Introduction

Calderas are the surface expressions of complex volcano-magmatic systems, with prolonged periods of unrest and eruption (Acocella et al. 2015; Nobile et al. 2017) punctuated by extensive periods of quiescence. Although much debated, caldera collapse is typically caused by the withdrawal of large volumes of magma from shallow magma chambers (Druitt and Sparks 1984; Lipman 1997; Cole et al. 2005; Cashman and Giordano 2014), and subsidence of a coherent block of crust into the underpressurised chamber(s) (Mori and McKee 1987; Lipman 1997; Cole et al. 2005; Acocella 2007), although collapse may also be triggered by overpressure within the chamber initiating fractures in the roof rocks (Gudmundsson 1988, 1998). Caldera-forming eruptions are typically silicic and can deposit extensive sheets of ignimbrite both within and beyond the collapse caldera, together with widespread fall deposits, and even the circulation of ash globally (e.g. Self and Rampino 1981; Self et al. 1984; Newhall and Dzurisin 1988; Hildreth and Fierstein 2012).

Understanding caldera-forming eruptions and syn-eruptive collapse mechanisms is notoriously difficult at both modern and ancient calderas. Monitoring unrest at active calderas using seismic and remote sensing methods is essential in forecasting volcanic activity, but understanding these relationships is complex. How much magma withdrawal is required to induce collapse and how does collapse vary (Geshi and Miyabuchi 2016)? Furthermore, how many collapse events may occur in the lifecycle of a caldera (e.g. due to evacuation of multiple magma reservoirs), and how might the deposits of these different eruption and collapse events vary both temporally and spatially (proximal and distal)? Answering these questions at active calderas is challenging, as only the latest stages in the caldera’s deposits and structures are preserved and access may be further obscured by water and sediments (Acocella 2007).

Therefore, we can look to ancient and more recent but inactive calderas. There are numerous field studies of such calderas (see reviews by Cole et al. 2005; Cashman and Giordano 2014) that attempt to resolve questions on caldera collapse and evolution, and these have been supported more recently by numerical and analogue modelling studies (e.g. Troll et al. 2002; Acocella 2007; Geshi et al. 2012). However, ancient calderas can typically be obscured by resurgent intrusions and volcanic deposits, thoroughly disrupted by faults and hydrothermal alteration, and/or obscured by water and later sediments. The level of erosion and preservation of the volcanic deposits and caldera-controlling structures can also represent a particular constraint to study. These constraints are particularly important with respect to the intra-caldera/proximal records of eruption, which are consequently less well studied than more distal, better preserved ignimbrite sheets (Smith and Kokelaar 2013).

Therefore, it remains a challenge to volcanologists to identify ancient calderas where access and preservation allow for detailed field observations that can be used to elucidate caldera collapse and caldera-forming eruption processes. We argue that, given modern advances in physical volcanology (in particular pyroclastic density currents, the sedimentation of ignimbrites and better understanding of lava-like ignimbrites; e.g. Branney and Kokelaar 2002; Branney et al. 2008; Brown and Branney 2013), it is therefore essential to revisit localities that have not received recent attention. In this paper, we present a field study of a Palaeogene caldera system on the Isle of Arran, western Scotland, as an example of the proximal record of caldera-forming eruptions and the nature of collapse within the caldera.

Why the Isle of Arran?

The Isle of Arran in the Firth of Clyde, western Scotland, hosts some well-preserved remnants of the British Palaeogene Igneous Province (Fig. 1). The British Palaeogene Igneous Province itself is part of the North Atlantic Igneous Province, a predominantly mafic Large Igneous Province that developed during the rifting of the North Atlantic Ocean in response to the arrival of the Iceland plume at the base of the lithosphere (e.g. Thompson and Gibson 1991; Kent and Fitton 2000; Storey et al. 2007). In the British Isles, the North Atlantic Igneous Province comprises the extensive lava fields of Skye, Eigg, Mull-Morvern and Antrim (Emeleus and Bell 2005), as well as localised intrusive and volcanic centres (Fig. 1), including Arran. Other Palaeogene remnants of the North Atlantic Igneous Province are preserved on Greenland, the Faroe Islands and offshore (Saunders et al. 1997). Although the majority of magmatism in the British Palaeogene Igneous Province is preserved as basaltic lavas and gabbroic and granitic intrusions, there is widespread evidence of extrusive silicic volcanism and explosive eruptions (Bell and Emeleus 1988; Brown et al. 2009). Due in part to its historical importance and relative ease of access, the British Palaeogene Igneous Province has been the source of many developments in the global understanding of volcanological processes.

Fig. 1
figure 1

Simplified geological map of the Isle of Arran, adapted from BGS (1987), with additional information from this study. The grid shows the 10 km eastings and northings of the British National Grid. Inset shows the onshore locations of magmatic rocks that make up the British Palaeogene Igneous Province in western Scotland and Northern Ireland. 1—Skye, 2—Rùm and Eigg, 3—Ardnamurchan, 4—Mull, 5—Arran, 6—Antrim, 7—The Mourne Mountains, 8—Slieve Gullion and Carlingford

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Pyroclastic rocks in the British Palaeogene Igneous Province are largely found within calderas (on Mull and Rùm, as well as Skye and Arran), often bound by arcuate faults and intrusions, originally interpreted as ring faults, ring dykes and cone sheets. Detailed studies of the pyroclastic successions on Skye and Rùm have revealed complex histories of caldera collapse in the British Palaeogene Igneous Province (Troll et al. 2000; Holohan et al. 2009; Brown et al. 2009), but exposures in these areas are limited due to later, cross-cutting layered intrusions. Recently, many silicic extrusive rocks in this province, previously described as lavas or shallow intrusions, have been re-interpreted as welded and rheomorphic lava-like ignimbrites, for example the rhyodacite sheets on Rùm (Troll et al. 2000; Holohan et al. 2009) and the Sgurr of Eigg Pitchstone (Brown and Bell 2013). These studies have allowed us to better understand the processes at work in, and make comparison with, other lava-like ignimbrite-producing volcanic provinces, for example the Snake River Plain (Knott et al. 2016), the Paraná Magmatic Province (Luchetti et al. 2017) and the Canary Islands (Sumner and Branney 2002).

