Early Devonian magmatism, Acadian Orogeny, Devonian, Northern England

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From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey.


Introduction

Porphyroblasts of andalusite/chiastolite in Skiddaw Group mudstone within the thermal aureole of the Skiddaw Granite; these examples are mostly postkinematic relative to the well-defined Acadian cleavage, x25. (P054639).
Contoured top surface of the Weardale Granite Pluton and other components of the North Pennine Batholith. The inset map shows the location of the batholith in northern England in relation to the other major intrusive bodies. P916066.
Outline geology of the Manx Group in the central and southern parts of the Isle of Man. P916042.

Granitic plutons were emplaced at a high structural level across the Iapetus Suture Zone, within both the Southern Uplands and northern England terranes, during the Early Devonian Acadian Orogeny. In the Lake District, these include the high heat-production Shap and Skiddaw granites, steep-sided equant plutons that were emplaced at the margin of the pre-existing, Caradoc subvolcanic granite masses; other concealed bodies may also have contributed to the expansion of the Lake District Batholith at this time. A separate high heat-flow granitic batholith, including the Weardale Pluton, was also emplaced to the east-north­east, below the northern Pennines. Farther north, forming the high moorlands of the Cheviot Hills, is the Cheviot Volcanic Formation, the remains of a large andesitic volcanic field. The co-magmatic and coeval, subvolcanic Cheviot Granite Pluton is the easternmost member of the Galloway Suite of granitic intrusions that also includes the Criffel and Fleet plutons. Associated with the granites are swarms of microgranite and lamprophyre dykes that cut Lower Palaeozoic rocks throughout northern England and southern Scotland. In the Isle of Man, several granitic plutons and associated minor intrusions have been intruded into the Manx Group.

Skiddaw Granite Pluton (399 ± 0.4 Ma, U-Pb, zircon)

The roof zone of the steep-sided, broadly cylindrical Skiddaw Pluton (about 4.5 km in diameter) is exposed in valleys within the Skiddaw Massif. It is a medium-grained, biotite granite, locally porphyritic and composed of orthoclase, oligoclase, quartz and biotite. The degree of greisen formation increases northwards to Grainsgill where the granite is pervasively converted into quartz–muscovite greisen. Emplaced into the Skiddaw Group, the mass is surrounded by a concentric thermal aureole that grades outwards through garnet, biotite-cordierite, chiastolite and chloritoid zones. Intrusion overlapped the formation of the main cleavage, with chiastolite both overgrowing and being weakly wrapped by the fabric (P054639). Mineralogical studies on the aureole rocks suggest that the granite was emplaced at a depth of 8 to 9 km, whereas the composition of fluid inclusions formed during greisenisation show that that process occurred at a depth of between 2.4 and 4.9 km; uplift and erosion must clearly have followed rapidly after intrusion. The granite is associated with the formerly economic tungsten vein mineralisation that cuts both the granite and the adjacent Carrock Fell Centre (see Chapter 10).

Shap Granite Pluton (404 ± 0.5 Ma, U-Pb, zircon)

In the eastern Lake District, the steeply conical Shap Pluton, approximately 8 km2 in outcrop area, cuts the Borrowdale Volcanic Group and adjacent Windermere Supergroup. The surrounding thermal aureole and interpretation of potential-field data indicate a more extensive subcrop. This decorative pink and grey biotite monzogranite contains up to 60 per cent Carlsbad-twinned orthoclase–perthite megacrysts, up to 5 cm in length, set in a coarse-grained groundmass composed of orthoclase, plagioclase, quartz and biotite (P519145). The megacrysts have been interpreted either as phenocrysts, or as porphyroblasts resulting from late-stage potassium metasomatism; the present consensus favours the former. Microdioritic enclaves and xenoliths of country rock are abundant locally. The main contact metamorphic effect in both the Borrowdale Volcanic Group and Windermere Supergroup was production of a biotite hornfels aureole, though within the latter, sillimanite, andalusite and cordierite occur in the more aluminous rocks, whilst the more calcareous lithologies contain an assemblage that includes vesuvianite, grossularite, diopside, anorthite, tremolite and actinolite. Hydrothermal mineralisation and fissure metasomatism are conspicuous features of the granite and its aureole respectively, the latter being dominated by garnet–epidote–hornblende veins. The aureole overprints cleavage in both the Borrowdale Volcanic Group and Windermere Supergroup, but some of the dykes believed to be cogenetic with the Shap Granite are cleaved at their margins.

