CLASSIC CORNISH FIELDWORK LOCATIONS
PORTHMEOR COVE
Access & location:
Porthmeor Cove (SW425374) lies on the northern coast of Penwith mid-way between Morvah and Zennor. It can be most easily be reached by parking at the Carn Galver engine houses (SW 42123644) and walking some 1.5 km eastwards along the coast path. Alternatively, it is possible to park closer to the cove by the roadside, but this is not advisible for anything larger than a small car, due to the narrow road and verge.
Pros & cons:
Porthmeor Cove is probably one of the most important geological localities in SW England; a SSSI, it is the only location where a fully exposed granite cupola can be seen (Floyd et al., 1993) and provides excellent exposures of the granite contact, internal granite sheets and extensive veining/stoping/sheeting of the Mylor Slate and greenstone cover rocks. The slates are well exposed on the foreshore and on Long Carn; they are well cleaved and show evidence of three phases of cleavage as well as extensive folding and quartz veining that pre- and post-dates granite emplacement. The site can be visited at all states of the tide (in fine weather), though is best seen at low tide, when the beach exposures can also be seen. Exercise care on the steep slopes and narrow paths leading down to the beach and rock platforms. Make sure you take a camera, but leave your hammer at home!
Note: This is a SSSI, no hammering of rock surfaces is allowed, nor should material be removed.
Geology:
The geology of the area between Carn Gloose and Great Zawn, with the granite contact and cupolas at Porthmeor Cove (after BGS 1:50,000 sheet 351/358 with added personal field data).
A panorama across the east side of Porthmeor Cove showing the main granite outcrops.
Porthmeor Cove cuts across the contact metamorphosed killas (greenstones and Mylor Slates) to expose the coarse-grained megacrystic granite of the Zennor Lobe, at the back of the cove. Within the metasedimentary/metabasic cover later granite cupolas are exposed, together with a series of dykes, post-dating the main granite. The Mylor Slates are medium-grey to dark-grey in colour and show variable development of cordierite porphyroblasts, which vary from 5 to 15 mm in length. They have a very well developed bedding-parallel (as can be picked out by colour variations in the original sedimentary laminae) S1 cleavage which dips gently to the NNW (15/322) or points around N (353 to 006 degrees). Across large areas of the exposure this is the dominant structural control, which together with jointing gives rise to the stepped topography (with extensive planar surfaces) of most of the outcrops. Locally there is the strong development of SSE verging folds that belong to the D3 event (Alexander, 1997). These F3 folds have an axial planar cleavage (S3) that is best developed in the more psammitic (silt to fine-grained sand) units that occur occasionally within the slates. The F3 folds have small (<0.50 m) short limb lengths and while their fold axes plunge predominantly to the NE, there is some variation, suggesting that some of the fold hinges may be curvilinear along strike. The S3 cleavage dips NNW at angles of 30°-40° (sometimes showing refraction either side of more pelitic units). Outside of those areas with obvious F3 folding, the more psammitic beds still display the S3 cleavage showing it to be more pervasive than it at first appears. Rare instances of a very steep to vertical NNW-trending cleavage also occurs, which has been equated with S4 of Alexander and Shail (1996).
The slates contain a number of generations of quartz veins, the earliest (largely S1 parallel) are 'sweat quartz' derived by the movement and accumulation of silica during metamorphism (Primmer, 1985a). Later veins are largely tensional infillings, and several tension gash arrays can be seen in outcrop. These generally have a top sense of shear to the NW and would appear to be related to D1 or D2. These arrays may be folded around D3 folds and they are also cut by later granite veins and a second set of shallow-dipping (~40°) quartz veins (with small sinistral offsets). Several joint sets are present within the slates, the most prominent sets trend ENE-WSW, NW-SE and roughly E-W. On Long Carn the slates are cut by a number of ENE-striking sub-vertical granite sheets (offshoots from the cupolas below on the foreshore), reaching 0.50 metres in thickness. These sheets, and their branching offshoots, occur relatively closely spaced (~every 2-4 metres, locally), and vertically displace marker quartz veins in the slates over a range of a few cm, block-faulting some outcrop exposures. The granite sheets also send cleavage-parallel veins (typically 1-5 cm thick) laterally for considerable distances (some reach several metres) into the slates, many of which are transgressive, jumping from (S1) cleavage plane to cleavage plane via short, transverse 'jogs'. The overall appearance of these sheet/vein arrays is 'tree-like', a pattern that is seen at several points around the coast. The strike and attitude of these sheets is primarily controlled by joints, a fact which is borne out when comparing their trend with the dominant joint sets in the host slates.
