东太平洋CC区玄武岩岩石成因:^(39)Ar/^(40)Ar年代学与地球化学证据

Petrogenesis of basalts in CC area of eastern Pacific:^(39)Ar/^(40)Ar chronology and geochemistry evidences

玄武岩作为地幔岩浆的产物,其岩石地球化学特征在反映地幔组分和深部动力学作用方面有着极其重要的意义。本文对东太平洋CC区西部和魏源海山两个潜次(Dive64和Dive66)的9个玄武岩样品进行了年代学、全岩地球化学和Sr-Nd-Pb同位素研究。结果显示,Dive64与Dive66玄武岩样品^(39)Ar/^(40)Ar年龄分别为83.1±0.3 Ma、74.7±0.3 Ma,属于晚白垩世,主体分别为N-MORB和OIB。Dive64为亚碱性玄武岩,REE总体相对平坦,轻重稀土分馏较弱,无明显Eu负异常;Dive66玄武岩为碱性玄武岩,富集轻稀土元素,亏损重稀土元素,无明显Eu、Ce异常,MgO含量与CaO/Al_(2)O_(3)比值呈反比,与Cr含量、Sr含量呈略微正相关。Sr-Nd-Pb同位素表明两者的形成可能不止单一源区的贡献,Dive64玄武岩Sr-Nd-Pb同位素比值更靠近DM(亏损地幔)、HIMU(高U/Pb比值地幔)端元,Dive66玄武岩Sr-Nd-Pb同位素比值较接近EMⅠ(Ⅰ型富集地幔)、HIMU端元。综上所述,Dive64玄武岩起源于尖晶石橄榄岩区,来源于亏损的软流圈地幔(DM)并混合有HIMU组分;Dive66玄武岩起源于石榴石橄榄岩区,岩浆上升过程中主要伴随着橄榄石的结晶分异作用,单斜辉石与斜长石的结晶分异作用相对微弱,岩浆来源于HIMU组分和EMⅠ组分,其形成可能与地幔柱有关。

Basalt,as the product of mantle magma,carries various information from the deep earth,and its lithogeochemistry characteristics are of great significance in reflecting mantle composition and deep dynamics.In this paper,the geochronology,whole rock geochemistry and Sr-Nd-Pb isotopes of nine basalt samples from two Dive(Dive64 and Dive66)in the Western CC area of the eastern Pacific Ocean and Weiyuan seamount are studied.The results show that the Dive64 and Dive66 basalt samples are N-MORB and OIB,respectively,and their^(39)Ar/^(40)Ar ages are 83.1±0.3 Ma and 74.7±0.3 Ma,respectively,which belong to the Late Cretaceous.The Dive64 basalts are subalkaline basalts,with relatively flat REE,weak fractionation of light and heavy rare earth elements and no obvious negative Eu element anomaly.The Dive66 basalts are alkaline basalts,enriched in LREE and depleted in HREE,with no obvious anomalies of Eu and Ce elements.The MgO content of Dive66 basalt is inversely proportional to CaO/Al_(2)O_(3)ratio,and slightly positively correlated with Cr content and Sr content.The Sr-Nd-Pb isotope indicates that the formation of the two may be contributed by more than a single mantle source.The Sr,Nd,and Pb isotopic ratios of Dive64 basalt are closer to DM and HIMU endmembers,while those of Dive66 basalt are closer to EMI and HIMU endmembers.In summary,the Dive64 basalt originated from the spinel peridotite region,and its magma was derived from the depleted asthenosphere mantle(DM)mixed with HIMU components.The Dive66 basalts originated from garnet peridotite area,and the crystallization differentiation of olivine was mainly accompanied by the rise of magma,while the crystallization differentiation of monocline and plagioclase was relatively weak.The magma of Dive66 basalts originated from HIMU component and EMI component,and its formation may be related to mantle plume.