GCR block - Igneous Rocks of South-west England (IGN-SW-E)

Igneous Rocks of South-west England

Block Description

UK map showing distribution of GCR sites of GCR block IGN-SW-E
Distribution of GCR sites of GCR block Igneous Rocks of South-west England

The GCR sites selected for this GCR Block represent the magmatic rocks of south-west England that fall within the northern European Variscan fold belt (see VAR-STR-SW); they are dominated by pre-orogenic volcanic rocks and post-orogenic granites, together with minor volcanic rocks, that span the Devonian and Carboniferous systems (from 417 to 292 million years ago, Ma).

Volcanic activity in Britain during Devonian–Carboniferous time can be broadly divided into two geographically separate areas that show contrasting eruptive and tectonic settings. The volcanic rocks of south-west England are dominated by medium- to deep-water submarine extrusives, shallow intrusive rocks and volcaniclastic rocks generated within rifted ensialic troughs and narrow ocean basins that appear to characterize the Variscides as a whole. Subsequently, they were extensively tectonized and metamorphosed during the different stages of the Variscan Orogeny and are thus characteristic of pre-orogenic volcanism. On the other hand, the foreland continental environment to the north in central-northern England and southern Scotland was outside the active orogenic belt and, as a consequence, deformation of volcanic rocks was relatively limited. The eruptive setting was also different. The calc-alkaline Old Red Sandstone volcanic rocks of southern Scotland (see ORS-IGN) are dominated by subaerial lavas and volcaniclastic rocks interbedded with thick sequences of intermontane sedimentary debris. Similarly, the extensive basaltic volcanic rocks of Carboniferous age in northern England and the Midland Valley of Scotland (see C-P-IG) are characterized by subaerial lavas and shallow, but often thick, intrusive complexes. Another significant difference is that volcanism continued throughout the Carboniferous in the northern area, whereas in South-west England it terminated in the Visean in response to thrust-generated crustal shortening.

The post-orogenic granite batholith volumetrically dominates the magmatic rocks found in south-west England.

Of no less importance is the Lizard Complex in south Cornwall, a fragment of obducted ophiolite with an attendant sedimentary melange. If the early Devonian age for this dismembered ophiolite is correct, then it has European significance as being one of the few remnants of ocean crust exposed in the external zone of the Variscides.

The regional setting –Variscan Orogeny

A large part of the geology of Europe from Poland to Iberia is represented by the broad, sinuous, roughly E–W-trending Variscan fold belt which has affinities with the Appalachian Belt on the other side of the Atlantic Ocean and the Uralide Belt in Russia.

The Variscan Orogen of northern Europe consists of a series of Ordovician to Carboniferous rift-generated basins, separated by metamorphosed crystalline ridges, which were progressively closed by the northward migration and subsequent collision of African Gondwanaland with northern Baltica. The Variscan orogenic belt is divided into a number of tectonic zones separated by major thrusts, ranging from the northern (external) Rhenohercynian zone to the (internal) Saxothuringian and Moldanubian zones of central Europe. The Rhenohercynian and Saxothuringian zones include basinal sedimentary sequences and volcanic rocks indicative of initial rifting and later synorogenic sandstones heralding subsequent closure. Although much of the basic–acid volcanism of these basins is typical of rifted continental crust, basaltic lavas with mid-ocean ridge (MORB) chemical characteristics are present and suggest that narrow oceans floored by oceanic crust were generated. Between these two zones in central Europe is the Mid-German Crystalline Rise largely composed of pre-Devonian magmatic and high-grade metamorphic rocks that have their counterpart in Armorica as the crystalline Normannia High. The Moldanubian zone is dominated by largely Precambrian magmatic and metamorphic rocks overprinted by Variscan tectonometamorphic events.

Essential to the Variscan story is the staged closure of the sedimentary basins by subduction of narrow oceanic segments and northwards-directed thrusting from late Ordovician through to Carboniferous time, such that many sequences are thrust bound and parautochthonous or allochthonous in character. In contrast to the general northwards convergence and progressive closure throughout much of the Palaeozoic, the basinal region that formed in the Rhenohercynian zone was not only late in development, but rapidly closed – opening in early Devonian and closing by the end of late Devonian time.

