Volcanic activity and magma genesis, Silurian and Devonian igneous activity, Midland Valley of Scotland

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Cameron, I B, and Stephenson, D. 1985. British regional geology: The Midland Valley of Scotland. Third edition. Reprint 2014. Keyworth, Nottingham: British Geological Survey.


Outcrop of late Silurian and Lower Devonian igneous rocks in the Midland Valley. P915524.

A widespread Caledonian calc-alkaline igneous province extends throughout Scotland and Northern Ireland. Early Caledonian activity in the general area of the Midland Valley is indicated by the presence of igneous clasts in conglomerates of the Lower Palaeozoic inliers. The earliest penecontemporaneous volcanic deposits occur in the Downtonian, near Stonehaven and activity increased in intensity, frequency and extent to a maximum in the early Lower Devonian. Volcanicity had ended by the late Lower Devonian. Radiometric ages of Midland Valley intrusions and volcanic rocks fall within the range of 411 to 407 ± 6 Ma (Thirlwall, 1983a).

Volcanic rocks constitute a major part of most Lower Devonian successions forming such prominent uplands as the Sidlaw, Ochil, Pentland and Carrick hills (P915524). Most of the volcanic sequences are cut by contemporaneous dykes and thin sills. Larger, basic to intermediate hypabyssal intrusions occur in the Sidlaw Hills and in south Ayrshire and thick sills and laccoliths of acid rocks are widespread in the southern Midland Valley. Small dioritic plutonic masses with significant metamorphic aureoles occur in Ayrshire (Distinkhorn, Tincorn Hill and Fore Burn), in the western Ochil Hills and in the Pentland Hills.

Volcanic activity[edit]

Most volcanic successions consist of basaltic and andesitic lava flows with some intercalated volcaniclastic sediments ranging from siltstones to coarse conglomerates. Lava flows usually occur as impersistent, interdigitating, concordant sheets, 3 to 15 m thick. Individual flows can rarely be traced far, although distinctive groups of flows can often be correlated over considerable distances. Pyroclastic rocks are rare, except in the Pentland Hills and western Ochil Hills where they constitute a major part of the succession, and very few central vents are recognised. It thus seems likely that many of the flows emanated from fissure eruptions, the feeders to which are rarely preserved as contemporaneous, comagmatic dykes.

In some areas intercalations of fluvial and lacustrine sediments indicate that the lavas were erupted upon flood plains and that basin subsidence and erosion generally kept pace with the growth of the volcanic pile. Flow tops are often slaggy, cavernous, autobrecciated and reddened, although boles are seldom developed, suggesting that subaerial flows were rapidly covered by detritus which restricted surface weathering and laterite formation. Intercalated torrential deposits of thick, coarse-grained, volcaniclastic conglomerate indicate that considerable relief existed locally at times as volcanic hills built-up above the plains. Fine-grained, upward-coarsening, laminated sediments were deposited in lakes possibly impounded by temporary volcanic barriers.

A common feature of many areas, but particularly well seen in the coast sections of south Ayrshire and Angus are irregular intercalations and cavity-or fissure-fillings of siltstone and sandstone within the lava flows. Lamination in

the sediments is often undisturbed and a wide variety of sedimentary structures and trace fossils have been recorded, even from within cavities. Most authors have regarded the sediment as having been washed into voids, and in some cases lava tunnels, during rapid burial by flood deposits. However, in some cases the evidence suggests that pre-existing sediment was incorporated in later lava flows. In the Ochil Hills and on the Angus coast, features at the base of flows suggest that lavas were erupted on wet, unconsolidated sediments, possibly in a lacustrine environment. In south Ayrshire, zones of lobate pillow structures, accompanied by brecciated fragments of chilled lava (hyaloclastite) in a sandstone matrix, occur at the top and base of many andesitic sheets. These have been interpreted as sills, intruded at shallow depth into weak, unconsolidated, water-saturated sediments which became fluidised in contact with the magma (Kokelaar, 1982).

Petrology of the lavas[edit]

The extrusive rocks range in composition from olivine-basalt to rhyolite, although the dominant type in most areas is andesite or basaltic andesite. Many areas contain a complete range of compositions (e.g. Pentland Hills), but more restricted ranges occur in Ayrshire and the Sidlaw Hills where in situ acid lavas are virtually absent. Acid intrusions are common, even in areas of basic and andesitic volcanicity and it is possible that acid material may have been erupted in such areas, not as lavas but as pyroclastic deposits which were subsequently removed easily by erosion.

Basaltic and andesitic rocks are often impossible to distinguish in the field and, where altered, in thin section. This is reflected by a common grouping in some Geological Survey maps. Most of the basalts and basaltic andesites contain plagioclase phenocrysts, often with a combination of olivine (usually pseudomorphed), orthopyroxene, clinopyroxene, and rarely magnetite. Basic rocks with mafic phenocrysts alone are rare. Hypersthene-, augite- and hornblende-andesites, with or without plagioclase phenocrysts, are common and biotite-andesites occur infrequently. Trachyandesites, trachytes, dacites and rhyolites contain phenocryst combinations of oligoclase/albite, potash feldspar, quartz, hornblende and biotite. Many are flow-banded and some dacitic ignimbrites are recognised. The basic and intermediate rocks are commonly seen to have minor amounts of potash feldspar, with or without quartz, in the groundmass and sanidine rims are sometimes observed on plagioclase phenocrysts. Quartzo-feldspathic segregation veins are also common in basic and intermediate intrusions.

Amygdales are a common feature of the lavas. The siliceous varieties weather out of their matrix to form pebbles which have been collected from river gravels and from the beaches of Ayrshire and Kincardineshire to be polished as semi-precious stones (‘Scotch Pebbles’). The principal decorative amygdales are agate, onyx, jasper, amethystine quartz and rock crystal but other recorded minerals include quartz, calcite, mixed carbonates, zeolites, celadonite, vermiculite and chloritic material.

Magma genesis and tectonic setting[edit]

The lavas of the Midland Valley, together with contemporaneous volcanic sequences in the Grampian Highlands, constitute a widespread province characterised by many petrological and geochemical similarities. In several areas many of the more basic lavas contain high levels of such elements as magnesium, nickel and chromium which suggests that they represent primitive magmas originating by partial melting of upper mantle material. However, relatively high levels of incompatible trace elements, present in only small amounts in mantle material, suggest that the melts may have been enhanced by material from some other source. A recent isotope and rare-earth element study suggests that altered basalts and/or sediments from oceanic crust may have provided such a source.

Lava suites throughout the province are of a calc-alkaline nature and many are of a relatively high-potash type with a trace-element chemistry characteristic of active, orogenic continental margins. Spatial variations in trace element concentrations and ratios, in certain isotope ratios and in some major elements have been recognised in a NW–SE direction, perpendicular to the Caledonian structural trends. Differences are most marked between the Grampian Highlands and north Midland Valley, and slight changes across the Midland Valley are in the same sense. The relationships suggest a model of magma genesis involving melting of mantle material above a descending slab of oceanic lithosphere in a NW-dipping subduction zone (Thirlwall, 1981, 1982).


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