Pre-Variscan extension and sedimentation.
Post-Variscan extension: the prelude to granite magmatism.
The origins of the Cornubian Batholith.
The development of Late- to Post-Variscan sedimentary basins.
The Variscan Orogen is a broad, sinuous, E-W trending fold belt, running from Portugal to Poland (Floyd et al., 1993). The orogen developed during the closure of the Rheic Ocean and subsequent collision of the Gondwana and Laurasian continents, to form the supercontinent of Pangaea, by the end of the Carboniferous (Bristow et al., 1998). Cornubia forms one small part of this fold belt. Presently a peninisula of the British Isles, Cornubia's history has been charted back to the early Ordovician, since when it has occupied positions on a number of continents and terranes and been at both destructive and passive plate margins (Bristow et al., 1998). The following review of the regional geological history provides the tectonic framework surrounding the development of post-Variscan magmatism and mineralisation in southwest Cornwall.
The Precambrian history of Cornubia is unknown but, in common with the rest of southern Britain, it is likely to have formed part of the Gondwanaland continent (Africa, Antarctica, Australia, India and South America) that occupied a position close to the South Pole (~70° S) during the Late Proterozoic/Early Paleozoic (Bristow et al., 1998). With the opening of the Rheic Ocean in the Ordovician, a section of Gondwanaland, the Avalonian Superterrane, broke away northwards carrying southern Britain with it (Durrance, 1998). Further fragmentation of this landmass gave rise to a number of terranes including Avalonia, Iberia and Armorica. Cornubia lay along the southern part of the Avalonian Terrane (Bristow et al., 1998), which had been uplifted (during Cadomian orogenic movements) prior to separation, to form an E-W trending topographic high with a series of basins and intervening shelf seas, called Pretannia. The basement comprised metamorphic, plutonic and volcanic rocks forming a magmatic arc, lying between the Rheic Ocean, to the south, and the Iapetus Ocean (extending to 20°S), to the north (Durrance, 1998).
Little is known about the character of these basement rocks apart from isolated outcrops (e.g. Eddystone Rocks in the English Channel) and studies of clasts from Devonian sediments, but rock types such as mica schists, gneisses and rhyolites have been recorded (Durrance, 1998). A small area of gneiss (the Man of War Gneiss) has been identified within the Lizard Complex (Flett, 1946; Green, 1964) of southern Cornwall. This is a relict piece of pre-Variscan basement and represents a suite of originally gabbroic to tonalitic, calc alkaline arc, plutonic rocks (Sandeman et al., 1997). They outcrop in the shoals off Lizard Head [SW700110] and represent an exotic suite of rocks seen nowhere else in the complex. The gneiss has been dated (Sandeman et al., 1997) at 499 Ma (Tremadoc, Lower Ordovician) which makes it the oldest dated rock in South West England.
Pretannia was deeply eroded throughout the Lower Paleozoic, supplying sediment to the Welsh Basin and to northern/central Germany (Zeh et al., 2001). The shelf sea sedimentation on the flanks of Pretannia is now recorded by exotic blocks of quartzite (of Ordovician age) that occurs trapped in Devonian melange sediments on the Lizard Peninsula (Flett, 1946), and similar pebbles found as clasts in Triassic conglomerate units in east Devon (Edmonds et al., 1975).
The accretion of Avalonia with Baltica (Fennoscandanavia and Denmark) during the Early Silurian (Late Llandovery), following the closure of the Tournquist Sea that formerly lay between them, was accompanied by considerable transcurrent movements along a series of major NW-SE trending wrench faults (Durrance, 1998). Such faults often penetrate deep within the crust and may persist as planes of weakness long after accretion has been completed. They may be reactivated by later stress fields to give vertical and horizontal shear displacements. These wrench faults (see Figure1), many of which cut across the whole Cornubian Peninsula, played a major role in the development of Devonian/Carboniferous, and later, sedimentary basins across the region (Dearman, 1963; Selwood, 1990) and have also influenced granite emplacement; their reactivation before, during and after the Variscan Orogeny has had a profound influence on the structure of southwest England that continues to the present day (Turner, 1984).
The closure of the Iapetus Ocean at the end of the Silurian saw the accretion of Avalonia/Baltica with Laurentia (North America, Greenland and northern Britain) and resulted in the Caledonian-Appalachian Orogeny (Powell and Phillips, 1985) which gave rise to the Caledonian foldbelt, the unification of northern and southern Britain, and the formation of the Old Red Sandstone continental landmass, straddling the equator.
Figure 1. The major fault systems of Cornubia, many of which are thought to be Pre-Devonian in age (after Camborne School of Mines, 1988).
Caledonian deformation was intense across northern Britain, but decreased southwards. Across Cornubia it is difficult to locate definitive Caledonian structures, but it has been noted that many mineral lodes have a Caledonian (NE-SW) or Cadomian (E-W) trend and their host fractures may have been influenced by underlying structures in the basement (Durrance, 1998).
Pre-Variscan extension and sedimentation.
