赣东北蛇绿混杂岩岩石地球化学特征及构造意义

赣东北蛇绿混杂岩是中国为数不多的新元古代蛇绿岩，对该蛇绿混杂岩带中的方辉橄榄岩和玄武岩类岩石进行元素地球化学研究，以探讨新元古代赣东北地区的构造动力学背景.研究表明，方辉橄榄岩具有低TiO₂(0.02%~0.37%)、低K₂O(0~0.02%)、低Na₂O(0.02%~0.10%)和富MgO(40.81%~44.58%)特征，烧失量普遍较高(10.09%~13.47%)，表明样品普遍遭受蚀变；稀土元素配分模式图和微量元素蛛网图显示，橄榄岩样品具有亏损地幔源区特征，同时兼有俯冲带流体交代的特征.玄武岩类SiO₂含量主要为52.63%~56.19%，平均为55.00%，属安山玄武岩范围.岩石具低MgO(3.66%~6.46%)，中等TiO₂(1.14%~2.66%)，高Al₂O₃(13.07%~15.27%)、Na₂O(3.42%~5.03%)的特征.安山玄武岩轻稀土富集((La/Yb)_N=1.84~2.61)，无明显Eu异常，稀土配分曲线呈右倾型；微量元素蛛网图中显示大离子亲石元素Th、Ba、U、Pb富集、高场强元素Nb、Ti亏损，可能系俯冲板片脱水产生的流体交代地幔楔所形成，TNT(Ta、Nb和Ti)的负异常指示赣东北蛇绿混杂岩的形成过程中可能遭受了陆源物质的混染，反映了消减作用的影响.方辉橄榄岩和安山玄武岩的地球化学特征表明赣东北蛇绿混杂岩的形成可能经历了多期演化，早期可能系古华南洋发生洋内俯冲作用，俯冲过程中释放的流体交代了俯冲带上的地幔楔，使地幔橄榄岩富集轻稀土.随后的弧后盆地扩张阶段，形成具有岛弧性质和MORB特征的玄武岩类，但由于弧后盆地发育不成熟，玄武岩类在地球化学上显示有大陆物质的特征，表现出高Pb、低Ti和轻稀土强烈富集. 

Geochemical Characteristics and Tectonic Significances of the Rocks from Northeastern Jiangxi Ophiolite

The harzburgites and basalts from northeastern Jiangxi ophiolite belt have been investigated in the present study, with the aim to review Neoproterozoic tectonic environment. The results show that harzburgites are characterized by low TiO₂ (0.02%-0.37%), K₂O (0-0.02%) and Na₂O (0.02%-0.10%), along with high MgO (40.81%-44.58%) and ignition loss (10.09%-13.47%), suggesting that the harzburgites suffered intensive alteration. The Chondrite-normalized REE patterns and primitive-normalized spider diagrams indicate that the harzburgites are sourced from a depleted mantle source suffered metasomatism in subduction zones. The data suggest that the meta-basalts are mostly characterized by high SiO₂ (52.63%-52.63%), Al₂O₃ (13.07%-15.27%), Na₂O (3.42%-5.03%) and low MgO (3.66%-6.46%), with medium TiO₂ (1.14%-2.66%) contents, indicating that they are of andesitic basalt rocks. In a Chondrite-normalized REE plots, andesitic basalt rocks are rich in LREE((La/Yb)_N=1.84-2.61) with smoothly right declined trend. The trace elements are characterized by enrichment in LILE such as Th, Ba, U and Pb and depletion in HFSE like Nb and Ti, indicating that andesitic basalts are derived from partial melting of enriched mantle wedge metasomatized by slab fluid in subduction belt, and the TNT (Ta, Nb, Ti) negative anomalies also imply those rocks may be contaminated by crustal materials. These geochemical characteristics indicate that the northeastern Jiangxi ophiolitic may have experienced multi-stage tectonic evolution. Harzburgites with LREE enrichments result from fluid metasomatism under intraoceanic subduction process in ancient Huanan ocean. Subsequently, island-arc tholeiite basalts and MORB-like basalts formed in the stage of back-arc basin evolution and those andesitic basalts contaminated with crustal materials, show strong enrichment of LREE, and with high Pb and low Ti.