Paleocene metamorphism along the Pennine–Austroalpine suture constrained by U–Pb dating of titanite and rutile (Malenco, Alps)

Abstract

We present in situ rutile and titanite U–Pb geochronology for three samples from the Ur breccia, which forms the boundary between the Malenco unit and the Margna nappe (Eastern Central Alps) near Pass d’Ur in southeast Switzerland. These sampled both oceanic brecciated material and a blackwall reaction zone in contact with a micaschist and serpentinized peridotite. Peak temperatures during Alpine metamorphism in these units were ~ 460 ± 30 °C. Textural observations combined with new geochronological data indicate that rutile and titanite both grew below their closure temperatures during Alpine metamorphism. We present a technique to calculate the most precise and accurate ages possible using a two-dimensional U–Pb isochron on a Wetherill concordia. Rutile from two samples gave a U–Pb isochron age of 63.0 ± 3.0 Ma. This age conflicts with previous 39Ar–40Ar data on heterogeneous amphiboles from which an age of 90–80 Ma was inferred for the high pressure part of the Alpine evolution, but is consistent with K–Ar ages and Ar–Ar ages on phengitic white mica. Titanite from three samples gave a U–Pb isochron age of 54.7 ± 4.1 Ma. This age is consistent with Rb–Sr isochron ages on mylonites along and in the footwall of the Lunghin–Mortirolo movement zone, a major boundary that separates ductile deformation in the footwall from mostly localized and brittle deformation in the hangingwall. Our ages indicate a Paleocene rather than upper Cretaceous metamorphism of the Pennine–Austroalpine boundary and permit at most ~ 15 Myr, and possibly much less, between the growth of rutile and titanite.

Deformation along the roof of a fossil subduction interface in the transition zone below seismogenic coupling: The Austroalpine case and new insights from the Malenco Massif (Central Alps)

“…Putting together recent findings in the broader Val Malenco area (e.g., Droop and Chavrit, 2014;Picazo et al, 2019) and the P-T-t results of this study, we highlight the existence, during the geodynamic evolution of the Central Alps, of a continuous subduction and underplating process along the active subduction interface, from the upper nappes (Austroalpine) toward the deeper ones (Malenco). The Val Malenco area sheds light onto a region below the seismogenic depths exposed in Arosa-St. Moritz and displays subduction-related deformation at a depth range in the transition zone, which is globally found to host both slow creep as well as more rapid silent slip events.…”

“…Since there is no correlation between mica grain size and apparent age for this sample, we consider this a robust age. Moreover, it is identical to the age obtained by Bachmann et al (2009b) for the youngest movements at the plate interface farther north in southern Switzerland and relatively close to the ages obtained by Picazo et al (2019). It has been particularly difficult to derive meaningful age information for the crystalline basement rocks, since the various mineral fractions separated reveal significant disequilibrium, including the low-Rb/ Sr phases such as apatite, albite, or titanite.…”

“…Rb/Sr geochronology on mica-rich metasediments deposited during Jurassic rifting and later metamorphosed during Alpine convergence yields a robust age of deformation and (re)crystallization of 48.9 ± 0.9 Ma. However, our Rb/Sr as well as 40 Ar/ 39 Ar dating of some Alpine, seemingly recrystallized white micas from mylonites and one foliated cataclasite of the crystalline basement reveal a wide range of ages, which, despite falling within the range obtained by previous studies (Handy et al, 1996;Villa et al, 2000;Picazo et al, 2019), can only be considered as mixed ages due to partial resetting of the isotopic signature of the system.…”

“…Mohn et al (2011) suggest that deformation observed in the rocks of the Malenco ultramafics, as well as of S Platta, Margna, and Sella units, is possibly related to pressure-dominated metamorphism of P ~0.5-0.6 GPa (Bissig and Hermann, 1999). Despite relatively large uncertainties, Picazo et al (2019) recently obtained ages of 63.0 ± 3.0 Ma for pressure metamorphism associated with nappe stacking and 54.7 ± 4.1 Ma for possibly peak temperature Alpine metamorphism.…”

“…Ductile deformation in the Malenco Massif led to pervasive mylonitic deformation of antigorite schists. The strong compositional and deformational gradient along the basal Margna shear zone between the crustal and mantle materials locally led to the formation of a metasomatized tectonic mixing zone comprising lenses of mafic tuffs, submetric to metric lenses of metasediments, as well as minor volumes of tremolite schists, and in parts deformed basement rocks exhibiting foliated cataclasites (named "breccias" in Picazo et al, 2019).…”