Multistage graphite precipitation through protracted fluid flow in sheared metagranitoid, Digana, Sri Lanka: evidence from stable isotopes
Multistage graphite precipitation through protracted fluid flow in sheared metagranitoid, Digana, Sri Lanka: evidence from stable isotopes
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DOI：
10.1016/s0009-2541(02)00400-x
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发表时间：
2003-06
期刊：
影响因子：
3.9
通讯作者：
S. S. Binu-Lal-S.;K. Kehelpannala;M. Satish-Kumar;H. Wada
中科院分区：
文献类型：
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作者：
S. S. Binu-Lal-S.;K. Kehelpannala;M. Satish-Kumar;H. Wada
Digana变质花岗岩，位于Digana剪切带，斯里兰卡的高地和万尼复合体之间的关键交界处，主机各种石墨类型的结构证据显示在不同阶段的形成之前和之后的变形事件影响这个地形。在这里，我们讨论它们的出现，协会和稳定同位素。在这项研究中报告的石墨的碳同位素从斯里兰卡高级变质地形显着不同的石墨在其他地方报道的前寒武纪地形的组成。来自随机晶体的稳定同位素组成描绘了一个富集的碳同位素组成，范围在−1.6‰到−8‰之间，表明来自富CO2流体的还原沉淀。在石墨晶体上进行的晶内微尺度测量表明，在变质花岗岩类样品中，边缘的δ 13 C组成逐渐变轻，变化可达1.5‰，这与石墨生长过程中温度降低引起的流体状态变化相一致，并表明核在较高温度下的保存。与变花岗岩相反，混合岩的碳同位素组成高度富集（-0.6 ‰），13 C向边缘富集，表明石墨的部分熔融和溶解/再沉淀导致重同位素优先富集。石英-方解石脉的氧同位素组成为18.5‰ ~ 20‰，为典型的地壳再循环物源。不同石墨类型同位素数据的严格约束表明矿床的流体来源是均一的。在这项研究中设想在该地区的长期流体流动，这是有关的地壳规模的剪切带的多阶段变形。
The Digana metagranitoid, situated within the Digana shear zone, a critical junction between the Highland and Wanni complexes of Sri Lanka, hosts a variety of graphite types with structural evidence showing formation during different stages prior to and after deformational events affecting this terrain. Here we discuss their occurrence, association and stable isotopes. The carbon isotopes of graphite reported in this study from the Sri Lankan high-grade metamorphic terrain differ significantly from the composition of graphite reported elsewhere in other Precambrian terrains. The stable isotope composition from random crystals depicts an enriched carbon isotopic composition in the range −1.6‰ to −8‰, suggesting precipitation by reduction from CO2-rich fluids. Intracrystalline microscale traverses made on graphite crystals indicate the δ13C composition of the rims becoming progressively lighter in13C, in metagranitoid hosted samples, with the variation reaching up to 1.5‰, consistent with the changing fluid regimes induced by decreasing temperature during the graphite growth, and shows the preservation of core at higher temperatures. A highly enriched carbon isotopic composition (−0.6‰) with a rimward enrichment of13C observed in migmatites, contrary to the values in metagranitoid, indicates preferential enrichment of heavy isotopes as a result of the partial melting and dissolution/reprecipitation of graphite. The oxygen isotopic composition of a quartz–calcite vein show values in the range 18.5‰ to 20‰, typical for recycled crustal source. The tight constraint in the isotopic data from different graphite types studied indicates a homogeneous fluid source for the deposits. Prolonged fluid flow in this region is envisaged in this study, which is related to the multistage deformation of the crustal scale shear zone.