Fluid-Rock Interaction in Subduction Zones: Mineralogical Evidence of Metamorphic Rock from Southwestern Tianshan
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摘要: 西南天山（超）高压变质带内发育有大规模被流体交代的变质岩，其脉体和围岩记录了重要的俯冲带流体信息.在以往对有脉体发育的俯冲带变质岩全岩地球化学研究的基础上，进一步对该样品脉体和围岩中的石榴石、帘石、绿辉石、多硅白云母等矿物成分进行原位分析，对比同一矿物在脉体和围岩中的成分区别，以深入理解脉体和围岩的全岩成分差异、俯冲带水岩相互作用中相关元素的地球化学行为及流体性质.根据岩石学特征，该样品分为富蓝闪石脉体、富帘石和碳酸盐的过渡部分及富绿辉石内部.元素储库计算揭示全岩的元素组成和元素赋存矿物及其丰度具有一致性，脉体中更高的中重稀土元素（M-HREE）含量显示该部分更多的石榴石对M-HREE的赋存，过渡部分更高的REE-Th-U-Pb-Sr等元素含量反映了大量存在的帘石族矿物、碳酸盐对这些元素的控制，富绿辉石内部更高的K-Rb-Ba-Cs等元素含量则反映了这部分更多的白云母对这些元素的控制.另外，同一矿物在样品不同部分的成分差异可直接反映元素在不同变质阶段和不同水化程度下的地球化学变化，相较于受流体改造较少的富绿辉石内部，富蓝闪石脉体和过渡部分的多硅白云母和绿辉石丰度降低但更富集K-Rb-Ba-Cs等元素；石榴石丰度升高但其所含的HREE含量更低；帘石丰度升高但其所含的Th-U-轻稀土元素（LREE）含量更低.结合过渡部分出现的大量碳酸盐矿物，推测外来流体为含碳酸盐的蛇纹岩衍生流体.水岩相互作用后，流体的LILE含量显著升高，碳酸盐组分明显降低，其余元素含量变化不大.Abstract: Southwestern Tianshan HP-UHP metamorphic belt developed subduction-zone metamorphic rocks affected by large-scale fluid flows, its veins and host rocks provide important informations about the infiltrating fluid in subduction zone. Based on the previous bulk-rock geochemical studies on metamorphic rocks developed veins in the subduction zone, We have conducted an in situ element study of glaucophane, garnet, omphacite, phengite and other minerals in the veins and host rocks of the metamorphic rock samples to compare the compositional differences of the same mineral in veins and host rocks. This study aims to deeply understand the differences of bulk rock compositions between veins and host rocks, and the geochemical behaviors of related elements in the fluid-rock interaction and fluid properties in subduction zones. According to the petrological characteristics, the samples can be divided into glaucophane-rich veins, epidote and carbonate-rich transitional part and the omphacite-rich interiors.The calculation of element budgets reveal that the relationship between the difference of bulk-rock compositions and host minerals and their abundances. The higher contents of medium-heavy rare earth element (M-HREE) in veins reflects the occurrence of M-HREE by more garnets in veins; and the higher contents of rare earth elements-Th-U-Pb-Sr in the transitional part reflects a large number of epidote group minerals and carbonates controlthese elements; the higher contents of K-Rb-Ba-Cs in omphacite-rich interiors reflects the control of these elements by phengite. On the other hand, the compositional differences of the same mineral in different parts of the rock samples can reflect the geochemical changes of elements in different metamorphic stages and different rehydration degrees directly.Compared with the omphacite-rich interior representing the least fluidinfiltration, the rehydration of external fluid decreased the abundances of phengite and omphacite in the veins and transitional part, and enriched K-Rb-Ba-Cs-Pb-Sr; increased abundance of garnet butdepleted HREE; increased abundance of epidote but depleted in Th-U-light rare earth elements (LREE). Combined with the large amount of carbonates in transitional part, it is speculated that the external fluid is carbonate-bearing serpentinite-derived fluid. After the fluid- rock interaction, the contents of LILE increased and the carbonate component of the fluid decreased significantly in fluid, but the contents of other elements show less variation.