The magmatic rocks covered by this GCR Block are all within the external Rhenohercynian Zone of the Variscides, some of which have features in common with their counterparts in the rest of the orogen. The sedimentary and volcanic record of the Rhenohercynian Zone reflects an early Devonian rifting event that was rapidly followed by late Devonian to late Carboniferous crustal shortening that produced stacks of northwards-converging nappes and accompanying low-grade regional metamorphism. Relatively high-level, post-orogenic, granite magmatism is a common feature of this zone.

Basic–acidic volcanism is a characteristic of all Variscan tectonic zones, although volcanic sequences are generally dominated by basalts (‘spilites’) with minor trachytes and rhyolites (‘keratophyres’ and ‘quartz keratophyres’). Early Devonian rhyolites and late Devonian–Dinantian basalt pillow lavas and intrusives in south-west England have their temporal counterparts throughout the Rhenohercynian and Saxothuringian Zones, although the Moldanubian zone also exhibits late Dinantian calc-alkaline andesite-dominated volcanism. Throughout the Rhenohercynian zone most of the basalts are incompatible element-enriched, intraplate tholeiites and alkali basalts – the latter of which are particularly characteristic of south-west England. However, Middle and Upper Devonian basalts within stratigraphically restricted melange sequences have chemical affinities to MORB and, together with the Lizard ophiolite and Start Complex in Cornubia, they provide evidence for the existence of Rhenohercynian oceanic crust. Another chemical characteristic that appears to be a common feature of the basalts within the Rhenohercynian (and possibly Saxothuringian) Zones is the change in degree of incompatible-element enrichment from the Devonian to the Carboniferous. This is probably a reflection of the change from generally depleted MORB-type basalts in mid-late Devonian rift zones to the more enriched intraplate basalts of late Devonian and early Carboniferous age on the margins of basins.

As far as the Variscan granites are concerned, all the tectonic zones contain granitic massifs, many of which include types resembling those of the Cornubian batholith. In addition to older pre-Variscan intrusives and metamorphics, two or three distinct intrusive phases are found.

GCR site selection

Two inter-related criteria were used for the selection of sites in south-west England:

1. to provide a full stratigraphical coverage of different magmatic activity throughout the Hercynian fold belt;

2. to illustrate the special or unique petrological and chemical characteristics of different magmatic units and their petrogenesis.

The sites can be conveniently grouped into four main networks (A–D) that roughly relate to stratigraphical age and major magmatic events within the Variscan Orogeny.

(A) Lizard ophiolite melange and Start Complex

The plutonic complex of the Lizard ophiolite includes the serpentinized peridotite, gabbro and basaltic dykes, together with heterogeneous acid–basic gneisses that play an important role in the interpretation of early Variscan basins in south Cornwall; they also provide evidence for subsequent northward obduction. Although a volcanic carapace to the ophiolite is not present in sequence, tectonically associated, metamorphosed lavas (now hornblende schists) chemically similar to mid-ocean ridge basalts are consistent with a Lizard ocean-crust model. The metavolcanic greenschists of the Start Complex also exhibit mid-ocean ridge chemical features and may represent another tectonized segment of the early Variscan ocean floor in this region.

(B) Pre-orogenic volcanic rocks

This unit comprises various stratigraphically localized volcanic rocks which were erupted contemporaneously with basinal sedimentation. They range in age from Devonian to early Carboniferous, but culminated in late Devonian–Visean times. Although they represent a bimodal basic–acid suite (the old ‘spilite-keratophyre association’), the volcanic rocks are dominated by basaltic pillow lavas and high-level intrusives of both tholeiitic and alkaline character. Basic and acidic tuffaceous volcaniclastics are also common throughout the Upper Palaeozoic. The volcanic rocks invariably have been altered subsequent to consolidation and deposition.

(C) Cornubian granite batholith

The culmination or late stages of the Variscan Orogeny were marked by the emplacement of the Cornubian batholith at the end of the Carboniferous Period.

(D) Post-orogenic volcanic rocks

Shortly after consolidation of the batholith and regional uplift, a post-orogenic volcanic episode began in late Carboniferous–early Permian times, comprising both suprabatholithic acide volcanism and mafic intrusive and extrusive rocks related to fault-bounded troughs.

