Petrogenesis of sector-zoned garnet in graphitic metapelite from the Danba dome, eastern Tibetan Plateau (SW China)

IF 3.5 2区地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS

Contributions to Mineralogy and Petrology Pub Date : 2024-05-09 DOI:10.1007/s00410-024-02139-8

Z. M.G. Li, F. Gaidies, Y.-C. Chen, Y.-L. Zhao, C.-M. Wu

{"title":"Petrogenesis of sector-zoned garnet in graphitic metapelite from the Danba dome, eastern Tibetan Plateau (SW China)","authors":"Z. M.G. Li, F. Gaidies, Y.-C. Chen, Y.-L. Zhao, C.-M. Wu","doi":"10.1007/s00410-024-02139-8","DOIUrl":null,"url":null,"abstract":"<div><p>Texturally and chemically sector-zoned garnet crystals in two contiguous metapelitic rocks from the Danba dome, eastern Tibetan Plateau (SW China) were investigated. A petrographic boundary in one of the rocks (sample 21DB103) separates a thin section into two zones. Whereas one zone containing sector-zoned garnet and fined-grained matrix is enriched in graphite and quartz, the other zone encompasses garnets with relatively regular habit in a coarse-grained matrix poor in graphite and quartz. The two zones are distinct with regards to the chemical compositions of biotite and plagioclase, as well as the major and trace element zoning patterns of garnet. Electron back-scattered diffraction analysis shows that all the investigated garnet crystals in this sample are single crystals. Relatively higher <i>P-T</i> conditions are estimated for the initial growth of sector-zoned garnet (~ 5.0 kbar / ~540 ℃) compared to the regular garnet (~ 3.8 kbar / ~510 ℃) in this rock, possibly indicating that growth of the sector-zoned garnet postdates growth of the regular garnet. Texturally and chemically radial sectors with garnet-quartz intergrowths and irregular sectors of garnet are preserved in the other graphite-rich rock (sample 21DB104). Isopleth thermobarometry applied to the core of the largest garnet crystal exhibiting sector zoning in this sample reveals <i>P-T</i> conditions of initial garnet crystallization (~ 4.4 kbar / ~512 ℃) that deviate far (~ 0.8 kbar/~45 ℃) from equilibrium, potentially indicating significant overstepping required for garnet nucleation. Plagioclase inclusions in garnet display varying trace element abundances, indicating their replacements of different preexisting phases. These results suggest that abundant graphite may play a pivotal role in changing fluid conditions and reducing the solubility of SiO<sub>2</sub> to grow sector-zoned garnet, as well as impeding matrix coarsening. Development of sector-zoned core and dodecahedral faces of garnet may be related to rapid growth with changes in crystal morphology. Irregular sectors may have developed through fluid infiltration and local chemical adjustments.</p></div>","PeriodicalId":526,"journal":{"name":"Contributions to Mineralogy and Petrology","volume":"179 6","pages":""},"PeriodicalIF":3.5000,"publicationDate":"2024-05-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"Contributions to Mineralogy and Petrology","FirstCategoryId":"89","ListUrlMain":"https://link.springer.com/article/10.1007/s00410-024-02139-8","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}

