Th‐Pb age dating of zoned hydrothermal monazite from alpine‐type fissures/clefts is a powerful tool for constraining polyphase deformation at temperatures below 350°C and presents an alternative to K/Ar and 40Ar/39Ar dating techniques for dating brittle tectonics. This study considers the relationship between cleft orientations in ductile shear zones and cleft mineral crystallization during subsequent brittle overprinting. In the Grimsel area, located in the Aar Massif of the Central Alps, horizontal clefts formed during a primary thrust dominated deformation, while younger and vertically oriented clefts developed during secondary strike‐slip movements. The change is due to a switch in orientation between the principal stress axes σ2 and σ3. The transition is associated with monazite crystallization and chloritization of biotite at around 11.5 Ma. Quartz fluid inclusion data allow a link between deformation stages and temperatures to be established and indicate that primary monazite crystallization occurred in both cleft systems at 300–350°C. While cleft monazite crystallization ceases at ~11 Ma in inactive shear zones, monazite growth, and/or dissolution‐reprecipitation continues under brittle deformation conditions in vertical clefts during later deformation until ~7 Ma. This younger shear zone activity occurs in association with dextral strike‐slip movement of the Rhone‐Simplon fault system. With the exception of varying Th/U values correlated with the degree of oxidation, there is only limited compositional variation in the studied cleft monazites.
SIMS Th-Pb dating of hydrothermal fissure-vein monazite-(Ce) has the unique potential to date multiple tectonic events under low-grade metamorphic brittle/ductile conditions over large time frames. Monazites-(Ce) from brittle fault systems in the Eastern Alps allow us to constrain their Cretaceous activity over 20 Ma within single crystals, recording all major tectonic events. Eo-Alpine formation of the fluid-filled fissure-veins occurred 90 Ma ago at 352 ± 19°C and 342 ± 42 MPa. This corresponds to peak conditions during regional metamorphism of the Cretaceous collisional nappe stacking. Several stages of dissolution-reprecipitation/recrystallization record fault activity between 84 and 70 Ma. Corresponding fluid inclusions indicate conditions of 229 ± 10°C and 143 ± 20 MPa. This correlates with the formation of sedimentary basins during post-orogenic extension associated with strike-slip movements. The results strengthen the hypothesis that many large fault systems in the Eastern Alps developed during the Cretaceous orogeny and became reactivated during Neogene Alpine tectonics.
Abstract. Zoned hydrothermal monazite-(Ce) from Alpine-type fissures and clefts is used to gain new insights into the tectonic history of the Lepontine Dome in the Central Alps and the timing of deformation along the Rhone-Simplon Fault zone on the dome's western end. Hydrothermal monazites-(Ce) (re)crystallization ages directly date deformation that induces changes in physicochemical conditions of the fissure or cleft fluid. A total of 480 secondary ion mass spectrometry (SIMS) spot analyses from 20 individual crystals, including co-type material of the monazite-(Nd) type locality, record ages for the time of ∼19 to 2.7 Ma, with individual grains recording age ranges of 2 to 7.5 Myr.
The combination of these age data with geometric considerations and spatial distribution across the Lepontine region gives a more precise young exhumation history for the area. At the northeastern and southwestern edges of the Lepontine Dome, units underwent hydrothermal monazite-(Ce) growth at 19–12.5 and 16.5–10.5 Ma, respectively, while crystallization of monazite-(Ce) in the eastern Lepontine Dome started later, at 15–10 Ma. Fissure monazite-(Ce) along the western limit of the dome reports younger ages of 13–7 Ma. A younger age group around 8–5 Ma is limited to fissures and clefts associated with the Simplon normal fault and related strike-slip faults such as the Rhone Fault. The data set shows that the monazite-(Ce) age record directly links the fluid-induced interaction between fissure mineral and host rock to the Lepontine Dome's evolution in space and time.
A comparison between hydrothermal monazite-(Ce) and thermochronometric data suggest that hydrothermal monazite-(Ce) dating may allow us to identify areas of slow exhumation or cooling rates during ongoing tectonic activity.
Abstract. Thorium–lead (Th-Pb) crystallization
ages of hydrothermal monazites
from the western, central and eastern Tauern Window provide new insights
into Cenozoic tectonic evolution of the Tauern metamorphic dome. Growth
domain crystallization ages range from 21.7 ± 0.4 to 10.0 ± 0.2 Ma.
Three major periods of monazite growth are recorded between
∼ 22–20 (peak at 21 Ma), 19–15 (major peak at 17 Ma) and
14–10 Ma (major peak around 12 Ma), respectively, interpreted to be
related to prevailing N–S shortening, in association with E–W extension,
beginning strike-slip movements and reactivation of strike-slip faulting.
Fissure monazite ages largely overlap with zircon and apatite fission track
data. Besides tracking the thermal evolution of the Tauern dome, monazite
dates reflect episodic tectonic movement along major shear zones that took
place during the formation of the dome. Geochronological and structural data
from the Pfitschtal area in the western Tauern Window show the existence of
two cleft generations separated in time by 4 Ma and related to strike-slip
to oblique-slip faulting. Moreover, these two phases overprint earlier
phases of fissure formation.
Highlights.
In situ dating of hydrothermal monazite-(Ce). New constraints on the exhumation of the Tauern metamorphic dome. Distinct tectonic pulses recorded from east to west.
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