Lizard complex

The Lizard Complex, Cornwall is generally accepted to represent a preserved example of an exposed ophiolite complex in the United Kingdom. The rocks found in The Lizard area are analogous to those found in such famous areas as the Troodos Mountains, Cyprus and the Semail Ophiolite, Oman. The Lizard comprises three main units; the serpentinites, the 'oceanic complex' and the metamorphic basement. Since the pioneering work of Bromley and Kirby 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. 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 ductile deformation and serpentinisation. In many cases the rocks have also been subject to varying degrees of later brittle deformation. Pre-deformation they would have been a combination of undepleted mantle in the form of lherzolite peridotite and depleted harzburgite mantle from whichbasaltic phases had been removed. On the 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 can be studied 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' 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. The oceanic complex consists of the Crousa Gabbro, locally intruded by a suite of dolerite dykes, and a number of schists, split into two broad groups: hornblende schist and mica schist.