Quantitative petrological evidence for the origin of plagioclase megacrysts: Constraints from the Wulong gabbro, North China Craton

The Paleoproterozoic Wulong porphyry gabbro dykes on the southern margin of North China Craton host uncommon plagioclase megacrysts that offer novel perspectives into the origin of megacrystic textures and the dynamics of their magmatic settings. U–Pb isotopic dating of zircon and titanite reveal that the Wulong gabbro was emplaced at ca. 1831 Ma. The plagioclase megacrysts (1–6 cm long) present petal-shaped aggregates, isolated islands, and flowing shapes, with oscillatory growth zonings and mineral inclusion (e.g., epidote and garnet) zonings. The Sr and Ba distributions within the plagioclase megacrysts exhibit a sawtooth-like pattern, which is interpreted as being caused by the thermal cycling resulting from the multi-batch replenishment of hot magma. Also as a result of thermal cycling, magmatic convection is induced, leading to the aggregation of the plagioclase megacrysts. Phase equilibrium modeling of these mineral assemblages suggests that the pressure and temperature during plagioclase crystallization were approximately 1,000 MPa and 800°C, respectively. The multicomponent diffusion model for garnet estimates the residual time of the magma chamber at ~2,000 years. According to our thermodynamic model, the temperature of the mafic magma involved in the multi-batch underplating process is estimated to be 940℃. The quantitative calculation model of the thermal cycling indicates that near-liquidus storage is not stable, and its rapid eruption or emplacement was easily triggered by underplating magmas.