TERMINATION OF MAJOR DUCTILE STRIKE-SLIP SHEAR AND DIFFERENTIAL COOLING ALONG THE INSUBRIC LINE (CENTRAL ALPS) - U-PB, RB-SR AND AR-40 AR-39 AGES OF CROSS-CUTTING PEGMATITES/

To constrain the age of strike-slip shear, related granitic magmatism, and cooling along the Insubric line, 29 size fractions of monazite an d 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 muscovi te and biotite. The three pegmatitic intrusions analyzed truncate high -grade metamorphic mylonite gneisses of the Simplon shear zone, a majo r Alpine structure produced in association with dextral strike-slip mo vements along the southern edge of the European plate, after collision with its Adriatic indenter. Pegmatites and aplites were produced betw een 29 and 25 Ma in direct relation to right-lateral shear along the I nsubric line, by melting of continental crust having Sr-87/Sr-86 betwe en 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 term inated before 26.4 +/- 0.1 Ma. During dike injection, temperatures in the country rocks of the Isorno-Orselina and Monte Rosa structural uni ts did not exceed approximate to 500 degrees C, leading to fast initia l cooling, followed by slower cooling to approximate to 350 degrees C within several million years. In one case, initial cooling to approxim ate to 500 degrees C was significantly delayed by about 4 m.y., with f inal 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 loca lities was non-uniform in space and time, with significant variations on the kilometre scale. These differences are most likely due to stron gly varying heat flow, and/or heterogeneous distribution of unroofing rates within the continuously deforming Insubric line. If entirely asc ribed to differences in unroofing, corresponding rates would vary betw een 0.5 and 2.5 mm/y, for a thermal gradient of 30 degrees/km.