Quantitative structural analyses and numerical modelling of ...

88 O. LEXA ET AL.Fig. 7. Results of 1D static thermal models to compare temperature evolution controlled by different radioactive heat production offelsic lower crust (FLC; curves labelled 1–6 according to heat production) and radioactive heat production within the lithosphericmantle (dotted line) with thermal evolution caused by lithospheric delamination at 90 km depth (dashed line).such as thermal diffusivity, specific heat, viscosity anddensity are recalculated according to temperaturedependence for each marker (see Appendix).The geometry of the model, the distribution ofmaterial layers and the boundary conditions of thenumerical simulations are depicted in Fig. 6. The initiallayered geometry introduces an artificial perturbationin the interface between felsic and mafic lowercrust to allow immediate relaxation of gravitationalinstability in the central part of the computationaldomain. The initial temperature distribution is calculatedas the steady-state solution of Eqn A.3, withheat sources located only in the upper crust. Thelower crustal radioactive heat production is accountedonly for transient development. One of theimportant features of the model is the ability toprogressively eliminate sources of radioactive heatproduction according to the temperature achieved. Asargued above using the available geochemical data, ata certain stage of the evolution of the lower crust, mostof the radioactive elements, in particular U and Th,were stripped from the felsic granulites into partialmelt or ÔfluidÕ and transported, together with K-richmagmas, into the middle–upper crust. In the models,two values are used to cut-off heat production tosimulate radioactive element evacuation via fluid or viamelt.Heat sourcesThe effect of such behaviour on thermal evolution wasfirst examined in terms of a 1D static thermal model(Fig. 7). A sharp change in heating rate in Fig. 7ccorresponds to the time when heat production isswitched off in most of the lower crust. To compare thescale and magnitude of temperature change as a resultof processes like delamination or heat productionwithin lithospheric mantle, the plot is overlaid withresults of two additional numerical simulations. Thedotted line represents the results of a 1D model tosimulate production of heat within a >60 km thicklithospheric mantle with a radioactive heat productionof 0.2 lW m )3 calculated on the basis of the wholerockgeochemical data of Ackerman et al. (2009) forthe Hornı´ Bory garnet peridotite (Table 1). It is evidentthat even such an enriched mantle cannot providesufficient heat to be responsible for the significantincrease of temperature within the lower crust (Fig. 7c).Similarly, we argue that the process of lithospheredelamination (simulated by instantaneous replacementof lithospheric mantle below 90 km by asthenospherein the model) cannot provide the necessary heat input.Results related to the mantle delamination process areshown by the dashed line in Fig. 7.A series of numerical experiments was set up to studythe influence of radioactive heat production locatedwithin the FLC (Fig. 7). Our calculations show thatthe temperature required for partial melting of micabearingfelsic crust located at a depth of 70 km (850–900 °C) are reached after 20 Myr for radioactive heatproduction of 2 lW m )3 and in 7 Myr for radioactiveheat production of 4 lW m )3 . At 60 km depth, whichis the assumed upper limit of the felsic layer, themelting temperature is reached in >50 Myr and inÓ 2010 Blackwell Publishing Ltd190