Constraining metamorphic dome exhumation and fault activity through hydrothermal monazite-(Ce)

Zoned monazite-(Ce) from Alpine fissures/clefts is used to gain new insights into the exhumation history of the Central Alpine Lepontine metamorphic dome, and timing of deformation along the Rhone-Simplon fault zone on the dome’s western termination. These hydrothermal monazites-(Ce) directly date deformation and changes in physiochemical conditions through crystallization ages, in contrast to commonly employed cooling-based methods. The 480 SIMS measurement ages from 20 individual crystals record ages over a time interval between 19 and 5 Ma, with individual grains recording ages over a 5 lifetime of 2 to 7.5 Ma. The age range combined with age distribution and internal crystal structure help to distinguish between areas whose deformational history was dominated by distinct tectonic events or continuous exhumation. The combination of this age data with geometrical considerations and spatial distribution give a more precise exhumation/cooling history for the area. In the east and south of the study region, the units underwent monazite-(Ce) growth at 19-12.5 and 16.5-10.5 Ma, followed by a central group of monazite-(Ce) ages at 15-10 Ma and the movements and related cleft monazites-(Ce) are youngest at the 10 western border with 13-7 Ma. A last phase around 8-7 Ma is limited to clefts of the Simplon normal fault and related strike slip faults as the Rhone and Rhine-Rhone faults. The large data-set spread over significant metamorphic structures shows that the opening of clefts, fluid flow and monazite-(Ce) stability is direct linked to the geodynamic evolution in space and time.

Th-Pb analyses were conducted at the Swedish Museum of Natural History (NordSIM facility) on a Cameca IMS1280 SIMS instrument. Analytical methods and correction procedures followed those described by Harrison et al. (1995), Kirkland et al. (2009) and Janots et al. (2012, using a -13kV O 2− primary beam of ca. 6nA and nominal 15µm diameter. The mass spectrometer was operated at +10kV and a mass resolution of ca. 4300 (M/∆M, at 10% peak height), with data collected in peak hopping mode using an ion-counting electron multiplier. Unknowns were calibrated against monazite-(Ce) standard  44069 (Aleinikoff et al., 2006). Lead isotope signals were corrected for common Pb contribution using measured 204 Pb and an assumed present-day Pb isotope composition according to the model of Stacey and Kramers (1975

Results
The complete ion-probe data set is given in the data Supplement

Hydrothermal monazite-(Ce) crystallization
Hydrothermal fissure monazite-(Ce) typically crystallizes at temperatures below 350 • C (Gnos et al., 2015;Bergemann et al., 2017Bergemann et al., , 2018 down to somewhere in the range of 200 • C or slightly below (e.g. Townsend et al., 2000). Crystallization and later reactions occur when the fissure fluid is brought into disequilibrium. This may be caused by tectonic events for a number in contact with the surrounding fluid. It is therefore not limited to grain rims, but commonly occurs along mineral inclusion interfaces, cracks and microcracks, that may be invisible in BSE images (Grand Homme et al., 2018).
These processes may be active as long as conditions in the cleft stay within the monazite-(Ce) stability field. Therefore, several (re-)crystallization or dissolution-precipitation cycles may occur over the active lifespan of a monazite-(Ce) crystal. Later reactions may be aided by secondary porosity and fracturing induced by the previous dissolution-reprecipitation/recrystallization 5 events, by bringing an increased crystal volume into direct contact with the fluid.

Monazite-(Ce) Th-Pb single and weighted mean ages
As detailed above, SIMS spot analyses were placed across the samples according to growth domains visible in BSE images ( Fig. 4). The derived spot ages were grouped together on the basis of chemical composition thought to represent crystallization under homogeneous chemical conditions, and spatial distribution across the sample according to zonation visible on BSE 10 images to calculate, whenever possible, weighted mean domain ages (Fig. 7). It appears that dissolution-precipitation may largely preserve the chemical composition of an affected crystal part, this would mean that areas with different chemical compositions may have reprecipitated simultaneously. Despite this, spots of different chemical groups were only in a few, clear cases grouped together for weighted mean age calculation. This is to avoid the risk of mistaking multiple mixing ages of different chemical domains as a distinct event. In areas that experienced few and discrete tectonic events, this approach allows ). However, large parts of the study area experienced more than two distinct deformation events and/or phases of prolonged activity. New growth on an existing crystal results in sharp boundaries between zones. But dissolution-reprecipitation processes may lead to irregularly shaped altered zones within a crystal, which may or may not be visible on a BSE image. If this happens multiple times the limited number of analyses per grain will result in many individual ages being discarded. Meaning that 20 events may not be recognized when looking only at the weighted mean ages. To avoid this, the entire dataset of each region was additionally plotted according to the number of ages per 0.5 Ma intervals to identify age clusters (Fig. 1, appendix). In the next step the peaks or plateaus of the age histogram were plotted according to their relative intensity. They were then combined with the weighted average ages (this study; Janots et al., 2012;Berger et al., 2013;Bergemann et al., 2017) to visualize distinct events or phases of tectonic activity (Fig. 8). As only a limited number of analyses are possible to obtain for each grain, some 25 weighted mean ages combine only a small number of individual ages. This is especially true for ages dating multiple late stage events that presumably happened at relatively low temperatures. In such cases only those weighted mean ages were kept whose geologic significance is also indicated by other dating techniques such as fault gouge dating, specifically close to the Rhone-Simplon line. Otherwise, these ages are included in the overall age range of the sample in question given in Tab. 2.
Another reason for a spread out age pattern may be a grain experiencing prolonged phases of low-intensity tectonic activity of  identify growth zones on BSE images (compare Gnos et al., 2015;Bergemann et al., 2018). As opposed to areas where crystals record individual, stronger deformation events that tend to show a sharper zonation (compare Janots et al., 2012;Berger et al., 2013;Bergemann et al., 2017Bergemann et al., , 2019.

Monazite-(Ce) ages and Lepontine history
Hydrothermal cleft monazite-(Ce) crystallization and dissolution-reprecipitation occurred over time in different parts of the 5 study region, as it passed through the monazite-(Ce) stability field. The time interval recorded within individual monazite-(Ce) crystals spans from 2.5 Ma to 7 Ma for individual grains (Fig. 7, Table ??). The recorded time interval within individual grains is generally longer in the South and East regions of the study area (Fig. 2). The total age range covers the time from ca. 19 to 5 Ma. The monazite-(Ce) chronologic record can be seen to start in the eastern-and southernmost regions (Fig. 9). The recorded activity then moves to the northeastern and central to the western area. Younger ages in the west progressively concentrate on Hydrothermal fissure monazite-(Ce) always dates crystallization and not cooling due to system closure and often shows complex recrystallization features. It provides an important record of the shifting tectonic activity associated with the regions exhumation history within the monazite stability field. A comparison between hydrothermal monazite-(Ce) samples from different parts of the Lepontine metamorphic dome shows that age clusters within individual crystals from a simply exhuming area 5 have a less clear age distribution than samples from fault zone areas, or fast exhuming areas. Monazite-(Ce) (re)crystallization/ dissolution-reprecipitation during exhumation is in these areas connected to repeated tectonic activity of small intensity, while distinct events or short periods of intense tectonic activity of fault zones appear to result in larger, more homogenous crystal zones that are easier to date.