Despite these advances in the region, the caldera system in central Arran has remained poorly studied. The caldera (approximately 9 km2) is well exposed in places, and contains a caldera-fill sequence of pyroclastic and minor sedimentary rocks. Critically, it has not been overly disrupted by alteration, faults, later intrusions and volcanic deposits, and the level of erosion is such that a relatively undisturbed sequence of the caldera stratigraphy and its structure is preserved. Therefore, this makes Arran an extremely important area in which to study volcanic and sedimentary processes, how these change through time and, in particular, proximal records of ignimbrite deposition at a caldera volcano and its collapse mechanisms. Despite all these advantages, central Arran has been the subject of few studies since the work of King (1955).

By way of example, the historically active Öskjuvatn caldera at Askja, North Iceland, is a comparable size (around 12 km2) to the Arran caldera and occupies a similar tectonic (rift) setting (Trippanera et al. 2018). However, observations of the caldera-fill succession at Askja are impossible due to the presence of a large lake, and the only information on collapse processes during and following the 1875 eruption are from historical sources, which can be unreliable (Hartley and Thordarson 2012). Given the similarities to such a nearby caldera that is close to significant population centres in Europe, we emphasise the need for detailed field study of localities such as Arran and the lessons that can be learned and applied to active calderas.

In this paper, we present new field and petrographic data from the pyroclastic rocks preserved in central Arran, propose a volcanological model for the formation, evolution and collapse of the caldera and discuss the nature of eruptions and proximal record of caldera-forming eruptions. We include discussion on what our analysis of this ancient centre shows in terms of large caldera-forming eruptions, their deposits and resulting structures in general.

Geological setting

The pre-Palaeogene rocks in the northern part of Arran (Fig. 1) comprise the Neoproterozoic to Lower Cambrian schists, phyllites and grits of the Southern Highland Group (BGS 1987), this being the youngest division of the Dalradian Supergroup (Stephenson et al. 2013). The pre-Palaeogene rocks in the southern half of the island are dominated by Devonian and Permo-Triassic red sandstones and conglomerates (containing clasts of quartzite, vein quartz and schist—most likely eroded from an exposed Dalradian landscape), separated by a thin succession of Carboniferous sedimentary rocks and lavas (Fig. 1). The basement upon which these sedimentary rocks were originally deposited is not exposed. The Highland Boundary Fault, the major crustal lineament that separates the Dalradian rocks of the Grampian Terrane to the north from the Midland Valley Terrane to the south, is known to cross Arran (Fig. 1), but its exact trace is uncertain (Young and Caldwell 2012).

The Palaeogene igneous rocks on Arran (Fig. 1) comprise the North Arran Granite, a caldera system in the centre of the island, and a suite of sills and minor intrusions in the south of the island, which vary in composition from dolerite to rhyolite. In addition, a mafic dyke swarm is exposed largely around the coast of the island and in the mountains in the north of the island (BGS 1987). These dykes intrude the Dalradian meta-sedimentary rocks, the Palaeozoic and Mesozoic sandstones and the Palaeogene intrusions (Tyrell 1928; BGS 1987). The North Arran Granite is a roughly circular laccolith that was likely intruded from the south or south-east (Stevenson and Grove 2014), suggesting that its emplacement could have been structurally controlled by the Highland Boundary Fault. It has been dated by 40Ar-39Ar at 57.85 ± 0.15 Ma (Chambers 2000). Composite sills, with mafic margins and more evolved cores, are common across much of southern Arran (Tyrell 1928; BGS 1987). The most well-known of these forms cliffs and coastal outcrops at Drumadoon Point (Fig. 1), and this intrusion, and adjacent dykes, have been dated at 59.04 ± 0.13 and 59.16 ± 0.17 Ma (Meade et al. 2009). In this contribution, the Palaeogene caldera system in central Arran is named the Central Arran Igneous Complex, while the pyroclastic and sedimentary intra-caldera succession is termed the Arran Volcanic Formation.

The Central Arran Igneous Complex includes a series of pyroclastic rocks and coarse-grained intrusions, and is approximately 4 km × 5 km in area. It was first mapped by Gunn et al. (1901) and has historically been known as the ‘Arran Central Ring Complex’. This term was first used in the Arran Memoir (Tyrell 1928), in which comparisons were drawn between the granitic intrusions around the edge of the complex and the Loch Bà felsite on Mull, which was interpreted as a caldera-bounding ring dyke (Bailey et al. 1924). The complex sits within Devonian red sandstones (to the north and south) and Permo-Triassic red sandstones (to the west and east) (BGS 1987). The volcaniclastic rocks of the caldera-fill sequence mostly crop out on and around the summit of Ard Bheinn (Fig. 2). King (1955) mapped the western part of the Central Arran Igneous Complex in detail and suggested that it was younger than the North Arran Granite, due to the fact that it appears to have been emplaced into sedimentary rocks that were previously domed during emplacement of the North Arran Granite, around 2 km to the north.

