摘要
对中国大陆科学钻探工程主孔两段榴辉岩与片麻岩互层的岩芯样品(Ⅰ:734.21-737.16m和Ⅱ:929.67-932.86m)分别进行了主要元素和微量元素以及Sr-Nd同位素分析。结果表明:(1)榴辉岩具有较大的成分变化范围(SiO2含量为46.8%-59.8%),表现出平坦或轻稀土(LREE)富集的分布模式和大离子亲石元素(LILE)富集或亏损以及Nb、Ta负异常特征。部分榴辉岩样品具有明显低的活动性元素(Rb、Ba和K)含量,指示了板块俯冲过程中的脱水变质效应;(2)两段岩芯中的片麻岩具有明显不同的主量和微量元素组成,其中第Ⅰ段具有相对较低的SiO2含量(61.3%～66.2%)和高的重稀土(HREE)含量,而第Ⅱ段具有明显偏高的SiO2含量(73.8%～74.1%)和低的HREE含量。但两段均表现出LREE富集和Eu负异常以及LILE富集和高场强元素(Nb、Ta、Ti)负异常特征;(3)榴辉岩和第Ⅰ段岩芯中的片麻岩具有相近的且偏高的εNd(750Ma)值(-3.6-+0.5),而第Ⅱ段岩芯中的片麻岩具有明显偏低的εNd(750Ma)值(-8.7)。部分片麻岩样品表现出较高的87Sr/86Sr比值(0.7070)和非常低的Rb/Sr比值(0.008),指示岩石Rb-Sr同位素体系受到了变质扰动。非活动性元素如Zr、Hf、Nb、Ta、Ti、Y、REE和Ti在脱水变质过程中没有受到明显扰动,因此可用于恢复榴辉岩和片麻岩的原岩性质及其形成的大地构造背景。根据分析样品的元素和Nd同位素特点,推测榴辉岩原岩具有板内玄武岩性质,第Ⅰ段岩芯中的片麻岩原岩为玄武质岩浆上升过程中同化地壳物质形成的中性岩,而第Ⅱ段岩芯中的片麻岩原岩为地壳物质重熔形成的酸性岩。因此,深钻榴辉岩和片麻岩原岩为新元古代华南陆块北缘裂谷构造带双峰式岩浆活动的产物,在岩浆侵位过程中经历了高温大气降水热液蚀变。
Two continuous core segments composing of eclogite and gneiss from the main hole of CCSD project, at depths of 734.21 to 737.16m (02CCSD - 1) and 929.67 to 932.86m (02CCSD - II) respectively, are systematically analyzed for major and trace elements as well as Sr - Nd isotopes. The results show the following Characteristic features: (1) the eclogites have variable chemical compositions with SiO2 concentrations of 46.8 to 59.8%, and exhibit flat to LREE enrichment patterns, and enrichment or depletion in LILE as well as Nb and Ta negative anomalies in the primitive mantle-normalized spidergrain; ( 2) the gneisses from both Cole segments show obviously different major and minor element compositions. The gneiss from the first core segment has relatively low SiO2 concentrations of 61.3 to 66.2% and high HREE contents, whereas the gneiss from the second core segment exhibits significantly high SA concentrations of 73.8 to 74.1 % and low HREE contents. Nevertheless, both of them are characterized by LREE and LILE enrichment as well as Eu and HFSE (Nb,Ta,Ti) negative anomalies; ( 3) both eclogite and gneiss from the first Cole segment display similar but relatively high epsilon(Nd) (750Ma) values of - 3.6 to + 0.5, but the gneiss from the second core segment has a significantly low epsilon(Nd)(750 Ma) value of -8.7. Some of the samples have relatively high(87)Sr/Sr-86 ratios but very low Rb/Sr ratios, indicating preferential Rb loss relative to Sr. Some of the eclogites have very low contents of mobile elements ( Rb, Ba and K), indicating the significant loss of these elements by dehydration metamorphism during continental subduction. to contrast, immobile elements Such as Zr, Hf, Nb, Ta, Ti, Y, REE and Ti show no evidence for significant loss, thus can be used to trace the geochemical nature of protoliths. Element and Nd isotope data indicate that the protolith of eclogites has an affinity to within-plate basalt; the protolith of gneiss from the first core segment is probably intermediate rock formed by crustal assimila
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第2期325-338,共14页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目(2003CB716501)资助
