Tighvein is a rather flat moorland summit around 5 km south of the Sheans. It is very poorly exposed, but the presence of intermediate, silicic, and mingled intrusions has been discussed by Tyrell (1928) and Herriot (1975). Samples were taken from the summit region and from the small crag Creag na h-Ennie which lies about a kilometre and a half north north east of the summit. The summit exposures show a medium- to coarse-grained, heavily weathered diorite, intruded by small dykes of silicic material (Fig. 2.31) similar to those seen on the East Shean or the Tir Dhubh exposures of the Glenloig Hybrids in the CAIC. The Creag na h-Ennie exposures show spectacularly mingled hybrid rocks with an intermediate host and highly abundant more mafic enclaves, much like the main body of the Sheans intrusion.
Petrography
The silicic intrusions visible in Fig. 2.31 are petrologically similar to the fine rhyolitic dykelets observed in several places intruding the Glenloig Hybrids (Fig. 2.3). Fig.
72 Intrusive Rocks
Fig. 2.31 – Photograph of the exposure at the Tighvein summit. It shows silicic dykelets (prominent) intruding a heavily altered dioritic host.
2.32a shows a fine rhyolitic groundmass made up of quartz, K-feldspar, and opaques, and larger crystals, usually found as glomerocrysts. These are mostly euhedral plagioclases with less abundant subhedral to anhedral clinopyroxenes. The euhedral shape and lack of disequilibrium textures in the plagioclases suggest that they may be phenocrysts.
The Creag na h-Ennie sample is a non-ophitic olivine dolerite (Fig. 2.32b) similar to that found on the summit of Sithein, but containing rounded enclaves of finer opaque-rich and amphibole-bearing material. The dolerite itself comprises <1 mm euhedral to subhedral plagioclase laths, small clinopyroxene and altered olivine crystals, and small but abundant opaque minerals. Larger plagioclases display disequilibrium features such as sieve-textured cores and so are presumably antecrysts or xenocrysts.
2.7.4
Summary
The North Arran Granite forms a large (13 km diameter) laccolith in the north of the island. It comprises biotite syenogranites or various grain sizes and textures.
2.7 Other Units 73
Fig. 2.32 – Photomicrographs of samples from the ‘Tighvein Intrusion Complex’. a) Sample BJG/15/21 from a rhyolite dyke intruding on the summit of Tighvein. P = plagioclase, Cpx = clinopyroxene. Bottom left = crossed polars, top right = plane polarised light. b) Sample BJG/15/18, olivine dolerite from the Urie Loch area on Tighvein. En = enclave of finer grained intermediate material, P = plagioclase, Cpx = clinopyroxene, Ol = altered olivine, Px = plagioclase xenocryst, Ox = opaque oxides.
74 Intrusive Rocks
Accessory phases of Fe-Ti oxides, zircon, apatite, and allanite have been previously described.
The Sheans and Tighvein are poorly exposed moor-covered plateaus in the south of the island. They are made up of mafic and intermediate intrusive rocks intruded by felsic veins, much like the Glenloig Hybrids of the CAIC. Both contain non-ophitic olivine dolerites, amphibole diorites, and microgranites/rhyolites like those seen in dykelets in the Glenloig Hybrids.
2.8
Summary
The CAIC and surrounding areas contain a diverse variety of igneous rocks. Mafic rocks are largely restricted to basalt/ophitic dolerite dykes and one sill intruding the Arran Volcanic Formation. These all contain groundmass plagioclase, clinopyroxene, Fe-Ti oxides, and altered olivine, with phenocrysts/xenocrysts of plagioclase and rarely olivine. The only ultramafic rock is an olivine picrite dyke which, in addition to plagioclase, clinopyroxene, and Fe-Ti oxides also contains large olivines and Cr-spinels. The other mafic rocks are the poorly-exposed small gabbro bodies dotted around the complex. The most striking of these is a plagioclase-magnetite cumulate that also contains clinopyroxene and orthopyroxene.