Fig. 2
figure 2

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—Ard Bheinn 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)

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The ‘vent agglomerates’ (Tyrell 1928) of the complex (interpreted here as lapilli tuffs and breccias) were originally thought to represent the products of a single explosive volcanic phase, which was followed by intrusion of ‘felsites’, and the growth of small resurgent volcanic edifices on the caldera floor (Gregory and Tyrell 1924; Tyrell 1928; King 1955). Large outcrops of basalt, dolerite and ‘basalt breccia’ were interpreted as the only remains of a Palaeogene lava pile that once covered Arran, which had collapsed into the vent during subsidence (King 1955). This caldera-collapse has also been proposed to explain the origin of the isolated exposures of Mesozoic sedimentary rocks in the complex (King 1955). The intrusions, which comprise granites, gabbros and intermediate hybrids, were thought to be mostly younger than the caldera-fill sequence based on poorly exposed field relationships (King 1955, 1959). King (1955) also discussed differences within the agglomerates, noting that some seemed to be sedimentary in origin (‘sedimentary agglomerates’) or different in colour due to compositional variation (‘andesitic and basaltic agglomerates’).

Sampling and methods

Fieldwork was carried out on Arran across several field seasons between 2015 and 2017. Most of the information presented in this paper is the result of detailed mapping and stratigraphic logging; however, a number of samples were collected for thin section petrography and geochemical analysis. Only ignimbrite samples that we judged to be representative of magmatic compositions (i.e. clast-free) were subjected to whole-rock geochemical analyses.

Nine samples of clast-free ignimbrite from four units were crushed in a manganese-steel jaw-crusher and ground to a fine powder in an agate planetary-ball mill. Loss on ignition (at 900 °C) was determined gravimetrically, before the samples were prepared using the methods outlined in McDonald and Viljoen (2006) at Cardiff University (UK). Major element analyses were carried out using inductively coupled plasma optical emission spectrometry (ICP-OES). Accuracy was constrained by subjecting the international reference materials BIR-1 (basalt) and JG-3 (granodiorite) to the same process. Precision was constrained by duplicates of unknown samples—duplicates generally provided uncertainties of < 5% for elements with concentrations > 2 wt%.

The central Arran igneous complex

As part of the present study, the western half of the Central Arran Igneous Complex has been re-mapped (Fig. 2). The most fundamental difference between this new map and those of Tyrell (1928) and King (1955) is the lack of a near-complete ring intrusion. This complex, therefore, should not be described as a ‘ring complex’. Although exposure of the caldera-fill rocks on the slopes of Ard Bheinn and Binnein na h-Uaimh (Figs. 2 and 3) is good, exposure in the majority of the rest of the complex is limited and largely restricted to stream beds. There is no exposure of a ring fault surrounding the complex, but the juxtaposition of Palaeogene igneous rocks at the same level as Palaeozoic sedimentary rocks indicates extensive downfaulting of the complex.

Fig. 3
figure 3

Overview of the hills Ard Bheinn and Binnein na h-Uaimh taken from the west. The coloured overlay shows the underlying geological units of the Arran Volcanic Formation. The uncoloured parts show areas underlain by the pre-caldera country rock. The summits of Ard Bheinn and Binnein na h-Uaimh are 670 m apart

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Intrusive rocks of the central Arran igneous complex

The north and east of the complex (Figs. 1 and 2) are dominated by the pre-caldera Glenloig Hybrids, named after the most accessible exposure under the bridge at Glenloig. They include texturally heterogeneous amphibole-bearing, fine-grained intermediate rocks, which vary in composition from basaltic-andesitic to dacitic, and lesser amounts of coarser grained granite and amphibole granite. These rock types display intrusive and mingling interaction textures with one another. In some stream sections, isolated exposures of quartz-bearing gabbro are found; however, due to poor exposure, it is not possible to deduce the relationship of these gabbros with the Glenloig Hybrids. A small outcrop of these Glenloig Hybrids, along with a thin outcrop of gabbro, is preserved as an inlier within the caldera-fill succession between Binnein na h-Uaimh and Creag Mhor (Figs. 2 and 3).

Several granitic bodies are found within the Central Arran Igneous Complex. The largest of these is the Glen Craigag Granite, which is mostly exposed in the upper parts of Glen Craigag and Ballymichael Glen, in the centre of the complex (Fig. 2). Its mineralogy principally comprises medium-grained quartz and perthitic K-feldspar, commonly in granophyric intergrowths, with minor plagioclase, amphibole and accessory minerals. In Ballymichael Glen, pyroclastic rocks overlie the Glen Craigag Granite on an eroded palaeosurface, suggesting intrusion and erosion of the granite occurred prior to the onset of explosive volcanism.

Several smaller granitic bodies crop out around the margins of the Central Arran Igneous Complex (Fig. 2). These are here collectively termed the Satellite Granites, and all comprise medium- to coarse-grained quartz, K-feldspar and plagioclase, with minor amphibole and biotite and accessory minerals. They were originally mapped as a near-continuous ‘ring intrusion’ (Tyrell 1928), but our mapping shows that they are isolated from one another at this level of erosion. Relationships to the other units of the Central Arran Igneous Complex are poorly exposed, but the Creag Mhor Granite appears to be a shallowly eastward-dipping sheet within the ignimbrites, and the granite in Ballymichael Glen has steeply dipping, complex contacts, with fingers protruding into the rocks of the Arran Volcanic Formation. This suggests that the granites intruded the lower pyroclastic units of the Arran Volcanic Formation. Their arrangement around the margins of the caldera could be a result of intrusion along a caldera-bounding ring fault.

A large outcrop of fine- to medium-grained basaltic rock on the western slopes of Ard Bheinn and Binnein na h-Uaimh (Figs. 2 and 3) was originally interpreted as the only remnant of an inferred Arran lava field, that had subsided into the caldera (King 1955). Petrographic work during this study shows that it is, for the most part, an ophitic dolerite. Mineralogy is consistent throughout the outcrop, and consists of small plagioclase laths, commonly embedded in clinopyroxene oikocrysts up to 3 mm in size, and patches of iddingsite show alteration of primary olivine. This unit displays a clear intrusive relationship with several of the pyroclastic units of the Arran Volcanic Formation, with small fingers of dolerite intruding into the overlying unit (White Tuff Member). In many places, especially near the contacts, the dolerite is heavily brecciated, with peperitic textures visible in several exposures. There is also one small outcrop of the underlying unit (Allt Ruadh Member) preserved above it. Other than this, it is largely concordant with stratigraphy (Figs. 2 and 3), so we interpret the unit as a sill.

