南兴安岭晚中生代火山岩的岩石成因(II):Pb同位素制约

 对南兴安岭地区晚中生代火山岩的Pb同位素进行了分析，结合其主微量元素和Sr-Nd同位素地球化学特征，探讨了火山岩的岩石成因。满克头鄂博组低钾拉斑-钙碱性玄武安山岩表现出富集LILE、LREE和HFSE亏损元素地球化学特征以及稍高放射性成因Sr、Pb (⁸⁷Sr/⁸⁶Sr=0.704 88～0.704 99; ²⁰⁶Pb/²⁰⁴Pb=18.18～18.23)和弱相对亏损至弱富集的Nd同位素组成(ε_Nd((t)=-0.12～+0.68)，其来源于俯冲沉积物参与改造富集地幔熔融源区。该组的英安岩-流纹岩较基性岩浆具有稍高放射性成因Sr、Pb (⁸⁷Sr/⁸⁶Sr=0.705 22～0.707 09; ²⁰⁶Pb/²⁰⁴Pb=18.16～18.33)和低Nd同位素组成 (ε_Nd(t))=-1.5～-0.4), 且在空间上与玄武岩共生，为玄武质岩浆结晶分异，并同化混染作用 (AFC)的产物。玛尼吐组英安岩具有与现代俯冲带adakite岩石相似的特征；在Sr-Nd-Pb同位素组成上，它们较同期或近期玄武安山岩更低Sr、Pb (⁸⁷Sr/⁸⁶Sr=0.704 09~0.704 25;²⁰⁶Pb/²⁰⁴Pb=18.15~18.17)和高Nd同位素比值(ε_Nd((t))=+0.9~+2.1)，为造山带下地壳镁铁质岩石的部分熔融产物。

Origin of Late Mesozoic Volcanic Rocks from the Southern Da Hinggan Mountain, NE China (II): Constraints of Pb Isotope

Major, Trace element and Sr-Nd-Pb isotope data of Late Mesozoic volcanic rocks from the northern Da Hinggan Mountain, NE China are reported in this paper. The basaltic-andesites occurring in the Manketou'ebo Formation ( Fm) belonging to low-K calc-alkaline series, Which show LILE, LREE enrichment and HFSE depletion. They have slightly high radiogenic Sr-Pb (~(87)Sr/~(86)Sr =0.704 88 ～ 0.704 99; ~(206)Pb/~(204)Pb = 18.18 ～ 18.23) and are slightly depleted to enriched in Nd isotope (ε_(Nd) (t) = -0.12 ～ +0.68) attributed to the participation of sediments in the mantle source. The dacites and rhyolites of the Manketou'ebo Formation have higher radiogenic Sr, Pb (~(87)Sr/~(86)Sr =0.705 22 ～ 0.707 09; ~(206)Pb/~(204)Pb = 18.16 ～ 18.33) and lower radiogenic Nd isotopic compositions (ε_(Nd) (t) = -1.5 ～ -0.4) than the contemporaneous basaltic lavas. This resulted from the differentiation of primary basaltic magmas and assimilation and fractional crystallization (AFC). The dacites of the Manitu Formation have the signatures of modem adakites from the subduction zones. Compared with the basaltic lavas, they have lower radiogenic Sr , Pb (~(87)Sr/~(86)Sr = 0.704 09～0.704 25; ~(206)Pb/~(204)Pb = 18.15 ～ 18.17) but higher radiogenic Nd isotopic compositions ( ε_(Nd) (t) = + 0.9 ～ + 2.1 ) , arguing against an origin of differentiation of the contemporaneous basalts. These rocks are considered as melts of mafic protoliths from the lower crust beneath the orogen under the heating of the underplating basaltic magmas.