Chevkinite-group minerals from the mantle-derived metaluminous Woshui syenite of the Emeishan large igneous province

Abstract

The composition of chevkinite-group minerals from the Late Permian Woshui pluton is presented. The Woshui pluton is one of the few mantle-derived metaluminous syenitic intrusions within the Emeishan large igneous province of SW China and contains four times less ∑REE (∑REE = 0.05 wt.%) than the spatially and temporally associated Baima peralkaline syenitic pluton (∑REE = 0.2 wt.%). The chevkinite-group minerals are relatively uniform in composition and observed chemical trends are consistent with substitution between major cationic sites defined as: Ca A 2 + + Ti C 4 + ⇌ REE A 3 + + M C 3 + , 2 + The chevkinite-group minerals from the Woshui syenites are unique in that they display positive chondrite-normalized Eu-anomalies ( i.e . Eu/Eu* = 2.3 to 23) which may be related to the conditions that produced the parental magma. The magmatic conditions of Woshui pluton were likely to be low pressure ( i.e . ≤ 1 kbar), moderate temperature ( i.e . 700–800°C) and relatively oxidizing ( f O 2 ≥ FMQ > 0). In comparison to chevkinite-group minerals reported from peralkaline rocks, those from metaluminous rocks tend to have higher SiO 2 , Al 2 O 3 , CaO and ZrO and lower FeO*, Nb 2 O 5 and Ce 2 O 3 . The compositional differences are attributed to parental magma composition and possible magmatic origin ( i.e . partial melting vs . crystal fractionation).