From melt to mush: dynamic assembly of the Carlingford Complex layered intrusion, Ireland

Mafic intrusions provide critical insights into the emplacement dynamics of upper-crustal magma reservoirs, yet their formation remains debated, with two competing models: (i) large, liquid-dominated magma chambers, in which fractionation is driven by crystal accumulation and separation, or (ii) crystal-rich mush systems, incrementally assembled by multiple small intrusions. Constraining which model best describes the development of any mafic intrusion is central to addressing fundamental questions of cumulate formation and subvolcanic chamber replenishment.The Carlingford Complex (Co. Louth, Ireland) preserves the shallow crustal architecture of a volcanic centre active during Paleogene plume-related rifting, presenting an ideal case study to evaluate the two proposed models of mafic intrusion assembly. Here, we present integrated field, microstructural, and geochemical data to develop a model for the emplacement of the Carlingford Complex.High-resolution sampling of outcrop and drill-core material reveal four stratigraphic zones, each characterised by distinct mineralogical and geochemical signatures. The basal sequence records pulsed replenishment and accumulation within an open, liquid-dominated environment, later transitioning into a phase of high melt flux approaching a closed system, facilitating large-scale crystal settling, flotation, and convection. The upper sequence preserves evidence of complex liquid-solid interactions, including late-stage infiltration into existing mush zones, and sporadic intrusion of laterally discontinuous sills of varied composition. Together, these zones preserve evidence of both liquid- and crystal-rich modes of magma replenishment, spanning the behavioural endmembers exemplified by other Paleogene intrusions. Our work demonstrates that subvolcanic systems can shift between contrasting emplacement regimes over relatively short spatial and temporal intervals, highlighting the sensitivity of magma systems to changes in melt flux and thermal state.