An analysis of the textures within the primary assemblage peridotite, the Lizard, Cornwall

Abstract

Rock samples from the primary assemblage peridotite of the Lizard, Cornwall, contain a range of textures. These textures, principally between olivine and pyroxene, range from those with a linear or planar fabric associated with evident penetrative deformation to those of considerable complexity in which only an initial deformation is apparent. The diverse textures are described in terms of a set of texture elements and by this method a semi-quantitative assessment is made of the varieties of texture elements present in a range of samples. New electron probe microanalyses of co-existing pyroxenes are presented from various sites across the primary assemblage peridotite. From these analyses, temperatures of re-equilibration are obtained. The new data, both textural and thermometric, illustrate the consequences upon an original texture set as the primary assemblage peridotite underwent penetrative deformation and re-equilibrated with declining temperatures and pressures within the spinel-lherzolite facies of the upper mantle. It is concluded that at 1200°C ± 70°C and 15 Kb ± 4 Kb a large part of the primary assemblage peridotite contained extensive sets of the texture elements described, in an undeformed state, and that these represent primary textures from an original equilibration within the upper mantle.