GEOCHEMICAL CONSTRAINTS ON THE PARTIAL MELTING OF DABIE SUBDUCTED BLOCKS DURING UHPM PROCESS
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摘要: 报道了大别超高压变质带西部麻城四道河榴辉岩-围岩剖面系统的元素和同位素地球化学研究成果, 对超高压变质榴辉岩在俯冲和折返过程中的部分熔融作用进行了探讨. 研究表明, 榴辉岩原岩性质类似于N-MORB, 其长英质围岩可分为TTG片麻岩和含石榴石花岗岩; 两类长英质围岩Sm -Nd同位素特征与其寄主的榴辉岩相似; REE特征、w (Nb)/w (Ta) 比值、Nd同位素组成及锆石U -Pb定年等地球化学证据支持了TTG片麻岩为大别地区陆壳俯冲过程中发生部分熔融作用而形成, 其与超高压榴辉岩的关系属特殊的异地关系; 含榴花岗岩为超高压榴辉岩折返过程中的部分熔融作用形成, 但因其形成环境为石榴石稳定场的深度, 故含榴花岗岩与超高压榴辉岩被视为近似原地的构造关系.Abstract: This paper presents a comprehensive geochemical study of rocks from the cross section of Sidaohe quarry in Macheng, one of the representative outcrops of eclogite and its host rocks in Dabie UHPM belt. It is revealed in this paper that the eclogite shares the geochemical features with N MORB, and its felsic host rocks can be categorized as TTG gneiss and garnet bearing granite. Evidences from REE and w (Nb) / w (Ta) ratios, Sm Nd isotopes and U Pb zircon dating favor the scenario that TTG gneiss originated from the partial melting of eclogite during the process of subduction, therefore, a special allochthonous relationship in tectonics arises between TTG gneiss and eclogite, the origin of TTG gneiss, while the garnet bearing granite would have resulted from the partial melting of eclogite during its exhumation, but the V type REE pattern denotes that HREE enriched mineral (s) was crystallized when the granite was forming, and the garnet would have been the most favorable candidate of these minerals. Low w (Nb) / w (Ta) ratios of garnet bearing granite indicate that the melting occurred in some extent due to fluid metasomatism, a product of the retrograde metamorphism of UHPM rocks. Therefore, the garnet bearing granite and eclogite would have been in a sub in situ relationship, as the granite may have occurred in the depth where garnet was stable.
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Key words:
- UHPM belt /
- partial melting /
- fluid matasomatism /
- geochemical constraint /
- western Dabie
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图 2 四道河剖面w (MgO)-w (FeOt)-w (Na2O+K2O) 图
Fig. 2. w (MgO)-w (FeOt)-w (Na2O+K2O) plot of samples from cross section of Sidaohe quarry
图 3 四道河剖面长英质围岩w (Or)-w (An)-w (Ab) 图
Fig. 3. w (Or)-w (An)-w (Ab) plot of felsic country rocks from Sidaohe quarry
图 4 四道河剖面样品REE组成
Fig. 4. Chondrite normalized REE plots of samples from Sidaohe quarry
表 1 四道河剖面样品岩石化学成分
Table 1. Chemical compositions of rocks from the cross section of Sidaohe quarry
表 2 四道河剖面样品微量元素组成
Table 2. Trace elemental contents of rocks from the cross section of Sidaohe quarry
表 3 四道河剖面样品Sm-Nd同位素组成
Table 3. Sm-Nd isotopic compositions of rocks from the cross section of Sidaohe quarry
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