Termination of major ductile strike-slip shear and differential cooling along the Insubric line (Central Alps): UPb, RbSr and [formula omitted] ages of cross-cutting pegmatites

Abstract

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 40Ar 39Ar 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 87Sr 86Sr between 0.7199 and 0.7244 at the time of melting. High-temperature dextral strike-slip shear was active at 29.2 ± 0.2 (2σ) 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 ≈ 500°C, leading to fast initial cooling, followed by slower cooling to ≈ 350°C within several million years. In one case, initial cooling to ≈ 500°C was significantly delayed by about 4 m.y., with final cooling to ≈ 300°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°/km.