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

This study combines experimental work and numerical simulations to reconstr uct the thermal history of the Frankenwald Transverse Zone, which was forme d by a granitic intrusion into a fault zone. Illite crystallinity, vitrinit e reflectance, and geobarometric investigations reveal a metamorphic and pa leo-temperature anomaly associated with the granitic intrusion. Results of numerical simulations adequately explain paleo-temperatures in that area. I n order to be able to obtain a quantitative comparison between numerical mo del results and paleo-temperature as observed in the field, we propose an e mpirical relationship between illite crystallinity and the maximum paleo-te mperature based on Literature data of illite crystallinity and a combinatio n of other temperature-dependent parameters like vitrinite reflectance, pha se petrology and smectite-to-illite transformation. Application of this str ategy to the Frankenwald Transverse Zone yields the following results: (1) The paleo-temperature anomaly can be explained by the cooling of a number o f plutons which intruded into the center of the zone. No additional heat so urces are required to explain the observed anomaly. (2) The diapiric shape of these plutons could be confirmed because, in contrast, dike-shaped bodie s 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. (C) 1999 Elsevi er Science B.V. All rights reserved.