Geology of Lizard, Cornwall

The Geology of The Lizard, Cornwall in the United Kingdom has been the subject of much study. It is a fascinating and unique area geologically as it is a rare example of an exposed ophiolite complex. The rocks found in the area are analogous to those found in such famous areas as the Troodos Mountains, Cyprus and the Semail 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 serpentinites, 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 onto continental 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 and serpentinization. In many cases the rocks have also been subject to varying degrees of brittle deformation. Pre-deformation they would have been a combination of undepleted mantle in the form of lherzolite peridotite and depleted harzburgite mantle from which basaltic 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 of amphibole (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 of Predannack, dunite, a highly depleted peridotite derivative which consists of almost pure olivine, 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 the foliations were the result of mass flux within the cooling magma body, and that the different types of serpentinite were the result of an igneous cooling alteration rim.

Oceanic Complex

The oceanic complex consists of a number of schists, split into two broad groups: hornblende schist and mica schist.

The hornblende schist, found in contact with the serpentine mass directly to the North (at Ogo Dour) and to the South (at Pentreath and Church Cove) is the metamorphic remnant of basaltic intrusives into the upper 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 of shear 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 of volcanic 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 the protolith.

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]