Giant plagioclase growth during storage of basaltic magma in Emeishan Large Igneous Province, SW China

Giant plagioclase basalts (GPBs) reflect the storage of flood basalt magma in subvolcanic magma chambers at crustal depths. In this study of the Late Permian Emeishan large igneous province in southwest China, we focus on understanding the plumbing system and ascent of large-volume basaltic magma. We report a quantitative textural analysis and bulk-rock geochemical composition of clustered touching crystals (CT-type) and single isolated crystal (SI-type) GPB samples from 5- to 240-m-thick flows in the Daqiao section. Both types of GPBs are evolved (<6 MgO wt%), but have high Ti/Y ratios (>500) and high total FeO content (11.5–15.2 wt%). The mineral chemistry of the two types of plagioclase displays a small range of anorthite content (<5 mol%), which is consistent with their unzoned characteristics. The two types of GPBs have S-type crystal size distributions but have quite different slopes, intercepts, and characteristic lengths. The characteristic lengths of the five flows are 1.54, 2.99, 1.70, 3.22, and 1.86 mm, respectively. For plagioclase growth rates of 10−11 to 10−10 mm/s, steady-state magma chamber models with simple continuous crystal growth suggest that CT-type plagioclase megacrysts have the residence time of about 500–6,000 years, whereas the residence time for SI-type plagioclase is significantly longer, about 1,000–10,000 years. By combining field geology, quantitative textural data with geochemistry, we suggest that CT- and SI-type crystals grew and were coarsened in the outer part and inner part of a magma chamber, respectively. Magma evolution during storage is controlled by crystallization, crystal growth, and magma mixing, and pulsating eruptions occur in response to the continuous supply of hot magma.