鹤壁新生代玄武岩源区及成因:地球化学和Sr-Nd-Hf同位素证据

摘要

大陆碱性玄武岩是研究大陆地慢、软流圈和岩石圈、壳幔相互作用的窗口.鹤壁碱性玄武岩为华北中部新生代玄武岩重要组成部分,其源区和成因对了解新生代期间华北克拉通中部岩石圈地幔改造提供了重要依据.鹤壁新生代玄武岩主要为碱性玄武岩,贫硅(SiO2 =45.0％～47.8％)富锾(MgO =7.3％～8.5％)和全碱(Na2O+K2O=4.6％～6.3％),富集轻稀土元素,轻、重稀土元素强烈分馏[(La/Yb)N=15.1～21.2],无明显Eu异常.富集大离子亲石元素(LILEs)和高场强元素(HFSEs).在原始地幔标准化蛛网图上,具有明显的Nb、Ta正异常,其中Nb/U=35 ～ 48、La/Nb=0.4～0.7、Ba/Nb=3.6 ～11.全岩的Sr、Nd、Hf同位素比值分别为87Sr/86 Sri=0.7036 ～0.7044,εNd(t)=+0.4～+5.4,εHf(t)=+5.0～+9.7,显示鹤壁新生代玄武岩具有洋岛玄武岩(OIB)的地球化学特征和同位素组成.岩相学特征和地球化学特征表明鹤壁新生代玄武岩是软流圈地幔(＞ 80km)小程度部分熔融形成的熔体与富集的岩石圈地幔相互作用后的结果,地壳物质混染和结晶分离作用不显著.%Continental alkali basalts are the key to understand the nature of the subcontinental lithospheric mantle and the interaction among the crust, lithospheric mantle and asthenosphere. Cenozoic alkali basalts, located in the central North China Craton, are main components of Cenozoic basalt province in the North China Craton. Their source and petrogenesis would provide complement evidence for studying the modification of lithospheric mantle beneath the central North China Craton during Cenozoic. The Hebi basalts are basanites and alkali olivine basalts, belonging to alkaline rocks, with SiO2 contents of 45. 0%～47. 8% , MgO concentrations of 7.3% ~8.5% , and Na2O + K2O abundances of4.6% ~6.3%. They are enriched in light rare earth elements (REEs) with highly fractionated LREEs and HREEs ( (La/Yb)N = 15. 1 ~ 21. 2) . In primitive mantle-normalized spidergrams, all rocks have evident positive Nb and Ta anomalies without Eu anomalies. They are enriched in LILEs and HFSEs, with Nb/U ratios of 35 ~ 48, La/Nb ratios of 0.4 ~0. 7, and Ba/Nb ratios of 3. 6 ~ 11. They have depleted Sr-Nd-Hf isotopic compositions, with initial 87Sr/86Sr ratios of 0. 7036 ~0. 7044, εNd(t) values of +0. 4 to +5. 4 and εHf(t) values of +5. 0 to +9. 7. All these characteristics are similar to those of the oceanic island basalts ( OIBs). All these geochemical features, combined with petrographic observations, suggest weak crustal contamination and fractional crystallization in their origin. It is concluded that the Hebi alkali basalts were derived from small degree partial melting of asthenospheric mantle at depths more than 80km, with interaction with lithospheric mantle.