The petrochemistry of rocks of the Lizard complex, Cornwall

Kirby, Gary Alan (1978) The petrochemistry of rocks of the Lizard complex, Cornwall. University of Southampton, Doctoral Thesis.

Abstract

The Lizard Complex of South Cornwall contains an assemblage of peridotites, gabbros, dolerite dykes, amphibolites,metasediments and felsic gneisses. The succession of the eastern coastal section conforms with the Penrose definition of an ophiolite. Most dykes trend NW-SE, are subvertical and, at Porthoustock, comprise greater than 50% of the section. This is interpreted as the base of a sheeted dyke complex. All are tholeiitic. Three geochemical magma groupings can be defined with chemistries varying from transitional to oceanic tholeiite. Much of the gabbro of the cast coast section was originally a plagioclase(-ollvine)cumulate deposited on a floor of tectonite peridotite. Primary igneous lamination and layering, locally gradedand younging to the north, dip at steep angles to the north. Deformation at conditions ranging from submagmatic to greenschist and lower has destroyed many of the primary igneous textures. Flaser zones were produced along low-angled normal faults under amphibolite facies conditions and are concentrated towards the base of the original cumulate pile. The troctolites of Coverack are a series of partially deformed and remobllised cumulates precipitated from gabbroic melts which segregated from the host peridotite. Geochemical variations in cumulate gabbros suggest that they were precipitated from a magma chamber which underwent periodic Influxes of more primitive magma. The most differentiated rocks occur at the top of the gabbro. Mornblende leucodlorltes, the extreme differentiates, may have resulted from addition of volatiles to the magma. The parental gabbroic magma was also oceanic tholelite but was not strictly cogenetic with dykes of the sheeted complex. The peridotite includes both lherzolite and harzburgite and is thought to be mantle-derived. Greenstones in the melange to the north of the complex are spatially and geochemically allied to rocks of the Lizard Complex and are distinct from others of South West England. Amphibolites, felsic gneisses and metasediments are thought to have been produced underneath the ophiolite. Two varieties of amphibolite have been distinguished. These were derived from chemically and lithologically distinct protoliths. The one is not the prograde metamorphic equivalent of the other. Both have oceanic chemical characteristics and may represent an older ophiolite sequence deformed during initial overriding of peridotite. Felsic gneisses are migmatites thought to have been produced synchronously with the deformation of the amphibolites by partial melting and metamorphic segregation of metasediments and metabasites below the overriding peridotite. A model for the evolution of the Lizard Complex is proposedin which the ophiolite was generated at a spreading centre. The complex was finally assembled in an oceanic region and obducted over the Devonian continental margin of South Cornwall.

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