The Origin and structure of the Cheviot Igneous complex

Past workers in the field

A.G. Jhingran, D.A. Robson, A.G. Green, M.K. Lee, N.M. Al-Hafdh and A.J. Miles together with geologists working for the British Geological Survey have been the main contributors to the current understanding of the origin and structure of the Cheviot igneous complex. This page provides a overview of their valuable and sometimes contradictory findings.

A.G. Jhingran

Jhingran’s fieldwork led to his suggestion that the Cheviot pluton is made up of three varieties of granite; the more basic and medium to fine-grained ‘Marginal ‘ variety that is located towards the boundary of the pluton, the more felsic and course-grained ‘Standrop’ variety that occupies the central area of the pluton, and a medium-grained ‘Granophyric’ variety located between them. This classification of types together with his map showing their distribution within the previously BGS mapped granite/lava boundary has proven very influential over the years. The current BGS map, shown here alongside Jhingran’s map, retains most of the boundary features shown in the earlier map but adds detail to the the complicated alternation of granite and meta-andesite that exists at locations north of the Harthope fault. The current map also confirms something that Jhingran thought might be the case – the granite in the Common Burn area is not isolated from the main body of the pluton, it is connected via a spur under the peat-covered area between Broadhope Hill and Broadstruther.

Map showing the distribution of granite types in the Cheviots – based on Jhingran

Map showing the plan distribution of granite types and lava after Jhingran

Key

Key to map after Jhingran

The mapped extent of the Cheviot pluton based on the current BGS online map

The surface extent of the Cheviot pluton based on the current BGS online map

Key

Key to BGS Map

D.A. Robson

D.A. Robson envisaged Cheviot igneous activity beginning with the rise and assimilation of magma followed by explosive eruptions of pyroclastic material from multiple vents – vents that might well have been aligned along a central zone of fracturing in the folded and faulted Silurian sedimentary bedrock that may now be marked by the Gyle/Harthope fault.
Each of the vents would have built an ash cone and each cone would have ejected material over an area several kilometres in radius. Subsequently, the vents poured out a great deal of lava, mostly of andesitic composition, punctuated by periods of more explosive eruption.
Robson suggests the next phase in the evolution of the Cheviot igneous complex saw the rise and emplacement of the granitic pluton followed by the intrusion of many dykes along lines of weakness and fracture.
Finally, much of the softer rocks consisting of ash and lava were eroded away, revealing the top portion of the pluton along with the remaining lavas, meta-lavas and some of original pyroclastic material.

Animation: The sequence of events in the evolution of the Cheviot igneous complex

Cheviot Volcanic Sequence

Robson’s map of the area followed Jhingran’s surface granite-lava boundary and represented the granite in the area of the Common Burn as a small body lying separate from the main body. However, he did write, ‘there can be little doubt that they unite as a single stock at shallow depth’.
He also identified bench features in the area, the result of unequal weathering of the softer outer and harder inner portions of lava flows, that tended to follow the dip of the underlying bedrock.
Regarding the faults that cut the central igneous zone of the Cheviots, Robson saw there had been some horizontal movement along the Harthope and Gyle faults as well as some considerable vertical movement. He wrote, the Gyle fault, ‘throws down to the north-west and is responsible for the steep scarp face on the Scottish side below Windy Gyle’ and the Harthope fault, ‘throws down on the south-east side and forms…the deep, steep valley of the Harthope Burn.’

Map showing the extent of the Cheviot pluton at surface level, granite types and the main faults – based on D.A. Robson

Map showing the distribution of rock types and main fault lines in the Cheviot Igneous area - following D.A. Robson

