Caradoc magmatism, Ordovician, Northern England

From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey.

Introduction

Geological map showing the principal Ordovician and Devonian igneous bodies and their relationship to major structural features. BrO Broad Oak Granodiorite Pluton, Butt Buttermere Intrusion, CF Carrock Fell Centre, CW Crummock Water Granite, Dun Dunmail Intrusion, En Ennerdale Granite Pluton, Esk Eskdale Granite Pluton, Ha Haweswater Gabbro-Microdiorite Swarm, HG Haweswater intrusion, LG Loweswater Intrusion, Ryd Rydal Intrusion, Sh Shap Granite Pluton, Sk Skiddaw Granite Pluton, Th Threlkeld Microgranite Intrusion, Ulp Ulpha Intrusion, Wa Wasdale outcrop of Esk, Wear North Pennine Batholith, Wen Wensleydale Granite Pluton. P916043.

Craggy peaks of Sca Fell (left), Scafell Pike, Ill Crag, Bowfell and Crinkle Crags (right) in the central Lake District lie at the heart of one of the major silicic caldera volcanoes within the Borrowdale Volcanic Group. (P005109).

During Caradoc times, the Lake District was the focus for one of the most intense episodes of igneous activity seen during the geological history of the British Isles. A chain of volcanoes lay along the margin of Eastern Avalonia, at least from eastern Ireland, through the Lake District, the English Midlands and into Belgium. In the Lake District, two subaerial volcanic successions were built up within opposing half-grabens, each originally 40 to 50 km wide. Preserved in the northern one is the Eycott Volcanic Group — more than 3200 m of basaltic, andesitic and dacitic lavas and sills with subordinate pyroclastic rocks (P916043). In the southern half-graben, the Borrowdale Volcanic Group comprises at least 6000 m of basaltic to rhyolitic lavas and sills, along with voluminous pyroclastic and volcaniclastic sedimentary rocks. The Borrowdale sequence contains evidence for the former existence of caldera volcanoes and local extensional basins (P005109). It is one of only a handful of volcanic suites worldwide with rocks that contain garnet phenocrysts. In association with the volcanism, components of the Lake District granitic batholith were emplaced beneath the central part of the region at this time (P916043). In the Isle of Man, mafic sheets at Poortown, east of Peel, are intruded into Arenig sedimentary rocks and appear to be geochemically similar to the Lake District’s Caradoc volcanic rocks. Also in the Isle of Man, the biotite-bearing Dhoon Granodiorite Pluton and associated Oatlands Complex may be a product of the Caradoc magmatic phase, but might equally have been emplaced later, at the time of the Acadian Orogeny (see Chapter 5).

The extensional tectonic regime and successive episodes of caldera collapse are seen as the main mechanisms for preservation of the subaerial volcanic rocks in the Lake District. Such occurrences are rare in the geological record and the Borrowdale Volcanic Group is a particularly well exposed example. Most subaerial volcanoes, particularly those constructed with abundant pyroclastic deposits, are more usually disintegrated explosively, weathered, eroded and redeposited in subaqueous sedimentary regimes. The depth of erosion present in the Lake District provides a three-dimensional model of the volcanic system that is rarely available in modern, silicic caldera volcanoes.

The Borrowdale Volcanic Group is also noteworthy in that it provides evidence for the emergence of terrestrial arthropods from a freshwater, rather than marine, environment. In the south-west of the outcrop bedding surfaces in sedimentary rocks exhibit trails and trackways attributed to a myriapod-like organism; this is one of the earliest known records of the existence of terrestrial arthropods. The traces were preserved in shallow-water, lacustrine sediments subjected to episodic drying-out, as demonstrated by the presence of desiccation cracks, and it would seem likely that the organisms survived subaerial conditions, at least temporarily.

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