Rhomb-porphyry eruption dynamics in the Oslo Rift recorded in pyroclastic fissure conduits

Dike-fed fissure vent complexes are commonly linked to basaltic eruptions but are also proposed for some intermediate to silicic eruptions in Large Igneous Provinces (LIPs) and continental rifts. Direct evidence of these complexes is scarce however, and our understanding of the associated eruptive processes remains limited. Basaltic and trachy-andesite (i.e., rhomb- or rectangle-porphyry: RP) eruptions across the Oslo Rift (part of the Skagerrak-Centered LIP) are assumed to have erupted from similar dike-fed fissures, although no direct evidence of eruptive vents has been identified. We present observations illustrating the presence of a shallow (<80 m paleo-depth) RP fissure conduit in the central Oslo Rift recording explosive eruptive activity. Previous workers interpreted these outcrops as a partially preserved lava, suggesting the basaltic unit was cross-cutting due to deposition in an erosional depression. Our studies of outcrops, hand-samples, and thin-sections illustrate three primary textural zones: 1) a core zone with ductile deformation and moderate fragmentation; 2) a heavily-fragmented ash-rich margin zone with inclusions of both rhomb-porphyry and host rock (basalt); and 3) an edge zone variably characterised by un-brecciated rock or brecciated rock with ash-filled fractures. Lateral textural gradations (over 1–3 m width) from core to brecciated host-rock and substantial overall width (~40 to <<100 m), are consistent with a near-surface (<80 m) paleo-depth, whilst heavily fragmented margins suggest mild to moderate explosive activity towards the end of the associated eruptive episode. An exposure of a slabby- to rubbly-pahoehoe RP8 flow top with mingled and overlying pyroclastic (i.e., ash and lapilli) components additionally high- lights explosive processes operating in the late stages of older RP eruptions. Our observations provide the first direct evidence of elongate eruptive-conduits (i.e., fissure-conduits) in the Oslo Rift, but textural characteristics illustrate that at least some rhomb-porphyry eruptions were punctuated by episodes of moderately explosive activity during waning eruptive activity.