A suite of mafic dykes, largely < 2-m wide and similar in morphology and composition to the dykes exposed all over Arran, intrudes the Glenloig Hybrids, the Glen Craigag Granite and the lowest extra-caldera ignimbrites (the Muileann Gaoithe Member) (Fig. 2). No mafic dykes are observed intruding the Satellite Granites or the caldera-fill succession of the Arran Volcanic Formation. The only dyke seen intruding an intra-caldera ignimbrite is a pitchstone (i.e. silicic and vitrophyric) dyke in a tributary to the Glen Craigag stream.

The Arran volcanic formation

The caldera-fill succession is made up dominantly of pyroclastic rocks, with some minor sedimentary packages. We assign all these rocks, as well as pyroclastic units that were deposited outside the caldera, to the Arran Volcanic Formation.

The Arran Volcanic Formation comprises a number of different mappable pyroclastic units which we interpret as ignimbrites (i.e. the deposits of pyroclastic density currents) (Fig. 2), 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. 2), with good exposure on the high ground around Ard Bheinn and Binnein na h-Uaimh (Fig. 3). This is the area that King (1955) described in detail. Exposures of these rocks are found over an elevation change of more than 400 m (Fig. 3), 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. Away from a small number of outcrops showing a possible caldera basement, it is impossible to estimate how far the Arran Volcanic Formation extends below the level of exposure. An unknown thickness of the Arran Volcanic Formation above the current level of exposure has been lost to erosion.

The general volcanic stratigraphy of the area is shown in Fig. 4, with stratigraphic and lithological information displayed in Table 1. 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 palaeo-topographic 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. 4), which all suggest volcanic hiatuses. Reddened units are tentatively used to identify either distinct members or inter-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 2 and follows the lithofacies code approach of Branney and Kokelaar (2002).

Fig. 4
figure 4

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

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Table 1 Descriptions and interpretations for the volcanic and sedimentary units that make up the Arran Volcanic Formation
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Table 2 Explanation of ignimbrite lithofacies codes used in this paper, following the terminology of Branney and Kokelaar (2002)
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The Muileann Gaoithe member

This is the lowest exposed unit within the caldera (Fig. 4), and is the only volcanic unit exposed outside the caldera. A stratigraphic log up the extra-caldera Muileann Gaoithe section is shown in Fig. 5a. It largely comprises a white-weathering flow-banded rhyolite tuff (77–79 wt.% SiO2, Table 3) with some layers of massive lapilli tuff (mLT) and thinly bedded red tuff (sT). In places near the base, the flow banding displays metre-scale folding (Fig. 5b). Abundant smoky quartz crystals up to 2 mm are characteristic of this unit. The glassy groundmass shows sub-mm (Fig. 5c) to continuous (Fig. 5d) compositionally distinct bands.

Fig. 5
figure 5

a Stratigraphic log of the Muileann Gaoithe Member on the south side of Muileann Gaoithe. b Folded flow banding in the lower part of the unit, looking north west. c) Photomicrograph of the rhyolitic lava-like ignimbrite from the top of the Muileann Gaoithe Member. Viewed in plane-polarised light. Q quartz, F feldspar. The micro-scale flow fabric is defined by texturally/compositionally distinct (different colours) sub-mm and near-continuous bands and prolate rods. d Photomicrograph of the rhyolitic lava-like ignimbrite from the base of the Muileann Gaoithe Member. Viewed in plane-polarised light. Q quartz, F feldspar. The flow fabric is defined by continuous bands

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Table 3 Anhydrous whole-rock geochemical data for various ignimbrite samples
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We interpret the Muileann Gaoithe Member as a rhyolitic parataxitic to lava-like ignimbrite. The layers of lapilli tuff may represent changes in flow-boundary conditions within the aggrading ignimbrite, which could reflect increased explosivity at the vent. The red tuff is thought to represent ash which capped the underlying eruptive unit and was exposed to the atmosphere before deposition of the overlying unit. This would separate the Muileann Gaoithe section into two eruptive units, each capped by a thin red tuff. At Dereneneach, red tuff is only observed at the top of the unit, suggesting only one eruptive unit is exposed here.

The Allt Ruadh member

The stratigraphy of the Allt Ruadh Member along the Allt Ruadh section is shown in Fig. 6a. It is dominated by orange- and grey-weathering massive lithic lapilli tuffs (mLT; Fig. 6b), containing clasts of pre-Palaeogene schist, sandstone and quartzite, and presumably Palaeogene basalt, dolerite and occasional granite (Fig. 6c). Thin layers of high-grade crystal tuffs (Fig. 6d) are exposed in the lower part of the Allt Ruadh section. Elsewhere in the complex where exposure is not as good, only the mLT are seen.

Fig. 6
figure 6

a Stratigraphic log of the Allt Ruadh Member along the Allt Ruadh section. b A typical exposure of orange-weathering massive lapilli tuff (mLT) found in the Allt Ruadh Member. c Photomicrograph of typical mLT showing clasts of schist (left), dolerite (right) and altered rhyolite (bottom). Viewed in plane-polarised light. d Photomicrograph of a glassy eutaxitic- to lava-like tuff containing crystals of quartz (Q), K-feldspar (F) and Fe-Ti oxides (ox). Viewed in plane-polarised light

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We interpret this unit as a series of non-welded ignimbrites representing a prolonged period of highly explosive volcanism, in which a large volume of magma was erupted. The vitrophyric and eutaxitic layers are interpreted as phases of lower explosivity in which the pyroclastic density current retained more heat. The purple glassy tuffs near the top of the section (Fig. 6a) may be basal vitrophyres to the overlying packages of mLT.

