超高压榴辉岩中高场强元素的分配——以大别山碧溪岭为例
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本文为国家自然科学基金项目(编号41972198)和中国科学院广州地球化学研究所矿物学与成矿学重点实验室合作基金(编号KLMM20180201)联合资助的成果。


Distribution of high field strength elements in UHP eclogites: a case study of Bixiling, Dabie Mountains
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    摘要:

    本文通过对来自大别山中部碧溪岭地区的3块含柯石英榴辉岩中石榴子石、绿辉石以及角闪石进行主量、微量元素分析,研究了榴辉岩中微量元素,特别是高场强元素在峰期变质作用阶段以及随后的角闪岩相退变质作用阶段的分配特征。榴辉岩中代表峰期变质作用的石榴子石与绿辉石颗粒的δEu呈现出相关性,线性拟合斜率为0.75,与前人结论相似,说明微量元素在它们之间的分配达到平衡。绿辉石颗粒中的Na2O含量(6.14%~7.92%)和硬玉组分含量(>50%)较高,且通过石榴子石单斜辉石地质温度计得到平均变质温度为T=699℃,表明这些绿辉石属于超高压榴辉岩相原生矿物。在超高压变质作用过程中,Zr (Kd=0.18~0.91)倾向于进入石榴子石,而Hf (Kd=0.60~3.92)相对于Zr更倾向于进入绿辉石。绿辉石中高场强元素(Zr)的含量与Mg、Fe2+含量之和呈正相关,说明Zr在绿辉石中占据八面体M1位置。Zr在绿辉石中的含量与硬玉组分含量呈负相关性,其原因为伴随着硬玉含量降低,绿辉石中M1O键长缩短从而更适合Zr进入。而中阿尔卑斯地区幔源榴辉岩中绿辉石的硬玉含量较低(28.4%~42.8%),碧溪岭地区榴辉岩中绿辉石的硬玉含量较高(44%~55%)。这解释了中阿尔卑斯地区幔源榴辉岩捕虏体中Zr倾向于进入绿辉石,而大别山地区榴辉岩中Zr倾向于进入石榴子石的原因,也说明了寄主矿物的化学成分是影响高场强元素分配行为的主要因素。具有明显转变关系的绿辉石与角闪石的微量元素组成表明,在角闪岩相退变质阶段,绿辉石中轻稀土元素与重稀土元素发生明显分异,其中轻稀土元素倾向于进入次生角闪石中,而重稀土元素则倾向于保存在绿辉石中。微量元素在角闪石与绿辉石之间虽然未达到平衡,但Zr、Ba倾向于进入角闪石而Sr倾向于保存在绿辉石中。

    Abstract:

    Based on the major and trace element analysis of garnet, omphacite, and amphibole in three coesitebearing eclogite samples from the Bixiling area, central Dabie Mountains, the distribution of trace elements (especially high field strength elements) in eclogite during peak metamorphism and subsequent amphibolite facies retrograde metamorphism has been studied. Garnet and omphacite grains, which represent the peak metamorphism, are selected from eclogite. Their Eu anomalies show a correlation with a linear fitting slope of 0.75, which is similar to previous conclusions, indicating that the distribution of trace elements between them has reached an equilibrium. The contents of Na2O in omphacite grains are 6.14%~7.92%, and the contents of jadeite are more than 50%. The average metamorphic temperature obtained by garnetclinopyroxene geothermometer is T=699℃, which indicates that these omphacite crystals are formed during ultrahighpressure eclogite facies metamorphism. During UHP metamorphism, Zr (Kd=0.18~0.91) tends to enter garnet, while Hf (Kd=0.60~3.92) is more likely to enter omphacite than Zr. The content of Zr in omphacite is positively correlated with the sum of contents of Mg and Fe2+, indicating that Zr occupies the octahedral M1 site in omphacite. The content of Zr in omphacite is negatively correlated with the content of jadeite. The reason is that with the decrease of jadeite content, the M1O bond length in omphacite is shortened, which is more suitable for the entry of Zr. The jadeite content of omphacite in mantlederived eclogites in the middle Alps is low (28.4%~42.8%), and the jadeite content of omphacite in eclogites in the Bixiling area is high (44%~55%). This is the reason why Zr in the mantlederived eclogite xenoliths from the middle Alps tends to enter omphacite, while Zr in eclogite from the Dabie Mountains tends to enter garnet. It also shows that the chemical composition of host minerals is the main factor affecting the distribution behavior of high field strength elements. The trace element compositions of omphacite and amphibole show that LREE and HREE are separated during the retrograde metamorphism of amphibole facies. LREE tends to enter secondary amphibole, while HREE tends to be preserved in omphacite. Although trace elements do not reach equilibrium between amphibole and omphacite, Zr and Ba tend to enter amphibole, while Sr tends to be preserved in omphacite.

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侯信高,刘磊.2022.超高压榴辉岩中高场强元素的分配——以大别山碧溪岭为例[J].地质学报,96(11):3834-3848.
Hou Xingao, Liu Lei.2022. Distribution of high field strength elements in UHP eclogites: a case study of Bixiling, Dabie Mountains[J]. Acta Geologica Sinica,96(11):3834-3848.

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  • 收稿日期:2021-06-13
  • 最后修改日期:2021-08-28
  • 录用日期:2021-08-28
  • 在线发布日期: 2022-12-21
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