Late Palaeozoic Ultramafic Lavas in Yunnan, SW China, and their Geodynamic Significance

Abstract

We report petrological and geochemical data on ultramafic pillow lavas from Late Palaeozoic marine sequences in Yunnan, SW China. These lavas have >26 wt % MgO, euhedral to subhedral olivine phenocrysts and acicular or quench clinopyroxene crystals with or without microlitic plagioclase in a devitrified and altered glassy matrix. These ultramafic lavas are compositionally komatiitic (>18 wt % MgO), but we term them high-Mg picrites because they lack spinifex-textured olivine. Although olivines in these picrites are cumulate crystals, causing the high MgO contents of the bulk rocks, the high forsterite content of these olivines, Fo = 0·902 ± 0·011, suggests that the primitive magmas parental to the picrites would have had ∼17–19% MgO, which is similar to the estimates for primary magmas of the Cretaceous Gorgona komatiites in the literature. The calculated liquidus temperature of the primitive magmas parental to the Yunnan picrites is ∼1400 ± 25°C, which would imply a mantle potential temperature of ∼1540 ± 30°C. This is inconsistent with magma generation beneath ocean ridges or in the mantle wedge above a subduction zone, but is consistent with a mantle plume origin, which is fully supported by the trace element characteristics of the magmas.