Creag Shocach conglomerates

The Creag Shocach conglomerates are a series of sandstones, gritstones and conglomerates which overlie the Allt Ruadh Member (Fig. 4). They are ‘mesobreccias’ in the classification of Lipman (1976). The clasts (pebbles and cobbles) comprise pre-Palaeogene country rock lithologies, i.e. red sandstones and quartzite conglomerates. The matrix largely comprises fine- to medium-grained quartz sand. We interpret this unit as representing erosion of Devonian and Permo-Triassic lithologies into the caldera from steep caldera walls left by subsidence related to the eruption of the Allt Ruadh Member.

Creag an Fheidh member

The Creag an Fheidh Member is only exposed in the east of the complex, and appears to be cut off by the same fault that truncates the intra-caldera outcrop of Glenloig Hybrids (Fig. 2). The lower parts of this unit (which are only well-exposed on Creag an Fheidh, and may not be laterally extensive) are predominantly lithic-poor tuffs and crystal tuffs (mT and mTcr) with some massive lapilli tuffs towards the top of the unit (Fig. 7a, b). Some of the crystal tuffs have a characteristically dark glass groundmass (Fig. 7c). In thin section, the eutaxitic tuffs show flattened wispy features interpreted as fiamme (Fig. 7d). The massive lapilli tuff at the top of the unit contains abundant clasts of rhyolite which resemble the Muileann Gaoithe lava-like tuffs. The upper surface contains rounded pebbles and cobbles of rhyolite and quartzite.

Fig. 7
figure 7

a Stratigraphic log of the Creag an Fheidh Member along the Creag an Fheidh section. b A massive lapilli tuff in the upper part of the unit. c Photomicrograph of a massive lapilli tuff from the upper part of the member. Viewed in plane-polarised light. d Photomicrograph of a eutaxitic tuff from the lower part of the unit, showing flattened clasts (fc) as well as quartz (Q), K-feldspar (F) and Fe-Ti oxide (ox) crystals. Viewed in plane-polarised light

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The unit is interpreted as a series of localised heterogeneous ignimbrites that were only deposited in the eastern part of the complex. The increasing lithic lapilli content suggests the eruptions became more explosive with time. The rounded pebbles and cobbles and presence of exotic clasts at the top of the unit suggest fluvial reworking of the upper surface of the ignimbrite in a period of volcanic quiescence before the eruption of the White Tuff Member.

Allt Beith tuff cone

On the upper surface of the Creag an Fheidh Member are several exposures of a very fine brown-grey thinly banded tuff (Fig. 4). It shows both cross-stratified and planar bedding features on scales of < 1–50 mm, with occasional graded bedding (Fig. 8). Unlike all other erupted products within the CAIC, it is basaltic-andesitic in composition (54.5 wt.% SiO2; Table 3). The surrounding exposures are dominated by the conglomerates that make up the upper surface of the Creag an Fheidh ignimbrite. We interpret these exposures as the remnants of a small basaltic phreatomagmatic tuff cone or ring. This interpretation is based on four main features: (1) the deposit is very fine grained, suggesting intense fragmentation; (2) phreatomagmatism can explain explosive mafic activity and the production of fine basaltic-andesitic ash; (3) it is surrounded by other deposits of fluvial facies; and (4) it is very localised, being seen nowhere else in the complex.

Fig. 8
figure 8

Finely laminated and cross-laminated basaltic-andesitic tuff that makes up the Allt Beith tuff cone. Hammer for scale is 400 mm long

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White tuff member

The White Tuff is a very homogeneous, white-weathering rhyolitic tuff. Most exposures of this tuff appear structureless, although in places a distinct planar fabric can be seen (Fig. 9a). This is most often sub-horizontal planar banding, but in places it is chaotic. These fabrics are variably expressed as pervasive fractures tens of centimetres apart, or as fine mm-scale colour variations (Fig. 9b). The White Tuff Member is petrologically and geochemically homogeneous. It is rhyolitic throughout (76–77 wt.% SiO2; Table 3), containing abundant plagioclase, K-feldspar and smoky quartz crystals 1–5 mm in size (Fig. 12c, d). The top of the unit comprises a localised conglomerate with rounded clasts of rhyolite, basalt and quartzite. The White Tuff Member is interpreted as a voluminous rhyolitic lava-like ignimbrite. The conglomerate at its upper surface, which contains exotic clasts as well as cobbles of rhyolitic tuff, suggests fluvial working on the exposed surface of the unit after deposition.

Fig. 9
figure 9

a Stratigraphic log of the White Tuff Member up the western slopes of Ard Bheinn. b The thinly banded unit shown at around 415 m in the log. The meso-scale flow fabric is clearly visible as bands of different colours, in this case yellow and grey. c Photomicrograph of typical massive lava-like ignimbrite from the base of the member, showing the euhedral quartz crystals (Q) that are so characteristic of this ignimbrite, as well as K-feldspar (F) and Fe-Ti oxides (ox). Viewed in plane-polarised light. d Photomicrograph of banded lava-like ignimbrite from the middle of the member, showing the flow fabric deformed around a large K-feldspar crystal (F). Viewed in plane-polarised light

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Pigeon cave member

The Pigeon Cave Member comprises glassy tuffs, massive lapilli tuffs and massive lapilli agglomerates (Fig. 10a, b). The agglomerates are characterised by weathering to a turquoise colour, and containing elongate ribbons of lava-like rhyolite (Fig. 10c), interpreted as deformed cognate spatter clasts. Many of the crystal tuffs have a distinct black glassy groundmass (Fig. 10d). Small spatter clasts can be recognised in thin section as wispy glassy features (Fig. 10e). The Pigeon Cave Member is interpreted as a series of heterogenous ignimbrites, with the massive lapilli tuffs recording highly explosive eruptions, and the agglomerates recording slightly less explosive spatter eruptions.

