Permian Magmatism

The latest Carboniferous marked the onset of bimodal magmatism in SW England characterised by ultramafic lamprophyre dykes and, slightly later, a Permian felsic granite batholith with associated quartz-porphyry dykes. The earliest lamprophyric rocks are intruded during the Stephanian into D3 extensional structures (Alexander and Shail, 1996). Later, Permian granite magmatism emplaced the Cornubian Batholith, exposed at surface as a suite of plutons, into the Devonian-Carboniferous sedimentary succession. The Cornubian Batholith comprises granites and quartz-feldspar porphyry dykes (locally referred to as “elvans”) which are illustrated in Figure 1.

The surface crop of the granite plutons have been studied extensively. Poor outcrop exposure and various mapping techniques have led to contrasting interpretations of these plutons. In this section, the most up-to-date mapping and interpretation by Simons et al. (2016, 2017) is presented as the state-of-the-art. The work by Simons et al. (2016) presents a revised geological classification of the granite plutons based on previous mapping which has been augmented through new geochemical, mineral and field evidence. Further work by Simons et al. (2017) helps elucidate magma fractionation and the relationships between different granite classes identified in Simons et al. (2016).

Lamprophyres

Ultramafic lamprophyre intrusions ranging from olivine-basalt, minette (K-feldspar > plagioclase) to biotitic-microsyenite compositions occur across the SW peninsula (Edmonds et al., 1968; Edwards, 1999; Leveridge et al., 1990, 2002; McKeown et al., 1973). The “Exeter Volcanic Rocks” is a term used to describe significant occurrences of lamprophyric and basaltic dykes, sills and lava flows associated with extensional structures west of Exeter (Edwards, 1999). Lamprophyric dykes are notable further west such as the Pendennis, Mawnan, Treliske, Mountjoy and Furzehill Bridge lamprophyres, however, extrusive rocks are not found in these areas (Leveridge et al., 1990, 2002; Reid and Scrivenor, 1906), most likely because of erosion. The mafic melt is considered to have a mantle source with only minor crustal contamination or mixing with contemporaneous felsic melt (Leat et al., 1987; Simons et al., 2017). It is however, postulated that minette compositions may be achieved from lamproite magmas mixing with >30% felsic crustal melt (Prelević et al., 2004). Geochronological data for lamprophyric intrusive and extrusive bodies are summarised in Table 1.

Table 1 Geochronlogical dates of lamprophyres magmatism in SW England including both extrusive lavas of the Exeter Volcanic Group and intrusive dykes. Sources: 1 - Miller et al. (1962); 2 - Miller and Mohr (1964); 3 - Rundle (1980); 4 - Rundle (1981); 5 - Roberts (1997); 6 - Edwards (1999), personal communication from Chesley (1992); 7 - Dupuis et al. (2015).

	Rock Type	Locality	Analytical Technique	Age (Ma)	Refs.
Exeter Volcanic Rocks	Basalt	Dunchideock [SX 876 873]	K-Ar whole rock	287 ±11	2
	Minette	Killerton [SS 975 005]	K-Ar whole rock	285 ±6	1
	Minette	Killerton [SS 975 005]	K-Ar biotite	283 ±7	4
	Minette	Killerton [SS 975 005]	Ar-Ar biotite	290.8 ±0.8	6
	Microsyenite	Knowle Hill	Ar-Ar biotite	281.8 ±0.8	6
Intrusive Lamprophyres	Minette	Bridford [SX 8263 8752]	K-Ar biotite	300 ±4.6	3
	Minette	Chyweeda [SW 613 326]	K-Ar phlogopite	292.9 ±3.4	3
	Kersantite	Fremington	Ar-Ar plagioclase	292.4 ±7.1	5
		Helford [SW 753 265]	Ar-Ar phlogopite	287.59 ±0.68	7
		Fisherman’s / Frenchman’s Creek [SW 749 258]	Ar-Ar phlogopite	284.38 ±1.07	7
		Pendennis Point [SW 766 601]	Ar-Ar phlogopite	292.15 ±1.5	7
		Trelissick [SW 827 316]	Ar-Ar phlogopite	286.9 ±1.54	7
		Holywell Bay [SW 834 387]	Ar-Ar whole-rock	262.1 ±1.3	7

The geochronology data are latest Carboniferous to Early Permian age and indicates that mafic magmatism was coeval with granite magmatism and intrusion of the main batholith (see Table 2; Chesley et al., 1993; Chen et al., 1993; Dupuis et al., 2015). Spatially, lamprophyres do not occur over or within the batholith and a “shadow zone” exists suggesting lower crustal melting may have precluded the assent of mafic magma and resulted in hybridisation of the two melt components as part of the batholith (Shail et al., 2014). However, this does not preclude known lamprophyres to be altered by later mineralisation events (e.g. Holywell Bay; Dupuis et al., 2015).