Concealed plutons in the Lake District

Concealed plutons in the Lake District To the north of the Ennerdale Microgranite, bleached and recrystallised Skiddaw Group rocks, accompanied by locally abundant tourmaline veins, occur in an elongate, east-north­east-trending zone, 24 km long and up to 3 km wide, adjacent to the Causey Pike Fault. The metasomatic event has been dated at around 400 Ma (Rb-Sr, whole rock) and its effects overprint thermal metamorphism that was probably caused by a concealed elongate granitic mass, referred to as the Crummock Water Intrusion. The geometry of this pluton, modelled from potential-field geophysical data, suggests emplacement along an active fault zone. Another metasomatic bleached zone affects Skiddaw Group rocks in the northern part of the Black Combe Inlier. A radiometric age has not been obtained, but the metasomatism may be related to a concealed Acadian pluton because the metasomatised rocks have very similar mineralogical characteristics and structural relationship to the host mudstones, as those in the Crummock Water zone adjacent to the Causey Pike Fault. An intrusion beneath Black Combe would, like the Shap and Skiddaw plutons, be marginal to the batholith; it could be the source of the Early Devonian microgranite dyke swarm that is concentrated in the metasomatised zone, and also of the tungsten, tin and bismuth mineralisation seen in adjacent areas (see Chapter 10).

Weardale Granite Pluton (399 ± 0.7 Ma, U-Pb, zircon)

Weardale Granite Pluton (399 ± 0.7 Ma, U-Pb, zircon) The northern Pennines are underpinned by an elongate, east-north-east-trending batholith, 60 by 25 km in extent, the existence of which was detected by gravity surveys and by the Rookhope Borehole (P916066). Interpretation of the gravity data suggests that the batholith comprises a cluster of broadly cylindrical plutons, the central one of which is referred to as the Weardale Granite Pluton. This granite, as sampled by the Rookhope and Eastgate boreholes, is aphyric, contains biotite and muscovite, has a shallow-dipping, gneissose­like foliation and is cut by pegmatitic and aplitic facies and by a few tourmaline-bearing veins. The Weardale Pluton is peraluminous in composition and geochemically similar to the Skiddaw Granite. In common with the Shap and Skiddaw granites, it has a high heat-flow value and this is believed to have had an important role in driving the convection cells responsible for the later formation of the zoned North Pennine Orefield. The granite may have been the source of elements such as Sn, F and Bi within the mineralisation (see Chapter 10).

Cheviot Volcanic Formation (395.9 ± 3.8 Ma, Rb-Sr, biotite)

The Cheviot Volcanic Formation is poorly exposed over an area of about 600 km2 and its present thickness is about 500 m. Since it is likely that the coeval granite pluton had a substantial cover, the original thickness of volcanic rocks probably exceeded 2000 m, making the Cheviot volcanic eruptions comparable in scale to the early phase of Caradoc volcanism in the Lake District. The volcanic rocks unconformably overlie steeply dipping, tightly folded sandstone and cleaved mudstone of the Riccarton Group (Wenlock) and are overlain by either Upper Old Red Sandstone Group conglomerates containing abundant andesite and granite clasts, or by Lower Carboniferous beds, only some of which contain andesite fragments. Small exposures of red sandstone within the volcanic outcrop may be intercalated with the volcanic sequence, but fossils have not been found to indicate the biostratigraphical age of the volcanism. The radiometric ages determined show that volcanism occurred at the end of Early Devonian time, during the Acadian Orogeny.

The succession comprises stacked trachyandesite and subordinate trachyte sheets that contain phenocrysts of plagioclase, hypersthene, augite, ilmenite and apatite. There are also a few sheets of biotite-feldspar-phyric trachyte (previously described as ‘mica-felsites’) and, near the base of the succession, one or more sheets of rhyolite that represent eruptions of more fractionated magma. The restricted compositional range and absence of basaltic trachyandesite are in contrast with other Old Red Sandstone volcanic successions farther north in the Midland Valley of Scotland.

The sheets are massive to amygdaloidal and scoriaceous; near horizontal platy jointing is characteristic of many. The more readily weathered tops of the units have been eroded in some areas to form a prominent bench and scarp landscape. The benches reflect the gentle eastward dip of the sequence. The sheets have been interpreted as lavas, erupted in a subaerial setting, though the presence of a significant proportion of sills cannot be ruled out. The nature of this sequence has parallels in the Caradoc volcanic complexes of the Lake District, with the Birker Fell Formation of the Borrowdale Volcanic Group and the main part of the Eycott Volcanic Group; all three may have resulted from similar clusters of lava-producing low-profile volcanoes.

The basal unit of the formation in the south-west of the outcrop comprises up to 60 m of breccia composed of rubbly, angular to subangular clasts of silicic volcanic rock, along with some mudstone fragments. This unit is most likely the product of initial phreatomagmatic eruptions, though a sedimentary origin cannot be entirely discounted. Other intercalations of pyroclastic and volcaniclastic sedimentary rocks are present higher in the succession, though they are sparse. In the uppermost parts of some sheets there are enclaves and fissure-fills of green fine-grained sandstone and siltstone. Fragments of these rocks are commonly seen in streams, suggesting that this lithology is more common in the unexposed parts of the succession.

Cheviot Granite Pluton (395.9 ± 2.9 Ma, Rb-Sr, biotite, whole rock)

The Cheviot Pluton has an outcrop of about 60 km2 and, at a depth of 4 km, a diameter of nearly 20 km. It intruded the Cheviot Volcanic Formation late in the volcanic episode, thermally altering the volcanic rocks, in places for up to 2 km away from the contact. The Cheviot Pluton comprises an outer zone of grey, quartz monzonite to quartz monzodiorite which was emplaced first, followed by an inner zone of medium to coarse monzogranite; the final phase of intrusion is represented by a pink medium-grained granophyric granite. The mineral assemblage includes some clinopyroxene as well as biotite, an unusual feature in rocks of this composition. Exposures of the pluton margin in Common Burn (NT 930 265) and Hawsen Burn (NT 953 225) show a complex of granite dykes and veins penetrating the volcanic rocks.

