EXTREMELY ENRICHED MANTLE BENEATH THE YANGTZE CRATON IN THE NEOPROTEROZOIC: CONSTRAINTS FROM THE QICHUN MAFIC-ULTRAMAFIC ROCKS

To decipher the geochemical properties of Precambrian subcontinental lithopheric mantle beneath the Yangtze Craton, we performed a geochemical and geochronological study for mafic-ultramafic igneous rocks at Qichun in the northern Yangtze Craton. The major rocks are medium to coarse grained pyroxenite, gabbro and serpentinite, with small fine grained schist veins interlayered in them. The petrography characteristics and high LOI values indicate significant metamorphism and alteration. All the Qichun mafic-ultramafic rocks have low concentrations in SiO₂ (43.19% ~ 54.19%) but high concentrations in MgO (19.65% ~ 29.36%). The Mg# range from 69 to 80. Two schists (metaandesite) have relatively higher SiO₂, TiO₂, Al₂O₃, Na₂O and K₂O but lower Fe₂O₃, CaO and MgO contents, looking like a complementary relationship between cumulate and melt in lithochemistry. The mafic-ultramafic rocks have flat REE patterns, slightly negative Eu anomalies, low concentrations in LILE and most incompatible elements. The metaandesite show significantly enrichment in LREE and LILE, higher concentrations of all incompatible elements, slightly positive Eu anomalies, negative Nb and Ta anomalies. The pyroxenes and hornblendes from the Qichun mafic-ultramafic rocks show similar oxygen isotope ranges of 5.1-6.2‰ except one sample. The equilibrated temperatures of pyroxene-magnetite or hornblende-magnetite fall in the range of 430-500 ^oC, consistent with amphibolite-facies metamorphism. Contemporaneous analyses of zircon U-Pb ages, Lu-Hf isotopes and trace elements for pyroxenite 04QC07 yield a crystallization age of 808±4 Ma, ε_Hf(t) values of -21.3±1.2, Ti-in-zircon temperatures of 673±11 ^oC. The results indicate that the mafic-ultramafic rocks are cumulates derived from a mafic magma originating from an enriched mantle. The enriched signal were realized by interaction between the ancient terrestrial sediment and the subcratonic lithospheric peridotite. On the compilation of Neoproterozoic mantle-derived rocks in the northern Yangtze Craton, it appears that the middle Neoproterozoic (~ 800 Ma) is an important episode for the enrichment of subcratonic mantle beneath the Yangtze Craton. And this enriched signal was preserved in the mantle for at least 170 Myr.