浙江新昌-嵊州新生代玄武岩的Ca同位素组成:对华南新生代玄武岩源区成分的启示

Ca isotope composition of Cenozoic basalts from Xinchang-Shengzhou,Zhejiang Province:Implications for source compositions of the Cenozoic basalts in the South China

本次工作首次报道了浙江中带新昌-嵊州新生代玄武岩的Ca同位素组成。新昌-嵊州新生代玄武岩具有相对均一的δ44/40Ca值(0.62±0.07)‰^(0.71±0.05)‰,平均值为(0.68±0.07)‰,这些值远低于硅酸盐地球(BSE)((0.94±0.05)‰)、正常型大洋中脊玄武岩(N-MORB)及绝大多数洋岛玄武岩(OIBs)的δ44/40Ca值。新昌-嵊州玄武岩Sr(Nd)同位素和全岩Ce/Pb、Zr/Hf与Ca/Al比值具有明显的负(正)相关关系,而与Ti*具有正(负)相关关系。这个现象意味着新昌-嵊州玄武岩起源于不均一的地幔源区,其成分变化可以通过两个不同的岩浆混合形成。本次工作推测两个源区分别为碳酸盐化岩石圈地幔和类似于海南岛玄武岩地幔源区的地幔集中带(FOZO)组分。新昌-嵊州玄武岩异常低的δ44/40Ca值可能继承自两个Ca同位素组成相似且具有较低δ44/40Ca值的岩浆。

We report the first estimations of the Ca isotopic compositions of Cenozoic basalts from Xinchang-Shengzhou,Zhejiang Province.Theδ44/40Ca values((0.62±0.07)–(0.71±0.05)‰)of these basalts,with averages of(0.68±0.07)‰,are abnormal as they are much lower than those of bulk silicate earth((0.94±0.05)‰),normal mid-ocean ridge basalts,and ocean island basalts.The Sr(Nd)isotopic compositions of the Xinchang-Shengzhou basalts are negatively(positively)correlated with whole rock Ce/Pb,Zr/Hf,and Ca/Al ratios and are positively(negatively)correlated with Ti*(Ti*=2×TiN/(EuN+GdN),where N represents the primitive mantle normalized value).This observation suggests that the Xinchang-Shengzhou basalts are derived from heterogeneous mantle sources and that their compositional variations can be explained by the mixing of two types of parental magma with similar or lowerδ44/40Ca values.We infer that the two mantle sources are carbonated lithospheric mantle and focus zone-like mantle components similar to the mantle source of Hainan basalts.The abnormally lowδ44/40Ca values of the Xinchang-Shengzhou basalts may be an inheritance from two types of parental magma with similar and lowerδ44/40Ca values.