Following the closure of Iapetus, the area now known as Cornwall, lay on the floor of an epicontinental extension of the Rheic Ocean. To the north lay the coastline (running through South Wales and the English Midlands, extending to Southern Ireland and South East Poland - a distance of over 2500 km) of the 'Old Red Sandstone' continent, known as 'Laurasia', composed of Scandinavia, Greenland, North America (though only Labrador, Northern Canada and the Appalachian Mountains were above water) and most of Britain (Figure 2).
Figure 2. Early Devonian (395 Ma) paleogeography of the Variscan orogenic belt (after Cook et al., 2000).
By late Lower Devonian times the relief of the continent had been much reduced by erosion; swamps and deltas drained south into the Rheic Ocean, which had progressively transgressed across central and northern Cornwall (Isaac et al., 1998). Extensional movements and differential subsidence/uplift on E-W (Cadomian) and NW-SE basement structures lead to the development of ensialic sedimentary basins and rift-related basic volcanism (Andrews et al., 1998). The area of the SW Peninsula was occupied by six E-W trending sedimentary basins (Figure 3) during the Devonian and Carboniferous. These were (from SW to NE) the Gramscatho Basin (with the associated Lizard and Start Complexes), the Looe Basin, the South Devon Basin, the Tavy Basin, the Culm Basin and the North Devon Basin (Leveridge & Hartley, 2006)
Figure 3. A map of the sedimentary basins of Cornwall. After Leveridge & Hartley, 2006.
The Gramscatho Basin occupying South Cornwall was limited to the North by an outer shelf within which the Looe Basin developed. The sedimentary basins show evidence of sequential development and infill from south to north, during the Devonian and Carboniferous, with sediment supplied by NW-moving stacked nappes (that were initiated in the Lower Devonian by the progressive closure of the Rheic Ocean), and by fluvial systems bringing in sediment from central Europe (Sherlock et al., 2000). Periods of deposition were punctuated by episodes of rift-related alkali basalt magmatism and volcanism within the basins (Goode and Taylor, 1988). The successions in the Looe and South Devon basins show along-strike variations due to a westward deepening and a greater submergence of the intervening highs. The successions in all basins are fault-bounded, dismembered or interdigitated by episodes of deformation brought on by convergence (Leveridge & Hartley, 2006). The relationship between the lithostratigraphic units in the various basins is shown below in Figure 4.
Figure 4. The chronostratigraphy of sedimentary basins across Cornwall and Devon during the Devonian and Lower Carboniferous (after Leveridge & Hartley, 2006, Leveridge et al. 2002).
The sediments of the Gramscatho and other basins (Figure 5) are characterised by thickly bedded, medium to coarse grained, disorganised sandstones, mudstones and turbidites (e.g. the Porthtowan Formation), repetitive graded siltstones and mudstones (e.g. the Mylor Slate Formation), chaotic gravel/mud deposits (including olistoliths, as in the Roseland Breccia Formation), mudstones and subordinate conglomerates and associated shelf sediments (Isaac et al., 1998). In the Carboniferous sequences, cherts and tuffs are locally important (Isaac and Thomas, 1998). Volcanic sequences are found within the sediments. They are most prolific in West Cornwall, accounting for 10-20% of the Mylor Slate Formation (Isaac et al., 1998). They were originally tholeiitic basalts and spilites and generally follow the strike of the host rocks. There is evidence that they were emplaced in several phases, though most show evidence of being affected by the first phase of deformation (with the development of cleavage planes) during the Variscan Orogeny. K-Ar dating (Dodson and Rex, 1971) of these basaltic rocks places them between 365-345 Ma; contemporaneity of some of these rocks with the host Mylor Slate Formation is evidenced by vesicular textures, pillows and autobrecciation (Leveridge et al., 1990).
Figure 5. An interpreted reconstruction of the Rhenohercynian Zone and the sedimentary basins and fault systems in the pre-Devonian basement c.397-375 Ma (though their geometry and dimensions have been considerably modified from the original by later Variscan folding). SPL, Start-Perranporth line fault. PBF, Plymouth Bay Fault. SF, Sticklepath Fault (after Cook et al., 2000).