Key

Key to Robson

D.A. Robson and A.G. Green

In collaboration with A.G. Green, Robson combined original ground magnetic anomaly data with existing aeromagnetic anomaly data so as to: estimate the extent of the metamorphic aureole around the granitic body, add to the understanding of the faulted and unfolded boundaries between granite types, and generate a three dimensional model of the upper part of the intrusion.
Their work confirmed that, for the most part, the meta-andesite extends no more than one kilometre outwards from the surface edge of the granite but also identified an increase in magnetic intensity over outcrops that lie almost three kilometres away from the granite/lava boundary. They concluded that this must be due to the presence of meta-andesite at depth, ‘indicating an outward inclination of the plane of the granite/lava contact’.
They also recorded high magnetic intensity over the larger blocks of outcropping meta-andesite in the area (e.g.Long Crags) as well as over numerous small isolated areas of peat-covered ground. They tentatively interpreted the latter as evidence for unexposed blocks of meta-andesite that had been parts of the pluton’s roof that were detached by magmatic processes and embedded in the granite.
A large area showing high magnetic intensity exists north of Broadhope Hill indicating that here too, meta-andesite or marginal granite lies close to the surface.
Along the Breamish fault, Robson and Green found a marked narrowing of the ring of high magnetic intensity. They suggesting this is the case because the meta-andesites have been thrown down on the south-west of the outer Breamish fault, out of range of the sensing magnetometer.

Robson and Green’s final three dimensional model of the Cheviot pluton, the one that showed the best fit with observed data, emerged out of almost one hundred alternative models. This model, ‘consists of a gently-sloping dome of meta-andesite and marginal granite, surrounded by lavas with low magnetic intensity. A poorly-defined shallow basin of the granites with low intensity sits in the centre of the dome.’

3D model of the upper portion of the Cheviot pluton – based on Robson & Green

Robson & Green

Section of the magnetic model of the Cheviots (c – d) across the Breamish fault showing the vertical distribution of rock types

Section of the magnetic model of the Cheviots across the Breamish fault

Robson and Green identified two problems arising out of their findings.
The first was the absence of meta-andesite and a corresponding magnetic low in the Lambden Burn north of Dunsdale even though there was no evidence of faulting.
The second concerned the Schil, a tor of meta-andesite isolated from the metamorphic aureole by the College Burn valley that is occupied by non-metamorphic andesite. They thought this might be a result of upthrow to the west of the College Valley fault bringing metamorphic lava from depth up to the surface, or it may indicate the presence of a separate underlying small body of granite.

M.K. Lee

M.K. Lee combined Robson and Green’s magnetic data with gravity anomaly data to provide us with a more accurate three dimensional structure of the pluton.

A map showing the Cheviot gravity anomaly data translated into contour profiles over a 20 square kilometre area – based on Lee (1982)

A map showing the Cheviot gravity anomaly data translated into contour profiles over a 25 square kilometre area

Pseudo-petrspective view of the central granite body viewed from the west

Pseudo-petrspective view of the central granite body viewed from the west

Pseudo-petrspective view of the central granite body viewed from the south

Pseudo-petrspective view of the central granite body viewed from the south

He interpreted the gravity and magnetic data in terms of two bodies of rock: a low gravity, weakly magnetised central granitic body, and a surrounding, denser body of highly magnetic granitic rock together with meta-andesite.
The first body ‘reaches the surface over the central part of the granite outcrop and extends to a depth of about nine kilometres,’ at which point it, ‘reaches a diameter of about 35 kilometres.’

Magnetic and gravity anomalies translated into depths for Cheviot granites and lavas

Magnetic and gravity anomalies translated into depths for Cheviot Granite and Hornfels

Cross Section a-b through Granitic Body and Hornfels

Cross Section a-b through Granitic Body and Hornfels

Cross Section c-d through Granitic Body and Hornfels

Cross Section c-d through Granitic Body and Hornfels

Key

Key to the Robson and Green map

Importantly, the gravity data suggested the density of the main granite body corresponded with that of the ‘Standrop’ variety of granodiorite and not the ‘Marginal’ granite as supposed by Robson and Green on the limited basis of magnetic data alone.
Lee identified the second, outer body of rock as consisting of two layers; ‘Marginal’ quartz diorite with an outer layer of meta-andesite. This body also slopes outwards but, ‘tapers to merge with the central granite at a depth of about three kilometres.’

