The Ordovician (Caradoc) volcanic rocks of NE Snowdonia constitute two major groups, the Llewelyn Volcanic Group and the Snowdon Volcanic Group, which accumulated predominantly in shallow-water marine conditions. The younger Snowdon Volcanic Group comprised a bimodal, basalt-rhyolite suite and included a major caldera-forming eruption of acidic ash-flow tuffs superseded by both Surtseyan and Strombolian basaltic volcanism. Rhyolite domes were intruded into the volcanic sequence in the vicinity of the caldera. The Snowdon Cu-Pb-Zn vein mineralization comprises five paragenetic mineral assemblages. The veins cut rocks deposited within and over the caldera and it is proposed that the dominant controls of mineralization were volcanogenic. Circulation in hydrothermal cells, involving both juvenile fluids and seawater, deposited the minerals at a late stage in the evolution of the caldera.
The Lower Rhyolitic Tuff Formation (up to 600 m thick) represents an eruptive cycle of acidic ash-flow tuff which is stratigraphically associated with marine sediments and subaqueously emplaced basalt lavas. The formation comprises volcaniclastic and pyroclastic megabreccias and breccias, massive welded and non-welded acidic ash-flow tuffs, reworked tuffs and tuffities, siltstones, rhyolite intrusions and extrusions. Its basal contacts vary from conformable, to disconformable and unconformable. The inter-relationships of these variations to pre-, syn- and post-emplacement structures define a submarine, asymmetric downsag caldera. The main eruptive centre, coincident with the thickest accumulation of intracaldera tuffs, lies close to its north margin, on the north side of the Snowdon Massif. To the SW, the intracaldera tuffs thin progressively and much of the formation comprises tuffs reworked in the vicinity of a Caradocian shoreline. To the NE and E, outflow tuffs escaped into a deeper marine basin. Many of the features of the caldera are similar to those of subaerial calderas, and it is concluded that the enveloping sediments and lavas, and the character of the reworked tuffs, hold the key to the recognition of its submarine development. Subsequent resurgence resulted in only local and short-lived emergence of the intracaldera tuffs.
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