GEOCHEMICAL AND ISOTOPIC SIGNATURES AS PROXIES FOR SOURCE MANTLE COMPOSITION IN A POST-COLLISIONAL TECTONIC SETTING: AN EXAMPLE FROM SW ENGLAND

The geology of SW England has long been interpreted to reflect Variscan collisional processes associated with the closure of the Rheic Ocean and the formation of Pangea. The Cornish peninsula is composed largely of Early Devonian to Late Carboniferous volcano-sedimentary successions that were deposited in pre- and syn-collisional basins and were subsequently metamorphosed and deformed during the Variscan orogeny. Voluminous Late Carboniferous granitic magmatism (Cornubian Batholith) is broadly coeval with the emplacement of ca. 290-300 Ma lamprophyric dykes and flows. Although these lamprophyres are well mapped and documented, the processes responsible for their genesis and their relationship with regional Variscan tectonic events are less understood.

Syn-rift basalts have intra-continental alkalic affinities, and have REE profiles consistent with derivation from the spinel-garnet lherzolite boundary. εNd values for the basalts range from +0.37 to +5.2 and T_DM ages from 595 Ma to 705 Ma. The lamprophyres are extremely enriched in LREE and LILE, and depleted in HREE suggesting a deep, garnet lherzolite source that was previously metasomatised. They display εNd values ranging from -1.4 to +1.4, I_Sr values of 0.706, and T_DM ages from 671 Ma to 1031 Ma, suggesting that metasomatism occurred in the Neoproterozoic.

Lamprophyres and coeval granite batholiths of similar chemistry to those in Cornwall occur in other regions of the Variscan orogen, including Iberia and Bohemia. By constraining the evolution of the mantle beneath SW England and the processes associated with the formation of these post-collisional rocks, we may be able to gain a more complete understanding of mantle processes during the waning stages of supercontinent formation.