The basalts originally had a mineralogy of plagioclase, clinopyroxene, ilmenite ± olivine. The rocks (which range from fine to coarse grained) have now been altered to sodic plagioclase, clinopyroxene, epidote and chlorite. Within the metamorphic aureole of the Cornubian granites, the mineralogy is more likely to consist of andesine, hornblende and biotite with some epidote and chlorite. These rocks have undergone a variety of structural and metasomatic alterations and are collectively known in Cornwall as 'Greenstones' (Robson, 1964). Most of these rocks occur in the lower half of the Mylor Slate Formation to the West of St Ives and in a NE-SW zone between Penzance and Camborne. High level sills predominate, but there are also pillow lavas and rare volcaniclastic debris flows (Goode and Taylor, 1988). The tholeiites and spilites are associated with minor trachytes and keratophyres (which show intraplate incompatible-element-enriched chemical characteristics); the presence of the Lizard Complex and MORB-type alkali basalts and dolerites (Isaac and Thomas, 1998) are evidence that true oceanic crust developed locally within the Rhenohercynian zone (and external Saxothuringian zone; Figure. 2.4) due to crustal attenuation. Geochemical studies of these rocks (Merriman et al., 2000) have shown a change from subalkaline and within-plate alkaline magmas (contaminated with crustal material) in the Lower Devonian, to within-plate OIB-type alkaline magmas in the Middle Devonian to Lower Carboniferous. The magmas display geochemical changes marking the onset of rifting (subalkaline magmas) to the establishment of mantle-sourced magmas during the main stage of rifting. Closure of the sedimentary basins lead, in some cases, to the tectonic emplacement of metabasic schists within nappe units, derived from decoupled oceanic lithosphere. Silicic volcanism associated with the basalts (in south Devon) shows a similar evolution from Lower Devonian calc-alkaline rhyolites (with crustal input) at the initiation of rifting, to Middle/Upper Devonian trachytes produced by extreme fractionation during the production of oceanic crust (Jones and Floyd, 2000).
The Lizard Complex (Flett, 1946; Andrews, 1998; Styles et al., 2000) is a preserved fragment of this oceanic crust and in parts displays the upwards sequence harzburgite - gabbro - sheeted dykes (Bromley, 1989), although much of the complex is converted to serpentinite, amphibolite and mafic granulites by later in-ocean deformation and Variscan metamorphism and deformation. The complex can be divided into three tectonic units (Jones, 1997) with a base marked by the Old Lizard Head Thrust; the basal unit, the Goonhilly Downs unit and the Crousa Downs unit. These units are divided by thrusts developed after initial magmatism. The history of the complex suggests an origin at a slow-spreading N-S oriented ridge axis (D1 fabric); a Lower Devonian U-Pb date of 397 ± 2 Ma (Clark et al., 1998) for a plagiogranite at Porthkerris [SW806230] has been suggested as the timing of initial magmatic emplacement at the spreading axis. Another interpretation of the complex by Cook et al. (2000) has suggested an origin as exhumed mylonised mantle peridotites (emplaced along lithosphere-scale shears during the episodes of crustal extension that saw the development of the rifted sedimentary basins) emplaced in a rifted pull-apart basin on the continental margin.
Throughout the Devonian Period (408-360 Ma) and the succeeding Carboniferous Period (360-286 Ma) the Rheic Ocean was in the process of closure due to the progressive northward movement of the African 'Gondwanaland' continent and its subsequent collision with Laurasia. This involved the accretion of the Avalon-Meguma and Austro-Alpine microcratons during the Middle Devonian and the Aquitaine-Iberian microcontinent during the Lower Carboniferous against the Southern margins of Laurasia (Andrews et al., 1998). By the Upper Carboniferous, the Western arm of the Rheic Ocean had been eliminated and the two macrocontinents came into collision.
The Rheic basin, trapped between the two continents, was compressed and folded to form a fold mountain belt - the Variscan Orogen, which can be subdivided into a number of tectonic zones separated by major thrusts, ranging from the northern (marginal) Rhenohercynian Zone to the (internal) Saxothuringian and Moldanubian Zones of Central Europe (Figure 6).
The history of the Variscan Orogen is one of the staged closure of sedimentary basins by the subduction of oceanic segments, together with the development of parautochthonous and allochthonous units bounded by northwards directed thrusts. These stacked nappes were associated with low-grade (greenschist) metamorphism (Phillips, 1964; Primmer, 1985a) and later, post-orogenic granite magmatism.
Unlike much of the Variscan Orogen, the Rhenohercynian Zone (including Cornwall) developed late, in the Early Devonian, and closed rapidly, by the end of the Devonian. It comprised a series of small deepwater basins, bordered to the north by the shallow marine shelves running through modern day Somerset and North Devon. The extensional development of the orogen during the Devonian and Early Carboniferous is compatible with an ensialic back-arc setting. Northward-dipping subduction was established in Southern Europe in Lower Devonian times. Thermal doming, related to mantle convection, lead to the development of a crustal rise (composed of pre-Variscan crystalline basement mica schists and gneisses; Durrance, 1998) called 'Dumnonaea' which supplied sediment to the southern part of the Rhenohercynian Zone.
Figure 6. A map of the megazones of the Variscan Orogen (Variscan Massifs coloured). 1. Rhenohercynian Zone 2. Saxothuringian Zone 3. Moldanubian Zone 4. South European Zone (after Bromley and Holl, 1986).
