The highest grade of metamorphism and associated structural elements in
orogenic belts may be inherited from earlier orogenic events. We
illustrate this point using magmatic and metamorphic rocks from the
southern steep belt of the Lepontine Gneiss Dome (Central Alps). The
U-Pb zircon ages from an anatectic granite at Verampio and migmatites at
Corcapolo and Lavertezzo yield 280-290 Ma, i.e., Hercynian ages. These
ages indicate that the highest grade of metamorphism in several
crystalline nappes of the Lepontine Gneiss Dome is pre-Alpine. Alpine
metamorphism reached sufficiently high grade to reset the Rb-Sr and K-Ar
systematics of mica and amphibole, but generally did not result in
crustal melting, except in the steep belt to the north of the Insubric
Line, where numerous 29 to 26 Ma old pegmatites and aplites had intruded
syn- and post-kinematically into gneisses of the ductile Simplon Shear
Zone. The emplacement age of these pegmatites gives a minimum estimate
for 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
represent 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 Zone.