The Organization and Internal Structure of Cyclic Units in the Honningsvåg Intrusive Suite, North Norway: Implications for Intrusive Mechanisms, Double-Diffusive Convection and Pore-Magma Infiltration

Abstract

The Honningsvåg suite was emplaced into the Magerey Nappe during Scandian (late Silurian-early Devonian) deformation and Barrovian regional metamorphism. Due to later orogenic events it is exposed in cross-section. The suite forms a lopolith consisting of 5 nested intrusions containing mainly olivine-chrome spinel (ocC), plagioclase — olivine (± chrome spinel) (pocC), plagioclase — Ca-rich pyroxene-olivine (paoC) and plagioclase — Ca-rich pyroxene — Ca-poor pyroxene (pahC) cumulates. The oldest cumulates are cut by a subcylindrical intrusion megabreccia which may have been a feeder to the younger parts of the suite. A 600–2000 m thick sequence of cumulates in the southern lobe of intrusion 2 can be subdivided into at least 20 cyclic units 15–450 m thick which repeat asymmetrical layer sequences. An ideal unit is defined as comprising ocC, pocC, paoC and pahC (in stratigraphic order). Uncompleted, reduced and interrupted sequences are recognized in which there are departures from the ideal unit. Cyclic units are stacked in mutual contact and separated by sharp, regressive discontinuities. Phase contacts within some units are discordant to unit boundaries such that lithological layers pinch out laterally. Layers wedge out consistently in the same direction, the most primitive cumulus parageneses being thickest towards the axis of intrusion 2 while the most evolved cumulates are found in the more distal portions of the southern lobe. Cyclic units overstep the base of the lobe. The cumulates crystallized within a periodically — recharged and -inflated magma chamber under a double — diffusive convective regime. The stratified magma column generated cumulates where it impinged on the inwardly — sloping chamber floor. Repeated slow collapse and rapid re-elevation of the magma column resulted in lateral migration of belts characterized by different cumulus assemblages across the floor of the chamber. New compositional zones, which arose above basal layers, are required to explain lateral variations in cumulate sequences of the interrupted type.