Constraints on the P-T conditions and age of emplacement of the Lizard ophiolite, Cornwall: amphibole-plagioclase thermobarometry and ⁴⁰Ar/³⁹Ar geochronology of basal amphibolites

The basal slice of the Lizard Complex incorporates strongly foliated amphibolitic metabasites (Lower Landewednack Schists), interpreted as the remnants of a dynamothermal aureole formed beneath the ophiolite during intraoceanic thrusting. Structurally overlying the basal slice and occurring predominantly within the overlying Lizard peridotite is the Kennack Gneiss, a controversial, banded and gneissose unit comprising intimately mixed amphibolitic and granitic components, interpreted to represent commingled melts. Thermobarometry of hornblende-plagioclase pairs indicates that both the Landewednack Schists and the mafic component of the Kennack Gneiss recrystallized synkinematically at ca. 600 degrees C and 300-400 MPa. Three amphiboles, two from the Kennack Gneiss and one from the Landewednack Schists, yield disturbed Ar-40/Ar-39 age spectra characterized by low-temperature overprinting, but with good high-temperature plateaux with ages of 366.1 +/- 4.2, 364.2 +/- 4.8, and 359.8 +/- 7.4 Ma, respectively. Theoretical diffusion modelling of the first of these spectra, based on a spherical geometry, indicates that it is best described by a 13% loss of argon during a thermal overprint, at similar to 220 Ma, on a primary cooling age (similar to 500 degrees C) of 370 Ma. As has been documented in many ophiolite massifs, displacement of the Lizard Complex probably occurred promptly after its initial formation, but the imprecise crystallization age (ca. 375 +/- 34 Ma) precludes construction of a precise cooling history.