Three-dimensional isograds from the Lukmanier Pass, Switzerland, and their tectonic significance

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Abstract
Four mineralogical zones, corresponding to the following ‘maximum’ metapelitic assemblageszone 1: chloritoid-chlorite-kyanitezone 2: staurolite-chloritoid-chlorite-kyanitezone 3: biotite-staurolite-chloritoid-chlorite-kyanitezone 4: biotite-staurolite-garnet-chlorite-kyanitehave been mapped in the Ultrahelvetic rocks of the Lukmanier area in Switzerland. Petrographic study suggests that, despite the ubiquity of ‘disequilibrium’, the zone boundaries accord with the simple univariant equilibriachloritoid+kyanite = staurolite+chlorite+quartz+H2Ochloritoid+chlorite+muscovite = staurolite+biotite+quartz+H2Oand chloritoid+muscovite+quartz = staurolite+garnet+biotite+H26Using structural contours, minimum isogradic dips of 35° north have been obtained.The Lukmanier parageneses require the Lepontine phase of Alpine metamorphism to have had a geotherma! gradient distinct from both Alpine blueschists and the Bergell contact metamorphites. The high minimum value for isograd and isotherm orientation here points to a convective heat source for the Lepontine event.