The Caradoc volcanoes of the English Lake District

Abstract

SUMMARY During Late Ordovician (Caradoc) time, the English Lake District was the focus for one of the most intense episodes of magmatism seen during the geological history of the British Isles. There, two thick subaerial volcanic successions aggraded within opposing half-graben. In the northern half-graben, the Eycott Volcanic Group (EVG) comprises basaltic, andesitic and dacitic lavas and sills with subordinate pyroclastic rocks having geochemical affinities that are transitional between medium-K, continental-margin tholeiitic and calc-alkaline suites. In the southern half-graben, in the central Lake District, the Borrowdale Volcanic Group (BVG) comprises more than 6 km of basaltic to rhyolitic lavas, sills and pyroclastic rocks of medium- to high-K calc-alkaline continental margin type. Initial BVG eruptions were phreatomagmatic, but once conduits were opened, the volcanism became dominated by andesite lava effusion from clusters of low-profile volcanoes. The EVG compares to this early phase of BVG volcanism. A switch then occurred to paroxysmal eruptions, which produced many widespread sheets of densely welded silicic ignimbrite. This phase saw the development of major silicic, piecemeal calderas at Scafell and Haweswater. One of the later units, the Lincomb Tarns Formation, which is 150–800 m thick, is the most voluminous ignimbrite preserved within the Lake District. Other large-magnitude silicic eruptions are recorded within depocentres that lay along the current southern margin of the BVG outcrop. During intervals between the major silicic eruptions, andesitic tephra deposits were reworked and re-sedimented by mass-flow and fluvial processes, and were deposited within fluvio-lacustrine basins developed through regional extensional faulting and caldera collapse. Throughout, substantial volumes of magma were intruded contemporaneously into the volcanic piles. Towards the end of the volcanic episode, the granitic Lake District batholith was emplaced beneath the succession. Subsidence during cooling of this mass allowed renewed marine sedimentation across the region.

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

“…Thermodynamic cal culations carried out in this previous work show that car bon-saturated fluids with #XC02 of 0.69 (type V fluid) and atjD2 corresponding to the FMQ buffer are stable at temperatures of ",500°C for a pressure of 2-3 kbar. This pressure interval was considered the most likely, as it is compatible with an emplacement of the graphite deposit in a subvolcanic setting (Millward, 2004). The isochores cal culated for selected V inclusions in this study are consistent with the estimated PT conditions and constrain the begin ning of graphite precipitation to a pressure close to 2 kbar (Fig.…”

“…The host BVG was emplaced during a brief magmatic episode that lasted no longer than 5 Ma (Millward and Evans, 2003). The subaerial, 6000 m-thick, caldera-related succession comprises basaltic to rhyolitic la vas, sills, and pyroclastic rocks of medium-to high-K calc alkaline, continental-margin, affinity (Beddoe-Stephens et aI., 1995;Millward and Evans, 2003;Millward, 2004). The volcanic rocks are underlain by the upper Cambrian to Middle Ordovician Skiddaw Group, a succession of mar ine turbiditic mudstones and sandstones at least 5000 m thick (Cooper et aI., 1995(Cooper et aI., , 2004Stone et aI., 1999).…”

“…A pre-Early Devonian age for the deposit is suggested by the presence of a pronounced Acadian cleavage in many hand specimens from the mine spoil heaps, but this fabric could not be traced in situ in the mine from the veins into the wall rock. The apparently direct association of the graphite with emplacement of the dioritic intrusion lead Millward (2004) to infer that the deposit was emplaced during Ordo vician time; with the exception of the suite of Early Devo nian lamprophyre dykes, all mafic intrusions in the Lake District are considered to have an age of about 450 Ma (Katian).…”

Deposition of highly crystalline graphite from moderate-temperature fluids

“…The Borrowdale graphite deposit consists of mineralized faults hosted by andesite lavas and sills belonging to the Upper Ordovician (Caradoc ) Borrowdale Volcanic Group, and by a probably contemporaneous hypabyssal dioritic intrusion (Millward, 2004). This epigenetic deposit is unique because it is the only economic concentration of graphite hosted by volcanic rocks.…”

“…The FMQ buffer from Ohmoto and Kerrick (1977) was used. These calculations were done for a pressure of 2-3 kbar, compatible with pressure estimates inferred from emplacement of the graphite deposit in a subvolcanic setting (Millward, 2004). A pressure below 2 kbar is not possible as graphite is not stable under these conditions for the given fl uid composition (Frost, 1979).…”