Dykes & Sills

Northumbrian Dykes and Sills

There are four distinct sets of dykes and sills in Northumberland. These are;
the lamprophyre dykes of Early Devonian times,
the Devonian Cheviot dyke swarm,
the Permian Whin Sill and its associated dyke swarm,
and the Palaeogene tholeiite basalt dyke swarm.

Lamprophyre dykes

Lamprophyre dykes are generally more scarce than other types and just two of them have been reported here in Northumberland although they are more numerous further south in Teesdale, the North Pennines and the Lake District. Both of the Northumbrian lamprophyre dykes are marked on the current BGS online map located just a few hundred metres apart on a broadly north-south axis in Silurian sediments at the south-western perimeter of the andesite flow, close to the Chew Green Roman fort. To date, we have been unable to locate either of these dykes.

The Cheviot dyke swarm

The dykes of the Cheviot swarm have been said to extend radially to the south of the pluton, and more haphazardly to the north. However, most of the dykes tend to run in either a north-north-east or a north-north-west direction.
They were intruded between 390 and 400 million years ago and, with the exception of a few pyroxene-porphyry dykes , they are mostly felsic in nature. Four types have been recognised: mica-porphyries, quartz porphyries, felsites and pyroxene porphyries.
Dykes occur within the pluton itself and penetrate out through the altered hornfels of the pluton’s metamorphic aureole into the unaltered andesites and agglomerates that form the perimeter of the massive. The composition and colour of Cheviot dykes can vary considerably from centre to edge as well as along their length making it difficult to recognise their continuity.

A map showing the general distribution of the Cheviot swarm of dykes.

The Cheviot dyke swarm

Key to bedrock types shown on the maps on this page

Key to the maps on this page

The Whin Sill and associated dykes

The Whin Sill and its associated dykes were intruded early in Permian times, between 310 – 290 million years ago. The Whin Sill is, in fact, four separate sills, three of which outcrop here in Northumberland. These are; the Farne Island sill, the Alnwick sill, the Great Whin Sill that runs along the Roman Wall as well as on the Pennine escarpment and locations in Teesdale, and finally the Little Whin Sill that outcrops only in the Rookhope Burn and in the disused quarry at Greenfoot, near Stanhope in Weardale. All these intrusions are tholeiite basalts but each of them is characterised by a different magma flow direction, petrology and palaeomagnetic signature.

Map showing the distribution of the Whin Sill and associated dykes

The Whin Sill and associated dykes

The dykes associated with the Whin Sill tend to be between three and ten metres wide and share a north-east to east-north-east trend that follows the grain of the underlying Lower Palaeozoic basement rocks. They occur at the margins of the main sill intrusions and the evidence of geochemical analysis and paleomagnetic testing suggests that they fed the sills. However, there are locations where the dykes cut through the Great Whin Sill suggesting that at least some of them post-date sill emplacement.

The Palaeogene tholeiite basalt dyke swarm

In Palaeogene times, 60-55 million years ago, a vast plume of mantle magma that had risen under the lithosphere bearing the present day west coast of Scotland, Northern Ireland and eastern Greenland gave rise to thermal uplift accompanied by intense volcanic activity. Vast quantities of magma spewed out to form volcanic complexes, sills and dyke swarms. The favoured theory until comparatively recently has been that pulses of this magma bore south-east from a centre located at Mull and, in as little as five days, cut a shallow curve through the rocks between their origin and what is now the coast of Northumberland and beyond. This magma solidified to form two major echelons of dykes, the Acklington Dyke and the Cleveland Dyke along with numerous other dykes that together make up the Palaeogene tholeiite basalt dyke swarm. However, a more recent theory has been proposed that aims to overcome problems associated with the lateral intrusion of these dykes; chiefly their continuity and resistance to solidification and plugging. The suggestion is that the dykes were intruded vertically from a common, low-level source. Regardless of the means of their intrusion, the Palaeogene dykes share a common east-south-east orientation and a very similar petrography.

A map showing the distribution of Palaeogene dykes in Northern England

The Palaeogene Dykes of Northern England

Classification of tholeiite dykes

Holmes and Harwood developed a useful system for classifying these tholeiite dykes that is represented here in table form.
A .pdf file of this table can be downloaded below.

Holmes & Harwood Classification of Tholeiite Dykes

No vestige of a beginning, – no prospect of an end