摘要
海山火山碎屑岩是水下爆发性火山作用的产物,研究发现,玳瑁海山火山角砾岩是南海发现的具有过渡性质的拉斑玄武岩,呈现相对较高的K2O+Na2O含量(3.10%~4.98%),富集大离子亲石元素和轻稀土元素,具有位于E-MORB与OIB之间的稀土元素配分模式,∑REE为80~148×10-6,(La/Yb)N比值为2.6~6.7.它们与南海分布更广的碱性玄武岩具有相似的构造环境,为南海扩张后的板内成因.南海中中新世以来的海山主要出现拉斑玄武岩,而晚中新世以后则主要为碱性、强碱性玄武岩,研究认为岩石圈"顶盖效应"起到一定影响,岩石圈厚度随着时间变化而增厚制约着岩浆源区熔体的演化.南海陆缘存在古老的交代岩石圈地幔,在海盆扩张前后发生的拆沉及热化学减薄,使得软流圈中加入了富集不相容元素组分,可能是玳瑁海山玄武岩富集组分的来源.
Volcaniclastic rock on seamount is the product of subaqueous explosive volcanism.This paper explores the geochemical characteristics of basalts in volcaniclastic rock from Daimao seamount,highlighting its geological significance.Samples obtained from Daimao seamount exhibit transitional tholeiites,which represent typical Cenozoic basalt in SCS(South China Sea),with high content of K2O+Na2O(3.10%-4.98%),enriched large ion lithophile elements,moderate enrichment of LREE relative to HREE(with(La/Yb)N ratio between 2.6to 6.7),E-MORB to OIB REE patterns.These basalts from Daimao seamount are in some way similar to wider extended alkali basalts in SCS,and they two formed in inner-plate environment after the cessation of SCS spreading.Seamounts from SCS predominantly consisted of tholeiites basalts since middle Miocene,turning into alkali Basalts after late Miocene.The authors suggest the change from tholeiites to alkali basalts with time may be attributed to the lithosphere Lid Effect because of increasing thickness of the lithosphere.Previous studies indicate the lithospheric mantle beneath the north part of SCS is similar to the Proterozoic/Archean mantle in composition.The lithosphere mantle in and around SCS metasomatized by melts or aqueous fluids,delaminated or thinned by asthenosphere upwelling.These age-old lithosphere mantle materials add into the asthenosphere with fertile incompatible elements,which may be the reason why the basalts from Daimao seamount display enrichment characteristics.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
2013年第S1期10-20,共11页
Earth Science-Journal of China University of Geosciences
基金
国家自然科学基金重点项目(Nos.41030853
41230960)
广州海洋地质调查局局控科研项目(No.GZHJK2011004)
