滇西双江县勐库地区退变质榴辉岩的岩石地球化学特征及其地质意义

滇西双江勐库地区新发现的退变质榴辉岩呈构造透镜体产于奥陶纪湾河蛇绿混杂岩的白云母构造片岩内，勐库地区是目前云南省境内唯一退变质榴辉岩产地。岩石地球化学分析结果显示，退变质榴辉岩的SiO2含量为4722%~5040%，全碱(Na2O+K2O)含量为228%~415%，Al2O3含量为1370%~1558%，TiO2含量为139%~395%，可分为中Ti(139%~183%)和高Ti(330%~395%)两种类型。退变质榴辉岩的主要原岩是拉班玄武岩，少部分具碱性玄武岩特征。多数原岩为拉斑玄武岩的样品具TiO2含量高、Eu异常不明显及缓右倾稀土元素配分模式特征，其稀土元素配分模式和微量元素蛛网图特征均与E MORB相似而与N MORB不同，投在MORB和E MORB范围，推测其原岩很可能形成于E MORB环境；少部分原岩为碱性玄武岩的样品，其TiO2含量中等，较原岩为拉斑玄武岩样品具更高的ΣREE和ΣLREE含量，投在OIB范围，推测其原岩可能形成于OIB环境。勐库地区退变质榴辉岩样品的岩石地球化学特征与其赋存的奥陶纪湾河蛇绿岩颇为相似，暗示可能是其俯冲消减作用的产物。此外，研究样品同时具有E MORB和OIB的特征，且与青藏高原羌塘中部榴辉岩相似，表明在特提斯洋演化过程中地幔柱岩浆和正常洋中脊亏损地幔岩浆的相互作用可能是广泛存在的。

Geochemical Characteristics and Geological Significance of Retrograde Eclogite in Mengku Area,Shuangjiang County,Western Yunnan Province,China

The newly found retrograde eclogite in Mengku in Yunnan Province occurs as lenses within muscovite schist. The results of geochemical analysis show that the contents of SiO2, total alkali (Na2O+K2O), Al2O3 and TiO2 range from 4722% to 5040%, 228% to 415%, 1370% to 1558% and from 139% to 395%, respectively, and the TiO2 can be divided into medium (139% to 183%) and high (330% to 395%) groups. Most samples with protolith as tholeiitic basalt are characterized by high contents of TiO2, inconspicuous Eu anomaly and slight LREE enrichment. The characteristics of REE patterns and trace elements spider diagrams of this type of samples are similar to those of E MORB but different from N MORB, and these samples are plotted into the fields of MORB and E MORB in tectonic discrimination diagrams, suggesting that the protolith was formed under the environment of oceanic basin. Some samples with protolith as alkaline basalt are characterized by medium contents of TiO2 and higher contents of ΣREE and ΣLREE than those samples metamorphosed from tholeiitic basalt. This type of samples is plotted into the field of OIB, indicating that the protolith was formed under the setting of OIB. The geochemical characteristics of retrograde eclogite are similar to those of their hosting Ordovician Wanhe ophiolite in Mengku area, implying that the retrograde eclogite is the product of the Ordovician oceanic subduction. Additionally, the samples from the study area have both characteristics of E MORB and OIB, which are similar to those of the central Qiangtang eclogite, implying that a interaction between mantle plume magma and normal mid oceanic ridge magma existed widely during the evolution of Tethys ocean.