U–Pb geochronology of hydrothermal epidote unveils pre-kinematic hydration of highly deformed granitoids

The Aar Massif is a mid-crustal basement section of the European plate and it was intensely deformed during the Alpine orogeny. Alpine deformation of Aar Massif granitoids is expressed by a pervasive network of ductile shear zones consisting of fine-grained polymineralic (ultra)mylonites dominated by viscous granular flow processes (Wehrens et al., 2016). Fluid circulation and hydration reactions are recorded by both granitic protoliths and shear zones. In particular, they are made evident by the alteration of feldspar into hydrous minerals like epidote and mica. The timing of this hydration event and, consequently, whether Alpine deformation was initiated in already altered granitoids or in fresh ones were unclear. This lack of time constraints led to a pivotal question: did deformation initiate in rocks that were altered by pre-kinematic hydration, or was hydration syn-kinematic and driven by the formation of Alpine shear zones?

 

Laser ablation ICP-MS U–Pb geochronology applied to epidote in hydrothermal veins provides new evidence for pre-Alpine hydration of granitoids in the Aar Massif. Two veining events are recognized: 1) one at ca. 276 Ma occurring during Permian transtension and rifting, and 2) another at ca. 14 Ma related to late Alpine exhumation phases. Initial ²⁰⁷Pb/²⁰⁶Pb ratios of all Permian epidote samples overlap within uncertainty, suggesting only one fluid source and equilibration path. Also, these ratios are more radiogenic than those of the host rocks at the time of vein formation. The hydrogen isotopic composition of the Permian fluids was calculated from measurements in bulk epidote separates by high-temperature conversion elemental analyzer. With a temperature range of epidote crystallization estimated between 200–300 °C, the calculated δD_fluid value is -57 to -44 ‰. An external source for the Permian fluids is suggested by the disequilibrium of Pb isotopes between hydrothermal epidote and host granitoids. Percolation of meteoric water along transtensional faults and interaction with syn-rift sediments before reaching the granitoids underneath is suggested by the Permian transtensional geodynamics and supported the hydrogen isotopic composition of the Permian fluids.

 

The occurrence of Permian fluid circulation in the Aar Massif granitoids indicates that these rocks were altered before the onset of Alpine deformation. In fact, it can be inferred that fluid circulation caused not only veining, but also pervasive flow and thus the alteration of magmatic feldspar into fine-grained hydrous minerals throughout the granitoids of the present-day Aar Massif. This enabled pre-Alpine storage of water and the creation of numerous additional grain boundaries, both favoring viscous granular flow during Alpine deformation. In this context, the localization of strain in polymineralic aggregates containing hydrous minerals can recycle stored pre-kinematic (i.e., Permian) water. Thus, the initiation of Alpine deformation did not necessarily require the addition of syn-kinematic (i.e., Alpine) fluids, although their presence is confirmed by this and previous studies (e.g., Ricchi et al., 2019; Peverelli et al., 2021).