Abstract Contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh-pressure (UHP) lenses surrounded by high-pressure (HP) and medium-pressure (MP) felsic country rocks are an enigmatic feature of UHP terranes. One of the major questions arising is whether the UHP lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced UHP conditions but equilibrated and/or re-equilibrated at a different metamorphic stage. Here we address this question to the central Saxonian Erzgebirge in the northwestern Bohemian Massif, Germany. In order to screen the variety of garnet from lithologies occurring in the study area, we analyzed the detrital garnet record from seven modern stream sands. In addition to 700 inclusion-bearing garnet grains previously studied from the 125–250 μm grain-size fraction, we analyzed the 63–125 and 250–500 μm fractions and extended the dataset to overall 2100 inclusion-bearing grains. The new findings of coesite and diamond inclusions in several garnet grains, which are in compositional contrast to garnet of the known UHP lenses but match with those of the felsic country rocks, show that considerable parts of the country rocks underwent UHP metamorphism. Melt inclusions containing cristobalite, kokchetavite, and kumdykolite in garnet derived from the country rocks point to partial melting and re-equilibration during exhumation at HP/HT conditions. Although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions, the mineralogical evidence for UHP conditions in the felsic country rocks surrounding the UHP lenses proves a largely coherent slab subducted to UHP conditions. Furthermore, the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle, which may explain the high exhumation rates of UHP terranes.

中文翻译：

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撒克逊厄尔士山脉中部包含 UHP 透镜体的长英质岩石的深俯冲：对 UHP 地体挖掘的影响

摘要 由高压 (HP) 和中压 (MP) 长英质围岩包围的超高压 (UHP) 透镜的化学地质温度气压测量确定的对比变质条件是 UHP 地体的一个神秘特征。出现的主要问题之一是超高压透镜和围岩是否是对应于不同最大深度的不同峰值变质条件的产物，或者围岩是否也经历了超高压条件但在不同的变质阶段平衡和/或重新平衡。在这里，我们向德国波西米亚地块西北部的萨克森州中部厄尔士山脉提出这个问题。为了从研究区发生的岩性中筛选石榴石的种类，我们分析了七种现代河流砂岩的碎屑石榴石记录。除了先前从 125-250 μm 粒度级分中研究的 700 颗含内含物石榴石颗粒外，我们还分析了 63-125 和 250-500 μm 级分，并将数据集扩展到 2100 颗内含物颗粒。在几种石榴石颗粒中发现柯石英和金刚石内含物的新发现，与已知 UHP 透镜的石榴石在成分上形成对比，但与长英质围岩相匹配，表明围岩的相当一部分经历了 UHP 变质作用。来自围岩的石榴石中含有方石英、kokchetavite 和 kumdykolite 的熔体包裹体表明，在 HP/HT 条件下的折返过程中部分熔化并重新平衡。虽然达到不同最大深度的岩石混合可能是造成一些对比峰值变质条件的原因，在超高压透镜周围的长英质乡村岩石中超高压条件的矿物学证据证明，在超高压条件下俯冲的大部分连贯板片。此外，俯冲大长英质地壳中柯石英的存在及其在折返过程中向石英的转变对变质循环期间的浮力发展具有重要意义，这可能解释了超高压地体的高折返率。