Th‐Pb ion probe dating of zoned hydrothermal monazite and its implications for repeated shear zone activity: An example from the Central Alps, Switzerland

Abstract

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.

The Late Paleozoic Tectonometamorphic Evolution of Patagonia Revisited: Insights From the Pressure‐Temperature‐Deformation‐Time (P‐T‐D‐t) Path of the Gondwanide Basement of the North Patagonian Cordillera (Argentina)

“…Th diffusion is approximately 3 times slower than Pb diffusion (Cherniak & Pyle, 2008), and therefore, the Th-U-Pb system should remain closed in monazite under low-grade metamorphic conditions. Nevertheless, monazite commonly records low-grade metamorphic and hydrothermal events (e.g., Bergemann et al, 2017;Just et al, 2010;Oyhantçabal et al, 2012;Poitrasson et al, 1996;Teufel & Heinrich, 1997). Such disturbances of the Th-U-Pb system seem to arise from mechanisms, such as neocrystallization, recrystallization, and dissolution/reprecipitation, instead from diffusive exchange (Cherniak et al, 2004;Cherniak & Pyle, 2008;Gardés et al, 2006;Harlov et al, 2011;Kelsey et al, 2008;Oriolo et al, 2018).…”

Constraining long‐term fault activity in the brittle domain through in situ dating of hydrothermal monazite

“…In situ SIMS or LA‐ICP‐MS dating of zoned hydrothermal cleft or fissure‐vein monazite‐(Ce) (hereafter simply monazite) offers the possibility to constrain the history of low‐grade metamorphic areas that experienced multiple phases of ductile to brittle deformation (e.g., Bergemann et al., ; Grand'Homme et al., ). The understanding of these areas is classically hampered by a variety of problems encountered when employing (thermo) chronological approaches based on Rb–Sr, K–Ar, Ar–Ar, fission‐track and U–Th/He techniques to polymetamorphic rocks.…”

“…Fissure‐vein monazite grows in fluid‐filled pockets with fluid‐enhanced reaction kinetics at temperatures at or below ~350°C (Bergemann et al., ; Gnos et al., ; Grand'Homme, ). It has been shown that strong tectonic movement disturbs the chemical equilibrium in the fluid‐filled pocket without deforming the crystals.…”

“…For this reason hydrothermal monazite generally provides crystallization ages for different growth domains (e.g., Janots et al., ). As long as conditions in the fissure remain within the monazite stability field, monazite will crystallize in an estimated temperature window of ~350 to ~200°C (Bergemann et al., ; Grand'Homme, ). Unlike in thermochronological systems, fluid‐enhanced heat advection in shear and damage zones will not disturb the Th–Pb isotopic system.…”

“…Hydrothermal monazite is best dated with the Th–Pb system, as U–Pb systems are commonly disturbed (Janots et al., ). Since monazite overgrowths on monazite and likely also crystallization are caused by tectonic activity, it has been used successfully for dating fault activity recorded in fissures located within damage zones surrounding master faults (Bergemann et al., ; Berger et al., ).…”