The earliest evidence, in the Gramscatho Basin, of NW thrusting (the advancing Normannian (nappe) High) is provided by the development of ductile shear zones and thrust slices along the line of the Mullion Thrust. This has been tentatively dated at around 380 Ma (Clark et al. 1998). The Lizard Complex was rapidly exhumed from ~40 km depth at 1250-1300°C (Nutman et al., 2001) and became progressively sheared and cut by a series of thrusts. The incorporation of this hot (>1000°C) 'slab' into the nappe pile lead to dynamothermal metamorphism, at ~600°C and depths of 10 - 14 km (Sandeman et al., 2000), inducing temperatures in excess of 800°C in the adjacent rocks (Nutman et al., 2001). Overthrusting (leading to the first signs of extensional collapse in the advancing nappe front; Leveridge et al., 1990) following decoupling along the Lizard Thrust, generated a series of mixed granitic/basic melts that were intruded into the nappe pile. These melts appear to be derived from a mixed sedimentary/basement (Gramscatho Group sediments and underlying basement; Nutman et al., 2001) and a MORB basic source and were emplaced at 376.4 ± 1.7 Ma (Sandeman et al., 2000). This age constrains the earliest point at which the Lizard Complex was becoming incorporated onto the Laurasian foreland as Givetian (Middle Devonian) and the point at which the Man of War Gneiss (Sandeman et al., 1997) became incorporated into the nappe pile. The intrusion of these melts (now known as the Kennack Gneiss) overlapped with the amphibolite facies dynamothermal metamorphism and subsequent uplift and cooling. The Lizard Complex, caught between the Normannian Thrust and the Lizard Thrust, was transported further north, cooling as it did so. By the time it reached the point of maximum convergence it was cold enough not to affect the 3 km of Late Fammenian sediments that it over-rode (Jones, 1997) and which underlie the 'thin-skin' 1 km thick slice.
The Lizard Thrust is a very important crustal feature. Dipping at 20-35° it can be traced down into the crust, on seismic profiles, to depths of 17 km (Evans, 1990) and along strike from Plymouth Bay to the St Mary's Wrench Zone, some 130 km west of Land's End, out in the Western Approaches (Figure 6). Other thrusts and nappes can be detected, including the Normannian Thrust and Carrick Thrust (the Carrick Nappe is 10-12 km thick offshore, twice its thickness onshore). These features, along with several other, lesser, structures are seen to converge at depth, to merge with the top of the lower crust reflector at around 20 km depth.
Progressive closure of the ocean saw the advancing Normannian Nappe overriding subducted oceanic crust and the development of thrust-related highs that fed the contracting basins with flysch and melange sediments. From the Early Frasnian to Late Fammenian the remaining major thrusts that dominate the structure of West Cornwall developed. Seismic studies suggest that Southern Cornwall formed an interior part of the Variscan orogenic wedge and that the thrusts define a Northern parautochthon, plus (in ascending order) the Carrick, Veryan, Dodman, Lizard and (offshore) Normannian nappes (Andrews et al., 1998; Leveridge et al., 1990; Evans, 1990).
The Parautochthon represents the lowest structural level in South Cornwall. It is composed of the Porthtowan and Mylor Slate formations. Its base is defined by the regional R2 reflector and it also shows evidence of internal thrusting at depths of 1.5 - 4 km (Leveridge et al., 1990).
Figure 7. Deep crustal structures in the Western Approaches (after Evans, 1990).
The Carrick Nappe comprises the Portscatho Formation (thickly bedded sandstones and mudstones) and is bounded on its sole by the major Carrick Thrust. This can be detected to depths of 13.6 - 15 km where it meets a regional, sub-horizontal (decollement) reflector. Seismic studies suggest that at depth the nappe comprises crystalline basement (Leveridge et al.,1990).
The Veryan Nappe, is composed of a mixture of mudstones, calciclastic turbidites, cherts and MORB signature metabasites. The rocks are older than those they overlie in the Carrick Nappe (Eifelian as opposed to Frasnian) and show evidence of much greater rifting and thrust-related burial (Evans, 1990).
The Dodman Nappe forms a small, limited exposure onshore, at Dodman Point [SX001393]. It comprises the Dodman Formation, which shows similar characteristics to the rocks of the Veryan Nappe. The Dodman Thrust can be traced offshore and has been interpreted as a branch of the Lizard Thrust (Figure 7; Evans, 1990).
The Lizard Nappe structurally overlies the Dodman Nappe and this is followed by the Normannian Nappe. This unit represents continental basement (mica schists and granitoid gneisses; Edmonds et al., 1975) that originally formed the Southern margin of the Gramscatho Basin. It is exposed offshore at Eddystone Rocks [SX382342] and has been seismically profiled in the English Channel (Evans, 1990).
The progressive stacking of these nappes and their impingement on the Laurasian Foreland lead to crustal thickening and shortening, metamorphism (Primmer, 1985a; Warr et al., 1991) and a number of distinct deformational and folding events (Matthews, 1977; Badham, 1982; Rattey and Sanderson, 1984; Alexander and Shail, 1995, 1996). This pattern is repeated for each of the basins in turn as the continental collision progressed. Each basin has its own distinct deformational and nappe history (Selwood and Thomas, 1985; Andrews et al., 1988; Pamplin, 1990).