引用次数: 0

Abstract

Texturally and chemically sector-zoned garnet crystals in two contiguous metapelitic rocks from the Danba dome, eastern Tibetan Plateau (SW China) were investigated. A petrographic boundary in one of the rocks (sample 21DB103) separates a thin section into two zones. Whereas one zone containing sector-zoned garnet and fined-grained matrix is enriched in graphite and quartz, the other zone encompasses garnets with relatively regular habit in a coarse-grained matrix poor in graphite and quartz. The two zones are distinct with regards to the chemical compositions of biotite and plagioclase, as well as the major and trace element zoning patterns of garnet. Electron back-scattered diffraction analysis shows that all the investigated garnet crystals in this sample are single crystals. Relatively higher P-T conditions are estimated for the initial growth of sector-zoned garnet (~ 5.0 kbar / ~540 ℃) compared to the regular garnet (~ 3.8 kbar / ~510 ℃) in this rock, possibly indicating that growth of the sector-zoned garnet postdates growth of the regular garnet. Texturally and chemically radial sectors with garnet-quartz intergrowths and irregular sectors of garnet are preserved in the other graphite-rich rock (sample 21DB104). Isopleth thermobarometry applied to the core of the largest garnet crystal exhibiting sector zoning in this sample reveals P-T conditions of initial garnet crystallization (~ 4.4 kbar / ~512 ℃) that deviate far (~ 0.8 kbar/~45 ℃) from equilibrium, potentially indicating significant overstepping required for garnet nucleation. Plagioclase inclusions in garnet display varying trace element abundances, indicating their replacements of different preexisting phases. These results suggest that abundant graphite may play a pivotal role in changing fluid conditions and reducing the solubility of SiO₂ to grow sector-zoned garnet, as well as impeding matrix coarsening. Development of sector-zoned core and dodecahedral faces of garnet may be related to rapid growth with changes in crystal morphology. Irregular sectors may have developed through fluid infiltration and local chemical adjustments.

Abstract Image

查看原文本刊更多论文

青藏高原东部（中国西南部）丹巴穹窿石墨化玄武岩中扇形分带石榴石的成岩过程

研究了青藏高原东部（中国西南部）丹巴穹隆两块毗连的元古宙岩石中的石榴石晶体的纹理和化学成分。其中一块岩石（样本 21DB103）的岩相边界将一个薄断面分为两个区域。其中一个区域包含扇形石榴石和细粒基质，富含石墨和石英；另一个区域包含石榴石，石榴石的习性相对规则，基质为粗粒，石墨和石英含量较低。这两个区域在生物橄榄石和斜长石的化学成分以及石榴石的主要元素和微量元素分带模式方面截然不同。电子反向散射衍射分析表明，该样本中所有被研究的石榴石晶体都是单晶体。与该岩石中的规则石榴石（约 3.8 千巴/约 510 ℃）相比，扇形分带石榴石最初生长的 P-T 条件（约 5.0 千巴/约 540 ℃）估计相对较高，这可能表明扇形分带石榴石的生长晚于规则石榴石的生长。在另一块富含石墨的岩石（样品 21DB104）中，则保留了具有石榴石-石英互生的放射状扇形纹理和化学成分，以及不规则的扇形石榴石。对该样品中呈现扇形分带的最大石榴石晶体的内核进行等温线测温，发现石榴石初始结晶的 P-T 条件（~ 4.4 千巴/~512 ℃）远远偏离平衡状态（~ 0.8 千巴/~45 ℃），这可能表明石榴石成核所需的时间大大超过了平衡状态。石榴石中的斜长石包裹体显示出不同的痕量元素丰度，表明它们取代了不同的原有相。这些结果表明，丰富的石墨可能在改变流体条件、降低二氧化硅的溶解度以形成扇带状石榴石以及阻碍基质粗化方面起着关键作用。石榴石扇形带核心和十二面体表面的形成可能与晶体形态发生变化的快速生长有关。不规则扇形可能是通过流体渗透和局部化学调整形成的。

本文章由计算机程序翻译，如有差异，请以英文原文为准。

求助全文

约1分钟内获得全文求助全文

来源期刊

Contributions to Mineralogy and Petrology 地学-地球化学与地球物理

CiteScore

6.50

自引率

5.70%

发文量

审稿时长

1.7 months

期刊介绍： Contributions to Mineralogy and Petrology is an international journal that accepts high quality research papers in the fields of igneous and metamorphic petrology, geochemistry and mineralogy. Topics of interest include: major element, trace element and isotope geochemistry, geochronology, experimental petrology, igneous and metamorphic petrology, mineralogy, major and trace element mineral chemistry and thermodynamic modeling of petrologic and geochemical processes.