W.A.G.K. Wickramasinghe , T.B.N.S. Madugalla , Bhathiya Athurupana , Lei Zhao , Mingguo Zhai , Xianhua Li , H.M.T.G.A. Pitawala
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The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO<sub>2</sub>-rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. 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引用次数: 0
摘要
斯里兰卡巴兰戈达地区超高温花岗岩面地层中出露的特殊方解石体(CBs)为地壳中碳酸盐岩的生成保留了一些证据。碳酸盐岩完全由块状方解石组成,以延伸数十米的和谐带状或中尺度孤立袋状形式出现在块状白云石大理岩带中。不同大小的黑云母和钙硅酸盐飞地作为旋转或倾斜结构出现在 CBs 内。CBs与寄主大理岩之间的接触在纹理和矿物学上是渐变的,而飞地与CBs之间的接触则是尖锐的。与寄主大理岩相比,CBs 的大离子亲岩元素和稀土元素含量呈富集态,而与典型的碳酸盐岩相比则呈贫化态。此外,CBs 中的锶含量和 C-O 同位素值与已知碳酸盐岩、热液碳酸盐岩或变质碳酸盐岩中的锶含量和 C-O 同位素值不同。我们认为,大理岩的地壳无性化应被假定为 CBs 的可能起源机制。方解石颗粒的显微氧化证据显示了寄主大理岩熔化的迹象。在超高温花岗岩变质过程中,HC对VC的碰撞和推移或相关变质事件释放出富含二氧化碳的流体,很可能降低了碳酸盐的固结度,引发了大理岩的地壳膨胀。生成的低粘度碳酸盐熔体可能快速移动,导致硅酸盐的低度混合以及飞地的破碎和错位。本研究的结果反映了极端地壳条件下碳酸盐岩的无性化现象,有助于更好地了解碳循环过程的来源、迁移路径和储层。
An unusual occurrence of carbonatites derived from the crust in the UHT granulite facies metamorphic terrain of Sri Lanka
The special calcite bodies (CBs) exposed within the ultrahigh temperature (UHT) granulite facies terrain in the Balangoda area of Sri Lanka preserve shreds of evidence for the generation of crust-derived carbonatite in the crust. The CBs are exclusively made up of massive calcites and appear as concordant bands extending tens of meters or as meso-scale isolated pockets hosting the massive dolomitic marble band. Various sizes of mafic and calc-silicate enclaves occur as rotated or tilted structures within the CBs. The contact between the CBs and the host marble is texturally and mineralogically gradational, while the contact between the enclaves and the CBs is sharp. The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO2-rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. The results of the present study reflect the existence of anatexis of carbonates under extreme crustal conditions and provide a better understanding of the sources, migration paths and reservoirs of the carbon recycling processes.
期刊介绍:
Precambrian Research publishes studies on all aspects of the early stages of the composition, structure and evolution of the Earth and its planetary neighbours. With a focus on process-oriented and comparative studies, it covers, but is not restricted to, subjects such as:
(1) Chemical, biological, biochemical and cosmochemical evolution; the origin of life; the evolution of the oceans and atmosphere; the early fossil record; palaeobiology;
(2) Geochronology and isotope and elemental geochemistry;
(3) Precambrian mineral deposits;
(4) Geophysical aspects of the early Earth and Precambrian terrains;
(5) Nature, formation and evolution of the Precambrian lithosphere and mantle including magmatic, depositional, metamorphic and tectonic processes.
In addition, the editors particularly welcome integrated process-oriented studies that involve a combination of the above fields and comparative studies that demonstrate the effect of Precambrian evolution on Phanerozoic earth system processes.
Regional and localised studies of Precambrian phenomena are considered appropriate only when the detail and quality allow illustration of a wider process, or when significant gaps in basic knowledge of a particular area can be filled.