Fig. 10
figure 10

a Stratigraphic log of the Pigeon Cave Member up the northern side of Binnein na h-Uaimh, where it is intruded by brecciated fingers of the basalt/dolerite sill. b Stratigraphic log of the Pigeon Cave Member up the western side of Binnein na h-Uaimh, where it is not intruded by the sill. Dashed lines from (a) show possible lateral bed correlations. c Photograph showing a typical green-weathering massive lapilli agglomerate from the lower part of the Pigeon Cave Member. It contains elongate streaks and bands of rhyolitic material (rh). d Photomicrograph of a crystal-rich lapilli tuff with a dark glassy groundmass, lithic lapilli (L) and quartz crystals (Q). Viewed in plane-polarised light. e Photomicrograph of a crystal-rich agglomerate. The sickle-shaped wispy features are interpreted as small glassy cognate spatter clasts (sc). Viewed in plane-polarised light

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The outcrop of ‘Cretaceous chalk’ at Pigeon Cave on Binnein na h-Uaimh (discussed by Tyrell 1928) is overlain by the Pigeon Cave Member and has been previously interpreted as a subsided megablock (Tyrell 1928; King 1955). Some pink white-weathering sandstones are exposed several metres to the south of Pigeon Cave. These were also mentioned by Tyrell (1928), and King (1955) suggested that these may also be Cretaceous due to similarities with sediments found on Mull and Morvern, as well as in Antrim. It is unclear whether these sedimentary units are part of the caldera floor, subsided megablocks or have a Palaeogene intra-caldera origin.

Binnein na h-Uaimh conglomerates

The Binnein na h-Uaimh conglomerates comprise a number of exposures of very coarse clast-supported conglomerates (Fig. 4). The contact with the underlying Pigeon Cave Member can be traced for tens of metres down the northern side of Binnein na h-Uaimh, suggesting that flow was towards the north. In places this contact is steep, and is interpreted as the side of a canyon eroded into the underlying ignimbrites. The larger clasts are predominantly schist, which resembles the Dalradian schists exposed several kilometres to the north, on the other side of the Highland Boundary Fault.

Ard Bheinn member

The Ard Bheinn Member is the youngest exposed volcanic unit in the Central Arran Igneous Complex (Fig. 4). It makes up the summit of Ard Bheinn, so anything above this level has been lost to erosion. Its lower parts are dominated by clast-poor glassy and crystal-rich tuffs (Fig. 11a) with some lithophysae and planar flow fabric. The upper section is less homogenous, with autobreccia, agglomerates (Fig. 11b), crystal tuffs (Fig. 11c; the ‘plagioclase porphyry’ of King 1955), and eutaxitic lapilli tuffs (Fig. 11d). The lowest exposures of this member comprise stratified, lava-like, coarse crystal tuffs (Fig. 11b) which appear to fill a valley eroded into the underlying units (Fig. 2). This valley can be traced towards the south, away from Ard Bheinn, suggesting that flow here was to the south.

Fig. 11
figure 11

a Stratigraphic log of the Ard Bheinn Member up the southern side of Ard Bheinn. b Massive lapilli agglomerate near the summit on Binnein na h-Uaimh, looking north showing lithic lapilli (L) and elongate bands of rhyolite (rh). c Photomicrograph of the feldspar (F) rich mTcr (‘plagioclase porphyry’ of King 1955) from the summit of Ard Bheinn. Viewed between crossed polars. d Eutaxitic massive lapilli tuff at 490 m elevation on the Ard Bheinn log, containing stretched out mafic clasts (mc). Pencil for scale is 150 mm long. e Photomicrograph of the coarse crystal-rich tuff at the base of the Ard Bheinn Member, showing large quartz (Q) crystals and smaller K-feldspar crystals (F). Viewed in plane-polarised light

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The Ard Bheinn Member is interpreted as a series of high-grade ignimbrites—most being ‘lava-like’ on the basis of completely agglutinated pyroclasts (Fig. 11c, e). The lower part of the Ard Bheinn section (360 to 440 m on Fig. 11a) displays consistent columnar jointing, suggesting that this section acted as a single cooling unit. The upper parts of the member were likely formed from a series of small pyroclastic fountaining eruptions of varying explosivity and emplacement temperature.

Bedded tuffs

Some isolated exposures around the upper parts of Ballymichael Glen comprise ignimbrites that display stratification and flow banding on a variety of scales. These are shown in Fig. 2. Due to their isolated nature, and poor exposure on flat, vegetated ground, it is impossible to discern their relationship with any other of the mapped units. For this reason, we cannot determine their position within the stratigraphy of the Arran Volcanic Formation.

Discussion

This work demonstrates that the Central Arran Igneous Complex represents a well-preserved caldera system filled by a pyroclastic succession at least 400 m thick, the Arran Volcanic Formation. A number of sequential mappable units have been recognised, most of which are preserved within a broadly circular area, interpreted as a caldera. We discuss the collapse of the caldera and the nature of proximal ignimbrites before presenting an eruption history.

Collapse (and resurgence?) of the caldera

Given that the Arran Volcanic Formation comprises Palaeogene surface-deposited rocks juxtaposed against Devonian and Permo-Triassic rocks, it must occupy a caldera which has experienced at least some degree of downfaulting and must therefore possess at least one ring fault. Although this fault is not exposed, the complex meets the other criteria of Brown et al. (2009) for recognising a caldera in the British Palaeogene Igneous Province: (1) a collapse succession of breccias; and (2) evidence of subsidence (i.e. displacement relative to country rocks).

