Geochronology of accessory allanite and monazite in the Barrovian metamorphic sequence of the Central Alps, Switzerland

Abstract

15 western part of the Lepontine dome. In the central area some of the classical kyanite--40 staurolite--garnet schists directly underlying the metamorphosed Mesozoic sediments 41 contain monazite that records only a pre--Alpine, Variscan metamorphic event of upper 42 greenschist to lower amphibolite facies--conditions dated at 330 Ma. 43 The new age data provide evidence that nappe stacking at prograde amphibolite--44 facies conditions and refolding of the nappe stack occurred between 32 and 27 Ma, only 45 a few million years after eclogite--facies metamorphism in the Adula--Cima Lunga unit. 46 Amphibolite--facies metamorphism lasted for about 10 My to 22 Ma, allowing for 47 multiple ductile deformation and recrystallization events. The long lasting amphibolite--48 facies metamorphism requires fast cooling between 20 and 15 Ma in the Central Alps. 49 This fast cooling was not related to an increase in sedimentation rates in the foreland 50 basins, suggesting that tectonic exhumation was responsible for termination of 51 amphibolite--facies metamorphism in the Lepontine dome. 52 53 Keywords 54 Accessory minerals; Barrovian metamorphism; Central Alps; ion--microprobe; 55 metamorphic petrology; SHRIMP; U--Pb dating. 56 concentration of initial Pb incorporated into the mineral during crystallisation (e.g. 81 Romer and Siegesmund, 2003). The problem of initial Pb in allanite has been addressed 82 by Gregory et al. (2007) and Smye et al. (2014). It has been demonstrated that 83 regression in a Tera--Wasserburg diagram or the Th--isochron technique (Gregory et al., 84 2007) eliminates the need to assume the isotopic composition of initial Pb and allows 85 determination of geologically significant ages even for allanite with high amounts of 86 initial Pb (Janots and Rubatto, 2014). Additional issues with allanite are the relative 87 mobility of Th and U resulting in Th--U fractionation (Smye et al., 2014). Monazite and 88 allanite potentially incorporate excess of 206 Pb produced by the decay of 230 Th, which 89can compromise the 206 Pb/ 238 U ratios. Therefore, calculation of isotopic ages and their 90 interpretation for these minerals demand special attention to these complications. 91At amphibolite facies conditions, the stable Ti--bearing phase can be rutile, titanite 92 or ilmenite; their relative stability is controlled by pressure, temperature and bulk rock 93 composition. Rutile has the advantage that it generally does not incorporate initial Pb at 94 formation, but can contain very low U concentrations that render precise measurement 95 of radiogenic isotopic ratios difficult (Kooijman et al., 2010, Warren et al., 2012 Smye et 96 al., 2014). 97

The Maggia nappe: an extruding sheath fold basement nappe in the Lepontine gneiss dome of the Central Alps

“…Most of the acicular or prismatic hornblende, staurolite and kyanite synkinematic crystals are oriented parallel to L2-3 stretching lineations, others postkinematic are non-oriented (Allaz et al 2005). Maximum pressures reaching 8-9 kbars and temperatures of 600-700°C where attended in the Maggia region (Todd & Engi, 1997;Allaz et al, 2005;Brower et al, 2005;Burri et al 2005;Boston et al 2017).…”

“…7, Burri et al, 2005;Steck et al, 2013). In situ migmatites were dated in the Bellinzona region by the age of Cenozoic synkinematic leucosome veins of the high amphibolite facies grade metamorphism of the K-feldspar-silimanite zone at 32-22 Ma with U-Pb SHRIMP zircon ages by Rubatto et al (2009), allanite U-Pb ages by Gregory et al (2012) and U-Pb monazite ages by Boston et al (2017). These Cenozoic migmatites are of the same age as cross cutting crustal derived pegmatites and aplites (Romer et al, 1996;Schärer et al, 1996).…”

Deciphering tectonic, eustatic and surface controls on the 20 Ma-old Burdigalian transgression recorded in the Upper Marine Molasse in Switzerland

“…During the Burdigalian, a short phase of fast exhumation (Boston et al, 2017;Schlunegger and Willet, 1999), which 30 occurred in the Lepontine area situated in the central Alps of Switzerland (Spicher, 1980), was associated with an uplift pulse of the Aar-massif situated farther north Fig. 2).…”

“…These processes are considered to have maintained the isostatic equilibrium between the crust and the subducted lithospheric mantle and thus the elevation of the topography (Schlunegger and Kissling, 2015), and they most likely balanced, through the stacking of the crustal root (Fry et al, 2010, Fig . 1b), the fast removal of upper crust material in the Lepontine area at that 5 time (Schlunegger and Willet, 1999;Boston et al, 2017). Delamination of crustal material was also invoked to explain the rapid exhumation and northward thrusting of the Aar-massif along steeply dipping thrusts at c. 20 Ma .…”

“…At 20 Ma, the phase of nappe stacking was superseded by a period of vertical extrusion of the Lepontine dome (Hurford, 1986) and the Aar-10 massif . This was also the time, when the slip along the Simplon detachment fault occurred at the fastest rates, resulting in the fast exhumation of deeper crustal levels to the surface (Mancktelow, 1985;Mancktelow and Grasemann, 1997;Boston et al, 2017).…”