On the foreshore below Long Carn the larger cupola of the pair is emplaced. Exposed in the steep cliffs, it is inaccessible and is best viewed from the west side of the cove, where it can be seen to have steep, sharp contacts with the host slates. A number of granite sheets and veins run off from the cupola into the surrounding country rocks and close to sea level a number of angular xenoliths have been described (Floyd et al., 1993). The smaller cupola (some 19 metres by 15 metres in area) is fully exposed on the foreshore on the SE side of the cove (SW42513770), near the main contact with the Zennor Lobe CGMG; this is exposed in the nearby cliffs, lying concordantly beneath a greenstone sill, the contact striking roughly E-W and dipping at 22°/352°.
The cupola and associated dykes exposed in the SE corner of Porthmeor Cove. The small zawn immediately to the south marks the line of a fault. Below the cupola a greenstone sill overlies a section of the granite roof which contains sub-horizontal fine-grained granite and medium-grained granite sheets. South of the fault the granite contact lies again beneath the greenstone sill, dipping to the NW.
A sketch section across the eastern granite contact in Porthmeor Cove, shown in the photograph above, illustrating the position of the smaller of the two cupolas, above the contact (Modified after Stone and Exley, 1984).
The Zennor Lobe Granite exposed in the cove is a coarse-grained and megacrystic biotite granite (Exley and Stone's Type B granite). Close to the contact with the overlying greenstone sill a roof complex of aplite, leucogranite and pegmatite is developed, typical of many SW granites (Floyd et al., 1993). Within the megacrystic granite the feldspars are aligned with their long axes parallel to (and 010 faces normal to) the contact. A further granite outcrop occurs beneath the cupola at sea level, cut off from the main outcrop by a fault trending 050° and marked by anastomosing thick quartz veins with earthy haematite; this would appear to be part of the main Zennor Lobe Granite rather than a later intrusion on the basis of cross-cutting relationships between dykes (of three separate generations) exposed around the cupola. This lower granite body, lying conformably beneath a continuation of the greenstone sill with the contact dipping 20°/350°, also has a thin roof complex zone. In the lower part of the outcrop a series of gently-dipping stacked granite sheets up to 0.50 m thick can be seen within the main CGMG. These sheets vary from coarse-grained granite with small (1-2 cm) megacrysts to fine-grained aphyric granite and have sharp contacts with each other and the host CGMG. Feldspar megacrysts in the CGMG are again aligned parallel to the contact. The granite is cut by (and the granite sheets displaced by) a number of ENE-WSW trending tourmaline-bearing fractures with tension gash vein-arrays indicating downthrow to the SE. On the south side of the outcrop (SW4256237618) two leucogranite sheets from the roof complex penetrate up into the overlying greenstone; one, 20 cm thick, pinches out after a few metres, the other, 30 cm thick and partially emplaced along a NW-trending listric fracture, becomes Dyke 1 of Stone and Exley (1984) and forms one of the cross-cutting dyke series exposed close to the cupola. The cupola itself has very sharp contacts and an angular outline controlled by joints in the host slates; the roof zone is also slightly domed. The cupola is composed largely of Type B megacrystic biotite granite (Stone and Exley, 1984), but towards the centre of the intrusion the granite becomes aphyric and has a banded appearance. Below the banded area the granite is only sparsely megacrystic. At the apex of the intrusion a 60-70 cm thick aplite/leucogranite/pegmatite complex occurs, directly below the roof contact. The pegmatite is composed of quartz, feldspar, tourmaline and muscovite and occurs in a number of parallel interdigitating sheets with leucogranite and tourmaline-rich granite.
The upper section of the cupola exposed in the SE corner of Porthmeor Cove, facing NE. Note the slightly domed roof and other sharp contacts, the prominent jointing and position of the roof complex. Several offshoot sheets from the cupola can be seen intruding the slates in the background.
