The depths of crustal reservoirs within volcanic systems may experience transitions over time. Here, we report the crystal and bulk rock compositions of the shield-forming basaltic lavas of the Tianchi composite volcano in the intraplate Changbaishan Volcanic Field, NE China to constrain the crustal magmatic evolution with time. We investigated samples covering the entire basaltic stratigraphic sequence, consisting of the Toudao (TD), Baishan (BS), and Laofangzixiaoshan (LFZ) units from bottom to top, respectively. The core compositions of olivine macrocrysts vary among the three units, i.e., the TD and BS olivine phenocrysts can both be divided into two populations: a high-Fo population (~Fo_76-80) and a low-Fo population (~Fo_72-74). The LFZ unit only exhibits a high-Fo population (~Fo_77-80). Phase equilibria modelling using rhyolite-MELTS suggests that the high-Fo populations were stored at depths of ~20 km for the TD and BS units and ~15 km for the LFZ unit. The low-Fo populations crystallized at shallow depths, i.e., ≤15 km for the TD unit and ≤13 km for the BS unit. We employ a dynamic Fe-Mg interdiffusion modelling with constantly adapting boundary conditions in zoned olivine macrocrysts to constrain the magmatic environments and timescales during the pre-eruption and post-eruption, enabling clarify the magmatic histories recorded by two olivine populations. The dynamic Fe-Mg interdiffusion modelling considers the variable boundary condition caused by crystal growth and composition variation of melts during magma cooling. Calculated results suggest that the high-Fo populations from the TD and BS units recorded prolonged timescales ranging from six months to more than two years with lower cooling rates and slower crystal growth 37 rates. These characteristics reflect a relatively hot and slow-cooling magmatic environment; and the modelled timescales correspond to the sum time including shallow storage, magma ascent, and further cooling within the lava flows. Conversely, the high-Fo population from the LFZ unit and the low-Fo populations from the TD and BS units record shorter timescales (<140 days) with higher cooling rates and faster crystal growth rates. These results indicate relatively cold and highly undercooling magmatic environments; hence the timescales record magma ascent in the conduits and further cooling during lava emplacements. Our study demonstrates that the Tianchi basaltic plumbing system experienced a structural transition over time. In detail, the TD and BS magmas experienced multi-stage stalling and ascent, first accumulating in deep reservoirs and then transferring to shallow reservoirs for storage before the eruption. The LFZ magmas accumulated in a mid-crustal reservoir, followed by a direct ascent to the surface without additional residence.

Article link: https://pubs.geoscienceworld.org/msa/ammin/article-abstract/doi/10.2138/am-2023-9296/638398/The-transition-of-the-magma-plumbing-system-of?redirectedFrom=fulltext