Otros medios de prueba del CPCM (reproducción de sonido,

The BPIP itself is part of the North Atlantic Igneous Province (NAIP), a predomi- nantly 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. Kent and Fitton, 2000; Storey et al., 2007; Thompson and Gibson, 1991). In the British Isles, the NAIP comprises the extensive lava fields of Skye, Eigg (one of the Small Isles), Mull, and Antrim (Emeleus and Bell, 2005), as well as localised intrusive and volcanic ‘central complexes’ (Fig. 1.1). Other Palaeogene remnants of the NAIP are preserved on Greenland, the Faroe Islands, and offshore (Saunders et al., 1997).

Following uplift caused by the arrival of the plume, magmatic activity in each volcanic area of the BPIP started with extrusion of a thick lava-pile through fissure eruptions, not unlike the volcanic landscape of present-day Iceland. These fissures are preserved as the NW–SE trending regional dyke swarm that cuts the underlying geology and the lava fields. From a combination of radiometric and palynological dating and field relationships, it is thought that eruption of the lava fields commenced 61–60 Ma (Chambers et al., 2005) in the Hebrides, and possibly prior to 62 Ma in Antrim (Ganerød et al., 2010). All magmatism preserved in the onshore BPIP was part of ‘phase 1’ (pre-sea floor spreading) of the NAIP (Saunders et al., 1997).

There are four lava fields recognised in the onshore BPIP (Bell and Williamson, 2002; Emeleus and Bell, 2005; Preston, 2009). The Antrim lava field is exposed over a large portion of Northern Ireland. The Eigg lava field is preserved on Eigg, Muck, and the south east part of Rùm. The Skye lava field is exposed on Skye, Canna, and in northern Rùm. The Mull lava field is also found on Ardnamurchan, and Morvern; it may connect to the Eigg lava field offshore (Fyfe, 1993). In addition to these

6 Introduction

preserved lava fields, an Arran lava-pile was proposed to be preserved only as blocks which subsided or tumbled into the caldera of the central complex (King, 1954).

The lavas were generally erupted onto flat, low-lying land in a tropical climate, as shown by palynological and palaeontological studies (Emeleus and Bell, 2005). The lowermost flows occasionally erupted into lakes as shown by pillow lavas and hyaloclastites, or flowed over waterlogged sediments as shown by rootless cones, for example at Ardtun and Carsaig Bay on Mull. There is evidence that in places such as Mull, Skye, and Antrim, the first activity was eruption of basaltic scoria cones and pyroclastic tuffs (Emeleus and Bell, 2005; Ganerød et al., 2010). Topography was steep in places, indicated by presence of conglomerates interbedded with the lavas, and the onlap of lavas against underlying Moine or Torridonian rocks on Mull and Rùm, respectively (Bell and Williamson, 2002).

The lava-piles of the Hebrides reached thicknesses of 2 km, but may have been thicker in the deeper parts of the still-active half grabens, and were up to 5 km thick in the Faroes (Anderton, 2009; Emeleus and Bell, 2005). On Ben More on Mull, 966 m of continuous lava stratigraphy can be seen. Evidence from geographical relationships and zeolite assemblages in amygdales (which can be used to indicate metamorphic conditions, and therefore temperature and depth) show that there must have been around another 1 km of lava above this (Bailey et al., 1924; Walker, 1970). The bulk of the lavas are basalt and hawaiite sheet flows, generally less than 5 m thick. Higher in the stratigraphy more evolved lavas are encountered. These take the form of less laterally extensive flows of mugearite and trachyte (Kerr et al., 1999).

Emplacement of the central complexes involved localisation of magmatism and uplift of several hundred metres. The location of the volcanic centres is likely to have been controlled by Caledonian or other pre-Palaeogene crustal structures: Skye and Rùm on the Skerryvore-Camasunary Fault, Mull on the Great Glen Fault, and the North Arran Granite (NAG) and Arran central complex on or near the Highland Boundary Fault. The Northern Irish volcanic centres appear to lie close to the Navan- Silvermines Fault, the Irish continuation of the Iapetus Suture (Anderton, 2009; Bluck et al., 1992). There are known to be deep crustal structures in the offshore regions around St. Kilda; these could be faults providing a structural control on the position of the central complex (Bell and Williamson, 2002).

Magmatic activity at the central complexes took the form of various types of intrusions and, in most cases, surface volcanism, which was often explosive. Intrusive bodies include 1) coarse-grained mafic and ultramafic rocks such as the Cuillin gabbro of Skye and the layered peridotites and troctolites of Rùm, 2) intermediate and granitic rocks such as the heavily dissected granophyres on Mull and the NAG, 3) cone sheet swarms such as those on Mull and Ardnamurchan which can locally

1.1 The British Palaeogene Igneous Province 7

make up almost 100% outcrop, 4) ring dykes such as the renowned Loch Bà felsite ring dyke on Mull, and 5) local basaltic dyke swarms, which may be radial around the central complexes (Emeleus and Bell, 2005). The focal point of activity can be identified as the central point of the cone sheets, radial dyke swarms, ring dykes, and other arcuate features. In this way, changes in the focal point of magmatic activity can be identified through time, such as on Mull, where the three magmatic centres get younger towards the north west (Bailey et al., 1924; Emeleus and Bell, 2005).

Although less common than mafic rocks, there is widespread evidence of ex- trusive silicic volcanism and explosive eruptions (Bell and Emeleus, 1988; Brown et al., 2009). Due in part to its historical importance and relative ease of accessibility, the BPIP has been the source of many developments in the global understanding of volcanological processes. Pyroclastic rocks 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 BPIP (Brown et al., 2009; Troll et al., 2000), but exposures in these areas are limited due to later, cross-cutting layered in- trusions. 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 (Holohan et al., 2009) and the Sgurr of Eigg Pitchstone (Brown and Bell, 2013).

Accompanying the injection of magma into the central complexes was a large degree of hydrothermal alteration. Fluid originated in the water-saturated upper crust (Taylor and Forester, 1971) and was heated to 400°C in zones up to 20 km around the central complexes. Temperatures in the centre of the system reached as high as 800°C. This alteration caused almost complete stripping of olivine and extensive alteration of amphiboles, pyroxenes, and biotite. Quartz, feldspar, and glass were also affected. Mineral veins containing chlorite, epidote, prehnite, and calcite were developed in the centres and extend into the country rocks (Emeleus and Bell, 2005).

Gravity studies of the Hebrides show steep-sided, highly positive anomalies below the centres of Mull, Skye, and Rùm. Smaller anomalies are seen under Ardnamurchan and central Arran (Fig. 1.2). This suggests that granitic rocks (low density) only occur in relatively thin sheets near the surface (Bott and Tantrigoda, 1987), and that the bulk of the volume of the volcanic centres consists of large bodies of basic and ultrabasic rocks (high density), such as gabbro and peridotite cumulates (Bott and Tantrigoda, 1987; Bott and Tuson, 1973).

Although most of the volcanic centres display some or all of these features, they each have a complex magmatic history, and different relative timing of events. They

8 Introduction

Fig. 1.2 – Bouguer (onshone) and Free-air (offshore) gravity anomaly over Scotland (BGS, 2007), showing the strong positive anomaly over the central complexes of the BPIP – blue = low, red = high.