METAMORPHIC EVOLUTION OF SPESSARTINE QUARTZITES (COTICULES) IN THE HIGH-PRESSURE, LOW-TEMPERATURE COMPLEX AT BAHIA MANSA, COASTAL CORDILLERA OF SOUTH-CENTRAL CHILE
METAMORPHIC EVOLUTION OF SPESSARTINE QUARTZITES (COTICULES) IN THE HIGH-PRESSURE, LOW-TEMPERATURE COMPLEX AT BAHIA MANSA, COASTAL CORDILLERA OF SOUTH-CENTRAL CHILE
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智利中南部沿海科迪勒拉山脉巴伊亚曼萨高压低温杂岩中锰铝榴石石英岩（小叶）的变质演化
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
F. Hervé
中科院分区：
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作者：
A. Willner;Sabine Pawlig;H. Massonne;F. Hervé
在巴伊亚曼萨海岸暴露的绿片岩夹层中发现了一种闪长岩石英岩透镜体（角质层），该透镜体位于主要为变砂泥质的西部系列中，形成了智利中部至南部沿海科迪勒拉山脉的大部分基底。角质层的化学成分可以解释为原岩形成的含铁和含锰的热液沉淀物与铝蚀变衍生的材料上的洋壳。变质作用的峰值条件是通过多元反应计算的：270-370 °C，6-8 kbar。逆行P-T演变的标志是在低于300°C的冷却期间减压至2kbar，外部流体的流入和逆行相的无应变结晶。角质层及周围变基性岩中出现的“铁纹石”、“铁巴氏石”和多硅白云母标志着绿片岩相向绿帘石蓝片岩相的转变。石榴石的振荡环带内的角质层以及在绿帘石和电气石的变基性岩似乎是由当地的化学环境，在矿物的生长过程中。尽管小鳞片中的石榴石是由连续的脱水-脱碳酸反应形成的，但由于减压早期大量流体的涌入，原生流体包裹体主要是含水的。晚期减压阶段被原生流体包裹体的等容线所包围，这些包裹体明显低于变质作用的峰值条件。这是由于减压过程中流体包裹体体积的增加和寄主矿物的拉伸。石榴子石中流体包裹体密度高于石英中的流体包裹体密度是由于两种矿物屈服强度的差异。由小鳞片和邻近的洋源变基性岩所揭示的约12 °C/km的瞬时变质梯度表明缓慢的俯冲作用。侵彻韧性变形主要局限于增生棱柱体中晚期的前进和早期的退变质P-T轨迹。
Lenses of spessartine quartzites (coticules) are associated with greenschist intercalations in coastal exposures at Bahia Mansa within the mainly metapsammopelitic Western Series, which forms most of the basement in the Coastal Cordillera of central to southern Chile. The chemical compositions of the coticules can be explained by protoliths formed from ferriferous and manganiferous hydrothermal precipitates mixed with aluminous alteration-derived material on top of oceanic crust. The peak conditions of metamorphism were calculated with multivariant reactions: 270–370 °C, 6–8 kbar. A retrograde P–T evolution was marked by decompression to 2 kbar during cooling below 300°C, influx of an external fluid, and strain-free crystallization of retrograde phases. The appearance of “ferriwinchite”, “ferribarroisite” and phengite in coticules and surrounding metabasic roc ks marks the transition from greenschist to epidote blueschist facies. Oscillatory zoning in garnet within the coticules as well a s in epidote and tourmaline in the metabasic rocks seems to be governed by the local chemical environment during prograde growth of minerals. Although the garnet in the coticules formed by a continuous prograde dehydration–decarbonation reaction, the primary fluid inclusions are predominantly hydrous owing to massive influx of fluid during the early stage of decompression. Th e late stage of decompression is bracketed by isochores of primary fluid inclusions that are clearly below peak conditions of metamorphism. This is attributed to a volume increase of the fluid inclusions and stretching of the host minerals during decompression. Higher densities of fluid inclusions in garnet compared to those in quartz are due to differences in yield stren gth between the two minerals. The transient metamorphic gradient of approximately 12 °C/km revealed by the coticules and adjacent metabasic rocks of oceanic origin suggest slow subduction. Penetrative ductile deformation is mainly restricted to the late pro grade and early retrograde P–T path within an accretionary prism.