In southern Cornwall the convergence between the Armorican Massif and South West England began in the Givetian, but was interrupted by Frasnian-Fammenian rifting. Renewed convergence brought about the closure of the basin by the Tournaisian (Lower Carboniferous). Continued NNW-SSE shortening above a regional decollement involved the various nappe piles (Andrews et al. 1998).
Three major deformational events (Rattey and Sanderson, 1984; Alexander and Shail, 1995, 1996; Shail and Alexander, 1997; Alexander, 1997) can be distinguished in the Gramscatho Basin (Table 1). The D1 event (Early Carboniferous) is related to thrust nappe emplacement and NNW-SSE shortening, with folds verging NNW. Fluid inclusion studies of syn-D1 quartz veins suggest that this event is coeval with peak metamorphic conditions (M1) at 3.2 ± 0.3 kbar and 320 ± 10°C. This equates to a maximum depth of 13 km (Harvey et al., 1994) and is consistent with crustal thickening by major nappe emplacement. Taking into account the thickness of the crust prior to convergence (32 km; Watson et al., 1984), this would give an orogenic crustal thickness of ~40 km.
The D2 deformation (D1 and D2 convergence lasted into the Visean - a period of some 65 Ma; Harvey et al., 1994) defines somewhat similar conditions to the D1 event, but fluid inclusion studies of syn-D2 quartz veins indicate that metamorphic conditions (M2) were 1.2 kbar fluid pressure at 270°C, which equates to a depth of 4.5 km (Shail and Wilkinson, 1994; Harvey et al., 1994. This suggests exhumation (M2 saw the overgrowth of the M1 fabric indicating continued heating during decompression on uplift; Primmer, 1985a) due to erosion or syn-convergence extension (Shail and Wilkinson, 1994).
The sedimentary pile, within the various basins, underwent diachronous regional metamorphism (resulting in the formation of slates from original mudstones), but at temperatures generally less than 350°C, which has allowed many of their primary features to be retained (Andrews et al., 1998). The general trend is one of increasing grade from the Culm Basin in Devon (diagenetic grade, <200°C), through E-W trending (structurally defined) belts of anchizone (200 - 300°C) and epizone (>300°C) metamorphism (Warr et al., 1991), to the epizone belt of southwest Cornwall. Basic rocks were metamorphosed to prehnite-pumpellyite facies in the anchizone areas, and to actinolite (lower greenschist) facies in the epizone areas (Primmer, 1985a). Thermal gradients during metamorphism exceeded 30°C km-1.
Table 1. The deformation history of the Gramscatho Basin area of southwest Cornwall (after Alexander and Shail, 1995).
The final closure of the Rheic Ocean was a transpressive event with the accretion of the Armorican and Iberian microcontinents with the Avalonian Terrane taking place along an E-W trending sinistral fault zone (reactivated Cadomian structure?) that marks the boundary between elements of the northern and southern shores of the ocean (Durrance, 1998); it is thought that this boundary is represented today by the Start-Perranporth Line.
The effects of the Variscan convergence were felt far beyond South West England; as far away as Lancashire and across the Midlands Late Westphalian NW-SE to NNW-SSE shortening of rift-generated sedimentary basins has lead to basin inversion (Corfield et al., 1996; Ruffell and Shelton, 1999; Burgess and Gayer, 2000) with the development of chevron folding, oblique slip and en-echelon periclines and open folds. These inverted, uplifted basins later supplied sediments to the surrounding areas.
Post-Variscan extension: the prelude to granite magmatism.
The close of Variscan convergence across Cornubia in the Late Carboniferous left the region as a mountainous terrain, with a crustal thickness of ~40 km, though in topographic terms it is unlikely that land surface elevations exceeded >3 km above sea level. The initiation of a period of orogenic collapse (extension) very soon after the close of convergence, meant that this topographic high was relatively short-lived and by the end of the Lower Permian crustal thicknesses had returned to their pre-orogenic state (~30 km) and by the Late Triassic the land surface was at, or near, sea level (Shail and Alexander, 2000). Extension was accompanied by renewed rifting, the formation of sedimentary basins and volcanism (Cornwell et al., 1990).
This phase of deformation (the D3 event) is related to NNW-SSE late orogenic extension (200°C, <0.5 kbar) and the reactivation of earlier structures. This movement is related to pre and syn-granite extension and the emplacement of mafic dyke rocks (Shail and Wilkinson, 1994). Later post-D3 deformation (140°C, <0.5 kbar) is related to NNW-SSE extension and ENE-WSW extension that eventually formed the fracture set utilised by elvan dykes and mineral lodes of the district (Alexander and Shail, 1995; 1996).