A transect from the western edge of the caldera to the summit of Binnein na h-Uaimh is relatively well exposed, and no significant breaks or duplications in stratigraphy are seen. This suggests that there is only one ring fault, i.e. at the contact between the Arran Volcanic Formation and the pre-Palaeogene sedimentary country rocks. Experimental studies and their comparison to real examples suggest that a caldera that displays one outward-dipping reverse ring fault is likely to have experienced subsidence in the range of 100 m to 1 km (Acocella 2007).

One fault is identifiable within the caldera (Fig. 2), but there are doubtless others. The change in stratigraphy across this fault suggests the sense of movement was ‘down to the east’, but there is no way to determine the magnitude of displacement. This fault is radial to the caldera.

Given the presence of at least one radial fault, an element of piecemeal subsidence can be assumed (Moore and Kokelaar 1998; Troll et al. 2002). However, there are no other places where the stratigraphy is noticeably disrupted at the scale of the available exposure, so this was clearly not a dominant collapse mechanism. There may also be an element of trapdoor subsidence (Lipman 1997), as massive lapilli tuffs of the Allt Ruadh Member are exposed in the east of the complex at the same elevation as the summit of Ard Bheinn. This could suggest greater subsidence of the caldera to the west, but could also be explained by lateral changes in deposit thicknesses (Brown and Branney 2013) or amount of erosion. There is no evidence of consistently inward-dipping beds, or significant evidence of slumping, so funnel-like subsidence has not occurred (e.g. Miyakejima, Japan—Geshi et al. 2012). Given the broadly horizontal nature of the caldera-fill units and the lack of significant intra-caldera faulting, the closest-approximated end-member subsidence style (by the classification of Lipman 1997; Acocella 2007) is piston subsidence, in which a coherent caldera floor is bounded by one or more steeply-dipping ring faults. It is uncertain whether the Arran caldera ring fault is a simple outward-dipping reverse fault (stage 2, Acocella 2007) or an inward dipping normal fault with volcanic deposits masking the internal earlier ring fault (stage 4, Acocella 2007). However, the relatively simple stratigraphy and lack of disruption likely indicate an outward-dipping reverse fault, supporting rare field and seismic evidence from calderas such as Rabaul, Papua New Guinea (Mori and McKee 1987; Saunders 2001).

The upper two units described here—i.e. the Binnein na h-Uaimh conglomerates and the Ard Bheinn Member—show some evidence of flow-directions towards the edges of the caldera. This suggests that at this stage in the caldera’s history, there was a palaeo-topographic high roughly in the location of the current Ard Bheinn summit. We tentatively suggest that this could be evidence for resurgent doming in the period following the main Allt Ruadh-related collapse event. Following Troll et al. (2002), doming may also explain the presence of the radial fault described above, and the northerly transport of Dalradian clasts within the caldera. Uplift associated with caldera resurgence has been linked to shallow magmatic intrusion of sills/dykes and laccoliths. This process has been identified through magnetotelluric imaging on the island of Ischia (Bay of Naples, Italy), where some 800 m of uplift, accompanied by volcanic activity, has occurred (Di Giuseppe et al. 2017).

The nature of proximal ignimbrites

The majority of ignimbrites in the Arran Volcanic Formation are high grade (lava-like to welded) with rarer low grade, non-welded examples. These ignimbrites are indicative of high temperature, high mass-flux pyroclastic density currents generated from low fountaining columns that retained heat (Branney and Kokelaar 2002). The lava-like ignimbrites typically display pervasive base-parallel flow banding, indicative of syn-depositional rheomorphism (Andrews and Branney 2011). There are relatively few examples of post-depositional rheomorphism such as extensive domains of contorted flow banding and refolded folds (Andrews and Branney 2011), and a general absence of autobreccia. Together, this indicates there was little slumping, sliding and ultimately brittle deformation of the cool(ing) ignimbrites (e.g. Moore and Kokelaar 1998; Andrews and Branney 2011).

Perhaps most noticeable is the general lack of lithic-rich lapilli- and block-layers in the ignimbrites. These types of breccias/lapilli tuffs are commonly found in intra-caldera proximal ignimbrites and are typically linked to climactic subsidence events and associated caldera wall/floor destabilisation. Modern examples of such units include Ischia (75 ka, Brown et al. 2008) and Pantelleria (46 ka, Jordan et al. 2018) in Italy, and Tenerife in Spain (273 ka, Smith and Kokelaar 2013). The absence of these units on Arran indicates that whilst subsidence clearly occurred, it was not always catastrophic and that explosive caldera-forming eruptions can occur without such ‘tracers’ of caldera collapse.

Overall, the general absence of slumping/sliding ignimbrites and the paucity of collapse-related lithic breccias, support a gradual piston-like collapse of the caldera, with only minimal disruption by faulting and/or later resurgence. In many respects, the Arran caldera is remarkable for its incremental but consistent collapse and the relative stability of the caldera floor.

Eruptive history

We now present an overall model for the eruptive history and caldera evolution of the Central Arran Igneous Complex which is consistent with the observations described in this paper (Fig. 12).

  • The lava-like ignimbrites of the Muileann Gaoithe Member are preserved in situ and as clasts within the later ignimbrite units. As the base is not seen within the caldera, we cannot say whether this was the first stage of volcanism in the area. It is possible that it overlies the erupted products of earlier volcanism that are now buried.

  • A period of highly explosive eruptions formed the Allt Ruadh Member (Fig. 12a). This blanket of mLT covers the entire area of the caldera (Fig. 2).

  • This evacuation of magma caused underpressure in the underlying magma chamber and caused the caldera to collapse. Collapse at this stage was piston-like, with a coherent caldera floor moving along a single ring fault, although an element of trapdoor subsidence with thickening to the west may have occurred (see Acocella 2007). The steep caldera walls left by the outward-dipping reverse ring fault collapsed, forming the Creag Shocach conglomerates (Fig. 12a).

