Platinum-group elemental and Sr-Nd-Os isotopic geochemistry of the ∼635 Ma mafic intrusions in the northern margin of the Yangtze Block: A link of metasomatized subcontinental lithospheric mantle and Ni-Cu-(PGE) sulfide mineralization

Abstract

A number of ∼635Ma mafic-ultramafic intrusions occur in the Nanyang region and Suizhou-Zaoyang (Suizao) region along the northern margin of the Yangtze Block. The intrusions in the Suizao region are mainly composed of troctolite, olivine gabbro and gabbronorite. The parental magmas of the intrusions are estimated based on the non-modal melting using two fresh samples with the lowest REE concentrations and the results show that the parental magmas may have La/Sm of ∼2.5, Sm/Yb of ∼2.5 and La/Yb of ∼6.2 and could be produced by 5–10% of partial melting of the mantle composed of spinel-garnet (50:50) lherzolite. The estimated compositions of the parental magmas also display the enrichment of light REE and weak depletion of HFSE (Th, U, Nb and Ta) on the primitive mantle-normalized trace element patterns and have Ba/Nb, Ba/La and Ba/Th distinctly higher than MORB and OIB, consistent with the derivation from metasomatized subcontinental lithospheric mantle (SCLM) rather than convecting mantle. The rocks of the intrusions have radiogenic 187Os/188Os(i) of 0.137–0.247, which are likely attributed to low degrees of crustal contamination and no involvement of convecting mantle-derived magmas. They consistently have εNd(t) values of −5.4 to −10.4, much lower than the value of +2.5 for the mantle inferred from Neoproterozoic evolutionary trend and could be interpreted as the mixing of the parental magmas with 7–10% of Archean basement of the Yangtze Block. The rocks of the intrusions commonly have <0.1wt% S and 0.08–17.5ppb PGE (Os, Ir, Ru, Rh, Pt and Pd). They have Cu/Pd (7900–6,500,000) much higher than the value of the primitive mantle (∼7000), which is likely resulted from the retention of sulfide in the mantle source due to the low degrees of partial melting of the mantle. We conclude that the low PGE concentrations of the rocks can be attributed to the sulfide retention in metasomatized SCLM source and no substantial interaction of metasomatized SCLM with the melts from convecting mantle. The ∼635Ma mafic intrusions in the Suizao region may thus have little potential for economically important Ni-Cu-(PGE) sulfide mineralization.