Formation of hybrid cumulates: melatroctolites in Intrusion 4 of the Honningsvåg Intrusive Suite, northern Norway

New mapping shows that the 1160-m-thick Layered Series of the mafic to ultramafic Intrusion 4 of the Honningsvåg Intrusive Suite, North Norwegian Caledonides, comprises approximately 25% feldspathic peridotite, 55% melatroctolite (olivine-rich troctolite), 5% troctolite, <2% olivine melagabbro (olivine-rich olivine gabbro) and 13–15% olivine gabbro. The cumulate sequence can be divided into eight macrocyclic units, the base of each unit resulting from magma replenishment. Compositions of olivine and plagioclase obtained from ca. 1700 electron microprobe point analyses, display complex and limited variations (Fo87–74 and An80–67) relative to stratigraphic height. The origin of olivine-rich cumulates, such as melatroctolites (52–72% olivine) and olivine melagabbros (29–37% olivine), in large quantities, is difficult to explain by either simple fractional crystallisation or conventional layer-forming processes. A new genetic model is proposed involving suspension of pre-existing cumulus crystals during magma recharge in a periodically replenished magma chamber, their partial resorption and simultaneous crystallisation of liquidus olivine. Cumulates formed in this way represent mixtures of crystals that crystallised from compositionally different magmas and are, therefore, termed hybrid cumulates. Apart from their distinctly non-cotectic bulk modal proportions, the hybrid nature of the olivine-rich cumulates is evidenced by partially resorbed plagioclases enclosed within cumulus olivines. The occurrence of resorbed plagioclase inclusions in olivine gabbros and large variations in modal plagioclase content suggest that these may also be hybrid cumulates.