Transient liquid- and solid-dominated inflation of an upper crustal magma chamber: insights from the Carlingford Complex (Ireland)

Jack Beckwith, Michael Stock, Marian Holness, Mark Cooper, Jens Andersen, Christian Huber, David Chew, Oliver Higgins, Elliot J Carter, Sam Broom-Fendley

Layered intrusions are the crystallised remnants of magma reservoirs and preserve a detailed record of magma storage, differentiation, and recharge processes in the upper crust. Their assembly is commonly categorised into two end-member emplacement regimes: long-lived liquid-dominated magma chambers, and incrementally assembled crystal-rich mush systems. These end-members are often presented as competing concepts despite intrusions necessarily spanning a range of crystal fractions as they cool and crystallise. The Carlingford Complex (Co. Louth, Ireland) preserves the shallow-crustal architecture of a volcanic centre emplaced during plume-related rifting of the North Atlantic, thus providing the opportunity to evaluate contrasting models of subvolcanic magma reservoir assembly. Integrated field observations, microstructural analysis, mineral and bulk-rock geochemistry define four stratigraphic zones that record systematic changes in emplacement behaviour. High-resolution sampling of outcrop and drill core material reveals a previously unrecognised Lower Zone that extends the intrusion downward and represents its most mafic portion, preserving evidence for pulsed replenishment and accumulation within an open, liquid-dominated environment. The overlying Middle Zone marks the transition to a phase of high melt flux in a largely closed system, with microstructural and geochemical trends indicating intrusion-scale crystal settling, flotation, and convection. Upper Zone A records chaotic emplacement of multiple, laterally discontinuous sills in an open, crystal-rich environment. Upper Zone B mirrors the geochemical trends of the Middle Zone but is locally disrupted by small-scale sills and microstructures consistent with late-stage fluid infiltration. Together, these variegated petrological zones provide compelling evidence that subvolcanic systems can alternate between liquid- and crystal-dominated emplacement regimes over relatively short spatial and temporal intervals, highlighting their sensitivity to changes in melt flux and thermal state. These findings have direct implications for understanding magma supply to volcanoes and the formation of associated mineral deposits.