Intermediate rocks are represented by the extensive and varied Glenloig Hybrids unit. This unit comprises a complex series of intermediate and silicic magmatic rocks, that show different degrees of intrusion, mingling, and mixing with one another. The intermediate examples all contain brown/green amphibole in addition to feldspars and opaque minerals, while the silicic examples are porphyritic rhyolites and microgranites.
A number of granite bodies are found within the CAIC. The Glen Craigag Granite in the centre of the complex appears to pre-date volcanic activity (depositional contact, and significant intrusion by mafic dykes, not seen in the ignimbrites), and comprises a granophyre containing quartz, K-feldspar, plagioclase, Fe-Ti oxides, zircon and sphene. The Satellite Granites around the margin of the complex show intrusive contacts with the ignimbrites, so must be younger, and also do not show evidence of intrusion by mafic dykes. They contain quartz and perthitic K-feldspar, with some plagioclase, Fe-Ti oxides, and zircon, but no sphene.
The other silicic intrusion in the CAIC is a glassy pitchstone dyke. It contains a large number of feldspar, pyroxene, and oxide xenocrysts.
The North Arran Granite to the north of the CAIC (across the trace of the High- land Boundary Fault) is a petrologically homogenous biotite bearing syenogranite.
2.8 Summary 75
The Sheans and Tighvein in the south of the island comprise complex series of intermediate and silicic intrusions, much like the Glenloig Hybrids.
Chapter 3
Volcanic Stratigraphy and Eruptive
History
The data and interpretations presented in this chapter have been published in the Bulletin of Volcanology (Gooday et al., 2018).
The exposure on Ard Bheinn, Binnein na h-Uaimh, the western side of A’ Chru- ach, and the lower parts of Glen Craigag are dominated by various pyroclastic rocks (Fig. 1.9). There are also some limited exposures of sedimentary rocks. As discussed, it is thought that the ellipsoidal extent of the main body of these pyroclastic rocks approximately defines the boundary of a caldera, meaning that these units are a caldera-fill succession. All of these intra-caldera pyroclastic and sedimentary units, as well as pyroclastic units that were deposited outside the caldera, are assigned to the Arran Volcanic Formation (AVF).
The Arran Volcanic Formation comprises a number of different mappable py- roclastic units (Fig. 3.1) which are interpreted as ignimbrites (i.e., the deposits of pyroclastic density currents), separated by erosional unconformities and sedimentary horizons. They are best exposed in the western third of the complex (i.e., west of Glen Craigag; Fig. 3.1), with good exposure on the high ground around Ard Bheinn and Binnein na h-Uaimh (Fig. 3.2). This is the area that King (1954) described in detail. Exposures of these rocks are found over an elevation change of more than 400 m (Fig. 3.2), giving the best estimate of total preserved thickness. Dips of units and other structural data are impossible to measure due to the lack of bedding seen at the scale of individual exposures. Where a sense of dip can be gleaned from following contacts, beds appear approximately horizontal. In one very poor, weathered exposure in Ballymichael Burn, pyroclastic rocks appear to lie on an eroded surface of the Glen Craigag Granite (Section 2.3). This is the only exposure
78 Volcanic Stratigraphy
of a possible caldera floor, and in the rest of the complex it is impossible to say how deeply the caldera floor – presumably made up of pre-Palaeogene sedimentary rocks and CAIC intrusions – is buried. An unknown thickness of the Arran Volcanic Formation above the current level of exposure has been lost to erosion.