Onshore extensional reactivation of Variscan structures, with the generation of SSE verging F3 folds (Alexander, 1997; Shail and Alexander, 1997; Shail and Wilkinson, 1994), began in the late (Stephanian) Carboniferous (NNW-SSE regional extension) and persisted into the Triassic (ENE-WSW regional extension and dip-slip reactivation along NNW trending high-angle faults). The amount of extension that took place within the relaxing orogen has been calculated at 15 - 20%, the crustal thinning effectively cancelling any topographic evidence of the Variscan convergence and the development of lithosphere-wide extensional fractures facilitating the intrusion of lamprophyre dykes related to crustal underplating (Clark et al., 1993). It is during this phase of extension (NNW-SSE), which lasted from approximately 295 - 270 Ma that the intrusion of the Cornubian granites, elvans and most main-stage mineralisation occurred. The interaction of the Variscan structures with a pre-Variscan, dextral (Badham, 1982), E-W, transform fault system (running from the Uralides, through Europe (Pitra et al., 1999), to the Appalachians) and the NNW-SSE trending wrench fault system produced a complex series of conjugate shear zones and pull-apart structures (Willis-Richards and Jackson, 1989) that remained active throughout the early Permian. These pull-aparts had a profound influence on the emplacement of the upper reaches of the Cornubian Batholith.
D3 deformation can be divided into three main styles: ductile distributed shear, brittle-ductile detachments and brittle listric faults (Alexander, 1997), which all display a top sense of shear to the SE and trend towards more brittle styles over time (Alexander and Shail, 1995), reflecting extensional exhumation which may, in part, be related to (and facilitated) the intrusion of the Cornubian Batholith. In the zones of distributed (ductile) shear the short limb lengths of F3 folds are generally <1 metre and the axial planar cleavage (where developed) dips gently to the NW (Alexander and Shail, 1996); these fold can be observed on both the north and south coasts of west Cornwall (in particular on the northern and southern margins of the Land's End Granite), in the footwall of the Carrick Thrust, where they are cut by later extensional faults (many of which have been utilised by the granite during emplacement) indicating that extension and exhumation into the brittle domain pre-dated granite emplacement.
Continuing (if punctuated) extension into the Permo-Triassic, with changing stress regimes, saw Lower Permian ENE-WSW strike-slip faulting (during ENE-WSW shortening; related to the formation of the 'caunter lodes' of the Camborne-Redruth District) followed by NNW-SSE (mesothermal lode mineralisation) and ENE-WSW (crosscourse and St Just District mineralisation) extension, fault development and reactivation of existing structures (Alexander, 1997; Alexander and Shail, 1995) before the Post-Variscan episode was finally brought to a close in the Lower Triassic.
Spatially associated with, but pre-dating, the Cornubian granites (in a belt up to 25 km from the granite contacts) are a series of mantle-derived basic (and ultrabasic) dyke rocks (Leveridge et al., 1990; Manning, 1998). These lamprophyres occur in steeply dipping sheets that run parallel to the locally dominant cleavage (though some sill-like bodies do occur) or occupy extensional faults or pre-existing joints (Selwood et al., 1998). The dykes generally trend NE-SW, but trend N-S around Falmouth [SW8032] and Truro [SW8345]. Abrupt changes in dip and strike are common and en-echelon arrays of dykes occur locally. Changes in thickness are common, but average widths are between 1 and 15 metres. Petrographically they are described as phlogopite minettes or olivine-phlogopite minettes and carry phenocrysts of phlogopite with biotite rims. They are fine-medium grained reddish brown rocks (Manning, 1998). They are particularly susceptible to weathering and fresh olivine rarely survives. They are typically enriched in 'mantle-incompatible' elements such as Ba, P and Sr and are peralkaline. They often carry xenoliths (slates, sandstones, quartz and non-megacrystic, reddened granitic rocks; Leveridge et al., 1990) and have brecciated contacts. A radiometric date of 296 ± 5 Ma (Darbyshire and Shepherd, 1985) places them roughly coeval with the earliest granitic intrusions in the Stephanian. Unfortunately there are no known exposures that show lamprophyres and granites in contact, so the exact relationship cannot be ascertained.
The lamprophyres are largely unaffected by Variscan deformation and are therefore post-orogenic. They are derived from magmas generated in the heterogeneous mantle lithosphere or asthenosphere (Exley et al., 1983). It is assumed that the extending Variscan orogen, being underplated by mantle-derived magmas, was crossed by a series of extensional faults that penetrated to the base of the crust and tapped these magmas. The heat flow from underplating, extensional decompression of already hot lower crust and heat supplied to the crust by the lamprophyric magmas would be enough to instigate crustal anatexis (estimated conditions of anatexis are 860°C ± 60°C, 5.2 ± 0.4 kbar with 3-5% H2O; Stone and Exley, 1986; Charoy, 1986) and give rise to the granitic magmas that went on to form the Cornubian Batholith.
The origins of the Cornubian Batholith.
