Not only beautiful crystals – Alpine-type fissures in the light of quartz fluid inclusion, structural and monazite age data

The retrograde evolution of an orogen is characterized by a variety of processes like tectonic activity in the form of doming, brittle-ductile shearing and a general decrease in temperature conditions. These processes are accompanied by hydrothermal activity along shear zones and cracks in brittle deformed rocks, inducing retrograde metamorphic reactions and new growth of hydrothermal minerals in mineral veins and open fissures. These open Alpine-type fissures form at peak to retrograde metamorphic conditions of 450-550 °C at 0.3-0.6 GPa, and considerably below. Fissure formation generally occurs at ductile to brittle conditions, often at pronounced competence contrasts, with fissure orientations approximately perpendicular to host rock lineation and foliation. Measured orientations of the youngest fissures yield information also obtainable from stress analysis of Late Miocene deformation, but the orientations of older fissures provide clues on older stress field orientations. After their initial formation, the open fissures remain fluid filled and continue to see mineral crystallization down to at least 150 °C. A possibility to identify phases of tectonic activity that that lead to the formation of, or affect fissures is the dating of hydrothermal monazite. Like most fissure minerals, it is strongly zoned due to repeated dissolution-precipitation cycles initiated by a disequilibration of the fissure chemistry through repeated deformation. This allows identifying multiple growth stages through extended periods of time and changing temperature conditions. Monazite growth overlaps with and continues after the late stages of quartz crystallization, thus allowing to at least partially constrain the timing of quartz growth and thus entrapment of aqueous fissure fluids within the quartz. By combining data sets from across the Alps of fissure orientations, hydrothermal monazite ages and quartz fluid inclusion data, we can show that, despite fissures forming mainly under retrograde metamorphic conditions, most fissure-filling fluids are produced close to peak metamorphic conditions. We thus see fluid composition zones across the Alps systematically changing from saline fluids dominated by higher hydrocarbons over methane, through water to CO2 dominated fluids, in relation to metamorphic grade from diagenetic to amphibolite facies metamorphic rocks. The fluid type further constrains the crystal habitus of quartz crystallizing in open fissures, and fluid inclusion patterns are identical in areas of Cretaceous and Cenozoic Alpine metamorphism.