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Journal of the Geological Society; 2000; v. 157; issue.5; p. 1049-1064
© 2000 Geological Society of London

Regular Article

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

C. A. COOK, R. E. HOLDSWORTH, M. T. STYLES & J. A. PEARCE

1 Department of Geological Sciences, University of Durham, Durham DH1 3LE, UK (e-mail: r.e.holdsworth{at}dur.ac.uk)
2 British Geological Survey, Keyworth, Nottingham NG12 5GG, UK

Scientific editing by Rob Strachan.

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.

Key Words: Lizard Complex • Variscan Orogeny • upper mantle • exhumation




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