Volume Introduction

Igneous Rocks of South-west England

Site List

Your selection found 54 GCR sites. Sites are sorted alphabetically by country, local authority and then by site name -
CodeNameCountryLocal AuthorityGrid RefGCR Site Account
1673Botallack Head to Porth LeddenEnglandCornwall and Isles of ScillySW359330Not available
2148Cameron (Beacon) QuarryEnglandCornwall and Isles of ScillySW704507Not available
2082Cape CornwallEnglandCornwall and Isles of ScillySW352313Not available
2143Carn Grey Rock and QuarryEnglandCornwall and Isles of ScillySX034551Not available
1671Carrick Du to Clodgy PointEnglandCornwall and Isles of ScillySW508413Not available
1685Clicker Tor QuarryEnglandCornwall and Isles of ScillySX285613Not available
2080Cligga HeadEnglandCornwall and Isles of ScillySW738535Not available
743Coverack Cove–Dolor PointEnglandCornwall and Isles of ScillySW783185Not available
1676Cudden Point to Prussia CoveEnglandCornwall and Isles of ScillySW553278Not available
2140De Lank QuarriesEnglandCornwall and Isles of ScillySX101753Not available
1679Dinas Head to Trevose HeadEnglandCornwall and Isles of ScillySW849764Not available
2147Folly Rocks (Praa Sands)EnglandCornwall and Isles of ScillySW573280Not available
1684Greystone QuarryEnglandCornwall and Isles of ScillySX366805Not available
1672Gurnard's HeadEnglandCornwall and Isles of ScillySW433387Not available
741Kennack SandsEnglandCornwall and Isles of ScillySW734165Not available
1686Kingsand, Cawsand BeachEnglandCornwall and Isles of ScillySX439508Not available
1017Kynance CoveEnglandCornwall and Isles of ScillySW684133Not available
2844LankiddenEnglandCornwall and Isles of ScillySW756164Not available
740Lizard PointEnglandCornwall and Isles of ScillySW695116Not available
2957Luxulyan QuarryEnglandCornwall and Isles of ScillySX005590Not available
2134Megiliggar RocksEnglandCornwall and Isles of ScillySW605267Not available
2922Mullion IslandEnglandCornwall and Isles of ScillySW661176Not available
1678Nare Head to The BlouthEnglandCornwall and Isles of ScillySW911377Not available
1675Penlee PointEnglandCornwall and Isles of ScillySW473269Not available
2268Pentire Point to Rumps PointEnglandCornwall and Isles of ScillySW931804Not available
2845Polbarrow to The BalkEnglandCornwall and Isles of ScillySW717135Not available
2269PolyphantEnglandCornwall and Isles of ScillySX260825Not available
2842Porthallow Cove to Porthkerris CoveEnglandCornwall and Isles of ScillySW798232Not available
2146Porthcew (Rinsey Cove)EnglandCornwall and Isles of ScillySW593270Not available
1677PorthlevenEnglandCornwall and Isles of ScillySW632252Not available
2083Porthmeor CoveEnglandCornwall and Isles of ScillySW425376Not available
2843Porthousstock PointEnglandCornwall and Isles of ScillySW810215Not available
2144Roche RockEnglandCornwall and Isles of ScillySW992596Not available
2145St Mewan BeaconEnglandCornwall and Isles of ScillySW986534Not available
1674Tater-duEnglandCornwall and Isles of ScillySW440231Not available
2267Tintagel Head to Bossiney HavenEnglandCornwall and Isles of ScillySX057893Not available
2081Tregargus QuarriesEnglandCornwall and Isles of ScillySW949541Not available
1680Trevone BayEnglandCornwall and Isles of ScillySW889763Not available
2141Wheal MartynEnglandCornwall and Isles of ScillySX002555Not available
2136Birch TorEnglandDevon CCSX687813Not available
1689Brent TorEnglandDevon CCSX471804Not available
2037Burrator QuarriesEnglandDevon CCSX549677Not available
1691Chipley QuarriesEnglandDevon CCSX808721Not available
2921Elender Cove to Black CoveEnglandDevon CCSX768356Not available
2035Hannaborough QuarryEnglandDevon CCSS529029Not available
2137Haytor Rocks and QuarriesEnglandDevon CCSX758771Not available
1693Killerton Park and QuarriesEnglandDevon CCSS969007Not available
2138Leusdon CommonEnglandDevon CCSX705729Not available
2139Meldon Aplite QuarriesEnglandDevon CCSX567921Not available
1688Pitts Cleave QuarryEnglandDevon CCSX502763Not available
1694Posbury Clump QuarryEnglandDevon CCSX815978Not available
2270Ryecroft QuarryEnglandDevon CCSX843847Not available
1690Trusham QuarryEnglandDevon CCSX848808Not available
1692Webberton Cross QuarriesEnglandDevon CCSX876873Not available
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