The granites of the Cornubian Batholith show a number of similarities to their earlier Caledonian (Cairngorm) counterparts in Scotland (Watson et al., 1984) and to near contemporary Variscan granites in Spain (Saavedra, 1982) and Bohemia (Stemprok, 1995). All are peraluminous, high SiO2, corundum normative and HHP granites. They have a close spatial association with lamprophyric dykes (and hence, a close association with extensional underplating mantle tectonics) and display similar tabular forms. However, unlike the Cairngorm Granites, those in Cornubia are rich in boron and highly mineralised (on a scale that far exceeds their mineralised Variscan counterparts elsewhere in Europe). Watson et al. (1984) suggested the ultimate source for both suites of rock was metasomatised mantle. Contemporary and more recent work refutes this as the source of the bulk of the granite itself (Darbyshire and Shepherd, 1994), but there is a growing body of evidence to show that the mantle played a major role in supplying heat to the relaxing orogen and also transferred fluids (Shail et al., 1998; Whitney, 1989) into the source region (and lower crust in general).
There is a strong spatial and temporal association between lamprophyre dykes and granite intrusions in Bohemia (Holub and Stemprok, 1999), though there is disagreement about their contribution in terms of granite magma generation; Gerdes et al. (2000), argue against a lamprophyre and mantle contribution to granitic melting (via heat and fluid input) within the Bohemian Massif and favour radiogenic heat generated in the thickened crust prior to orogenic collapse as the main anatectic heat source. There are very strong similarities between the Bohemian granites and those of Cornubia, both in terms of tectonic setting and geochemistry (Forster et al., 1999; Gerdes et al., 2000), and also in terms of mineralisation (Stemprok, 1995; Holub and Stemprok, 1999). There is, however, no established link between the lamprophyre suites present in Bohemia and mineralisation, though, as in Cornubia (Shail et al., 1998), there is a strong suggestion that they acted as pathways for mantle-derived fluids to enter the orefield (Holub and Stemprok, 1999). There is as yet no unequivocal evidence of mantle magmas mixing with the granite at depth (Clark et al., 1993; cite igneous mafic microgranular enclaves in some of the granites as potential lamprophyric inclusions, resulting from incomplete mixing).
Isotopic studies by Darbyshire and Shepherd (1994) and others have constrained the source region and parent rocks of the Cornubian magmas. Nd model ages for the granite suggest that the parent rocks are Proterozoic in age (1289 - 1788 Ma) and metapelitic in character, with intermediate volcanic rocks and recycled clastic sediments, and that anatexis took place in the lower crust. The isotopic data indicates more than one distinct source for partial melting and also indicates mantle involvement The parent rocks (Manning, 1998) are suggested as being Brioverian metapelites (similar age rocks outcrop in the Channel Islands); either poorly hydrated garnet granulite (anatexis at 7-8 kbar and 800°C; Floyd et al., 1983), or cordierite-sillimanite-spinel gneiss (anatexis at 5 kbar and 800°C; Charoy, 1986).
Variscan convergence was followed by post-orogenic collapse (Alexander, 1997; Alexander and Shail, 1995, 1996; Shail and Alexander, 1997), exemplified by the D3 event, in which the extending crust would have been heated from below by decompression and mantle underplating. Faulting of the crust, sufficient to generate fractures capable of tapping the upper mantle, lead to the widespread intrusion of lamprophyre dykes in the late Carboniferous which would have supplied further heat to the crust. These conditions would have been sufficient to initiate large-scale anatexis, at around 800°C, accompanied by volatile streaming from the mantle (supplying REE's, He, U, Th, Cl, F, Cs, Rb, Cu? etc). Various estimates place the depth of anatexis at around 19-24 km (Charoy, 1986), in a ~40 km thick crustal section.
These anatectic events are envisaged as taking place at a number of sites throughout Cornubia. They also encompass the offshore Haig Fras (Western Approaches; Evans, 1990) and Granite Cliff 'granites' (on the continental slope), which have a broad 'Cornubian' trend along, strike (indicating that the tectonic regime seen in onshore Cornwall and Devon is far more extensive than can be ascertained from our limited exposure) and also extend to granites of similar age and characteristics in Iberia and elsewhere in Europe. Some of the Iberian granites show many similarities with those in Cornubia, with emplacement in extensional tectonic regimes, accompanied by pull-apart development (exemplified by extensional shear zones within the granite) and mantle underplating and magmatism (Barbero, 1995), while others formed in compressional environments (Evans et al., 1998).
The development of Late- to Post-Variscan sedimentary basins.
In the southwest, under the present English Channel a number of sedimentary basins, exploiting Variscan structures were developed from the late Carboniferous onwards. The largest of these, the Plymouth Bay Basin (Evans, 1990: Harvey et al., 1994; Ruffell et al., 1995) developed following Variscan convergence and subsequent extension (D3 as recorded onshore) to create space and form the earliest depocentre. Some 10 km of Carboniferous-Permian-Triassic sediments collected in the basin (Figure 8). A marked change in depocentre orientation from E-W to NW-SE, mirroring tectonic regime changes onshore, coupled with potassic basic volcanism (the Exeter Volcanic Series) marks the Carboniferous-Permian boundary and is related to tectonic regime that resulted in the intrusion of the Cornubian Batholith (Evans, 1990).
Figure 8. Offshore and onshore Post-Variscan sedimentary basins around Cornubia and the Western Approaches (after Evans, 1990).
