Hydrothermal transients in Variscan crust: paleo-temperature mapping and hydrothermal models

Abstract

This study combines experimental work and numerical simulations to reconstruct the thermal history of the Frankenwald Transverse Zone, which was formed by a granitic intrusion into a fault zone. Illite crystallinity, vitrinite reflectance, and geobarometric investigations reveal a metamorphic and paleo-temperature anomaly associated with the granitic intrusion. Results of numerical simulations adequately explain paleo-temperatures in that area. In order to be able to obtain a quantitative comparison between numerical model results and paleo-temperature as observed in the field, we propose an empirical relationship between illite crystallinity and the maximum paleo-temperature based on literature data of illite crystallinity and a combination of other temperature-dependent parameters like vitrinite reflectance, phase petrology and smectite-to-illite transformation. Application of this strategy to the Frankenwald Transverse Zone yields the following results: (1) The paleo-temperature anomaly can be explained by the cooling of a number of plutons which intruded into the center of the zone. No additional heat sources are required to explain the observed anomaly. (2) The diapiric shape of these plutons could be confirmed because, in contrast, dike-shaped bodies would produce much smaller paleo-thermal anomalies. (3) The resolution of paleo-temperatures obtained from the illite crystallinity data is not good enough to discriminate precisely between advective and conductive modes of heat transfer. According to our preferred model, conductive heat transport is more likely than fluid-driven advective heat transport.