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首页> 外文期刊>Contributions to Mineralogy and Petrology >Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration
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Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration

机译：前寒武纪俯冲带晚期（赞比亚）辉长岩的部分生态化：流体渗透引发的前移变质作用

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Gabbros and eclogites occur closely associated in a 200-km-long and up to 40-km-wide area of the Zambezi Belt in central Zambia. This area is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust. Gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions. This resulted in kyanite-omphacite-bearing assemblages for eclogites that have Al-poor bulk compositions. Undeformed eclogites typically preserve features of a former gabbroic texture, reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals. Textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic. Relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre-scale domains in most of the rocks. No evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites. In contrast, the fine grained intergrowth of omphacite, garnet, kyanite and quartz, which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics, indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions. Eclogitisation took place under P-T conditions of 630-690 deg C and 26-28 kbar, suggesting a large overstepping (>10 kbar) of reaction boundaries. Eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large, subhedral grains of omphacite, kyanite and garnet. The gabbro-to-eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution-precipitation mechanism that characterises the metamorphic mineral replacements. The process of eclogitisation was limited by reaction kinetics and dissolution-precipitation rates rather than by the metamorphic P-T conditions. Even though ductile deformation occurred and equilibrium phase boundaries were overstepped, the infiltration of fluids was necessary for triggering the gabbro-to-eclogite transformation.
机译：辉长岩和榴辉岩在赞比亚中部赞比西河带200公里长至40公里宽的地区紧密相连。该区域被解释为代表前寒武纪缝合带的一部分，镁铁质岩石是俯冲洋壳的遗迹。从辉长岩到榴辉岩的渐进转变的渐进阶段通过不完全反应的不平衡结构得以保留。这导致块状铝含量低的榴辉岩中含有蓝晶石-方英石组合。未变形的榴辉岩通常保留以前辉长岩质地的特征，这反映为榴辉岩相矿物替代斜长石和岩浆辉石。变形榴辉岩的质地范围从剪切的卟啉碎屑到卟啉碎屑。岩浆辉石的遗迹很普遍，在大多数岩石中，仅在毫米至厘米级范围内就发生了完全的岩化作用。在榴辉岩中没有发现促进蓝晶岩或角闪岩相矿物组合的证据。相比之下，绿泥石，石榴石，蓝晶石和石英的细粒共生体代替了以前的斜长石，或在岩浆辉石遗迹的压力阴影下形成，这表明，岩化作用可能直接影响了辉长岩的原生质体，而没有任何明显的中间变质反应。在630-690摄氏度和26-28 kbar的PT条件下进行了凝固，这表明反应边界存在较大的跨步（> 10 kbar）。凝结开始，并伴随着通道化的流体流动，导致静脉中形成大的亚辉石，蓝晶石和石榴石的半面体颗粒。辉长岩到榴辉岩的转化通过一种流体得以增强，该流体为溶蚀沉淀机制（表征变质矿物替代物）提供了必要的物质传输。发炎的过程受反应动力学和溶出沉淀速率的限制，而不是受变质P-T条件的限制。即使发生了延性变形并且超过了平衡相的边界，流体的渗透对于触发辉长岩到榴辉岩的转变也是必要的。

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