Tertiary and carboniferous magmatism around Lundy Island and the outer Bristol Channel - a geophysical and geochemical perspective

Roberts, Clive Lynton (1997). Tertiary and carboniferous magmatism around Lundy Island and the outer Bristol Channel - a geophysical and geochemical perspective. PhD thesis The Open University.

Abstract

The British Tertiary Volcanic Province comprises central volcanic complexes with positive gravity and magnetic anomalies, some centres associated with both large volumes of continental flood basalts and regional dyke swarms. The Lundy Island is the southern most expression of Tertiary volcanism and consists of granite intruded by around 200 dykes and associated with positive gravity and magnetic anomalies. The Lundy Dyke Swarm comprises basalt/dolerite and trachyte to rhyolite intrusions within host Tertiary granite (58.7 ± 1.6 Ma) and Devonian sediments. Outcrops of dykes are confined to coastal exposures on Lundy as they are veneered by peat over most of the island. Dykes present paired magnetic anomaly profiles, which allows their trends tobe determined by proton magnetometry. The dykes have a radial disposition superimposed on a ENE-WSW regional trend. Anisotropy of magnetic susceptibility studies indicate that magma for the radial component was emplaced at shallow to moderate inclinations, suggesting a relatively shallow origin near to the western border of Lundy Island. The regional component was emplaced at shallow to sub-horizontalangles, suggesting lateral movement of magma from a possible source 12 km to the northwest.

Geochemical signatures indicate that the Lundy Dyke Swarm was transitional between plume-related magmatism and partial melting of the lithospheric mantle, the magma being stored in several small storage bodies at differing depths in the upper continental crust, rather than in one large magma chamber. Basic dykes at Lee Bay (60 ± 0.6 to 63.1 ± 0.7 Ma) pre-date the Lundy Dyke Swarm and were derived from a discrete magma chamber, possibly near to Morte Point. Conversely, other dykes in North Devon (Fremington dyke - 292.4 ± 1.7 Ma; Horse-Shoe Rocks - 339.6 ± 7.4 Ma) are not directly related to Tertiary magmatism, even though the Horse-Shoe Rocks have a Tertiary palaeomagnetic overprint.

The Lundy Igneous Complex (comprising granite, dykes and sub-surface basic rocks) is situated close to the intersection of the Variscan Front and the Welsh Caledonides massif where the continental thickness is between 25 and 27 km. Emplacement of magma was assisted by the heavily fractured nature of the host sediments. However, a large positive gravity anomaly to the northwest of Lundy Island does not have a corresponding magnetic anomaly and so is interpreted as the response to relatively dense uplifted basement in the Lundy Horst rather than a large volume of basic rocks.Thus, the Lundy Igneous Complex probably did not produce sub-aerial volcanic activity, as pressure in the magma chamber would not have exceeded the overlying litho static load, despite the fractured nature of the host rocks.

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