中国东北软流圈地幔中的原始橄榄岩质地幔:来自大兴安岭地区新生代玄武岩的地球化学证据

为了进一步了解中国东北新生代玄武岩地幔源区的物质属性，报道了大兴安岭哈拉哈河-柴河地区新生代玄武岩的全岩主量、微量元素和Sr、Nd、Pb、Hf同位素组成.哈拉哈河-柴河玄武岩属钠质碱性系列，具有与洋岛玄武岩相似的微量元素特征，如富集大离子亲石元素（LILEs）、明显的Nb、Ta正异常等.它们具有中等亏损的Sr-Nd-Hf同位素组成（87Sr/86Sr=0.703 5~0.703 9、ε_Nd=5.21~6.55、ε_Hf=10.00~11.25），接近中国东部新生代玄武岩的亏损端元.这些玄武岩具有中等的放射成因Pb同位素组成（206Pb/204Pb=18.37~18.57、207Pb/204Pb=15.52~15.54和208Pb/204Pb=38.24~38.43），在206Pb/204Pb-207Pb/204Pb相关图上位于4.42~4.45 Ga的地球等时线之间.它们在Sr-Nd-Pb同位素相关图中均落入地幔柱来源的、高3He/4He比（>30R_a）的洋岛玄武岩范围内，暗示其源区可能存在来自深部地幔的古老原始地幔物质.此外，这些玄武岩具有高MgO（8.49%~11.58%）、高Ni（174×10-6~362×10-6）和高Mg#（59.1~66.9）的特征，表明它们接近于原始岩浆的成分.反演的哈拉哈河-柴河玄武岩的原始岩浆组成具有中等的SiO₂、低Al₂O₃以及高CaO/Al₂O₃比的特征，与石榴子石橄榄岩高压（>2.5 GPa）实验熔体的成分相当，暗示玄武岩的源区岩性最可能为橄榄岩.对以原始地幔（而不是亏损地幔）的微量元素为初始成分的饱满石榴子石二辉橄榄岩进行低程度（1%~2%）部分熔融的模拟计算，产生的熔体与哈拉哈河-柴河玄武岩具有一致的微量元素特征，这进一步支持了上述推断.综上所述，认为大兴安岭地区哈拉哈河-柴河玄武岩的源区含有来自深部地幔的古老的橄榄岩质原始地幔组分. 

Primordial Peridotitic Mantle Component in Asthenosphere beneath Northeast China: Geochemical Evidence from Cenozoic Basalts of Greater Khingan Range

In order to further explore the nature of mantle source beneath the Northeast China, it presents major, trace element, and Sr-Nd-Pb-Hf isotopic compositions for the Cenozoic intra-plate volcanic rocks from the Halaha-Chaihe field in the Greater Khingan Range. These volcanic rocks are mainly alkaline (sodic) basalts, and generally exhibit OIB-like incompatible trace element characteristics, e.g.enrichment in large lithophile elements (LILEs) and positive Nb-Ta anomalies. They show moderate depleted Sr-Nd-Hf isotopic compositions (87Sr/86Sr=0.703 5-0.703 9, ε_Nd=5.21-6.55, ε_Hf=10.00-11.25) and almost represent the most depleted mantle end-member among the Cenozoic basalts in eastern China. Their Pb isotopic compositions (206Pb/204Pb=18.37-18.57, 207Pb/204Pb=15.52-15.54, 208Pb/204Pb=38.24 -38.43) range between 4.42 Ga and 4.45 Ga geochrons on the 207Pb/204Pb versus 206Pb/204Pb diagram. They also show similar Sr-Nd-Pb isotopic compositions with those mantle plume-derived ocean island basalts (3He/4He>30 R_a), which implies a deep mantle source. The high MgO (8.49%-11.58%), Ni(174×10-6-362×10-6) contents and high Mg# values (59.1-66.9) of these basalts imply that their compositions are close to those of the primary magmas. The calculated primitive compositions of Halaha-Chaihe basalts show moderate SiO₂, low Al₂O₃ contents and high CaO/Al₂O₃ ratios, which are accordant with the compositions of experimental melts of garnet peridotite under high pressure (>2.5 GPa) conditions, suggesting a garnet peridotitic mantle source. Moreover, trace-element modeling suggests low-degree melts from a primitive mantle (rather than a depleted mantle) are consistent with these basalts. In summary, it is suggested that the mantle source of the Halaha-Chaihe basalts from the Greater Khingan Range contains ancient, primordial, peridotitic component from the deep mantle.