雷琼地区晚新生代玄武岩地球化学：EM2成分来源及大陆岩石圈地幔的贡献

对雷琼地区21个晚新生代玄武岩样品的主量、微量元素和Sr、Nd、Pb同位素分别用湿化学法、ICP-MS和MC-ICPMS进行了测定.这些玄武岩主要为石英拉斑玄武岩,其次为橄榄拉斑玄武岩和碱性玄武岩.大多数样品的微量元素和同位素成分与洋岛玄武岩(OIBs)相似,而且随着SiO_2不饱和度增加,不相容元素含量也增加.除R4-1可能受到地壳混染外,其他样品相对均一的Nd同位素(ε_(Nd)=2.5-6.0)以及变化明显但范围有限的Sr同位素(0.703106～0.704481),可能继承了地幔源区的特征.~(87)Sr/~(86)Sr与~(206)Pb/~(204)Pb的正相关和~(143)Nd/~(144)Nd与~(206)Pb/~(204)Pb的负相关特征暗示DM(软流圈地幔)与EM2(岩石圈地幔)的混合.地幔捕虏体的同位素特征暗示EM2成分不可能存在于尖晶石橄榄岩地幔,而La/Yb和Sm/Yb系统表明岩浆由石榴石橄榄岩部分熔融产生,这意味着EM2成分可能存在于石榴石橄榄岩地幔.雷琼地区玄武岩的地球化学变化可以用软流圈地幔为主的熔体加入不同比例石榴石橄榄岩地幔不同程度熔融产生的熔体来解释:碱性玄武岩和橄榄拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较低程度(7%～9%)熔融体混合,而石英拉斑玄武岩是软流圈熔体与石榴石橄榄岩地幔较高程度(10%～20%)熔融体的混合.

Geochemistry of Late-Cenozoic basalts from Leiqiong area: The origin of EM2 and the contribution from sub-continental lithosphere mantle

The major-, trace-element and Sr-Nd-Pb isotopic ratios of 21 Late-Cenozoic basalts from Leiqiong area were determined by wet chemical method, ICP-MS and MC-IPCMS, respetively. Normative mineral calculation suggested that Leiqiong basalts were dominated by quartz tholeiites with subsidiary olivine tholeiites and alkaline basalts. The trace elements and isotopic composition of these basalts resembled OIBs. In addition, the incompatible elements abundance increased with increasing SiO_2-undersaturation. The chemical and isotopic compositions of most basalts might be inherited from their mantle source except that sample R4-1 showed characteristics of crustal contamination. The positive correlation between ~(87)Sr/~(86)Sr and ~(206)Ph/~(204)Pb as well as negative correlation between ~(143)Nd/~(144)Nd and ~(206)Pb/~(204)Pb implied a mixing of a depleted asthenospheric mantle and an EM2 composition (existing in the sub-continental lithosphere mantle beneath this region). The even more depleted Sr and Nd isotope of mantle xenoliths from Leiqiong than their host basalts indicated that the spinel peridotite mantle represented by the xenoliths could never produce these basaltic rocks. The La/Yb-Sm/Yb system of Leiqiong basalts required that the melting occurred in the garnet peridotite and in turn implied the EM2 composition should be derived from the lower lithosphere mantle (garnet phase stable field). The geochemical diversity of Leiqiong basalts could be interpreted as follows: The low degree of melt (7% ～ 9%) from upwelling asthenospheric mantle and garnet peridotite mantle produced the alkaline basalts and olivine tholeiites while relatively higher degree of melt (10% ～ 20%) from asthenospheric mantle and a higher proportion of garnet peridotite mantle made the quartz tholeiites.