Volcanology of the upper cone of Rockeskyller Kopf, West Eifel Volcanic field, Germany

Abstract

Rockeskyller Kopf in the W.est Eifel volcanic field of Germany is a composite volcanic complex that was erupted on to Devonian dolomite at around 500 ± 100 ka BP. The youngest deposits are a scoria cone and lava flow that are exposed in a near perfect radial cross section. The scoria cone was erupted in three main stages: a) an initial crater wall building stage (-10m thick) that is characterized by poorly-layered to massive, subangular, welded scoria, that average 1.25 cm in size; b) main stage deposits, - 14 m thick, exhibit well-developed, meter-scale bedding, with the majority of the units containing lapilli up to 6 cm and bombs as large as 1m; c) waning stage deposits (-12m thick) are characterized by a reduction in lapilli size and smaller scale, better defined layering. The crater wall deposits are disconformably overlain by crater fill deposits, composed of poorly layered and poorly sorted lapilli that pinch out at the edge of the crater. Two welded scoria with ash layers are draped over the entire crater wall unit, from the bottom of the crater to the outer most exposure of the wall. The next stage of eruption was dominated by effusive eruption producing spattered material including fragments with flow structures as well as spindle bombs that filled and spilled over the crater wall. Occurrence of lower crustal xenoliths and basement rock fragments varies throughout the eruption. Whole rock geochemical data and petrographic observations indicate that the rocks from the scoria cone and spatter material of Rockeskyller Kopf are F-suite, leucititic lavas. The samples however, become alkali enriched as the eruption proceeded, opposite to the observed alkali depletion typical of F-suite magma eruptions. Major element oxide values are scattered but show no patterned variation as the eruption proceeded. Average La/Lu ratios are approximately 250 and indicate that the magma was generated in the garnet stability field. Trace element concentrations and petrographic observations indicate that the magma was not contaminated by crustal material. The source magma for the scoria cone and lava flow of Rockesykller Kopf rose quickly from the mantle as one pulse. The magma chamber was tapped from the bottom or side causing the reverse trend in alkali enrichment. The crater wall deposits formed by Strombolian activity and sporadic minor phreatomagmatic pulses. The crater fill deposits are interpreted to be the result of fire-fountaining with the formation of spatter deposits and local bomb-rich horizons; this culminated in the formation of a thick lava flow.