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图 2 变质岩样品野外照片（a~b）、三部分的关系图（c）和岩石薄片照片（d~f）
a~c. 根据矿物组合将变质岩块体依次分为富蓝闪石脉体、过渡部分和富绿辉石内部；d~f. 根据三部分的薄片照片可以清楚地看到由富蓝闪石脉体（d）到过渡部分（e）到富绿辉石内部（f），蓝闪石（蓝色）和帘石（黄绿）的含量逐渐减少，而绿辉石（绿色）的含量逐渐增加
Fig. 2. Fieldphotographsof metamorphic rock samples (a-b), sketch for relationship of three sections (c)and relevant slides(d-f)
图 3 显微镜下照片
a~c. 富绿辉石内部，多硅白云母和绿辉石以基质的形式出现，帘石较为常见；d. 富绿辉石内部（右）与过渡部分（左）的分界；e~f.过渡部分以帘石和蓝闪石为主，出现碳酸盐矿物；g. 富蓝闪石脉体部分（右）与过渡部分（左）的分界；h~i. 富蓝闪石脉体部分，以蓝闪石和石榴石为主. Omp.绿辉石；Ph.多硅白云母；Ap.磷灰石；Ep.帘石；Ca.碳酸盐矿物；Gln.蓝闪石；Grt.石榴石；Ttn.榍石
Fig. 3. Microscopic features
图 4 不同部分的矿物组成模式图
Omp.绿辉石；Grt.石榴石；Ttn.榍石；Gln.蓝闪石；Ep.帘石；Ca.碳酸盐矿物；Ph.多硅白云母；Ap.磷灰石
Fig. 4. Modal amounts of mineral assemblages
图 6 球粒陨石标准化矿物成分元素蛛网图（标准化值来自Sun and McDonough, 1989）
Fig. 6. Chondrite normalized mineral composition distributed diagrams(normalization values from Sun and McDonough, 1989)
图 7 西南天山变质岩块体流体-岩石相互作用示意图
第一期流体使得变质岩块普遍富集多硅白云母，第二期流体使得变质岩块发生不同程度的水化，A部分的蓝闪石、石榴石含量占比增加，多硅白云母与磷灰石含量占比降低；B部分的蓝闪石、帘石、石榴石和碳酸盐矿物占比增加，多硅白云母和磷灰石含量占比降低；C部分几乎未受到第二期流体的影响；据Xiao et al.（2017）
Fig. 7. Schematic illustration of fluid-rock interaction of metamorphic rock block from Southwestern Tianshan
图 8 样品三部分的元素储库示意图
本图中将碳酸盐矿物（Ca）分为白云石（Dol）和方解石（Cal）
Fig. 8. The diagrams of element budgets for three parts
图 9 三部分多硅白云母及绿辉石的元素含量变化图
a~c. 多硅白云母的K2O-Ba，Cs-K2O和Pb-Sr含量对比图；d~f. 绿辉石CaO-Sr、Cs-Ba和Ba-K2O的含量变化；图9c中有两个C部分的多硅白云母样品（2-C-2和2-C-4）的Pb含量低于检测限
Fig. 9. Co-variation diagrams for element contents of phengite and omphacite from three parts
图 10 三部分石榴石和帘石族矿物的元素含量变化图（a~d）和帘石核边部元素分配对比图（e~f）
a~b. 石榴石HREE-Y的含量变化图及中重稀土元素配分图；c~d. 帘石族矿物的Pb-Th，Th-La含量对比图；e. 帘石核-边的球粒陨石标准化稀土元素蛛网图；f. 同一帘石颗粒核部、边部微量元素变化图
Fig. 10. Co-variation diagrams for element contents of garnet and epidote group minerals from three parts (a-d) and comparison diagrams of element distribution at the edge and core of epidote (e-f)
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