超高压变质过程中的元素地球化学行为——CCSD主孔榴辉岩的矿物化学研究

Element geochemical behavior during the ultrahigh-pressure metamorphism:Mineral compositional study of eclogites from the CCSD main drill hole.

中国大陆科学钻探主孔位于江苏东海县,苏鲁超高压变质带的南部。该钻孔的0～2050m深度获取了六种不同类型的榴辉岩和少量石榴石辉石岩岩心,它们是典型的基性超高压变质岩,为研究大陆深俯冲过程中的元素地球化学行为提供了非常好的样品。本文对各种超高压变质矿物的微量元素成分进行了系统的原位微区分析,结合全岩化学成分和矿物主量元素成分,深入地研究了超高压变质岩的微量元素赋存特征、分配规律、控制因素,及其对变质条件和流体-岩石相互作用的限定意义。结果表明,超高压榴辉岩中的LREE和Sr主要赋存在磷灰石、帘石和单斜辉石中,HREE赋存在石榴石中,Ba、Rb和Cs等LILE赋存在多硅白云母中,Ti、Nb和Ta等HFSE主要赋存在金红石、钛铁矿中,V、Sc、Co和Ni等元素大多赋存在石榴石和单斜辉石中。研究表明,全岩化学成分和矿物组成、及其含量的变化明显控制着超高压矿物的微量元素含量和分布形式。本研究也获得了如下重要的认识:超高压变质矿物之间的微量元素分配达到了化学平衡,并具有与地幔榴辉岩矿物之间类似的分配系数,表明榴辉岩的峰期变质温度很可能达到900℃～1000℃。部分高Ti和高Fe-Ti榴辉岩中的石榴石和绿辉石有明显的稀土元素成分环带,表明超高压变质岩经历了快速折返过程。金红石的Zr含量明显受到全岩成分和退变质作用影响,并不仅仅与形成温度有关,不是可靠的温度计。在超临界流体的作用下,榴辉岩中金红石的Nb、Ta发生了明显的分异,导致其Nb/Ta比值增大,由此推测俯冲到地幔深处的大量榴辉岩是地球内部高Nb/Ta比值的物质源区。在榴辉岩的不同程度退变质阶段,参与变质反应的流体具有不同的来源、成分和流体活动规模。

The main hole of the Chinese Continental Scientific Drilling Project (CCSD) is located at Donghai, in the southern part of the well-known Sulu ultrahigh-pressure (UHP) metamorphic terrane. Six types of eclogites and garnet pyroxenintes were collected from the depth interval between 100 to 2050 m of the hole. These typical UHP rocks can provide critical information for element chemical behavior during the continental deep subduction. In this study, trace element compositions of UHP minerals in the eclogites and garnet pyroxenites are carefully analyzed by in situ LA-ICP-MS. Using these data together with whole-rock compositions of UHP rocks and major element compositions of UHP minerals, the authors discussed the distribution and partitioning of trace elements in various minerals, and the UHP metamorphic P-T conditions and the fluid-rocks interaction. The results show that LREE and Sr are mainly concentrated in apatite, epidote and omphacite, HREE are mainly concentrated in garnet, LILE such as Ba and Rb are strongly partitioned in phengite, HFSE in rutile and ilmenite, while V, Sc, Co and Ni in garnet and omphacite, and that the trace element contents and their distribution pattern in UHP minerals are controlled by the whole rock compositions, the mineral assemblages and contents of the host rocks. In addition, the following conclusions are reached: (1) the distributions of trace elements among UHP minerals are of mutual equilibrium, and the partition coefficients are very similar to those in mantle eclogites, and probably indicating that the peak-stage UHP metamorphic temperatures of eclogites were higher than 900 degrees C similar to 1000 degrees C. (2) some garnets and omphacites from the high-Ti eclogite and high Fe-Ti eclogite show obvious growth compositional zoning both in major and trace elements, suggesting that the host UHP rocks were very rapidly uplifted. (3) Zr contents in eclogitic rutiles are not only controlled by the metamorphic temperature but also by other factors such as the Zr contents in the whole rocks and the compositional diffusion during the retrograde metamorphism, so the Zr-in-rutile geothermometer is not always credible. (4) During the UHP metamorphism, supercritical silicate-rich aqueous fluids have resulted in major fractionation between Nb and Ta in rutiles, so that the eclogitic rutiles have superchondrite Nb/Ta value. Therefore, voluminous eclogites that had been subducted to the mantle depth provide probably a hidden reservoir with higher than chondrite Nb/Ta ratio in the Earth. The fluids in the various stages of retrogression of UHP eclogites are different in origin, composition and flow scale.