Basaltic fissure eruptions of the Mull lava field, British Paleogene Igneous Province

Abstract

Linear fissure zones commonly feed modern basaltic eruptions, yet direct evidence for such fissure eruption sites within ancient large igneous provinces remains scarce. We present a detailed examination of a well-preserved sequence of vent-proximal basaltic deposits from the Isle of Mull within the British Paleogene Igneous Province. We interpret these deposits as a segment of a dissected dyke-fed fissure zone. By combining photogrammetry and fieldwork, we identify a >5 km coastal zone near Calgary Bay, Isle of Mull hosting eruption vent-proximal basaltic deposits. In two main locations, the pyroclastic deposits exceed 5 m in thickness and contain basaltic airfall tephra, bombs and agglutinated spatter, in places forming spatter ramparts – features indicative of vent-proximal explosive basaltic activity. These deposits are intersected by numerous irregular intrusions with ragged edges, which terminate beneath the top surface of the pyroclastic layers, suggesting a complex network of partially drained feeder dykes. Vent-proximal effusive basaltic lava deposits in the area include agglutinated clastogenic lavas, spongy pahoehoe, inflated pahoehoe and, less commonly, ‘a‘ā lavas, along with multiple instances of lava tubes. This study provides novel insights into the eruptive architecture and processes within the North Atlantic Igneous Province, documenting a previously uncharacterized dissected large igneous province fissure system.