Pseudotachylites are generally considered to be produced by flash melting of the most fusible minerals, with compositions often dominated by biotite mica. We present phase fraction calculations for 237 pseudotachylite analyses from 28 localities spanning a range of host rock compositions from granites to peridotites. Pseudotachylite matrix compositions cannot be reproduced by a simple linear mixture of the minerals in their host rocks and commonly show a potassium deficit in pseudotachylites which contain significant contributions from biotite or muscovite. This is strongly indicative of peritectic melting of mica under fluid‐absent conditions. Occasionally, a negative contribution from an aluminosilicate phase is required but with a positive contribution from potassium feldspar. This is most consistent with peritectic melting of micas under fluid present conditions. The present data therefore suggest that, while fluid absent flash melting might be the most common mode of formation of pseudotachylite, in some instances pseudotachylites can be produced in the presence of a free fluid. Potassium mass balance of pseudotachylites might therefore be a diagnostic indicator of fluid‐absent conditions in their source regions during seismogenic rupture. We give some examples of observational evidence of peritectic liquidus phases in pseudotachylites and suggest that the significance of these phases might have been overlooked in previous studies.

中文翻译：

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断层相关的假速滑石熔体中云母的包晶熔融和钾的质量平衡，作为流体缺乏源条件的指标。

伪速滑石通常被认为是通过最易熔矿物的闪蒸来生产的，其成分通常以黑云母云母为主。我们介绍了28个地区的237个假速滑石分析的相分率计算，这些地区涵盖了从花岗岩到橄榄岩的一系列宿主岩石成分。伪速闪石基质组合物不能通过其宿主岩石中矿物的简单线性混合物来复制，并且通常在伪速闪石中显示出钾缺乏，而伪速闪石中黑云母或白云母的贡献很大。这有力地表明了在缺乏流体的情况下云母的包晶熔化。有时需要铝硅酸盐相产生负面影响，而钾长石则具有正面影响。这与在流体存在条件下云母的包晶熔解最一致。因此，目前的数据表明，尽管缺少闪速熔化的流体可能是假速晶石的最常见形成方式，但在某些情况下，假速晶石可以在自由流体的存在下生产。因此，假速晶石的钾质量平衡可能是震源破裂过程中其源区无流体条件的诊断指标。我们给出一些假速晶中包晶液相线相的观察证据，并建议在以前的研究中可能忽略了这些相的意义。在某些情况下，假速滑石可在自由流体存在下产生。因此，假速晶石的钾质量平衡可能是震源破裂过程中其源区无流体条件的诊断指标。我们给出一些假速晶中包晶液相线相的观察证据，并建议在以前的研究中可能忽略了这些相的意义。在某些情况下，假速滑石可在自由流体存在下产生。因此，假速晶石的钾质量平衡可能是震源破裂过程中其源区无流体条件的诊断指标。我们给出了假速晶的包晶液相线相的观察证据的一些例子，并建议这些相的重要性可能在以前的研究中被忽略了。