The Cheviot granitic pluton has received only patchy attention since the 19th century. This has initiated disagreements between geologists over the origins of the pluton and the various types of ‘granite’ present. This has much to do with the nature of the terrain. Much of the pluton is fairly remote from road access. There are some good tracks but generally field workers have to be prepared to walk for long distances over rough and, sometimes boggy ground. The terrain is largely covered with superficial deposits such as blanket bog and glacial drift so that bedrock exposure is limited to perhaps 5%.
Geological Studies of the Cheviot Hills
This subject is dealt with more fully on the page, ‘The Origin and Structure of the Cheviot Complex’
In 1885, Teall noted the presence of both clino- and ortho-pyroxene which was at that time considered unusual for a granite. (Teall, J.J.H.(1885). On some Quartz-Felsites and Augite-Granites from the Cheviot District. Geol. Mag., Dec III, v. II, 106.)
In 1899, Kynaston suggested that augite was very widespread in the ‘granite’ (Kynaston, H., (1899). Contributions to the Petrology of the Cheviot Hills. Trans. geol. Soc. Edin., 7, 390.)
In 1932, Carruthers disputed this on the grounds that the greater part of the granite was a granophyre which did not carry pyroxene. He also argued that the pyroxene had been formed by contamination by the surrounding andesites. (Carruthers, R.G., Burnett, G.A., Anderson, W. and Thomas, H.H. (1932). The Geology of the Cheviot Hills. Mem. geol. Survey. U.K.)
In 1943, Jhingran made a thorough exploration of the pluton and noted three types of granite. He distinguished between a more basic ‘Marginal’ which occurred around the edges of the pluton; ‘Standrop’ and ‘Granophyric’ which occurred more centrally and were more acid than the ‘Marginal’.
About the map
Jhingran distinguished between three types of granite, each tending to occur in defined areas of the Cheviot hills. His varieties were:
Marginal – dark grey in colour and fine-grained in texture – the most basic type that occurred around the edges of the pluton.
Standrop – very light white to grey in appearance and medium to course-grained in texture with many phenocrysts. This variety was similar to the ‘Marginal’ in composition but richer in quartz and occurring in the central area of the hills..
Granophyric – pink in colour and medium to course-grained in texture – the most acid of the three varieties occurring in an area between the other two.
Within each area occupied by a particular variety, or mix of two, Jhingran found samples that did not conform to expectations and noted these on his map.
He also mapped the location of the many inclusions that he found in the granite. Some were large blocks of lava while some were tiny dark, fine grained patches in the lighter rock.
Jhingran agreed with Carruthers that the pyroxene was xenocrystal in the marginal granite. He also found that there were frequent exceptions to the main types in all areas, and he noted these together with the typical samples on his survey map. (Jhingran, A.G., (1942). The Cheviot Granite. Q. JL geol.Soc. London, 98, 241.)
In 1968, Haslam cast doubt on the xenocrystal origins of the pyroxene. He argued that pyroxene could be stable in a low pressure magma with as much as 67% silica at a high level in the crust. This was based on his research on the Ben Nevis pluton. (Haslam, H.W, (1968). The Crystallization of Intermediate and Acid Magmas at Ben Nevis, Scotland. Journ. Petrology, 2, pp 84-104.)
In 1986 Haslam followed this up with a detailed chemical analysis of the Cheviot pyroxenes. (Pyroxenes and Co-existing Minerals in the Cheviot Granite. Mineralogical magazine 1986 Vol50 pp 671-4)
In 1979 Robson & Green’s A magnetic survey of the aureole around the Cheviot granite showed how the andesites metamorphosed by contact with the granite, and the marginal ‘granite’ gave much higher readings than the remainder of the pluton. They argued that the main body of the pluton consisted of the Marginal variety but that there was a shallow basin of Granophyric and Standrop varieties in the central part which gave the lower magnetic readings there.
In 1982, Lee summarised both the magnetic survey work and the gravity anomaly studies in A Regional Geophysics of the Cheviot Area. He argued that the pluton extended to 9 km in depth and sloped outwards from the visible surface area. The central part was composed of ‘Standrop’ with ‘Granophyric’ outside it, and finally surrounded with ‘’Marginal’
In 1985, Al-Hafdh submitted a PhD thesis (Alteration Petrology of the Cheviot Granite). This is the most detailed general survey since Jhingran’s in 1942. His work has proved invaluable to us by providing a guide on where to search and what to look for, although we remain unconvinced about some of his findings. His thesis provides detailed chemical and mineralogical analysis of the main rock types, and a detailed description of alteration processes in the Cheviot pluton. He distinguished six types of granite or granodiorite in two cycles of intrusion. He suggested that the intrusion took the form of a series of ring dykes.
Cycle 1:
1. Marginal (as Jhingran) – a relatively mafic and basic medium-grained rock
2. Dunmoor – a more acid pink porphyritic rock just inside the Marginal boundary
Cycle 2:
3. Standrop – a coarser porphyritic rock typically at Great Standrop
4. Linhope – very similar to the Standrop type and only occupying a small area in the upper Linhope Burn
5. Hedgehope – similar to Dunmoor, outcropping high on Hedgehope Hill and at the summit of Dunmoor Hill.
6. Woolhope – the last stage, being a more acid rock than any of the others, and fine-grained. This is found extensively to the North of the pluton around the summit plateau of the Cheviot itself, but also in small quantities in the burns which flow down the south flanks of Hedgehope Hill.