Journal of Earth System Science ( IF 1.3 ) Pub Date : 2020-09-19 , DOI: 10.1007/s12040-020-01463-4 Gayatri N Akolkar , Manoj A Limaye
Abstract
In this work, the calc-silicate rocks affiliated to the ‘Kadana Formation’, a youngest formation of the Lunavada Group have been investigated. These rocks are found to be embedded within associated rock types, viz., quartzites and metapelites in the form of isolated lensoidal bodies. Contact metamorphic textures and the typical mineral assemblage, viz., Act +Di + Cal + Qtz + Ttn ± Mc ± Pl ± Bt ± Ep ± Scp ± Chl with minor proportion of apatite, zircon and opaques can be observed in these calc-silicates. Major oxides, trace and rare earth elements were analysed to investigate the protolith type, provenance and tectonic setting of these rocks. Protolith must be calcareous sandstone with varied proportion of clay and deposited in shallow water environment as revealed by CaO, Al2O3 and FeO+MgO and Al–Zr–Ti ternary diagram, respectively. Low to moderate weathering of source rocks has been indicated by A–CN–K ternary diagram. Th/Sc vs. Zr/Sc and Th/Co vs. La/Sc plots confirm the continental source with felsic nature for these rocks and the calc-silicate samples fall within the active continental margin region of Sc–Th–Zr/10 diagram which also justify the kind of provenance for primary sediments.
Highlights
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The calc-silicate rocks show major oxides, trace and rare earth element compositions which are very similar to those of their post-archean counterparts. Similarly, in the REEs vs. sample/REE chondrite normalized pattern, a moderate negative Eu anomaly is observed; these observations suggest that the chemical composition of the continental crust of the study area was similar to that of the post-archean continental crust.
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Data plotted in Al2O3, CaO and FeO + MgO diagram of Barton et al. (1991) indicated that the protolith of calc-silicate rocks of the study area is calcareous sandstone with small and varied amounts of clay within it, as most of the samples fall within or very close to the greywacke zone.
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CIA studies are pointing towards the low to moderate weathering of the source under the cold and arid conditions.
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The Th–Sc characteristics (Th/Sc > 1) support a predominantly continental source for these rocks; similarly, values of Zr/Sc indicate a moderate amount of sediment recycling.
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Higher abundances of incompatible elements imply predominantly felsic rocks in the source, which is also supported by LREE-enriched patterns. As per the Th/Co vs. La/Sc plot, mostly of granitic (felsic) composition is inferred for primary sediments for these calc-silicate rocks.
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The study concludes that the primary sediments for the calc-silicates of the Lunavada Group were deposited in an active continental margin setting.
中文翻译：
古吉拉特邦卢纳瓦达地区附近的钙硅酸盐岩石的地球化学：对其原生质，物源和构造环境的影响
摘要
在这项工作中，对Lunavada组最年轻的地层“ Kadana组”的钙硅酸盐岩进行了研究。发现这些岩石以孤立的类透镜体的形式嵌入相关的岩石类型中，即石英岩和变质岩。在这些钙硅酸盐中，可以观察到接触变质质地和典型的矿物组合，即Act + Di + Cal + Qtz + Ttn±Mc±Pl±Bt±Ep±Scp±Chl，且磷灰石，锆石和不透明物的比例较小。 。分析了主要氧化物，痕量和稀土元素，以研究这些岩石的原生岩类型，物源和构造环境。如CaO，Al 2 O 3所示，原生岩必须是钙质砂岩，具有不同粘土比例，并沉积在浅水环境中分别为FeO + MgO和Al-Zr-Ti三元图。A–CN–K三元图指示了烃源岩的低度至中度风化。Th / Sc与Zr / Sc以及Th / Co与La / Sc图确认了这些岩石具有长质性质的大陆来源，并且钙硅酸盐样品落在Sc–Th–Zr / 10图的活跃大陆边缘区域内这也证明了原始沉积物的起源。
强调
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钙硅酸盐岩石显示出主要的氧化物，痕量和稀土元素组成，与后古生代的岩石非常相似。同样，在REE与样品/ REE球粒陨石的归一化模式中，观察到中等程度的负Eu异常；这些观察结果表明，研究区大陆壳的化学成分与后古陆相大陆壳的化学成分相似。
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Barton等人在Al2O3，CaO和FeO + MgO图中绘制的数据。（１９９１）指出，研究区的钙硅酸盐岩石的原生石是钙质砂岩，其中有少量的粘土，因为大多数样品都落入或接近格雷瓦克带。
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CIA的研究表明，在寒冷和干旱条件下，源的气候处于中低水平。
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Th–Sc特征（Th / Sc> 1）为这些岩石提供了主要的陆源。同样，Zr / Sc值表明有中等程度的沉积物回收。
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大量不相容元素暗示着烃源岩中主要是长英质岩石，这也得到了富含LREE的模式的支持。根据Th / Co与La / Sc图，可以推断出这些钙硅酸盐岩的主要沉积物的花岗岩（长英质）成分大部分。
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研究得出的结论是，Lunavada组钙硅酸盐的主要沉积物沉积在活跃的大陆边缘环境中。
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