Alpine and pre-Alpine magmatism in the root-zone of the western Central Alps |
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| Authors: | R L Romer U Schärer Albrecht Steck |
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| Institution: | (1) Laboratoire de Géochronologie, Université Paris 7 et IPG-Paris, 2 place Jussieu, F-75251 Paris Cedex 05, France, FR;(2) Institut de Minéralogie, Université Lausanne, BFSH2, CH-1015 Lausanne, Switzerland, CH |
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| Abstract: | 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.
Received: 23 May 1995/Accepted: 12 October 1995 |
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