TheLizard complex,Cornwall is generally accepted to represent a preserved example of an exposedophiolite complex in the United Kingdom. The rocks found inThe Lizard area are analogous to those found in such famous areas as theTroodos Mountains,Cyprus and theSemail Ophiolite,Oman.
The Lizard comprises three main units; theserpentinites, the 'oceanic complex' and themetamorphic basement. Since the pioneering work of Bromley[1][2] and Kirby[3] these suites have been understood to represent a slice through a section ofocean crust, including the upper level of themantle,thrust ontocontinental 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 ofductiledeformation andserpentinisation. In many cases the rocks have also been subject to varying degrees of laterbrittle deformation. Pre-deformation they would have been a combination of undepleted mantle in the form oflherzolite,peridotite and depletedharzburgite mantle from whichbasaltic phases had been removed.
On the Lizard these two types of peridotite are represented by a heavily foliatedorthopyroxene (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 can be studied atKynance 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 ofOgo 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"[4] that summarised thinking up to the point of publication, proposed that the serpentinite body represented an intruded mass ofultra-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.
The oceanic complex consists of the CrousaGabbro, locally intruded by a suite of dolerite dykes, and a number ofschists, 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 (atPentreath 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 ofepidosite. These bands can extend laterally for many metres 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 atmid-ocean-ridge andback-arc settings.
The basement comprises a group of schists andgneisses of the Old Lizard Head Series and the Man of War gneisses. The Man of War Gneiss is interpreted as a sequence of metamorphosed igneous rocks, possibly intruded as part of the break-up associated with the formation of the ocean. U-Pb dating gives a Late Cambrian age for both the Man of War Gneiss and for intrusions cutting earlyfabrics in the Old Lizard Head Series[5][6]
The current outcrop pattern of the various units of the Lizard Complex is mainly controlled byCarboniferous age normal faulting. The earliest structures seen in the ophiolitic rocks are steeply-dippingfoliations thought to represent deformation inlithosphere scaleshear zones, associated with continental break-up in the early Devonian.[7] Locally thrust contacts can be seen showing evidence of northwestward movement between parts of the ophiolite and between the ophiolitic rocks and the metamorphic basement.[8] It has been suggested that the Kennack Gneiss (a mixture of basic and acidic igneous rocks) was formed by partial melting during the obduction of the ophiolite onto the continental crust.[9] Although an earlier stage of 'hot' emplacement is not ruled out, it is now generally accepted[10] that in the final stage of emplacement, during theVariscan orogeny, the ophiolite was relatively 'cold'.
The northward dip of thrusts at the base of, and within, the ophiolitic rocks is interpreted to be caused by rotation of initially south-dipping thrust planes due to the dominantly south-dipping post-Variscan extensional faults.
The formation of the oceanic crust found at the Lizard, its obduction and final emplacement are thought to have happened over a short period of approximately 35 million years during theDevonian period as theRheic Ocean closed, around 350-400Mya. This is based on U-Pb dating of zircons from various parts of the complex.[11]
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