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 °C and 300–400 MPa. Three amphiboles, two from the Kennack Gneiss and one from the Landewednack Schists, yield disturbed 40Ar/39Ar 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 dif...