Evidence of Partial Melting of Eclogites from Dabie-Sulu Ultrahigh-Pressure Metamorphic Belt: Trace Elements and Pb Isotope
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摘要: 俯冲玄武质洋壳的部分熔融作用是一种重要的大陆生长作用, 但是陆壳物质在俯冲过程中能否发生部分熔融缺少直接的证据.笔者利用已有的微量元素分配系数, 讨论了岛弧玄武岩在脱水和部分熔融2种过程中微量元素行为和铅同位素演化的差异.Pb在2种过程中都属于不相容元素, 但在脱水过程中其不相容性明显高于Rb、Ba、Th、U, 而在熔融过程中则低于上述元素, 在w (Rb) /w (Pb) ~w (Rb)、w (Ba) /w (Pb) ~w (Ba)、w (Th) /w (Pb) ~w (Th)、w (U) /w (Pb) ~w (U) 相关图上2种过程的演化方向差别明显.此外, Pb和Th、U的相容性的差异也使得石榴石和绿辉石的单矿物铅同位素在2种过程中明显不同.在此基础上, 结合大别—苏鲁榴辉岩的实测数据, 认为大别—苏鲁榴辉岩可能部分是岛弧玄武岩部分熔融后的残余体, 部分为单纯脱水的产物.大陆玄武质岩石在俯冲过程中也可能发生部分熔融作用.Abstract: Partial melting of subducted oceanic crust is one of the most important processes of continental growth. However, we see little direct evidence of partial melting of the continental during subduction. The difference in trace elements and Pb isotope between dehydration and partial melting of island arc basalt (IAB) is discussed based on the distribution coefficients of trace elements published. Although Pb was incompatible in both dehydration and partial melting, the activity of Pb was higher significantly than other incompatible elements (Rb, Ba, Sr, Th and U) in dehydration, and lower in partial melting. As a result, the evolution pathlous of dehydration and partial melting shows a large difference in the w (Rb) /w (Pb) -w (Rb), w (Ba) /w (Pb) - w (Ba), w (Th) /w (Pb) -w (Th), w (U) /w (Pb) -w (U) figure. Furthermore, the difference of incompatibility among Pb, Th and U leads to a significant distinction of single mineral isotope of garnet and omphacite between dehydration and partial melting. Moreover, the composition of trace elements and Pb isotope of actual measurements and that of analog calculation was compared. Most eclogites from Dabie-Sulu ultrahigh pressure belt might be residual after partial melting of IAB, however, the others were products of dehydration of IAB. Therefore, continental basaltic rocks could also be partially melted during subduction like the oceanic crust.
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Key words:
- dehydration /
- partial melting /
- trace element /
- Pb isotope /
- eclogite
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图 1 大别—苏鲁超高压变质带地质简图
★榴辉岩采样点
Fig. 1. Regional map of the Dabie-Sulu ultrahigh-pressure metamorphic belt
图 2 大别—苏鲁榴辉岩铅同位素组成和分配
虚线代表Geochron; C.单斜辉石; G.石榴石; W.全岩; 88T35和GE2为Australpine地区榴辉岩的结果[9]; 小图为榴辉岩D95-24单矿物铅同位素的模拟计算结果
Fig. 2. Pb isotopic composition of eclogites and distribution from Dabie-Sulu ultrahigh-pressure metamorphic belt
图 3 榴辉岩铅同位素演化三阶段模式示意
Fig. 3. Sketch map of three-stage model of Pb isotopic evolution of eclogite
图 4 微量元素的全岩/熔体(流体) 分配系数
a.30%石榴石+70%单斜辉石; b.30%石榴石+69%单斜辉石+1%金红石; c.30%石榴石+69%单斜辉石+1%钛铁矿; d.30%石榴石+50%单斜辉石+20%斜方辉石
Fig. 4. Distribution coefficients of trace elements between whole rock and melt (fluid)
图 5 大别—苏鲁超高压变质带榴辉岩的w (Th) /w (Pb) -w (Th), w (U) /w (Pb) -w (U), w (Rb) /w (Pb) -w (Rb), w (Ba) /w (Pb) -w (Ba) 图解
★.扬子岛弧玄武岩; ■.世界平均岛弧玄武岩(IAB), 熔融和脱水曲线根据残余体组成为30%石榴石+70%单斜辉石的模拟结果作出
Fig. 5. Plot of w (Th) /w (Pb) vs. w (Th), w (U) /w (Pb) vs. w (U), w (Rb) /w (Pb) vs. w (Rb) and w (Ba) /w (Pb) vs. w (Ba) of eclogites from Dabie-Sulu ultrahigh-pressure metamorphic belt
表 1 榴辉岩及相关变质岩的铅同位素组成
Table 1. Pb isotopic composition of eclogites and related metamorphic rocks
表 2 榴辉岩铅同位素的三阶段演化计算结果
Table 2. Calculated results of Pb isotope of eclogite in three-stage evolution model
表 3 模拟计算所用的微量元素分配系数和初始物质的微量元素质量分数
Table 3. Distribution coefficients of trace elements and composition of elements of the starting material used in analog calculation
表 4 全岩μ值变化模拟结果
Table 4. Results of μ from analog calculation of whole rock
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