Geochemical characteristics and comparison of the basic rocks of the Lizard Complex and the basaltic lavas within the Hercynian troughs of SW England

Abstract

The S Cornish basaltic rocks constitute a separate chemical province relative to the rest of the SW England Hercynian volcanic suites. In particular they are characterized by (a) lower incompatible element abundances, (b) generally lower large-ion-lithophile/high field strength element ratios, and (c) being mildly enriched in the light REE. Within this province some of the basalts have chemical features akin to incompatible element enriched MORB. Tectonically the S Cornish province can be divided into (a) the S Cornish mélange zone (N Lizard-Roseland area), and (b) the Mylor sedimentary trough (Penwith-Camborne area), both of which contain localized basaltic lavas. These basalts are compared with the Lizard ophiolite and associated hornblende schists which have compositions ranging from depleted to enriched MORB. The exotic volcanic megaclasts within the mélange zone are not chemically related to the ophiolite and as such do not represent the dismembered volcanic portion of the Lizard ophiolite. If the Lizard ophiolite and the hornblende schists represent different segments of a Palaeozoic oceanic area lying to the S, the ocean-floor type Hercynian basalts of S Cornwall are interpreted as an abortive attempt to produce oceanic crust in a rifted continental margin during the early Devonian.