Minor intrusion suites of northern England

Sporadic calc-alkaline lamprophyre dykes cut all Lower Palaeozoic units in the Lake District, Cross Fell, Cautley, Dent and Teesdale inliers; further south they occur in the Ingleton inlier and to the north the lamprophyre swarm spans the Iapetus Suture with widespread dykes seen in the Southern Uplands terrane. Two groups have been recognised: clinopyroxene– phlogopite–Ca-amphibole-phyric kersantite, and strongly biotite-phyric kersantite and minette. Some dykes show no cleavage whilst others are cleaved, suggesting that intrusion was both pre- and post-tectonic. Their radiometric ages, spanning the range 420 to 402 Ma (late Silurian to Early Devonian), suggest that they were emplaced before the granites.

Minor intrusions and dyke swarms comprising feldspar and quartz-feldspar-phyric microgranite, rhyolite and microdiorite occur in the Lake District and Cross Fell inliers and within the Cheviot massif. In the first of these areas, the minor intrusions are concentrated particularly around Scafell, in Black Combe, in the Duddon valley, and in the vicinity of the Shap Pluton. Some of the Lake District dykes have radiometric ages within error of the Shap and Skiddaw plutons, making them broadly contemporaneous with the Acadian magmatic event. To the south of the Cheviot Pluton a similar swarm of felsic dykes emanates in a broadly radial pattern.

Isle of Man

Intrusive rocks are abundant within the Manx and Dalby groups in the Isle of Man, though the igneous bodies are mostly dykes or small minor intrusions. There are three larger masses: the Dhoon Granodiorite and Foxdale Granite plutons, which have outcrop areas of about 2 km2, and the smaller Oatlands complex (P916042). Granite may underlie much of the island at depth.

The Dhoon Granodiorite Pluton is biotite-bearing and locally porphyritic but it is strikingly altered, with plagioclase replaced by zoisite and muscovite, probably as a result of greenschist-facies metamorphism. There is no radiometric age determination on the granodiorite, but relationships with the deformation sequence affecting the Manx Group rocks indicate that the pluton was emplaced early in the sequence of events. Together with the volcanic-arc affinity suggested by its trace element geochemistry, this could mean that the Dhoon Pluton was emplaced contemporaneously with the Ordovician arc magmatism in the Lake District, though a later age is equally possible. The granitic through to gabbroic Oatlands complex is now very poorly exposed but has been generally considered to have more features in common with the Dhoon Granodiorite than with the Foxdale Granite.

The Foxdale Granite Pluton is muscovite bearing and the accessory mineral assemblage includes garnet; porphyritic microgranite and pegmatitic varieties occur locally. The relationship to the deformation sequence, the relatively high content of radioactive elements, and the likely radiometric age of around 400 Ma, all indicate an association with the Early Devonian granites of northern England.

Many of the dykes and minor intrusions, which range in composition from mafic to felsic, are deformed and metamorphosed and may therefore have been emplaced early in the tectonic history. Altered basalt and basaltic andesite sills at Poortown (P916042) have a volcanic arc signature and, though the age of these sills is uncertain, they may be equivalent to the Caradoc volcanic rocks in the Lake District. A few mafic dykes cut the Wenlock strata of the Dalby Group and so establish that at least some dykes were emplaced during, or later than, the late Silurian. The suite of microgranite sheets occurring along the central spine of the island has been linked to the Dhoon Pluton.

Petrogenesis

The lamprophyric magmas were emplaced during Early Devonian times whilst a transtensional tectonic regime was in operation. The magmas were large-fraction melts of depleted oceanic lithosphere that had been metasomatised by aqueous fluids derived from the dehydration of Iapetus oceanic crust during subduction and by a CO2-rich phase from a deeper mantle source. Variable but small degrees of fractional crystallisation were involved before final emplacement.

By contrast, the broadly calc-alkaline granites and the Cheviot volcanic rocks were emplaced towards the end of the Acadian Orogeny. The Cheviot volcanic rocks show some similarities with other Old Red Sandstone examples in the Midland Valley of Scotland, but there are also differences. For example, the Cheviot rocks are more potassic and, despite having strong enrichment in light rare-earth elements, the Sr content is low.

Despite the orogenic setting of northern England towards the end of the Acadian events, a sedimentary protolith is unlikely as the main source of the Early Devonian granites. Rather, the parental magmas were probably derived from a mafic source that contained residual garnet. These calc-alkaline granites are compositionally evolved rocks, yet show little evidence of an extended history of crystal fractionation. However, as with the Ordovician magmas before them, assimilation of Skiddaw Group sedimentary material is implicated, particularly by the Sr, Pb and O isotope compositions.

Bibliography

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