The pyroclastic rocks on Muileann Gaoithe record a thickness of over 130 m, and lie upon in situ Devonian Old Red Sandstone. The extra-caldera nature of the Muileann Gaoithe exposures is apparent, as they lie around 1.5 km from the proposed ring fault. This is not so apparent with the isolated exposures of ignimbrites in the south east of the complex, on Tir Dubh and Creagan Liatha. These are also in close association with in situ pre-Palaeogene sandstone country rocks, but due to the poor nature of the exposure in this flat, peaty part of the complex, it is impossible to determine a structural relationship between these ignimbrites and the caldera-fill succession or the ring fault. However, they are interpreted to be extra-caldera. The contact between the AVF and the Glenloig Hybrids at Glenloig is also not clear. The contact is not well exposed, but it appears as though the pyroclastic rocks overlie the hybrid rocks with a sub-horizontal contact, suggesting deposition on this surface. This could result from one of three situations, depending on the relative position of a possible caldera-bounding ring fault: 1) This exposure at Glenloig is inside the caldera and shows a caldera-floor comprising Glenloig Hybrids. 2) This exposure is outside the caldera and shows extra-caldera deposition of the ignimbrites. 3) There was little or no caldera subsidence in this part of the complex.
79
Fig. 3.1 – Geological map of the western half of the Central Arran Igneous Complex. Approximate locations of stratigraphic logs presented in this paper are shown in red. 1 = Allt Ruadh Member, 2 = Creag an Fheidh Member, 3 = White Tuff Member, 4 = Pigeon Cave Member, 5 = Ard Bheinn Member. The grid shows the 1 km eastings and northings of the British National Grid. Inset shows the location of the Central Arran Igneous Complex (CAIC) on Arran in relation to the North Arran Granite (NAG).
80 Volcanic Stratigraphy Fig . 3.2 – Ov ervie w of the hills Ard Bheinn and Binnein na h-Uaimh ta k en from the west. The coloured o v erlay sho ws the underlying geological units of the Arran V olcanic F ormation. The uncoloured parts sho w areas underlain by the pre -caldera country rock. The summits of Ard Bheinn and Binnein na h-Uaimh are 670 m apa rt.
81
Table 3.1 – Explanation of ignimbrite lithofacies symbols used in this chapter, following the terminology of Branney and Kokelaar (2002).
Symbol Meaning Example lithofacies
m massive mT - massive tuff
s stratified sLT - stratified lapilli tuff
db diffusely bedded dbTcr - diffusely bedded crystal-rich tuff L lapilli mLT - massive lapilli tuff
T tuff mT - massive tuff
Br breccia mBr - massive breccia
Ag agglomerate mLAg - massive lapilli agglomerate l-l lava-like mTl-l - massive lava-like tuff cr crystal-rich mTcr - massive crystal-rich tuff
v vitrophyric mTv - massive vitrophyric tuff e eutaxitic mLTe - massive eutaxitic lapilli tuff
82 Volcanic Stratigraphy
Fig. 3.3 – Generalised stratigraphic log of the Arran Volcanic Formation, showing the relationships between the pyroclastic and sedimentary units and major hiatus events. Vertical thicknesses are not to scale. Map symbols are consistent with those in Fig. 3.1.
3.1 Arran Volcanic Formation 83
3.1
Stratigraphy of the Arran Volcanic Formation
The general volcanic stratigraphy of the area is shown in Fig. 3.3. We assign the mappable pyroclastic units as individual members within the Arran Volcanic Formation, based on lithological variations between units, and the presence of mappable palaeotopographic surfaces. The general characteristics of each member (weathering colour, lithology, clast composition, etc.) are generally distinct enough to allow isolated exposures to be assigned to the appropriate unit. However, the upper surfaces of all members show evidence for fluvial reworking, erosion, deposition of sedimentary units, and/or prolonged contact with the atmosphere (Fig. 3.3), which all suggest volcanic hiatuses. Reddened units are tentatively used to identify either distinct members or intra-member eruptive/flow units whose surfaces have undergone prolonged exposure to the atmosphere (no features of true palaeosols such as rootlets or bioturbation were identified). Within members, deposition is assumed to be sustained with lithological differences reflecting variations in mass-flux and temperature during progressive aggradation of the ignimbrite (Branney and Kokelaar, 2002). Within certain members, cooling joints are used to identify distinct cooling units. The terminology used to describe the different lithofacies mapped here is given in Table 3.1 and follows the lithofacies code approach of Branney and Kokelaar (2002).