Detrital coesite-bearing garnet is the final product of a complex geological cycle including coesite entrapment at ultra-high-pressure conditions, exhumation to Earth’s surface, erosion and sedimentary transport. In contrast to the usual enrichment of high-grade metamorphic garnet in medium- to coarse-sand fractions, coesite-bearing grains are often enriched in the very-fine-sand fraction. To understand this imbalance, we analyse the role of source-rock lithology, inclusion size, inclusion frequency and fluid infiltration on the grain-size heterogeneity of coesite-bearing garnet based on a dataset of 2100 inclusion-bearing grains, of which 93 contain coesite, from the Saxonian Erzgebirge, Germany. By combining inclusion assemblages and garnet chemistry, we show that (1) mafic garnet contains a low number of coesite inclusions per grain and is enriched in the coarse fraction, and (2) felsic garnet contains variable amounts of coesite inclusions per grain, whereby coesite-poor grains are enriched in the coarse fraction and coesite-rich grains extensively disintegrated into smaller fragments resulting in an enrichment in the fine fraction. Raman images reveal that: small coesite inclusions of dimension < 9 µm are primarily monomineralic, whereas larger inclusions partially transformed to quartz; and garnet fracturing, fluid infiltration and the coesite-to-quartz transformation is a late process during exhumation taking place at c. 330°C. A model for the disintegration of coesite-bearing garnet enables the heterogeneous grain-size distribution to be explained by inclusion frequency. High abundances of coesite inclusions cause a high degree of fracturing and fracture connections to smaller inclusions, allowing fluid infiltration and the transformation to quartz, which in turn further promotes garnet disintegration.

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含柯石英石榴石的生命周期分析

含碎屑柯石英石榴石是复杂地质循环的最终产物，包括超高压条件下柯石英的截留、地球表面的挖掘、侵蚀和沉积输送。与高品位变质石榴石通常富集在中粗砂部分相比，含柯石英的颗粒通常富集在极细砂部分中。为了理解这种不平衡，我们基于 2100 个含包裹体颗粒的数据集分析了烃源岩岩性、包裹体尺寸、包裹体频率和流体入渗对柯石英石榴石粒度非均质性的作用，其中 93 个含柯石英。 ，来自德国萨克森厄尔士山区。通过结合包裹体组合和石榴石化学，我们表明：(1) 镁铁质石榴石每个晶粒的柯石英包裹体数量较少，并且在粗粒部分中富集；(2) 长英质柘榴石在每个晶粒中含有不同数量的柯石英夹杂物，因此贫柯石英颗粒在粗粒中富集。馏分和富含柯石英的晶粒广泛分解成较小的碎片，导致细粒馏分富集。拉曼图像显示：尺寸 < 9 µm 的柯石英小夹杂物主要是单矿物，而较大的夹杂物部分转化为石英；石榴石压裂、流体渗透和柯石英向石英的转变是在 由此，贫柯石英晶粒在粗粒部分中富集，而富含柯石英的晶粒广泛分解成较小的碎片，导致在细粒部分中富集。拉曼图像显示：尺寸 < 9 µm 的柯石英小夹杂物主要是单矿物，而较大的夹杂物部分转化为石英；石榴石压裂、流体渗透和柯石英向石英的转变是在 由此，贫柯石英晶粒在粗粒部分中富集，而富含柯石英的晶粒广泛分解成较小的碎片，导致在细粒部分中富集。拉曼图像显示：尺寸 < 9 µm 的柯石英小夹杂物主要是单矿物，而较大的夹杂物部分转化为石英；石榴石压裂、流体渗透和柯石英向石英的转变是在C。330℃。含柯石英石榴石的解体模型可以通过夹杂物频率来解释异质粒度分布。柯石英包裹体的高丰度会导致高度的断裂和断裂连接到较小的包裹体，从而允许流体渗透并转化为石英，这反过来又进一步促进了石榴石的解体。