White mica has been widely used to date microstructures and tectonic events in faults, shear-zones and folds because of its suitability for radiogenic dating. However, complex (i) microstructural evolution, (ii) individual chemical evolution of the K-bearing phases, (iii) mixing of ‘detrital grains’ with newly formed and/or recrystallized or chemically reset grains as well as (iv) volume diffusion may result in apparent K-Ar ages. Here, specimens from a prograde sediment sequence of the exhumed fossil European Alpine accretionary wedge were used to investigate resetting processes of white mica by the type and intensity of deformation as well as peak metamorphic conditions. We combine the K-Ar system with mass and mineral quantities from grain size fractions to calculate the amount of recrystallized white mica in each grain size fraction along the metamorphic gradient. Increasing recrystallization with increasing metamorphic grade is related to thermally activated pressure solution and dissolution-precipitation creep, as seen by the formation of a spaced cleavage of recrystallized phyllosilicates documented through Synchrotron X-ray Fluorescence Microscopy and Scanning Electron Microscope imaging techniques. Increasing recrystallization by dissolution-precipitation processes induces chemical resetting of the isotopic system, resulting in a prograde decrease of apparent K-Ar ages. We demonstrate that Ar volume diffusion does not play a significant role for the low-temperature samples, promoting recrystallization as the important physico-chemical process for age resetting. However, white mica chemistry reveals that no simple relation between isotopic resetting and grain size exists along the prograde path. Reliable age information can therefore only be obtained in the case of (nearly) complete resetting, which accounts only for the smallest grain size fraction at the highest metamorphic temperature. These findings could shed new light on accurate dating of mica-rich fault rocks, where the time constraints depend not only on the temperature, but also on the amount and type of deformation.

白云母由于适用于放射成年测年，已被广泛用于对断层，剪切带和褶皱中的微结构和构造事件进行测年。但是，复杂的过程（i）微观结构演变；（ii）含钾相的单独化学演变；（iii）“碎屑晶粒”与新形成和/或重结晶或化学复位的晶粒混合，以及（iv）体积扩散可能会导致明显的K-Ar年龄。在这里，使用来自化石欧洲高山增生楔的渐进沉积序列的标本，通过变形的类型和强度以及峰值变质条件来研究白云母的复位过程。我们将K-Ar系统与晶粒度分数中的质量和矿物量结合起来，以计算沿变质梯度的每个晶粒度分数中重结晶的白云母的数量。再结晶随着变质等级的增加而增加，与热活化压力溶液和溶解-沉淀蠕变有关，这是通过用同步加速器X射线荧光显微镜和扫描电子显微镜成像技术记录的重结晶层状硅酸盐的开裂而形成的。通过溶解-沉淀过程增加的重结晶会引起同位素系统的化学复位，从而导致表观K-Ar年龄的逐步降低。我们证明了Ar的体积扩散对低温样品没有显著作用，促进重结晶是年龄重置的重要物理化学过程。但是，白云母化学显示，沿着前进路径，同位素复位与晶粒尺寸之间不存在简单的关系。因此，仅在（几乎）完全重置的情况下才能获得可靠的年龄信息，这仅说明了在最高变质温度下最小的晶粒尺寸分数。这些发现可能为富含云母的断层岩的精确测年提供了新的依据，这里的时间限制不仅取决于温度，而且取决于变形的数量和类型。因此，仅在（几乎）完全重置的情况下才能获得可靠的年龄信息，这仅说明了在最高变质温度下最小的晶粒尺寸分数。这些发现可能为富含云母的断层岩的精确测年提供了新的依据，这里的时间限制不仅取决于温度，而且取决于变形的数量和类型。因此，仅在（几乎）完全重置的情况下才能获得可靠的年龄信息，这仅说明了在最高变质温度下最小的晶粒尺寸分数。这些发现可能为富含云母的断层岩的精确测年提供了新的依据，这里的时间限制不仅取决于温度，而且取决于变形的数量和类型。