Later, lower Triassic (Scrivener et al., 1994), movements along wrench faults intersecting the basins, channelled basinal brines (carrying Pb, Zn, Ag, U, etc) into the Cornubian Massif and into the N-S to NNW-SSE trending 'crosscourse' fractures (Jackson et al., 1989). These brines were responsible for the mesothermal to epithermal phase of mineralisation that was responsible for the bulk of Pb/Ag deposition across the Tamar Valley and eastern Cornwall; as well as contributing U, Co, Ni and Fe to isolated mineralised faults across the western half of the peninsula (Dines, 1956).
The Cornubian Batholith is associated with a major (formerly World class) metallogenic province (Taylor, 1979). Much of this mineralisation is directly related to the granite batholith itself, which contains above World average amounts of tin (17-58 ppm) and tungsten (20-30 ppm) and appreciable amounts (4-108 ppm) of copper (Thorne and Edwards, 1985). Much of this metal content is thought to have been inherited from the source region of the granite, though some workers (Hutchison and Chakraborty, 1979) suggest that some of this material may have originated in the mantle; a possibility made more plausible by the discovery of mantle-derived helium in fluid inclusions from the main-stage lodes (Shail et al., 1998). The mechanism of volatile transfer in this case may be by direct fluid input into the lower crust (thereby aiding anatexis) by the dehydration of alteration phases in the upper mantle (Whitney, 1989) or by volatile transfer from intrusive mantle-derived magmas, such as the lamprophyre suite. Some mafic enclaves in the granites of Cornubia may represent assimilated or trapped lamprophyric material and there is a possibility that there may be some sort of 'interdigitating' zone at depth (though the bulk composition of the exposed granites mitigates against any large-scale direct mixing) where transfer may have occurred. Such a mechanism is not unusual and both volatile transfer and magma mixing have both been recognised as pathways in which mantle-derived components can be added to silicic systems (Whitney, 1989; Maughan et al., 2002; Hedenquist and Lowenstern, 1994).
The granites comprising the batholith were rich in water, boron (Charoy, 1986) and other volatiles, with high trace levels of alkali metals such as Li and Cs (Stone, 1982); a feature which they have in common with other tin granites around the world (Saavedra, 1982; Dietrich et al., 2000). This enabled them to transport large quantities of metal as halogen (principally chloride) brine complexes (Taylor, 1979; Rankin and Alderton, 1985; Hedenquist and Lowenstern, 1994), complex silico acids (Horsnail, 1979) and vapour complexes (Candela, 1989a). Further material was added to the granites at high crustal levels, during stoping, by the assimilation of xenoliths (Clarke et al., 1998).
The onset of deformation in the Variscan Orogeny resulted in the trapping of large volumes of connate/meteoric water by pore space collapse and loss of permeability. This resulted in the development of supra-hydrostatically pressurised formations containing a variety of Br/Cl brines (with SMOW characteristics). These brines (capable, under the right conditions, of scavenging base metals from the formations they passed through) were released with the onset of orogenic extension (Meere and Banks, 1997) via extensional faults. In Cornubia there is isotopic evidence (Primmer, 1985b) to show that some of these formational and metamorphic fluids contributed to the hydrothermal mineralisation (Wilkinson, 1990) and may also have played an important role in mineralisation in some districts (St Endellion) prior to the onset of orogenic extension (Clayton et al., 1990).
With continued crystallisation leading to the concentration of incompatible elements, a series of residual volatile reservoirs formed in the apical sections of the various plutons (Dines, 1956; Halls 1994). Subsequent vapour pressure increases lead to hydraulic fracturing and the formation of high temperature greisen veins (Halls et al., 2000), breccia pipes (Allman-Ward et al., 1982) and eventually, in conjunction with extensional and strike-slip faulting (Farmer, 1991), the formation of the main-stage lodes of the region. The initiation of large convection cells around the granite and the influx of meteoric fluids into the hydrothermal system (Jackson et al., 1982) allowed metals leached from the surrounding slates and metabasic rocks (principally copper and zinc, with minor lead) to be incorporated into the lode system (Hosking, 1979) and, later, allowed significant remobilisation and concentration of ores in some of the lodes (Dines, 1956).
Reactivation of NNW-SSE trending faults in the late Permian - early Triassic allowed basinal brines, from the Plymouth Bay Basin, etc, to enter the Cornubian Massif and contribute a second phase of mineralisation to the orefield (Scrivener et al., 1994; Gleeson et al., 2000). This resulted in the mineralisation of some of the fault structures (known as crosscourses) with lead, silver and a variety of minor metals, as well as uranium. This phase was most important in east Cornwall and the Tamar Valley where extensive Pb/Ag mineralisation took place (Dines, 1956).
Hydrothermal activity continues within the granites to this day, maintained by the high radiogenic content of the granite, with the circulation of warm brines (Burgess et al., 1982) at depth, charged with a variety of salts (Beer et al., 1978).
This page last updated 01/01/2007
