole-rock isochron age of 400 ± 9 Ma (early Devonian) is strongly discordant with the U-Pb age, and is interpreted to reflect thorough post-emplacement hydrothermal disturbance of the Sr isotope system, either associated with the development of the tectonic foliation during the Acadian phase of the Caledonian Orogeny or subsequent uplift. Geochemical data suggest that the affinities of the Moorby Microgranite lie with the arc-related Ordovician intrusions of the Lake District (e.g. the Ennerdale and Eskdale plutons) rather than with the early Devonian high heat production (HHP) granites such as Shap, Skiddaw and Weardale. While the physical properties of the Moorby Microgranite are comparable to those of the HHP granites, gravity modelling suggests that the microgranite is unlikely to be a major component of the inferred Caledonide Wash - north Norfolk Batholith.