The Petrology and Origin of the Tertiary Lundy Granite (Bristol Channel, UK)

The British Tertiary Volcanic Province (BTVP) comprises within-plate central igneous complexes associated with plateau lavas and regional dyke swarms. Lundy is the southernmost complex of the BTVP and comprises granite (small tilde90%) emplaced into deformed Devonian sedimentary rocks within the Hercynian Cornubian granite province of southwest England. The complex is intruded by a northwest-southeast trending dyke swarm. In common with other BTVP igneous complexes, Lundy is associated with positive gravity and magnetic anomalies which are interpreted in terms of the presena of an underlying basic intrusion at shallow depth, with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granite containing up to 20% alkali feldspar megacrysts in a coarse-grained groundmass composed of alkali feldspar, quartz, lithium-bearing muscovite, and 'biotite' (lithian siderophyllite), with a range of aaxssory minerals. The main granite has a coarse-grained (locally miarolitic) pegmatitic facies and is intruded by thin sheets and veins of fine-grained aplite and microgranite. The mineralogy indicates crystallization of the Lundy granite from a highly fractionated H2O- and halogen-rich magma at a relatively shallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na2O=3–4%, K2Osmall tilde5%, low TiO2, MeO, CaO, Zr, and Sr, and high Rb and Rb/Sr in comparison with many other peralurninous granites, including those from the Cornubian batholith and the BTVP. A new Rb-Sr whole-rock isochron for the granite yields an age of 58·7±1·6 Ma with an initial 87Sr/86Sr of 0·715±0·006. Nd values for the granite (i?½0·9 to '1·9) plot between contemporaneous mantle (positive Nd and Cornubian granites (Nd=ca.i?½11). The trace element data (Rb, Y, Nb) show affinities with syn-collision and within-plate granites. As the Sr isotope data indicate a major crustal component, and the Nd isotope data suggest both mantle and crustal components, we propose that the Lundy granite is derived from a parental magma comprising crustal components (derived from a similar source to that of the Cornubian granite batholith) and a mantle-derived component (derived from a differentiate of contemporaneous basaltic magma This magma experienced fractional crystallization of plagioclase, alkai feldspar, Fe-Mg minerals, and REE-bearing accessory minerals before emplacement, and the Lundy granite experienced further in situ fractional crystallization, associateded with crustal contamination by the Devonian shale after emplacement.