Geology of Lizard, Cornwall
Encyclopedia
Cornwall
Cornwall is a unitary authority and ceremonial county of England, within the United Kingdom. It is bordered to the north and west by the Celtic Sea, to the south by the English Channel, and to the east by the county of Devon, over the River Tamar. Cornwall has a population of , and covers an area of...
is the best preserved example of an exposed ophiolite complex in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
. The rocks found in The Lizard
The Lizard
The Lizard is a peninsula in south Cornwall, England, United Kingdom. The most southerly point of the British mainland is near Lizard Point at ....
area are analogous to those found in such famous areas as the Troodos Mountains
Troodos Mountains
Troodos is the biggest mountain range of Cyprus, located in the center of the island. Troodos' highest peak is Mount Olympus at 1,952 metres.The Troodos mountain range stretches across most of the western side of Cyprus...
, Cyprus
Cyprus
Cyprus , officially the Republic of Cyprus , is a Eurasian island country, member of the European Union, in the Eastern Mediterranean, east of Greece, south of Turkey, west of Syria and north of Egypt. It is the third largest island in the Mediterranean Sea.The earliest known human activity on the...
and the Semail Complex, Oman
Oman
Oman , officially called the Sultanate of Oman , is an Arab state in southwest Asia on the southeast coast of the Arabian Peninsula. It is bordered by the United Arab Emirates to the northwest, Saudi Arabia to the west, and Yemen to the southwest. The coast is formed by the Arabian Sea on the...
.
Lithologies
The Lizard comprises three main units; the serpentiniteSerpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
s, the 'oceanic complex' and the metamorphic
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
basement. Since the pioneering work of Bromley and Kirby these suites have been understood to represent a slice through a section of ocean crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
, including the upper level of the mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
, thrust
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
onto continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
.
Serpentinites
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 ductileDuctility
In materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
deformation and serpentinization. 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
Lherzolite
Lherzolite is a type of ultramafic igneous rock. It is a coarse grained rock consisting of 40 to 90% olivine along with significant orthopyroxene and lesser calcic chromium rich clinopyroxene. Minor minerals include chromium and aluminium spinels and garnets. Plagioclase can occur in lherzolites...
peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
and depleted harzburgite
Harzburgite
The ultramafic igneous rock, harzburgite, is a variety of peridotite consisting mostly of the two minerals, olivine and low-calcium pyroxene ; it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral...
mantle from which
basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic phases had been removed.
On the Lizard these two types of peridotite are represented by a heavily foliated orthopyroxene (enstatite
Enstatite
Enstatite is the magnesium endmember of the pyroxene silicate mineral series enstatite - ferrosilite . The magnesium rich members of the solid solution series are common rock-forming minerals found in igneous and metamorphic rocks...
)-rich serpentinite and less foliated, less orthopyroxene rich serpentinite which is typified by the presence of amphibole
Amphibole
Amphibole is the name of an important group of generally dark-colored rock-forming inosilicate minerals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures.-Mineralogy:...
(tremolite
Tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
).
The boundary between these two types of serpentinite can be studied at Kynance Cove
Kynance Cove
Kynance Cove is a cove in southwest Cornwall, United Kingdom. It is situated on the Lizard peninsula approximately two miles north of Lizard Point...
, 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
Ogo dour
Ogo-dour Cove is a small cove located at . It is at the northern end of Predannack Downs on the west side of the Lizard peninsula in Cornwall, United Kingdom....
at the Northern reaches of Predannack, dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...
, a highly depleted peridotite derivative which consists of almost pure olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
, 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.
Oceanic complex
The oceanic complex consists of the Crousa GabbroGabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
, locally intruded by a suite of dolerite dykes, and a number of schist
Schist
The schists constitute a group of medium-grade metamorphic rocks, chiefly notable for the preponderance of lamellar minerals such as micas, chlorite, talc, hornblende, graphite, and others. Quartz often occurs in drawn-out grains to such an extent that a particular form called quartz schist is...
s, split into two broad groups: hornblende
Hornblende
Hornblende is a complex inosilicate series of minerals .It is not a recognized mineral in its own right, but the name is used as a general or field term, to refer to a dark amphibole....
schist and mica
Mica
The mica group of sheet silicate minerals includes several closely related materials having highly perfect basal cleavage. All are monoclinic, with a tendency towards pseudohexagonal crystals, and are similar in chemical composition...
schist.
The hornblende schist, found in contact with the serpentine mass directly to the North (at Ogo Dour) and to the South (at Pentreath
Pentreath
Pentreath is a small cove with a beach in south-west Cornwall, United Kingdom. It is situated on the west side of the Lizard Peninsula 200 metres south of Kynance Cove...
and Church Cove
Church Cove
Church Cove is a hamlet in the civil parish of Landewednack in Cornwall, England. Its nearest town is Helston, which lies approximately north-west from the hamlet. The parish is notable for being the most southern point on British mainland....
) is the metamorphic remnant of basaltic intrusives into the upper crust
Upper crust
"The upper crust" is an English-language idiom that refers to a society's social or economic elites. It is derived from the housekeeping practices of the pre-industrialization British gentry, whose servants baked bread for the household but were permitted to eat only the pan-scorched bottom...
. 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
Folding
Fold or folding may refer to:* Paper folding, the art of folding paper* Book folding, in book production* Skin fold, an area of skin that folds* Fold , in the game of poker, to discard one's hand and forfeit interest in the current pot...
of the schist at Housel Bay indicates that the formation was also subject to more than one subsequent stage of shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
.
On the South-east tip of the Lizard the hornblende schist is "inter-bedded" with pale yellow/green veins and pods of epidosite
Epidosite
Epidosite is a highly altered epidote and quartz bearing rock. It is the result of slow hydrothermal fluid alteration of basalt that occurs below the black smokers within mid-oceanic ridge spreading centers. Similar altered rocks have been identified within ophiolite complexes and associated...
. 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
Volcanic ash
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions, less than in diameter. There are three mechanisms of volcanic ash formation: gas release under decompression causing magmatic eruptions; thermal contraction from chilling on contact...
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
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
.
Chemical analysis of the schist draws parallels between it and mantle-derived material found at mid-ocean-ridge and back-arc settings.
Basement
The basement comprises a group of schists and gneisses 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 early fabricsFabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
in the Old Lizard Head Series
Structure
The current outcrop pattern of the various units of the Lizard Complex is mainly controlled by CarboniferousCarboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
age normal faulting. The earliest structures seen in the ophiolitic rocks are steeply-dipping foliations
Foliation (geology)
Foliation is any penetrative planar fabric present in rocks. Foliation is common to rocks affected by regional metamorphic compression typical of orogenic belts. Rocks exhibiting foliation include the standard sequence formed by the prograde metamorphism of mudrocks; slate, phyllite, schist and...
thought to represent 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 . 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 . Although an earlier stage of 'hot' emplacement is not ruled out, it is now generally accepted that in the final stage of emplacement, during the Variscan orogeny
Variscan orogeny
The Variscan orogeny is a geologic mountain-building event caused by Late Paleozoic continental collision between Euramerica and Gondwana to form the supercontinent of Pangaea.-Naming:...
, 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.
Timing
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 Ma during the DevonianDevonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period. This is based on U-Pb dating of zircons from various parts of the complex.