中国大陆科学钻探主孔540-600m榴辉岩中赤铁矿-钛铁矿固溶体出溶结构的特征及对榴辉岩折返动力学过程的意义

梁凤华 曾令森 许志琴 陈方远 陈晶

中国大陆科学钻探主孔540-600m榴辉岩中赤铁矿-钛铁矿固溶体出溶结构的特征及对榴辉岩折返动力学过程的意义

梁凤华曾令森许志琴陈方远陈晶

- [1]中国地质科学院地质研究所国土资源部大陆动力学重点实验室,北京100037 [2]北京大学物理学院电子显微镜实验室,北京100871
基金项目:
科技部973重大基础研究项目（2003CB716504）及中国地质科学院重点实验室专项资金（KL05-3）的资助.致谢样品的采集工作由中国大陆科学钻探现场工作人员协助完成,电子探针测试在德国海德堡大学矿物系完成,拉曼和X-ray能谱测试在中国地质科学院地质研究所大陆动力学重点实验室完成,论文成文过程中得到刘福来研究员、张泽明研究员等的悉心指导,在此表示诚挚的感谢!

收稿日期: 2006-03-20

修回日期: 2006-05-18

刊出日期: 2006-07-31

Nature and significance of haematite-ihnenite solid solution in 540 -600m eclogite of Chinese Continental Scientific Drilling main borehole: implications for the exhumation of the CCSD eclogites.

HANG FengHua, ZENG LingSen, XU ZhiQin, CHEN FangYuan, CHEN Jing

- 1. Key Laboratory for Continental Dynamics, MLR, Institute of Geology, CAGS, Beijing 100037, China; 2. Electron Microscope Laboratory, School of Physics, Peking University, Beijing 100871, China

Received Date: 20 March 2006

Revised Date: 18 May 2006

Publish Date: 31 July 2006

摘要

摘要: 在中国大陆科学钻探主孔540-600m榴辉岩中集中发育了一种具特殊出溶结构的不透明金属矿物——具高度精细共生结构的赤铁矿-钛铁矿固溶体。在这段榴辉岩中，这种固溶体少部分以细小圆粒状包裹体分布于单斜辉石和石榴石内部，大部分则呈海绵陨铁状充填于单斜辉石和石榴石空隙之间。出溶分多个不同的期次，相对粗粒的赤铁矿质钛铁矿和钛铁矿质赤铁矿出溶片晶中又包含有更细粒的出溶片晶，在次一级的赤铁矿质钛铁矿域出溶钛铁矿质赤铁矿，同时在钛铁矿质赤铁矿域又出溶赤铁矿质钛铁矿，出溶片的大小可以从毫米级一直变化到纳米级，以至于很难测到纯的端元组分。出溶片在单个颗粒中按照平行（001）的方向定向排列，但在整个样品中则没有规律。出溶片中钛铁矿的摩尔百分数从23．80变化到89．84，反映了从不具明显磁性的赤铁矿和钛铁矿端元组分向具有强磁性的出溶体演化的过程。这样的出溶过程需要一个缓慢降温的环境，说明在超高压地体快速折返的整个过程中可能存在一个近等温降压或滞留在高温状态下（近等温等压）的阶段。这种固溶体在矿物的磁性特征方面十分特别：（1）可以包含异常强且稳定的天然剩磁；（2）所保存的剩磁即使在强的退磁磁场中也不会消退；及（3）能引起区域上显著的磁异常。因此，这种固溶体可能是导致CCSD主孔540～600m段榴辉岩较高磁化率、区域高度航磁异常及高度V、Ti异常的主要因素。赤铁矿-钛铁矿固溶体出溶结构在540～600m岩性段集中分布的现象与这一岩性段在各方面均有异常表现的奇特现象相吻合，结合前人的研究，认为赤铁矿-钛铁矿固溶体的成因与导致这段岩性段各种异常的成因有着直接的联系。赤铁矿-钛铁矿固溶体出溶结构的出溶成分和所具磁性强烈依赖于榴辉岩温压条件的变化特征，对它的继续深入研究，将会为精确限定苏鲁超高压变质带变质岩石形成中的温压变化，解释CCSD主孔附近区域的磁性异常及钒钛矿的勘探等问题起到重要的指示作用。
- 赤铁矿-钛铁矿固溶体 /
- 出溶结构 /
- 榴辉岩 /
- 中国大陆科学钻探 /
- 苏鲁超高压变质带
Abstract: We document the occurrence of hematite-ilmenite solid solutions in the 540-600m eclogites recovered by the Chinese Continental Scientific Drilling ( CCSD) project. These solid solutions have a very fine intergrowth structure from millimeter to nanometer scale, and can amount up to 30 vol. % of the whole-rock mineral composition. They occur either as fine rounded inclusions in clinopyroxenes and garnets or as interstitial phases among clinopyroxene and garnet boundaries and form a seemingly sponge-like structure. In either case, they have multiple generations of exsolution lamellae, with widths ranging from millimeters to nanometers. Within any single grain, the exsolution lamellae are parallel to the (001 ) plane of the host, but they do not define any overall systematic trend throughout any sample. Chemically, the molar fraction of ilmenite in these solid solutions varies from 23. 80% to 89. 84% , which may lead to the formation of a series of intergrowths with varying magnetic properties from paramagnetic hematite-ilmenite solid solution to intergrowths of canted antiferromagnetic ( CAF) hematite and paramagnetic ilmenite. To form these extraordinary hematite-ilmenite intergrowths, it requires an extremely slow-cooling progress during the tectonic evolution of the CCSD eclogites. This is at odd with commonly acclaimed rapid subduction and exhumation for the formation of Dabie-Sulu UHP rocks, which raises a critical question whether most of the eclogites have lost such key high temperature memories that have obscured our understanding on the tectonic processes responsible for the subduction and exhumation of continental materials. A number of studies have shown that hematite-ilmenite intergrowth has an extraordinary magnetic property. These intergrowths impart an unusually strong and stable remnant magnetization to the host rocks that is extremely difficult to be erased even in a strong alternating demagnetization field. The presence of such intergrowths may be a major factor that contributes to the anomalously high magnetic susceptibilities, high P-wave velocities, and geochemical anomalies (e. g. high TiO2, V, and V/Sc ratio, but low Nb and Ta) in these eclogites. Whether or not this type of eclogite is more common than what we know at present deserves further studies. Answers to this question have important implications for our understanding on the mechanisms for the subduction and exhumation of large mass of continental materials and regional exploration of vanadium and titanium deposits.
- Hematite-Ilmenite /
- Solid exsolution /
- Eclogite /
- Chinese Continental Scientific Drilling /
- Sulu UHP metamorphic belt

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中国大陆科学钻探主孔540-600m榴辉岩中赤铁矿-钛铁矿固溶体出溶结构的特征及对榴辉岩折返动力学过程的意义

Nature and significance of haematite-ihnenite solid solution in 540 -600m eclogite of Chinese Continental Scientific Drilling main borehole: implications for the exhumation of the CCSD eclogites.

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