超高压榴辉岩金红石中高场强元素变化的控制因素及其地球动力学意义  

Distributions and Geodynamic Implications of High Field Strength Elements in Rutile from Ultrahigh Pressure Eclogites

作　　者：高长贵[1] 刘勇胜[1] 宗克清[1] 陈海红[1] 胡兆初[1] 

机构地区：[1]中国地质大学地质过程与矿产资源国家重点实验室,湖北武汉430074

出　　处：《地球科学（中国地质大学学报）》2008年第4期487-503,共17页Earth Science-Journal of China University of Geosciences

基　　金：国家重点基础研究发展规划项目(No.2003CB716501);国家自然科学基金项目(Nos.40521001;40673026);教育部长江学者和创新团队发展计划(IRT0441);教育部新世纪优秀人才支持计划(NCET05-0664)

摘　　要：利用LA-ICP-MS对CCSD-MH超高压榴辉岩中金红石进行了详细的原位微区微量元素组成分析.金红石中高场强元素Nb和Ta含量主要受全岩Nb、Ta和TiO2含量控制,Zr、Hf含量比较稳定基本不受全岩含量影响.粒间金红石中,同一颗粒金红石核部Zr含量系统高于边部,而边部则出现了明显的Pb和Sr富集特征.CCSD-MH榴辉岩中金红石与全岩的Nb/Ta比值呈现明显的不一致性.全岩Nb/Ta比值明显低于金红石且与全岩TiO2含量负相关,而金红石的Nb/Ta比值与全岩Nb、Ta含量和Nb/Ta比值没有明显的相关关系.金红石和全岩之间非完全耦合的Nb/Ta组成表明,金红石并非形成于原岩的结晶过程中而是在超高压变质作用过程中形成,尽管金红石是榴辉岩中Nb、Ta含量的主要载体矿物,但金红石的Nb/Ta比值并不一定能完全代表全岩的特征,而与全岩Nb、Ta和TiO2的含量有关.粒间金红石核部Zr含量所记录的温度与粒径之间具有明显的正相关性,反映金红石中的Zr在其形成后没有封闭.粒间金红石所表现出的明显的边部富集Pb和Sr的特征,反映了后期流体活动对金红石组成的影响.这些研究结果为金红石中Zr在高温下的扩散作用和后期流体活动的影响提供了重要证据,这可能是利用金红石Zr含量地质温度计计算的苏鲁-大别榴辉岩变质温度(598～827℃)偏低的主要原因.Trace element compositions of rutiles in eclogites from the Chinese Continental Scientific Drilling （CCSD） main hole were analyzed using LA-ICP-MS. The results indicate that Nb and Ta contents of rutiles are significantly controlled by whole rock compositions, while Zr and Hf show no obvious dependence on the whole rock compositions. Coupled with enrichments of Pb and Sr at the rim of the interstitial rutiles, Zr contents decrease from the core to the rim. Nb/Ta ratios of rutiles are much higher than those of the parent eclogites. Moreover, Nb/Ta ratios of rutile show no correlation with Nb and Ta concentrations and Nb/Ta ratios of the parent eclogites, while Nb/Ta ratios of eclogites correlate negatively with the TiO2 contents. These observations suggest that rutiles were metamorphic products of the Triassic ultrahigh pressure （UHP） metamorphism, rather than formed by crystallization in equilibrium with melt. The partially decoupling between rutiles and parent eclogites in terms of Nb/Ta ratios implies that although rutiles are the dominant carriers of Nb and Ta in eclogites, they might not completely reflect the Nb/Ta ratios of the parent eclogites, especially for low-Ti eclogites （TiO2〈 0. 5%）. Nb/Ta ratios of rutiles are dominated by the combination of Nb, Ta and TiO2 contents of the parent eclogites. The positive correlations between temperatures recorded by Zr contents and the size of rutile indicate that Zr in rutile was not immediately closed after futile formation. On the other hand, the enrichments of Pb and Sr at the rim of the interstitial rutiles suggest influence of subsequent fluids on the compositions of rutiles, which could have affected Zr of the rutiles as well. This study provides the evidence for the Zr diffusion at a high temperature and fluid activity that could have contributed to the lower temperatures （598-827℃） recorded by Zr-in-rutile for the eclogites from the Dabie-Sulu UHP terrane.

关 键 词：CCSD-MH 榴辉岩 金红石 微量元素 Nb/Ta比值 

分 类 号：P588.3[天文地球—岩石学]