Heat sources and trigger mechanisms of exhumation of HP granulites in Variscan orogenic root

Abstract

The structure of the Moldanubian domain is marked by felsic granulites of Ordovician protolith age forming the cores of domes that are separated from mid-crustal Neoproterozoic and Palaeozoic metasedimentary rocks that occur in synclines by a late Ordovician to Silurian metabasic unit. Reflection and refraction seismic sections combined with gravity inversion modelling suggest the presence of a low density layer at the bottom of the crust (interpreted as felsic granulite) overlain by a denser layer (interpreted as amphibolite) with layers of intermediate density at the top (interpreted as metasedimentary rocks). It is proposed that the granulite domes surrounded by middle crustal rocks reflect transposed horizontal layering originally similar to that preserved in the deep crust and imaged by the geophysical surveys. This geological and geophysical structure is considered to be a result of Vise´an gravity redistribution initiated by radioactive heating of felsic crust tectonically emplaced at the bottom of a Palaeozoic orogenic root. The radioactive layer with heat production of 4 lW m )3 corresponds geochemically and isotopically to Ordovician felsic metaigneous rocks of the Saxothuringian domain that have been emplaced at Moho depth under thickened crust during late Devonian-early Carboniferous continental subduction. Part of the continental crust continued to be subducted and produced fluids ⁄ low-volume melts which directly contaminated and enriched the local lithospheric mantle by lithophile elements, most notably Cs, Rb, Li, Pb, U, Th and K. Thermal incubation of 10-15 Myr was sufficient to heat and convert the underplated felsic layer into granulites via dehydration melting and melt segregation. The process of melt loss was responsible for the removal of radioactive elements and for switching off the heat at the beginning of the exhumation process. At the same time, the metasomatized underlying mantle was heated producing characteristic ultrapotassic magmas. Gravitational instability was then induced by the density contrast between the light granulites and the overlaying denser mafic lower crustal layer and a viscosity drop related to thermal weakening and partial melting of the latter.

Role of strain localization and melt flow on exhumation of deeply subducted continental crust

“…Such a small difference suggests a relatively similar protolith. However, depletion in the incompatible elements can be considered as a result of partial melting of granitic sources, responsible for dissolution of accessory, REE-bearing minerals such as zircon, monazite, xenotime, and apatite in melt and its escape from the system (Janoušek et al, 2004;Lexa et al, 2011). We note that similar chemical changes can result from hydrous fluid percolation (Marquer et al, 1994;Goncalves et al, 2012).…”

How well do pseudosection calculations reproduce simple experiments using natural rocks: an example from high-P high-T granulites of the Bohemian Massif

“…Felsic granulites with the assemblage quartz + ternary feld-1.2 High-/high-granulites in the Bohemain Massif P T spar (mesoperthite) + garnet + rutile ± kyanite are a prominent lithology of the Moldanubian domain (Schulmann et al, 2005(Schulmann et al, , 2008Guy et al, 2010;Lexa et al, 2011). The protolith rock was most likely an effectively dry granite (e.g.…”

Uranium distribution in the Variscan Basement of Northeastern Sardinia

“…Several processes might have enhanced the Variscan geotherm, such as (i) shear heating Maino et al, 2015), (ii) the advection of hot, mantle-derived melts, (iii) the break-off of the mantle lithosphere (Li, Faure, & Lin, 2014) or (iv) the selective enrichment of radiogenic heat-producing elements, such as U, Th and K, in the crust (Lexa et al, 2011). The widespread late-Variscan magmatism in the Corsica-Sardinia Batholith (C-SB) has been occasionally explained in terms of enhanced radiogenic heating.…”