Garnet compositional re-adjustment: cooling rate constraint in metapelites from the Lepontine dome (Central European Alps)

This contribution focuses on the timing of metamorphism within the Lepontine dome, located in the Penninic domain of the Central European Alps (Switzerland). The Lepontine dome is formed by crystalline basement nappes bent towards the south in the migmatites of the Southern Steep Belt. The Lepontine nappes are formed by metamorphic rocks, mainly ortho- and para-gneisses, whose foliation dip-direction together with the attitude of thrust sheets define a dome shape. The Lepontine dome is characterized by a widespread Barrovian metamorphism of Tertiary age whose expressions are: an asymmetric concentric zonation of mineral-zone boundaries, locally dissecting the tectonic nappe contacts, and a NW-SE directed mineral and stretching lineation developed during peak metamorphic conditions, which suggests non-coaxial deformation during thrusting.

In a recent work, we dated the upper amphibolitic non-coaxial deformation. We performed U-Pb zircon dating on multiple samples which resulted in two groups of ages at ca. 31 Ma and 22 Ma. We attribute the development of the amphibolite facies syn-kinematic metamorphism to the widespread-recorded event at 31 Ma. This time constraint still lacks of specific information on the duration of the temperature peak, the subsequent cooling and the nature of the cooling process. To solve the temporal character of the formation and evolution of the high-grade metamorphic rocks, we applied a method to determine cooling rates calculated using post-peak-T estimates as initial temperature in the metapelites of the Lepontine dome. We selected garnet-paragneisses from the core of the Lepontine dome at different levels in the nappe pile, being the structural lowest one at the base of the Simano nappe and the uppermost in the Cima Lunga unit. Their mineral assemblage is marked by quartz, feldspar, garnet, biotite, white mica, kyanite, local staurolite, rutile and minor phases. Garnets are pre- to syn-kinematic with respect to the amphibolite facies metamorphic foliation. Furthermore, in the migmatitic paragneisses of the Southern Steep Belt we analysed one sillimanite-rich sample, where we found textural evidences of the presence of melt and k-feldspar.

We exploited garnet compositional re-adjustment due to major-element diffusion at the borders of the crystal to extract cooling rates, whose estimates where constrained by temperatures obtained via geothermometry and phase equilibria modelling. The post-peak temperatures of re-equilibration were estimated at ca. 600 ± 50 °C at the border garnet-biotite, where a step in garnet major element composition was seen. The diffusion time necessary to fit garnet-rim profiles along short transects (less than 1 mm length) was calculated as a preliminary result, giving a value < 2 Ma for most of the samples.

Note that a cooling time < 1 Ma is typical of transient thermal regimes, however the type of thermal regime can be properly evaluated only with the calculation of the cooling rate. High cooling rates are consistent with high temperatures in a localized area developed in a small time frame, such in the case of thrust-related shear heating during metamorphism. Slow cooling rates indicate instead a regional thermal history. Our preliminary results suggest high cooling rates for the high-grade metapelitic rocks of the Lepontine dome.