Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China
Elemental responses to subduction-zone metamorphism: Constraints from the North Qilian Mountain, NW China
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对俯冲带变质作用的元素响应：来自中国西北祁连山的约束
DOI：
10.1016/j.lithos.2012.11.012
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发表时间：
2013-02
期刊：
影响因子：
3.5
通讯作者：
Xiaoming Liu
中科院分区：
文献类型：
--
作者：
Yuanyuan Xiao;Yaoling Niu;Shuguang Song;Jon Davidson;Xiaoming Liu
俯冲带变质作用及其对应的化学元素行为对弧岩浆活动和地幔成分非均质性具有重要意义。本文报道了北祁连山两个不同变质史变质亚带沉积原岩和玄武岩原岩的蓝片岩和榴辉岩相岩的岩石学和地球化学研究结果。低品位蓝片岩的原岩成分为玄武岩，最可能产自弧后环境，而高品位蓝片岩/榴辉岩的原岩地球化学特征类似于现今正常和富集的中洋脊玄武岩以及一些火山弧岩。变质沉积岩包括变质灰岩、变质泥质岩、变质燧石岩和大理岩，其地球化学特征与全球大洋（俯冲）沉积岩具有相似性。假设高场强元素（hfse）是相对不动的，稀土元素（ree）和Th与高场强元素的相关变化表明这些元素也可能是不动的，而Pb和Sr在SZM期间的玄武岩和沉积原岩中都是可动的。Ba、Cs、Rb在沉积原岩中不活动，在玄武原岩中活动。玄武岩原岩中铀的明显流动性可能是由海底蚀变继承而来，而不是由SZM引起的。在原位矿物组成（主微量元素）分析的基础上，这些俯冲带变质岩中最显著的微量元素存储矿物为：稀土元素主要为lawsonite、pumpellyite、磷灰石、石榴石和绿帘石群矿物，大离子亲石元素主要为白色云母（包括云母和paragonite）， hfse主要为金红石和钛矿。这些矿物的存在和稳定性对这些元素在SZM过程中的地球化学行为起主要控制作用。稀土、钍和铀在新形成的矿物中重新分布而形成的不动性表明，俯冲带脱水变质作用不会导致这些元素在弧岩浆活动中富集。这些观测需要超临界流体或含水熔体的形成和贡献（它们可以有效地将上述不相容元素输送到更深的岩浆活动中）。此外，含金红石榴辉岩中保留的整体亚球粒Nb/Ta比值表明，通过SZM的俯冲/俯冲残余洋壳不可能是整体硅酸盐土中丢失Nb（相对于Ta）的原因。
Subduction zone metamorphism (SZM) and behaviors of chemical elements in response to this process are important for both arc magmatism and mantle compositional heterogeneity. In this paper, we report the results of our petrographic and geochemical studies on blueschist and eclogite facies rocks of sedimentary and basaltic protoliths from two metamorphic sub-belts with different metamorphic histories in the North Qilian Mountain, Northwest China. The protolith of low-grade blueschists is basaltic in composition and is most likely produced in a back-arc setting, while the protoliths of high-grade blueschists/eclogites geochemically resemble the present-day normal and enriched mid-oceanic ridge basalts plus some volcanic arc rocks. The meta-sedimentary rocks, including meta-graywacke, meta-pelite, meta-chert and marble, show geochemical similarity to global oceanic (subducted) sediments. Assuming that high field strength elements (HFSEs) are relatively immobile, the correlated variations of rare earth elements (REEs) and Th with HFSEs suggest that all these elements are probably also immobile, whereas Pb and Sr are mobile in rocks of both basaltic and sedimentary protoliths during SZM. Ba, Cs and Rb are immobile in rocks of sedimentary protoliths and mobile in rocks of basaltic protolith. The apparent mobility of U in rocks of basaltic protolith may be inherited from seafloor alterations rather than caused by SZM. On the basis of in situ mineral compositional analysis (both major and trace elements), the most significant trace element storage minerals in these subduction-zone metamorphic rocks are: lawsonite, pumpellyite, apatite, garnet and epidote group minerals for REEs, white micas (both phengite and paragonite) for large ion lithophile elements, rutile and titanite for HFSEs. The presence and stability of these minerals exert the primary controls on the geochemical behaviors of most of these elements during SZM. The immobility of REEs, Th and U owing to their redistribution into newly formed minerals suggests that subduction-zone dehydration metamorphism will not contribute to the enrichment of these elements in arc magmatism. These observations require the formation and contribution of supercritical fluids or hydrous melts (these can effectively transport the aforementioned incompatible elements) from greater depths to arc magmatism. In addition, the overall sub-chondritic Nb/Ta ratio retained in rutile-bearing eclogites indicates that the subducting/subducted residual ocean crust passing through SZM cannot be responsible for the missing Nb (relative to Ta) in the bulk silicate earth.
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