| 大兴安岭北段中—新生代玄武岩成分变异:对地幔热演化过程意义 |
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| 引用本文： | 唐杰,许文良,李宇,孙晨阳.大兴安岭北段中—新生代玄武岩成分变异:对地幔热演化过程意义[J].地球科学,2019,44(4):1096-1112. |
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| 作者姓名： | 唐杰 许文良 李宇 孙晨阳 |
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| 作者单位： | 1.吉林大学地球科学学院, 吉林长春 130061 |
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| 基金项目： | 国家自然科学基金项目41772047国家自然科学基金项目91858211国家自然科学基金项目41702051 |
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| 摘 要： | 近年来，东北地区地幔热演化过程的相关研究相对较少，而揭示东北地区地幔热演化过程的有效手段就是研究东北地区玄武岩的成分变异特征.系统总结并对比了大兴安岭北段早白垩世玄武质岩石和新生代玄武质岩石的化学成分变异，以便揭示研究区中生代晚期-新生代的地幔热演化过程.大兴安岭北段早白垩世玄武岩在化学上属于拉斑玄武岩系列，以亏损Nb、Ta、Ti等高场强元素为特征，它们的La/Nb和La/Ta比值分别介于1.8~5.6和30~87，暗示岩浆起源于岩石圈地幔；它们的初始87Sr/86Sr值、εNd（t）和εHf（t）值分别介于0.704 5~0.706 9、-1.52~+3.60和+1.74~+7.77，表明岩浆源区属于弱亏损-弱富集的岩石圈地幔；早白垩世玄武质岩石的Sr-Nd-Pb同位素成分指示岩浆源区是由DM和EMⅡ型地幔端元混合而成，并经历了俯冲流体的交代.表明大兴安岭北段早白垩世玄武质岩浆源区为受早期俯冲流体交代的岩石圈地幔.新生代超钾质和钾质玄武岩具有Nb-Ta的弱负异常，87Sr/86Sr值为0.704 7~0.705 7、εNd（t）值为-6.3~-0.8，而地幔捕掳体具有Sr-Nd同位素亏损特征；钠质玄武岩具有Nb-Ta的正异常，较超钾质和钾质玄武岩具有低的87Sr/86Sr（0.703 5~0.704 2）以及高的εNd（t）值（+3.4~+6.6），类似MORB的同位素组成，这些特征说明大兴安岭北段新生代玄武质岩石起源于软流圈地幔.综上所述，大兴安岭北段早白垩世和新生代玄武质岩石成分的差异不仅指示其岩浆源区从岩石圈地幔转变为软流圈地幔，更为重要的是它揭示了研究区地幔的热演化过程——从早白垩世高的地温梯度到新生代低的地温梯度的转变.这一过程也是岩石圈从中生代晚期到新生代逐渐增厚的过程.结合区域构造演化，可以得出大兴安岭北段早白垩世的玄武质岩浆作用与岩石圈伸展、减薄形成的裂陷作用相关，而新生代玄武质岩浆作用则与陆内裂谷作用相关.
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| 关 键 词： | 大兴安岭北段 早白垩世 新生代 玄武质岩石 地幔热演化 岩石学 |
| 收稿时间： | 2019-01-14 |
Composition Variations of Mesozoic and Cenozoic Basalts in Northern Great Xing'an Range: Implications for Thermal Evolution of Mantle |
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| Abstract: | Recently, few researches have been made on the thermal evolution of the mantle in Northeast China. An effective means to solve this issue is to study the variation characteristics in composition of basalts in Northeast China. In this paper, it summarizes and discusses the composition variations of the Mesozoic and Cenozoic basalts in the northern Great Xing'an Range, with the aim of revealing the thermal evolution of the mantle within the study area. The Early Cretaceous basalts in the northern Great Xing'an Range geochemically belong to tholeiitic series, which are characterized by depletion in high field strength elements (e.g., Nb, Ta and Ti).Their La/Nb and La/Ta ratios range from 1.8 to 5.6 and from 30 to 87, respectively, implying the basaltic magmas originated from the partial melting of the lithospheric mantle. Their initial 87Sr/86Sr ratios of 0.704 5-0.706 9, εNd(t) values of -1.52-+3.60 and εHf(t) values of +1.74-+7.77 further indicate that the magma source is weakly depleted-weakly enriched lithospheric mantle. Additionally, the Sr-Nd-Pb isotope compositions of the Early Cretaceous basalts suggest that their magmatic sources are characterized by mixing between DM and EMⅡ modified by subduction-derived fluids.Taking the above-mentioned into consideration, it is suggested that the Early Cretaceous basaltic magma was derived from partial melting of a lithospheric mantle metasomated by subduction-related fluids.The Cenozoic ultrapotassic and potassic basalts have weakly negative Nb-Ta anomalies, 87Sr/86Sr ratios of 0.704 7-0.705 7 and εNd(t) values of -6.3 to -0.8 whereas the mantle xenoliths entrained by Cenozoic ultrapotassic and potassic basalts show the depleted Sr-Nd isotopic characteristics. The Cenozoic sodium basalts have positive Nb-Ta anomalies, and lower 87Sr/86Sr ratios of 0.703 5-0.704 2 and higher εNd(t) values of +3.4-+6.6 than those of the ultrapotassic and potassic basalts, similar to those of MORB. These geochemical features suggest that the Cenozoic basaltic magmas in the northern Great Xing'an Range were primarily produced by melting of asthenospheric mantle. The composition variations of the Early Cretaceous and Cenozoic basalts in the northern Great Xing'an Range not only indicate that the basaltic magma sources had changed from lithospheric to asthenospheric mantle, but also reveal the thermal evolution of the mantle in the study area, i.e., high geothermal gradient in the late Early Cretaceous changed to low geothermal gradient in the Cenozoic. Combined with the regional tectonic evolution, it is concluded that the Early Cretaceous basaltic magmatism in the northern Great Xing'an Range is related to the taphrogeny caused by the lithospheric extension and thinning, while the Cenozoic basaltic magmatism is related to the intracontinental rifting. |
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