Pre-emplacement structural history recorded by mantle peridotites: an example from the Lizard Complex, SW England

Abstract

The Lizard Complex of SW England includes thrusted units of peridotites that were initially exhumed from upper mantle ( c. 52 km) to lower crustal ( c. 24 km) depths during a period of Early Devonian rifting and break-up. This basin closed during the Late Devonian, when the Lizard Complex was thrust towards the NNW along a major low-angle detachment and became incorporated within a series of Variscan thrust nappes. In the peridotites, a primary high- T and high- P spinel lherzolite mineral assemblage ( c . 1119°C and c. 15.7 kbar) was progressively exhumed and re-equilibrated to conditions of lower T and P ( c . 991–1010°C and c . 7.5 kbar) during the development of kilometre-scale mylonitic plagioclase- and amphibole-bearing mantle shear zones. These fabrics demonstrably pre-date emplacement related structures. The new structural and geochemical evidence from the peridotites also strongly suggests that the Lizard Complex formed in a rifted, non-volcanic continental margin setting, possibly in a pull-apart basin, rather than at a mid-ocean ridge. The P–T and textural evolution of the Lizard peridotites supports growing evidence that shear zones in the lithospheric upper mantle may to some extent accommodate large-scale displacements associated with crustal extension and continental breakup.