ALPINE AND PRE-ALPINE MAGMATISM IN THE ROOT-ZONE OF THE WESTERN CENTRAL ALPS

The highest grade of metamorphism and associated structural elements i n orogenic belts may be inherited from earlier orogenic events. We ill ustrate this point using magmatic and metamorphic rocks from the south ern steep belt of the Lepontine Gneiss Dome (Central Alps). The U-Pb z ircon ages from an anatectic granite at Verampio and migmatites at Cor capolo and Lavertezzo yield 280-290 Ma, i.e., Hercynian ages. These ag es indicate that the highest grade of metamorphism in several crystall ine nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine metamorp hism reached sufficiently high grade to reset the Rb-Sr and K-Ar syste matics of mica and amphibole, but generally did not result in crustal melting, except in the steep belt to the north of the Insubric Line, w here numerous 29 to 26 Ma old pegmatites and aplites had intruded syn- and post-kinematically into gneisses of the ductile Simplon Shear Zon e. The emplacement age of these pegmatites gives a minimum estimate fo r the age of the Alpine metamorphic peak in the Monte Rosa nappe. The U-Pb titanite ages of 33 to 31 Ma from felsic porphyritic veins repres ent a minimum-age estimate for Alpine metamorphism in the Sesia Zone. A porphyric vein emplaced at 448 +/- 5 Ma (U-Pb monazite) demonstrates that there existed a consolidated Caledonian basement in the Sesia Zo ne.