Granite Magmatism

Granite plutons characterise many of the topographic highs across Cornwall and Devon such as Dartmoor and Bodmin Moor. These plutons were intruded into the Devonian-Carboniferous sedimentary succession during erosion, extension and exhumation of the Variscan orogen (Shail and Leveridge, 2009). The region has 6 major plutons, Land’s End, Tregonning-Godolphin, Carnmenellis, St Austell, Bodmin and Dartmoor with numerous smaller exposures, and occur in an approximately ENE-WSW trend, offset by NW-SE dextral strike-slip faults (Leveridge et al., 2002; Scrivener, 2006). These plutons are generally poorly exposed, with most outcrops forming weathered hilltops (tors) or are exposed in quarries. Limited exposure exists from underground mine workings whilst the Land’s End Granite, and a short section of the Tregonning-Godolphin Granite, are the only plutons with coastal outcrops which provide high-quality exposure. Much of the unexposed inland areas are mapped from field brash or “float” which is not in-situ. Simons et al. (2016) classified the plutons into 5 granite classes (G1-G5) and mapped the spatial distribution of these, and various subclasses over the region; illustrated in Figure 2.

The interpretations by Simons et al. (2016) are based on whole-rock geochemical analyses and mineralogical data supported by geochronology and field-evidence where permissible. It should be noted that the whole-rock geochemistry and mineralogy is only collected from areas of outcrop and thus is limited to exposed areas. Therefore, whilst a robust classification can be made for known outcrops, much of the spatial distribution is inferred and harmonised with previous mapping.

The classification reconciled many other previous works that often focussed on a single pluton such as that of Booth and Exley (1987), Knox and Jackson (1990), Manning et al. (1996), Exley (1996) and Powell (2004). The major revisions made by Simons et al. (2016) were the classification of two-mica (G1) and muscovite (G2) granites and the grouping of Li-mica and tourmaline granite types into a single tourmaline (G4) class. The G4 class is contrary to the work by Manning et al. (1996) who considered Li-mica and tourmaline granites as geochemically distinct. The work also reflected the separation of two major magmatic stages that intruded G1 and G2 before G3 to G5; geochronology to support this is presented in Table 2. The timing of these magmatic events is supported and developed further by Tapster (2019, in preparation) and linked to the timing of lamprophyre intrusion.

Table 2i> Geochronlogical dates of lamprophyres magmatism in SW England including both extrusive lavas of the Exeter Volcanic Group and intrusive dykes. Sources: 1 - Miller et al. (1962); 2 - Miller and Mohr (1964); 3 - Rundle (1980); 4 - Rundle (1981); 5 - Roberts (1997); 6 - Edwards (1999), personal communication from Chesley (1992); 7 - Dupuis et al. (2015)..

Granite pluton/stock	Lithofacies	Locality	Method	Age ± 2σ Ma	Source
Land’s End	CGMBG	Lamorna Cove Quarry	U-Pb monazite	274.7 ±0.4	4
	CGMBG	Newmill Quarry	U-Pb monazite	274.9 ±0.5	2
	CGMBG	Cripplesease	U-Pb monazite	277.1 ±0.4	4
	FGBG	Castle-an-Dinas Quarry	U-Pb monazite	276.7 ±0.4	4
	FGBG	Castle-an-Dinas Quarry	U-Pb xenotime	279.3 ±0.4	2
	FGBG	Castle-an-Dinas Quarry	U-Pb zircon	300 ±5	5
	FGBG	Polgigga	U-Pb monazite	274.4 ±0.4	4
Carnmenellis	CGMBG-SM	Carnsew Quarry	U-Pb monazite	293.1 ±1.3	3
	CGMBG-SM	Carnsew Quarry	U-Pb monazite	293.7 ±0.6	2
	CGMBG-SM	Carn Brea	U-Pb monazite	292.0 ±1.0	4
	FGBG	Boswyn	U-Pb monazite	281.7 ±0.8	4
	CGMBG	Rosemanowes Quarry	U-Pb zircon	313 ±3	5
St Austell Granite	CGMBG	Luxulyan Quarry	U-Pb monazite	280.6 ±0.7	3
			U-Pb monazite	281.8 ±0.4	2
			U-Pb zircon	305 ±5	5
Bodmin Moor	CGMBG-SM		U-Pb monazite	291.4 ±0.8	2
	CGMBG	De Lank Quarry	U-Pb zircon	305 ±4	5
Dartmoor	CGMBG	Pewtor Quarry	U-Pb monazite	281.0 ±0.6	4
	CGPMBG	Haytor Quarry	U-Pb monazite	285.3 ±0.6	4
	CGMBG	Yellowmeade Farm	U-Pb monazite	278.2 ±0.8	2
	FGBG	Throleigh Common	U-Pb monazite	286.2 ±1.0	4
St Michael’s Mount	CGMBG-SM		Ar-Ar	281.4 ±1.5	3
Tregonning-Godolphin	FGBG	Godolphin Granite	Ar-Ar muscovite	281.6 ±1.6	4
	Topaz	Tregonning Granite	Rb-Sr whole rock	280 ±4	1
	Topaz	Tregonning Granite	Ar-Ar zinnwaldite	281.0 ±1.3	4
Carn Marth	CGMBG-SM	Carn Marth Quarry	U-Pb monazite	288.0 ±1.0	4
St Agnes	CGPMBG	Cameron Quarry	Ar-Ar muscovite	≥278	3
Cligga Head	CGPMBG	Cligga Head	Ar-Ar muscovite	279.9 ±0.8	3
Castle-an-Dinas	FGBG	Castle-an-Dinas	Rb-Sr whole rock	270 ±2	1
	FGBG	Castle-an-Dinas	Ar-Ar muscovite	≥273	3
Kit Hill	CGMBG-SM		Ar-Ar muscovite	284.3 ±1.0	3
Hingston Down	FGBG		Ar-Ar muscovite	283.1 ±1.0	3
Gunnislake	CGPMBG	Old Gunnislake Mine	Ar-Ar muscovite	283.1 ±0.9	2

