Estándares de Seguridad

To determine the volcano-tectonic conditions active during cone sheet emplacement, structural trends and local morphological variations both need to be considered. Identifying and mapping the extent of individual cone sheets is therefore vital and also provides a context for the data collected. Where the Ardnamurchan cone sheets are well exposed along several coastal sections, particularly along the southern shore, they collectively and individually display highly variable dips, anatomising strike trends and complex cross-cutting relationships (cf. Richey and Thomas, 1930; Keunen, 1937; Day, 1989). Figure 4.6 is a 1:1000 scale map of Mingary Pier (part of Area 1), which highlights the local variation in cone sheet strike and dip. As the majority of cone sheets are equigranular dolerites, that consistently weather to an orange–brown or pale grey colour (depending on whether they are located below the storm line or not), they are often difficult to differentiate from both field observations and hand specimens. Mapping discrete cone sheets inland (~>50 m from the coast) is further complicated as outcrops are sporadic and the host rock, as well as contact relationships, are often unexposed. Tracing chilled margins and identification of subtle variations in dolerite

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Fig. 4.6: A) Detailed 1:1000 scale map of small area at Mingary Pier (Area 1) highlighting the variation in cone sheet strike and dip as well as the complex cross-cutting relationships they exhibit. Inset: A full colour map provided to allow individual cone sheets to be better

distinguished. B and C) A photograph and interpreted photograph (including cone sheet strike and dips), respectively, of a gully wall exposed in the mapped area that clearly portrays the numerous local variations in cone sheet behaviour that are often encountered. For example, one dyke is observed to abruptly rotate into a sill where it seemingly deflects part of a sill into an inclined sheet (i). The geology in (C) is colour coded to the legend in (A).

grainsize may allow separate cone sheets to be distinguished. However, the paucity of exposure and internal grainsize variations often nullifies these techniques. To map the cone sheets a method of correlating individual sheet segments without relying on subtle textural variations is therefore required.

Many cone sheets (69 in total; ~26 %) analysed in the field throughout this study, and also noted by Day (1989), were observed to contain a volumetrically minor (<5 vol. %) population of Fe-sulphides; identified through reflected light microscopy to consist of pyrite and minor chalcopyrite (Fig. 4.7a). The grainsize of pyrite ranges from ~0.1–2 mm, whilst chalcopyrite is restricted to grainsizes <0.2 mm. Typically, the coarse-grained pyrite has a euhedral cubic crystal habit, although some irregular crystal aggregates occur, compared to the finer grainsize Fe-sulphides, which are anhedral and interstitial (Fig. 4.7a and b). Within individual cone sheets the distribution of Fe-sulphides is seemingly random, although they are occasionally arranged into layers (~1.5 cm thick) or associated with amygdales containing chlorite, zeolite, quartz and calcite (Fig. 4.7b). In one cone sheet in Area 1 [locality 447, NM 49362 62606], cooling fracture surfaces are coated with oxidised iron and sporadic, coarse (<2 mm) cubic pyrite crystals (Fig. 4.7c and d). Importantly, significant textural variation is observed between the Fe-sulphide populations of distinct cone sheets but is remarkably consistent within single cone sheets. This suggests the characteristics of Fe-sulphide populations are unique to each host cone sheet.

Assuming that the cone sheets have a common source (e.g. Anderson, 1936;

Phillips, 1974; Schirnick et al., 1999), the observed variation in distribution and abundance of Fe-sulphides may reflect changes in the source evolution of the magma or injection of cone sheets from different sections of a stratified magma chamber. However, the majority (68 %) of the cone sheets observed to contain Fe-sulphides were intruded into Mesozoic metasedimentary successions whilst those within the Neoproterozoic Moine metasedimentary rocks, Early Palaeogene volcaniclastics, the Glas Bheinn Dolerite and older cone sheets accounted for only 23 %, 4 %, 2 % and 5 % respectively. No Fe-sulphides were observed in cone sheets emplaced into the major intrusions of Centre 2 and 3. This correlation between

sulphide presence and host rock type cannot be explained if a magmatic origin to the Fe-sulphides is assumed. Instead, it is suggested that the Fe-Fe-sulphides are secondary mineral phases (Day, 1989) that were incorporated into the magma during cone sheet propagation

Cl Cl

Cl Cl

Cl Cl

Z Z Qtz

F Py Py

B

0.8 mm

1 mm

1 mm

Cubic pyrite

A

Chp

0.4 mm

C

D

Fig. 4.7: A) Reflected light photomicrograph from locality 95 (Area 2) showing the range of pyrite (Py) grainsize and the propensity towards subhedral to euhedral crystal habits of coarser pyrite crystals. A small bleb of chalcopyrite (Chp) is also highlighted. B) A plane polarised light (left) and reflected light (right) photomicrograph mosaic of a section through an amygdale within a cone sheet observed at locality 17 (Area 14). Fine to medium, subhedral to euhedral pyrite is disseminated throughout the amygdale and is spatially associated with calcite (Cl), quartz (Qtz) and zeolite (Z). C and D) Cooling joint planes within a cone sheet (locality 447; Area 1) are often observed to contain precipitated cubic pyrite crystals (C) or be coated in weathered iron oxide.

or post-emplacement, following either the partial melting of country rock or hydrothermal leaching. This is supported by the identification of a biological signature within pyrite through geochemical analyses (e.g. standard mass spectrometry and in situ laser combustion; Prof.

John Parnell, pers. comm.) and the association of some Fe-sulphides with amygdales. The majority of the Fe-sulphides likely originated from the Mesozoic successions, given the correlation between Fe-sulphide presence and host rock type as well as the observation of occasional minor pyrite within the limestones and shales. As the Fe-sulphide populations appear texturally unique to the host cone sheet and are likely secondary mineral assemblages acquired from the host rock (i.e. not from the source reservoir), it is suggested that

comparison between adjacent cone sheet outcrops allows the exposed extent of most cone sheets to be identified. For example, correlation of Fe-sulphide populations was used to produce the map displayed in Fig. 4.6a.