THE MOORBY MICROGRANITE - A DEFORMED HIGH-LEVEL INTRUSION OF ORDOVICIAN AGE IN THE CONCEALED CALEDONIAN BASEMENT OF LINCOLNSHIRE

A granophyric microgranite has been proved in the sub-Carboniferous ba sement at Moorby, near Horncastle in south Lincolnshire. The microgran ite is interpreted as a high level intrusion which has suffered strong alteration and deformation subsequent to emplacement, including the d evelopment of a spaced pressure-solution foliation and brecciation. Th e microgranite has yielded a U-Pb zircon age of 457+/-20 Ma (late Ordo vician) interpreted as the age of emplacement. A Rb-Sr whole-rock isoc hron age of 400+/-9 Ma (early Devonian) is strongly discordant with th e U-Pb age, and is interpreted to reflect thorough post-emplacement hy drothermal disturbance of the Sr isotope system, either associated wit h the development of the tectonic foliation during the Acadian phase o f the Caledonian Orogeny or subsequent uplift. Geochemical data sugges t that the affinities of the Moorby Microgranite lie with the arc-rela ted Ordovician intrusions of the Lake District (e.g. the Ennerdale and Eskdale plutons) rather than with the early Devonian high heat produc tion (HHP) granites such as Shap, Skiddaw and Weardale. While the phys ical 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 - nor th Norfolk Batholith.