An east to west cross section of the Cheviots derived from gravity and magnetic anomaly data

An east-west cross section of the Cheviots showing its geology of the Cheviots derived from gravity and magnetic anomaly data

Although Lee felt justified in asserting that main body had a density corresponding to the ‘Standrop’ variety, he did say that he discovered in the granitic rocks, ‘a particularly wide range of density values,’ reflecting ‘the wide lithological variations observed over relatively restricted parts of the outcrop’. His calculations were complicated still further by the numerous inclusions of denser meta-andesite xenoliths in the limited number of samples that he analysed (17), together with the effects on them of hydrothermal alteration and weathering.
The limited number of samples available to him emphasised, he thought, ‘the need for a comprehensive study of rock densities throughout the volcanic region.’

N.M. Al-Hafdh

Al Hafdh’s description of structural relations between the Cheviot rocks and his explanation of how they came to be so are the most detailed and radical of those relating to the area. He described the Cheviot complex as comprising a series of concentric ring structures in which distinct internal contacts between intruded granodiorites, together with marked differences in potassium and zirconium levels between units implied there had been two major cycles of magmatic activity. His map of granite types is based on Jhingran’s, but departs from it in details of boundary shape in certain locations e.g. the area adjacent to the the Breamish fault.

Map showing the distribution of granite types in the Cheviots with lines of cross section – based on Al-Hafdh

Map showing the distribution of granite types and lines of cross section - based on Al Hafdh (1985)

Key to rock types shown in the map, cross sections and intrusive sequence

Key to Al Hafdh map and Intrusion Sequence

Cross-section of the Cheviot granitic body showing the proposed relationships between rock types – after Al Hafdh

Al Hafdh

The continuous horizontal line between a, b and c represents the current surface level along the line of cross section. Everything above this line has been eroded away and is therefore, hypothetical.
The vertical broken line at b represents the Harthope fault. Note that land to the south (right) is shown to be uplifted.

An amended version of Al Hafdh’s proposed cross section so as to conform to the distribution of granite types as shown on his map

An amended version of Al Hafdh

We have amended the distribution of rock types at surface level between a and b so as to conform to the sequence as shown along the line between a and b on Al-Hafdh’smap.

Origins of the Lower Devonian magmas

Al-Hafdh argued that it was unlikely that subduction processes associated with the closure of the Iapetus Ocean had any direct link to the origin of Cheviot complex on account of the former being completed by 408Ma whereas the Cheviot lavas date from around 380Ma.
He favoured the view that this complex, along with other Lower Devonian volcanic and plutonic suites in Northern England and the Southern Scotland, resulted from the decent of a remnant slab of oceanic lithosphere so as to underlie the mantle. The heat and pressure on the slab released fluids that rose and caused metasomatic alteration to the mantle wedge above, leading to its partial melting and ultimately to volcanic activity at the surface.
He regarded this explanation as being in step with current explanations of the origins and geochemical characteristics of the present-day arc volcanoes that Cheviot was seen to resemble and it also provided an explanation for the variation in magma observed across the Lower Devonian complexes. Variation in chemical composition within and across the descending slab would have given rise to variation in the amounts of alteration fluids released at different locations as well as differences in their chemical composition, resulting in subtle to substantial differences in the magmatic mix for each volcano and pluton.
Observing a close similarity in the amounts of major and trace elements in the Cheviot lavas and granodiorites, he argued for the existence of a ‘long-lived magma chamber of nearly constant composition which was responsible for the early part of the Cheviot igneous history, and continued in being through certainly the first and perhaps also the second magmatic cycle seen in the granitic complex…’ He explained the presence of six distinct types of granitic rock as largely being the outcome of crystal fractionation in the magma, each type evolving from the previous type.

Proposed phases of intrusion of Cheviot granites – Al Hafdh

First stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh
Second stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh
Third stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh
Fourth stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh
Fifth stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh
Final stage in the proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Schematic diagrams of the intrusive sequence

Al-Hafdh’s proposed that the evolution of the Cheviot complex began with the formation of a cauldron subsidence at depth when, ‘a block of greywacke and argillites perhaps overlain unconformably by a sequence of Old Red Sandstone rocks, subsided on a ring fault such as in the Glencoe Cauldron…’ As the block sank, it displaced the less dense magma so that it welled up its sides and out as an extrusion of lavas that created the early caldera with flows stretching at least 20 km from its centre. Given the size of present-day volcanoes with such a base, he suggested that the volcano could have grown to between two and a half to four kilometres in height. This phase ended with pyroclastic eruptions evidenced by the ignimbrite deposits at Knock Hill.
There followed an immense out-pouring of highly viscous, silica-rich, andesite lavas that even now cover 600 square kilometres. There must have been multiple feeders to enable this degree of coverage, but none have been found and Al-Hafdh suggests that ‘subsequent events in the geological history of the cauldron would have destroyed the evidence whereby these feeders could be recognised.’

First stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 1, Stage 1: Marginal granodiorite

The first stage of the first cycle of granitic emplacement into the lava then began.
Al Hafdh envisaged the block of country rock sinking deeper into a much depleted magma chamber forcing magma up towards the surface so that it came into contact with the andesite and began the lava’s metamorphosis. Citing the evidence of observed chilled contacts between the outer ‘Marginal’ variety of granodiorite and the meta-andesite, he suggested the magma flowed steeply upward ‘along ring fractures such as that cutting the andesite at Cunyan Crags’. Fluids were released, the magma crystallised to form the equigranular Marginal granodiorite and then the fluids migrated still higher where they too crystallised to form ‘the quartz veins associated with this body such as those at Linhope Spout Waterfall.’

Second stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 1, Stage 2: Dunmoor granodiorite

After a hiatus that enabled complete crystallisation of the Marginal granodiorite and further fractionation of the remaining magma, stage two of the first phase of intrusion began with another ring-fracture subsidence accompanied by radial fracturing of the Marginal granodiorite. Water-unsaturated magma rose to fill these spaces and partially solidified within the Marginal variety so that plagioclase, biotite and pyroxene phenocrysts formed. This caused the liquid magma that encompassed the phenocrysts to become more water-saturated until it abruptly released its water as a fluid phase and crystallised as a fine-grained groundmass resulting in a porphyritic rock that Al Hafdh called the ‘Dunmoor’ granodiorite. The hydrothermal fluids released moved along fractures and permeated the main body as well as the Dunmoor dykes that had become radially emplaced in the Marginal granodiorite. This brought about the first episode of hydrothermal alteration.

Third stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 2, Stage 1: Standrop granodiorite

The Dunmoor granodiorite completely consolidated as the magma chamber was replenished with fresh injections of magma that returned the overall composition to ‘one very close to that of the Marginal Granodiorite’ but with lower potassium and zirconium content. This new mix constituted the ‘Standrop’ variety of granodiorite.
Al Hafdh’s second cycle of intrusion began with another subsidence on a ring fault that caused a very large mass of rock from the earlier phases to collapse into the magma chamber. This event allowed the Standrop granodiorite to intrude into the space, filling the centre of the complex and inducing stoping on the roof of the chamber so that large chunks of the meta-andesite, roof pendants, were detached from above and became included in the cooling magma as massive zenoliths, as at Housey Crags.
Only a crescent of Dunmoor granodiorite remained. The Standrop magma had already begun to crystallise slowly at depth so the resulting rock is the coarsest grained of all the Cheviot rocks. The intrusion of the Standrop type is the largest of the intrusions and Al Hafdh refers to localities where it can be clearly seen in contact with the Dunmoor variety e.g. at Shielcleugh Edge.

Fourth stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 2, Stage 2: Linhope granodiorite

The second cycle continued with stage two: a relatively small intrusion of ‘Linhope’ granodiorite in what Al Hafdh thinks may have been a ring dyke into the Standrop type, the top of which is near the present level of erosion. It, ‘is only seen at two places in the floor of the valley of the Linhope Burn, in one of which (948172) a flat horizontal contact is seen with the Linhope Granodiorite chilled from below against the Standrop Granodiorite above.’
Being of a similar composition and granularity as the Standrop but being chilled against it, Al Hafdh places its emplacement and complete consolidation after the Standrop type but before the next phase.

Fifth stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 2, Stage 3: Hedgehope granodiorite

The third stage of cycle two again involved subsidence on a ring fracture and the emplacement of ‘Hedgehope’ granodiorite in a ‘horse-shoe shaped ring dyke that is entirely enclosed by the Standrop Granodiorite.’
This variety followed a crystallisation path similar to the Dunmoor type resulting in a similar rock with plagioclase, biotite and pyroxene phenocrysts set in a fine-grained matrix. Al Hafdh links its final crystallisation with the release of hydrothermal fluids that
caused a second hydrothermal alteration cycle.