From sections of the outcrop where the side walls of the cupola are visible, it can be seen that the outer contacts are very steep to sub-vertical, cutting across the inclined greenstone and slate units without disturbance (there is no deflection of S1 or S3 cleavages and F3 folds are cleanly cut at the contacts). The main Zennor Lobe granite appears to have tilted the host rocks, by some 20°-25°, during emplacement; the field relations therefore suggest that the cupolas post-date that tilting and belong to a later phase of granite magmatism. The time interval between the two phases is constrained somewhat by the fact that both cupolas and main granite are cut by the same series of ENE-WSW trending first-generation tourmaline (schorl) veins. In other areas of Cornubia the onset of main stage (blue tourmaline) mineralisation has been bracketed within 2-4 million years of emplacement (Chen et al., 1993; Chesley et al., 1993; Clark et al., 1993); it seems likely, therefore, that the cupolas were emplaced within a geologically short time frame after the main granite and that they may also be coeval with some of the sheet intrusions seen along the coast outside of the immediate area of Porthmeor Cove. Along the granite/slate contact (best viewed on the steep side contacts of the cupola) is a layer of quartz (with minor feldspar) ~ 1 cm thick and on the outer surface of this layer, abutting the slates, is a 1 mm thick veneer of schorl. Whether this layer represents a volatile 'reaction rim' from the intrusion, or represents material drawn out of the country rocks (Yoshinobu and Girty, 1999) is unclear; the contact with the slates is sharp, with the granite it is somewhat diffuse. The slates are baked close to the contact and lose their fissility, although the S3 cleavage in particular can be picked out by the presence of arrays of small cordierite porphyroblasts lying on the S3 surfaces. Around the intrusion the slates are cut by a number granite dykes and sheets that pre- syn- and post-date the emplacement of the cupola itself. The history of emplacement can be deduced by examining an area of the outcrop adjacent to the cupola where three cross-cutting dykes are present. Dyke 1 (of Stone and Exley, 1984) is ~0.30 m thick and composed of pale leucogranite; it rises from the roof complex of the CGMG beneath the greenstone on the north side of the fault and dips SE at 20°-30° at its extremity. This is cut (with an apparent sinistral offset of 0.60 m) by Dyke 2 which rises from the cupola and is ~0.50 m thick. The dyke is, like its parent, composed of megacrystic biotite granite and trends sub-vertically ENE-WSW. At other points around the cupola margin other sheets and granite vein networks extend into the host rocks. A third dyke (Dyke 3 of Stone and Exley, 1984), of tourmaline microgranite from 0.10-0.30 m thick, cuts across the cupola and cuts, slightly displacing, Dyke 2, before turning (transgressively) to run along the axis of Dyke 1, from which a branch again turns, back into the slates.
Dyke 2 (centre of picture, running left to right) cutting and displacing Dyke 1. Dyke 3 (slightly darker, with a band of 'ponded' tourmaline on its upper margin) can be seen running concordantly within Dyke 1 on the lower right margin of the photograph. Hammer (30 cm) for scale.
A perspective sketch showing the relationships between dykes 1, 2 and 3 in outcrop at Porthmeor Cove. Modified after Stone and Exley, 1984.
The emplacement of the cupola therefore post-dates the intrusion of the Zennor Lobe CGMG and the contemporaneous tilting of the country rocks which accompanied its emplacement. The development of a roof-complex in the main granite was followed by late-stage internal sheeting and the intrusion of Dyke 1. A second phase of granite magmatism saw the essentially passive, joint-controlled, emplacement of the two cupolas, by some form of small-scale cauldron subsidence (Stone and Exley, 1984). It has been postulated that the rhythmic banding seen in the roof complex of the southern cupola, is due to punctuated pressure changes brought about as the foundered block sank into the magma chamber (Bromley and Holl, 1986). Cupola emplacement was accompanied by widespread granite sheet and vein intrusion at a variety of scales along tensionally-dilated joint sets in the metapelites, forming interconnecting networks of sheets. Presumably if this level of connectivity between sheets and underlying magma had proceeded further, more areas of the metapelites would have been 'blocked out' on all sides and more blocks would have foundered, resulting in the intrusion of further cupolas or the extension of existing ones.
Limited late-stage tourmaline-rich magmatism (Dyke 3) was a precursor to the tourmaline-rich mineralisation that marked the end of the magmatic cycle and the start of hydrothermal processes. Some of the tourmaline veins have greisenised margins (to a maximum of ~8 cm each side of the vein) within the granite and are characterised by tourmaline-rich haloes (to ~2.5 cm each side of the vein) in the slates. There is some recognisable 'ponding' of tourmaline at the contact and the proportion of tourmaline in the veins drops rapidly in the slates to give monomineralic quartz veins within 1 metre of the contact. Many veins pinch out altogether within 0.50 metres of passing into the country rocks. On the west side of the cove (SW423374) the contact dips at around 15° to the NW, beneath hornfelsed slates with porphyroblasts of cordierite that reach up to 3 cm in length. In places the porphyroblasts are so profuse that they obscure the host rock almost completely. In a zawn on the west side of the cove (SW42283748) the granite contact can be seen dipping at a very shallow angle to the NW, at a point close to sea level, adjacent to what appears to be a NE-SW trending fault or master joint. On the nearby foreshore ENE-WSW trending granite sheets cut the slates and associated greenstones.
This page last updated 22/09/2003
