- Geology of Lizard, Cornwall
The
Geology ofThe Lizard ,Cornwall in theUnited Kingdom has been the subject of much study. It is a fascinating and unique area geologically as it is a rare example of an exposedophiolite complex. The rocks found in the area are analogous to those found in such famous areas as theTroodos Mountains ,Cyprus and theSemail Complex ,Oman . As ophiolite generation is such a complex (and hotly debated) subject, Lizard has a number of features which are unique to it.Lithologies
The Lizard comprises three main units; the
serpentinite s, the 'oceanic complex' and the metamorphic basement. Since the pioneering work of Bromley [Bromley,A.V. 1976. A new interpretation of the Lizard Complex, S.Cornwall, in the light of the ocean crust model. Proc.geol. Soc. London, 132, 114.] and Kirby [Kirby,G.A. 1979. The Lizard Complex as an ophiolite. Nature, London, 282, 58-61.] these suites have been understood to represent a slice through a section of ocean crust, including the upper level of the mantle,thrust ontocontinental crust .erpentinites
The serpentinites are actually the metamorphosed and deformed remains of the upper layers of the mantle. The metamorphosis has in most cases taken the form of
shear deformation andserpentinization . In many cases the rocks have also been subject to varying degrees ofbrittle deformation . Pre-deformation they would have been a combination of undepleted mantle in the form oflherzolite peridotite and depletedharzburgite mantle from whichbasalt ic phases had been removed.On Lizard these two types of peridotite are represented by a heavily foliated
orthopyroxene (enstatite )-rich serpentinite and less foliated, less orthopyroxene rich serpentinite which is typified by the presence ofamphibole (tremolite ).The boundary between these two types of serpentinite lies at
Kynance Cove , and geologically represents the boundary between shallow mantle peridotites from which material has been extracted by melting and deeper peridotite from which no material has been removed.In the area of
Ogo dour at the Northern reaches ofPredannack ,dunite , a highly depleted peridotite derivative which consists of almost pureolivine , is found.Earlier theories, most notably the BGS publication "Lizard & the Meneage" [FlettJ.S. 1947. Lizard and the Meneage. Memoir 359, British Geological Survey] that summarised thinking up to the point of publication, proposed that the serpentinite body represented an intruded mass of
ultra-mafic material. They believed that thefoliations were the result of mass flux within thecooling magma body , and that the different types of serpentinite were the result of an igneouscooling alteration rim .Oceanic Complex
The oceanic complex consists of a number of
schist s, split into two broad groups:hornblende schist andmica schist.The hornblende schist, found in contact with the serpentine mass directly to the North (at Ogo Dour) and to the South (at
Pentreath andChurch Cove ) is the metamorphic remnant of basaltic intrusives into theupper crust . It is typified by a schistose texture and visible crystals of black or dark green hornblende. Structural studies of the hornblende schist indicate that it has been subject to at least three stages of deformation.Folding of the schist at Housel Bay indicates that the formation was also subject to more than one subsequent stage ofshear stress .On the South-east tip of the Lizard the hornblende schist is "inter-bedded" with pale yellow/green veins and pods of
epidosite . These bands can extend laterally for many meters and lie in line with the schistose foliations of the surrounding rock. It is unclear what the provenance of these bands is, but theories include that they are the remains ofvolcanic ash fall during the deposition of the schist protolith, or the calcic remains of thin beds of ocean floor material deposited during less active periods of emplacement of theprotolith .Chemical analysis of the schist draws parallels between it and mantle-derived material found at mid-ocean-ridge and back-arc settings.
References
External links
* [http://www.projects.ex.ac.uk/geomincentre/06The%20Lizard.pdf A 'virtual field trip' to the Lizard, from the Camborne School of Mines Virtual Museum]
Wikimedia Foundation. 2010.