Encounters with non-uniformitarianism: a very personal voyage amidst Ordovician volcanoes of Wales

11/10/2016, 18:30 - 19:30

Fragmentary structure of the Precambrian basement of Wales controlled Ordovician development of sedimentary basins and sites of magma ascent so that the volcanoes depict the underlying discontinuities. Onset of subduction at ~478 Ma, with rapid opening of the Rheic Ocean and closure of the Iapetus Ocean, marked initiation of a remarkable episode of volcanic activity. Oblique extension pulled apart the flawed crust and facilitated ascent of amphibole basalts and andesites to form a subaerial arc volcano (Rhobell Fawr) with associated porphyry copper. Then several narrow graben within the Welsh Basin subsided to accumulate thick, mainly submarine, volcanic piles and sills mainly of basalt and rhyolite. Silicic ‘super-eruptions’ formed caldera volcanoes in or close to the sea.

During the 1970s and 80s detailed studies found that some caldera-related rhyolitic ash-flow tuff deposits (ignimbrites) underwent hot-state welding-compaction (≥ 530°C) where the parent ash flows had entered the sea and continued to flow under water. Subaqueous high-temperature deposition perhaps was counterintuitive then and the Welsh Ordovician ignimbrites became renowned for the controversy they engendered, even dismissed as the protagonists clearly were ‘nuts’. Further detailed studies found more peculiar rock characters and emplacement processes, the likes of which were unknown elsewhere.

The marine settings of most of the volcanoes facilitated robust correlation of the strata, while successive improvements in radiometric dating of the rocks gave ages that at last linked the volcanoes to the deeper intrusions. Just as systematic mapping of the volcanoes and their subjacent plumbing was curtailed, it became clear that there was a particularly intense phase of magma productivity, at around 453 Ma (Caradoc in old money, now Katian). Sea level was generally high during the Ordovician, but at about this time it stood at its highest ever in Phanerozoic times. The exceptional sea level, peak magma productivity and unusual volcanic processes plausibly were linked to exceptional plate-tectonic activity, so that this geological episode in Wales is unique.

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The Making of Wales