To constrain the age of strike-slip shear, related granitic magmatism,
and cooling along the Insubric line, 29 size fractions of monazite and
xenotime were dated by the U-Pb method, and a series of 25 Rb-Sr and
Ar-40/Ar-39 ages were measured on different size fractions of muscovite
and biotite. The three pegmatitic intrusions analyzed truncate
high-grade metamorphic mylonite gneisses of the Simplon shear zone, a
major Alpine structure produced in association with dextral strike-slip
movements along the southern edge of the European plate, after collision
with its Adriatic indenter. Pegmatites and aplites were produced between
29 and 25 Ma in direct relation to right-lateral shear along the
Insubric line, by melting of continental crust having Sr-87/Sr-86
between 0.7199 and 0.7244 at the time of melting. High-temperature
dextral strike-slip shear was active at 29.2 +/- 0.2 (2 sigma) Ma, and
it terminated before 26.4 +/- 0.1 Ma. During dike injection,
temperatures in the country rocks of the Isorno-Orselina and Monte Rosa
structural units did not exceed approximate to 500 degrees C, leading to
fast initial cooling, followed by slower cooling to approximate to 350
degrees C within several million years. In one case, initial cooling to
approximate to 500 degrees C was significantly delayed by about 4 m.y.,
with final cooling to approximate to 300 degrees C at 20-19 Ma in all
units. For the period between 29 and 19 Ma, cooling of the three sample
localities was non-uniform in space and time, with significant
variations on the kilometre scale. These differences are most likely due
to strongly varying heat flow, and/or heterogeneous distribution of
unroofing rates within the continuously deforming Insubric line. If
entirely ascribed to differences in unroofing, corresponding rates would
vary between 0.5 and 2.5 mm/y, for a thermal gradient of 30 degrees/km.