Reappraisal of Early Cretaceous highly enriched mantle beneath the Dabie Orogen: evidence from mineralogy and geochemistry of the Xiaohekou complex
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An Early Cretaceous highly enriched mantle beneath the Dabie Orogen, previously proposed on the basis of exceedingly enriched isotopic signatures documented in intermediate–mafic–ultramafic complexes, has been re-evaluated by new field, mineralogical and geochemical data from the best exposed Xiaohekou complex. The Xiaohekou complex consists of three pyroxenite cores intruded by olivine gabbronorite dykes and successively surrounded outward by gabbroic rocks, leucogabbros and monzonites. Textural and compositional data for pyroxenes and plagioclase as well as whole-rock geochemistry suggest two-stage crystallizations. The Sr–Nd isotopic values correlate with the SiO2 and MgO contents, and one gabbronorite sample from the margin of the complex has sodic plagioclase and highly enriched Sr–Nd isotopic ratios, also pointing to fractional crystallizations and crustal contamination. One gabbro sample from the centre of the complex and three from olivine gabbronorite dykes have the lowest Sr but highest Nd isotopic values close to those of typical Enriched Mantle I (EMI). These data together with isotopic and MELTS modelling suggest that an EMI-like mantle source plus crustal assimilation and crystallization can explain all field, mineralogical and geochemical characteristics of the Xiaohekou complex, without the need to invoke a mantle source more enriched than EMI beneath the Dabie Orogen.