  • The Creag an Fheidh Member was erupted in the eastern part of the caldera (Fig. 2) and ponded against a radial fault (Fig. 12b). The presence of this fault could be due to some degree of resurgent doming (Troll et al. 2002).

  • The upper part of the Creag an Fheidh Member was fluvially reworked in a period of volcanic quiescence. A small pulse of mafic magmatism interacted with this surface water and/or groundwater, and the resulting phreatomagmatic eruption built the Allt Beith tuff cone (Fig. 12b).

  • The rhyolitic lava-like ignimbrites of the White Tuff Member were erupted (Fig. 12c). These ignimbrites are lithologically and petrographically almost identical to the Muileann Gaoithe Member ignimbrites, but were erupted after a significant period of non-homogeneous lower-grade volcanism. This stratigraphy of variable pyroclastic rocks stratigraphically ‘sandwiched’ between two thick rhyolite units is very similar to that observed at Sabaloka, Sudan (Almond 1971). This eruption was followed by a period of volcanic quiescence in which the upper surface was reworked.

  • The Pigeon Cave Member was erupted (Fig. 12c), and intruded by a dolerite sill (Figs. 4 and 10a). The nature of the pre-Palaeogene sedimentary rocks (Cretaceous chalk and possibly Cretaceous sandstone) is unclear as contacts are not exposed, but work on Rùm suggests that supposed intra-caldera ‘megablocks’ may in fact be broadly coherent pieces of caldera floor (Holohan et al. 2009; see also Lipman 1976). If this is the case on Arran, these caldera floor segments may have been exposed by subsidence-related faulting and/or resurgent doming (see below).

  • The intrusion of a dolerite sill, or overpressure from the underlying magma chamber, or both, caused resurgent doming to form a palaeo-topographic high (Fig. 12d) in the vicinity of the modern Ard Bheinn summit.

  • Debris flows comprising material from outside the caldera flowed away from this palaeo-high and were deposited as the Binnein na h-Uaimh conglomerates in steep-sided canyons eroded into the upper surface of the Pigeon Cave Member (Fig. 12d).

  • The heterogeneous high-grade ignimbrites of the Ard Bheinn Member were erupted (Fig. 12d). The lowest of these—coarse crystal tuffs—flowed away from the palaeo-high and were deposited in valleys eroded into the underlying members (Fig. 2).

  • An unknown thickness of the Ard Bheinn Member and any overlying units were lost to erosion.

Fig. 12
figure 12

A series of generalised cross sections through the Arran Volcanic Formation showing the history of eruption and caldera collapse. Lithologies other than the caldera-fill succession have been omitted for clarity

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Conclusions

The Central Arran Igneous Complex is a good example of a well-preserved caldera volcano, as previously established by King (1955), and provides an excellent opportunity to investigate an intra-caldera sequence. Its caldera-fill succession, the Arran Volcanic Formation, was eroded to a relatively shallow level, and generally escaped modification by later intrusions/volcanism and significant faulting. Given the erosion levels, the intra-caldera sequence provides an outstanding record of proximal ignimbrite deposition, which is often unavailable at other ancient calderas that have been heavily intruded and faulted, or modern calderas that are filled with later sediments and/or water, and have not been incised sufficiently. Detailed mapping and field observations have allowed us to interpret the caldera-fill rocks in terms of a stratigraphic sequence of successive eruptive units, which had not previously been attempted. This stratigraphy allows us to propose a chronological model for the formation of the complex and infer the processes which occur as small predominantly silicic calderas collapse. The ignimbrites are accompanied by sedimentary rocks recording intra-caldera fluvial, mass flow and lacustrine deposition, which attest to significant eruption hiatuses. The ignimbrites are preserved almost exclusively as intra-caldera units, with only one example, the Muileann Gaoithe Member, found beyond the caldera. The general absence of extra-caldera ignimbrites elsewhere on Arran is presumably due to erosion.

The ignimbrites of the Arran Volcanic Formation are dominated by high-grade lava-like and welded ignimbrites. The Allt Ruadh Member is the only surviving product of a highly explosive phase of eruptions and period of caldera collapse. The other members typically record rapid deposition from high temperature (>900 °C), high mass-flux pyroclastic density currents generated from low fountaining columns that do not entrain much atmospheric air, and therefore retain large amounts of heat (Branney et al. 1992). The members do however show considerable variation and typically transition from lithoidal and flow-banded lava-like tuffs, through eutaxitic tuffs and cognate spatter-bearing agglomerates, to occasional poorly to non-welded lapilli tuffs. These variations indicate that the pyroclastic density currents were sustained but unstable and subject to variations in mass-flux and temperature.

Although some examples are recorded, there is a general paucity of autobreccia, and this indicates that little slumping and sliding of cool(ing) high-grade ignimbrite units occurred during pauses in deposition (e.g. Moore and Kokelaar 1998). Furthermore, lithic-rich lapilli- and block-layers are rare, indicative of a general absence of climactic subsidence events and associated caldera wall/floor destabilisation, which may be anticipated in intra-caldera proximal successions (e.g. Smith and Kokelaar 2013). Together, these data suggest that although deposition occurred rapidly, caldera subsidence was relatively incremental and piston-like and that eruption of high-grade ignimbrites can occur without catastrophic collapse (see also Lavallée et al. 2006). In eruption hiatuses, however, subsidence continued and exotic material was introduced from outside the caldera by sedimentary processes. Possible resurgent doming events contributed to these processes and influenced deposition of later pyroclastic density currents.

The Arran Volcanic Formation shows that a relatively small caldera can be active through a large number of eruptive periods, separated by significant time gaps, yet can escape large-scale intra-caldera faulting and formation of nested caldera structures, while accommodating significant, but not catastrophic, collapse. We argue that further detailed field investigation of similar calderas (including previously mapped examples), using modern physical volcanology methods and terminology, is essential to elucidate observations from active calderas and modelling experiments.