Quantitative Petrological Constraints on the Origin of Plagioclase Megacrysts

Abstract Giant plagioclase aggregates, which exhibit enigmatic crystal structures, provide critical yet poorly constrained insights into the thermochemical dynamics and evolution of magma chambers, as their genesis is still unclear. The c. 1.83 Ga Wulong porphyritic gabbro dykes of North China Craton (NCC) host plagioclase megacrysts measuring 1–6 cm in size. These plagioclase megacrysts further exhibit diagnostic ‘flower-like’ petaloid clusters and flow-aligned morphologies as well as oscillatory zonings and epidote–garnet inclusion bands. Quantitative in situ elemental distribution profiles within plagioclase megacrysts, particularly for Sr and Ba, exhibit a sawtooth-like shape that are interpreted to reflect thermal perturbations from multistage magma recharge events. During thermal spikes, partial dissolution of metastable plagioclase grains liberated Sr and Ba into the melt, modulating element partitioning during reprecipitation. Convection currents induced by recurrent thermal fluctuations drove aggregation of plagioclase megacrysts into the observed macrotextures. Phase equilibrium modeling results reveal that the crystallization process occurred at 1000 MPa/880 °C. Diffusion chronometry of garnet inclusions in plagioclase megacrysts indicates a very short magma chamber residence time of ~2.0 kyr. Thermal–kinetic simulations reveal inherent instability in near-liquidus storage, where mafic underplating (ΔT > 140 °C) readily triggered either rapid eruption or shallow emplacement. This multiscale analysis from μm-scale zoning to km-scale dyke architecture establishes that plagioclase megacrysts constitute sensitive recorders of transcrustal magma transport dynamics, reconciling their paradoxical coexistence of their textural maturity with brief crustal residence through pulsed open-system processes.