Oxidation zonation within the Emeishan large igneous province: Evidence from mantle-derived syenitic plutons

摘要

The mantle-derived felsic plutonic rocks of the Emeishan large igneous province consist of peralkaline and metaluminous varieties. The peralkaline rocks appear to be formed by fractional crystallization of mafic magmas whereas the metaluminous plutonic rocks appear to be derived by partial melting of underplated mafic rocks. The metaluminous Woshui pluton is 259.6 ± 0.5 Ma and contains alkali feldspar, clinopyroxene, magnesiohornblende, titanite, magnetite and quartz and was relatively volatile- and trace element-rich and had an initial magmatic temperature of ～720 ℃ + 2σ 2σ. The metaluminous Huangcao pluton is slightly younger (258.9 + 0.7 Ma), contains alkali feldspar, fayalite, ferrosillite, ferrohornblende, quartz, magnetite and ferrohedenbergite, is volatile-poor and likely had higher magmatic temperatures (≥850 ℃). The mineral assemblages and trace element compositions (i.e. Eu/Eu' > 1, high V/Sc, high V/ Ga) suggest relatively oxidizing magmatic conditions (i.e.fO_2 > 0 FMQ) for the two plutons. The primary chemical differences between the two plutons are their volatile content and source fertility. Neighboring the metaluminous plutons is a peralkaline pluton which also shows evidence for relatively oxidizing magmatic conditions whereas other mantle-derived granitic plutons to the north and south show evi-dence for reducing conditions. The syenitic magmas within the central part of the Panxi region were likely oxidized due to variability of the oxidation state of the original source material and therefore the region may represent an 'oxidized zone' within the Emeishan large igneous province.