Large-Scale Flow of Metamorphic Fluids in a Continental Subduction Zone: Evidence from Coesite-Bearing Jadeite Quartzite in the Dabie Orogen
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摘要: 硬玉石英岩是一种稀少且与流体作用相关的变质岩，同时出露于高压或超高压洋壳和陆壳俯冲带中.通过对中国东部大别造山带中出露达50 km2的含柯石英的超高压硬玉石英岩进行研究，综合全岩主微量元素、矿物Mg-O同位素和锆石学研究.结果表明，硬玉石英岩的原岩为古元古代TTG岩石，经历过弱化学风化和强物理风化作用，然后在三叠纪时期受到围岩富黑云母片麻岩分解脱水而产生的大量重Mg同位素流体交代，从而形成硬玉石英岩.考虑到这种受流体交代成因的硬玉石英岩在大别山广泛出露，表明其在三叠纪大陆深俯冲过程中存在着大规模的变质流体活动，这项研究首次报道了大陆俯冲带有大规模的流体活动存在，同时也挑战了传统观点认为的大陆俯冲带缺乏岛弧岩浆作用主要原因是缺乏足够量的流体活动.Abstract: Jadeite quartzite is a rare and fluid-related metamorphic rock, usually occurring as tectonic blocks of high-pressure (HP) to ultrahigh-pressure (UHP) metamorphic zones in continental and oceanic subduction zone.A combined study of whole-rock geochemistry, Mg and O isotopes, zircon U-Pb ages and trace elements was carried out for coesite-bearing jadeite quartzites from the Dabie orogen. The results indicate that the Middle Paleoproterozoic protolith of jadeite quartzites was weathered to a kind of sedimentary rocks in a passive continental margin and then underwent significant metasomatism by metamorphic fluids with high δ26Mg values during the continental subduction in the Triassic. The metamorphic fluids were produced by the breakdown of biotite in the metasedimentary rocks. The regional occurrence of jadeite quartzites over an exposure area of about 50 km2 indicates that the metamorphic fluids would have flowed on a large scale in the continental subduction channel. Such widespread fluid flow is evident in the continental subduction zone, with the metamorphic fluids would have flowed on a large scale at the slab-mantle interface in the continental subduction channel.
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Key words:
- continental subduction /
- fluid metasomatism /
- jadeite quartzite /
- UHP metamorphism /
- biotite breakdown /
- petrology
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图 1 硬玉富集的岩石全球分布
Fig. 1. World map of jadeite-bearing rock occurrences and tectonic settings
图 2 大别山超高压硬玉石英岩中锆石显微结构图像(a，b)和Hf同位素特征(c)
Fig. 2. Photomicrographs showing the texture and mineral inclusions for zircon from jadeite quartzites in the Dabie orogen
图 3 大别山硬玉石英岩O同位素(a)和Mg同位素(b)
Fig. 3. Mineral and whole-rock O (a) and Mg isotope data (b) for jadeite quartzites in the Dabie orogen
图 4 (a) 大别山硬玉石英岩地形分布图；(b)在大陆俯冲过程中古元古代TTG岩石受到流体交代形成硬玉石英岩；(c)MgO-Mg同位素两端元混合模拟计算图; (d)俯冲带流体-岩石反应的卡通图
Fig. 4. (a) Topographic map of jadeite quartzite distribution in the Central Dabie mid-T/UHP eclogite-facies zone; (b) schematic diagram showing the large-scale formation of jadeite quartzites due to metasomatism of TTG-derived sediments by metamorphic fluids during the continental subduction in the Dabie orogen; (c) binary mixing modal calculations based on MgO contents and Mg isotopes in jadeite quartzites; (d) schematic diagram showing fluid-rock interaction in a continental subduction channel
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