Final stage in a proposed sequence of intrusions of Cheviot granite - based on Al Hafdh

Cycle 2, Stage 4: Woolhope granite

Al-Hafdh’s final igneous phase began once again with ring-dyke subsidence followed this time, with the intrusion of a highly evolved magma with the composition of a true granite that cooled relatively quickly to form a fine, equigranular rock that he called the ‘Woolhope’ variety. South of the Harthope fault it is poorly exposed with no visible contacts with the granodiorites that preceded it but its evolved chemical composition argued for its last-stage status.
However, north of the Harthope fault it is more widespread where Al-Hafdh understood it to occur , together with the ‘Hedgehope’ granodiorite, in ‘flat intrusive sheets evidenced by contacts keeping close to contours. Al-Hafdh argued that this overspreading would also have been the case in the southern part of the pluton but that its present-day absence here was evidence of this part’s uplift and erosion. The idea of this area having been subject to uplift is in direct contradiction to Robson’s observation that the Harthope fault has a down throw to the south-east.
Al-Hafdh went on to suggest that the veins of the Woolhope granite found, ‘in the Marginal granodiorite at Dunmoor Hill 967174 and at Bellyside Hill 907222 as well as with in the Dunmoor Granodiorite at Dunmoor Hill 967177’ may be, ‘the feather edge of intrusion of the Woolhope Granite’ in south-easterly and north-westerly directions.

A.J. Miles

Map showing the locations of the late Caledonian intrusive and extrusive rocks in Northern England and Southern Scotland

Map showing the locations of the late Caledonian intrusive and extrusive rocks in Northern England and Southern Scotland

Miles shared the view that the direct convergence of Laurentia and Avalonia had ended by 418Ma but drew attention to the regional folding and faulting resulting from the oblique convergence of the Laurentian and Avalonian plates.
This sinistral movement, accompanied by the forces exerted by the collision of Armorica at the eastern end of Avalonia. could have created transtensional forces on each side of the suture zone that disrupted bedrock and created conditions for magma to rise and assimilate in magma chambers.
As the ‘Sequence of events in the evolution of the Cheviot igneous complex’ animation demonstrates, there may have been a variety of sources for the rising magma that produced a variety of magma types. These types may have accumulated in a number of magma chambers in which magma mixed to varying degrees. Such a process could account for the metre-scale variety in Cheviot rock that we see in our fieldwork today.

Animation: The closure of the Iapetus Ocean and the building of the British Isles

Closure of the Iapetus Ocean and the subsequent strike-slip movements that could have engendered the formation of the Cheviot volcano

Animations: The creation of transpression and transtension forces at bends in a strike-slip fault

Transpression at a restraining bend
Transtension at a releasing bend

References

Jhingran, A.G .1943. The Cheviot Granite. Reprint from the Quarterly Journal of the Geological Society of London, Vol. 98, pp, 240-254.
BGS, 2016. Online Geology Maps at http://mapapps.bgs.ac.uk/geologyofbritain/home.html
Robson, D.A.. 1976. A Guide to the Geology of the Cheviot Hills. Natural History Society of Northumbria, The Hancock Museum, Newcastle Upon Tyne, Vol. 43, 1.
Robson, D.A. and Green, A.G. 1979. A magnetic survey of the aureole around the Cheviot granite, http://sjg.lyellcollection.org
Lee, M.K. 1982. Regional Geophysics of the Cheviot Area. Environmental Protection Unit, Institute of Geological Science – Natural Environment Research Council.
Al-Hafdh, N.M. 1985. The Alteration Petrology of the Cheviot Granite. Thesis submitted for PhD. at Newcastle University.
Miles, A.J. 2012. Genesis of Zoned Granite Plutons in the Iapetus Suture Zone: New Constraints from High-precision Micro-analysis of Accessory Minerals, Thesis submitted for PhD. at Edinburgh University.

No vestige of a beginning, – no prospect of an end

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