Syn- to late-orogenic quartz veins marking a retrograde deformation path in a slate belt: Examples from the High-Ardenne slate belt (Belgium)

An integrated structural, petrographic and microthermometric methodology has been applied on syn- to late-orogenic quartz veins from the High-Ardenne slate belt (Belgium) to define their relative timing. The quartz precipitates, which do not show any mutual cross-cutting relationships, represent the meso-scale brittle accommodation during fold initiation, amplification and locking. Crystal-plastic deformation structures and P–T trapping conditions indicate that the different processes accommodating folding occurred in a progressive manner along a retrograde deformation path, associated with the gradual exhumation of the slate belt from ca. 7.5 to 6 km depth. Successive veining occurred from peak metamorphic conditions (ca. 300 °C and 190 MPa), measured in extrados veins, to subsequent lower P–T conditions in the periphery of a lenticular vein (ca. 275 °C and 180 MPa), late-orogenic saddle reef filling (ca. 245 °C and 160 MPa), and the core of a lenticular vein (ca. 220 °C and 150 MPa). Open-cavity growth and fluid-assisted brecciation indicate that veining occurred under overpressured fluid conditions. It is therefore fair to assume that the High-Ardenne slate belt acted – at least episodically – as a mid-crustal overpressured fluid reservoir, not only in the earliest and latest stages, but also during the main stage of orogeny.