The work by Simons et al. (2016) provides an excellent foundation for comparing granitic rocks from across the batholith. It provides an insight into the internal variations of plutons, but also demonstrates the intra-class variability, primarily based on mineralogical analyses, within each defined class. The classification provides a contemporary, regionally consistent and up-to-date framework for granite classification.

Granite at Depth

The interpretations by (Simons et al., 2016) of the outcrop of the granite are considered robust, however, there is a significant assumption that the surface exposure of the granite batholith is representative of what lies at depth. Whilst the classification of different granite types within the batholith remains a difficult problem to resolve, some estimates can be placed on the shape and volume of the granite mass through geophysical interpretation.

Bott et al. (1958) demonstrated through gravity modelling that the granite plutons were linked in the subsurface and more extensive modelling showed that the batholith is approximately tabular (Willis-Richards and Jackson, 1989). Gravity modelling has been applied in various guises to define the extent of the granite at depth (Beer et al., 1975; Rollin, 1988; Rollin et al., 1982; Tombs, 1980, 1977), however, the modelling of the granite surface by Willis-Richards and Jackson (1989) provided a regional model for the upper surface of the granite, shown in Figure 3. There have been few occasions where boreholes have been drilled to validate these various models of the granite surface at depth, however, previous holes drilled based on smaller predictions include the intersection of a granite body at Bosworgey [SW 58060 33670] approximately 173 m (prediction; 350 m) and the failed intersection at Parbola [SW 61570 36370] where the hole stopped at 665.5 m following a predicted intersection at 540 m (Burley et al., 1978). When compared to depths in the regional surface model generated by Willis-Richards and Jackson (1989), these values indicate deeper values of 545 m and 530 m for Bosworgey and Parbola, respectively, and that they lie proximal to a sharp edge to the batholith. Therefore, it can be considered that current gravity modelling, either regional or locally, is insufficient. To-date, no new gravity measurements have been acquired and significant scope remains for a high-resolution gravity survey to elucidate the granite surface a depth which remains a target for mineral exploration and geothermal energy potential.

Despite the model by Willis-Richards and Jackson (1989) targeting the upper surface of the granite, with little constraint on deeper structure, a volume for the batholith was calculated and estimated to be of the order of 68,000 km^3^ with a thickness of 8 to 22 km.

More recent gravity modelling by Taylor (2007) confirm the tabular nature of the granite batholith but infer that the tabular emplacement model is likely to be more complicated. Furthermore, the thickness of granite plutons is revised significantly to approximately 5 to 8 km with the exception of Dartmoor at 8 to 10 km. Moreover, Shail et al. (2014) compared seismic reflection data from SWAT5 and SWAT6 (Scheirer and Hobbs, 1990) and seismic refraction data by Brooks et al. (1984) with gravity modelling of Edwards (1984) to underline that geophysical data most likely suggests a thickness to the batholith of <10 km which would result in a drastically reduced overall volume. Again, acquisition of new, high-resolution gravity measurements would enhance these interpretations.

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