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

摘要

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℃) and relatively oxidizing (?O2 ≥ FMQ>0). In comparison to chevkinite-group minerals reported from peralkaline rocks, those from metaluminous rocks tend to have higher SiO_2, Al_2O_3, CaO and ZrO and lower FeO~*, Nb_2O_5 and Ce_2O_3. The compositional differences are attributed to parental magma composition and possible magmatic origin (i.e. partial melting vs. crystal fractionation).