Timing of HP/HT alpine metamorphism: new data from Cima di Gagnone (Central Alps)

Keywords: HP-HT metamorphism, microstructures, U-Pb-Th dating, P-T-t-d path.

The occurrence of (ultra)high pressure and high temperature mineralogical assemblages developed during the Alpine phases makes the Cima di Gagnone area (Cima Lunga unit) one of the most studied area in the Central Alps. It consists of continental basement rocks (orthogneisses, paragneisses and metapelites) enveloping (ultra-) mafic bodies of oceanic crust (eclogite, amphibolites and peridotites) which record pressure and temperature up to 3 GPa and 800 °C, respectively (e.g. Nimis and Trommsdorff, 2001; Scambelluri et al., 2015). This high-grade metamorphism is constrained between 40 and 35 Ma by U-Pb dating from the ultra-mafic and mafic rocks (e.g. Gebauer, 1999). The metamorphism peak of the surrounding gneiss complex is instead constrained at considerably lower conditions (up to 0.8 GPa and 660 °C; Grond et al., 1995). The temperature peak in the felsic rocks is dated at ca. 32 Ma (Gebauer, 1996), coeval with the Bergell emplacement. Several models have been proposed to explain the coupling between ultrahigh- and middle- pressure rock pairs resulting in a large uncertainty in the adopted subduction-exhumation models.

We performed new petrological, micro-structural and geochronological data from the gneissic rocks, with the aim to investigate how the pressure and temperature conditions experienced by the felsic and mafic rocks are truly different. We explored the spatial variation of the metamorphic record through sample collection the structural control of the inclusion-matrix couples. Petrological and microstructural (SEM-EBSD) analyses are performed to define the deformation and metamorphic patterns of samples collected. Our results indicate that some portions of the gneissic matrix preserve relicts of higher pressure and temperature than previously suggested. The high-T conditions are temporally constrained by U-(Th)-Pb dating of monazite and zircon, which provides peak age estimations similar to the mafic rocks. The new data shed a light on heterogeneous metamorphism recorded by different rocks, providing new elements for the discussion on the most fitting geodynamic models.

REFERENCES

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