Direct dating of mid‐crustal shear zones with synkinematic allanite: new in situ U‐Th‐Pb geochronological approaches applied to the Mont Blanc massif

Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U‐Th‐Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA‐ICP‐MS U‐Th‐Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common‐Pb component. The use of measured Pb‐isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common‐Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.


Introduction
Resolving the timing and rates of crustal deformation is fundamental to our understanding of tectonic and orogenic processes. Direct dating of deformation features by isotopic techniques has great potential to provide new insights into the complexities of orogenesis and tectonics in general. Of particular interest are shear zones, which can accommodate vast lateral and vertical crustal motions, control the development of tectonic features, and provide important pathways for orogenic and oreforming fluids.
In contrast to constraining deformation ages through cross-cutting structural relationships, the direct-dating approach focuses upon isotopic analysis of synkinematic minerals that belong to the metamorphic assemblage associated with deformation. Previous efforts at directly dating deformation focused upon Rb-Sr dating of high Rb/Sr phases (e.g. Freeman et al., 1997) and 40 Ar/ 39 Ar dating of white mica, amphibole and feldspar (e.g. Simon-Labric et al., 2009). In situ 40 Ar/ 39 Ar dating of recrystallised micas by laser-heating techniques has successfully been applied to mylonites in low temperature conditions, but potential cmscale mobility of Ar and excess-Ar may compromise successful thermochronology (Kelley, 2002;Mulch et al., 2005). Texturally controlled Rb-Sr dating also has great potential, but is prone to resetting by postdeformation fluid circulation (e.g. Wickman et al., 1983) and isotopic disequilibrium on the thin-section scale (Frey et al., 1976).
Uranium and thorium rich accessory phases, such as zircon and monazite, provide robust ages in many geological settings. Although mechanical modification of zircons in shear zones is relatively common, chemical modifications, and hence age resetting, are not (e.g. Wayne and Sinha, 1992;Moser et al., 2009). To a lesser extent, U-Pb techniques have been successfully applied to dynamically recrystallised titanite and monazite (Resor et al., 1996;Storey et al., 2004). Another accessory phase that is a prime target for geochronology in these environments is allanite, which is a rare earth element (REE) rich end-member of the epidote solid solution series ([Ca, REE,Th] 2 [Fe,Al] 3 Si 3 O 12 [OH]).
The mineral is a key to the storage and mobility of REE, Th and U (Hermann, 2002;Gier e and Sorensen, 2004), and offers geochronological information that can be linked with physico-chemical conditions (e.g. pressure, temperature conditions), based upon petrological observations. Allanite is also commonly found associated with monazite (Janots et al., 2008). Epidote and clinozoisite generally surround allanite. As thermodynamic stabilities of these phases can be calculated, it is possible to make quantitative links between the timing of monazite/allanite/clinozoisite/epidote growth and PT conditions (e.g. Smye et al., 2010).
A number of studies have reported on the textural and chemical relationships of allanite to rock-forming minerals, and on evidence of its stability in PT-space (Gregory et al., 2009;Hermann, 2002;Janots et al., 2008;Gregory et al., 2012). Recent advances in allanite in situ U-Th-Pb dating yield reliable ages, despite the fact that allanite contains significant amounts of common-Pb (Gregory et al., 2007;Smith et al., 2009;Darling et al., 2012a). Whereas the petrology and geochemistry of allanite have been addressed repeatedly, little is known so far regarding the effects of deformation and recrystallisation on allanite U-Th-Pb systematics (Cenki-Tok et al., 2011).
Accordingly, we have investigated allanite-bearing samples from a greenschist facies shear zone of the Mont Blanc massif (MBM). The principal aims of this study are to: (i) investigate U-Th-Pb isotope systematics of synkinematic allanite using newly developed methods, taking into account the common-Pb composition reflecting the crystallisation environment; and (ii) test whether allanite may be used to directly date the age of deformation and metasomatism.

Geological setting of the Mont Blanc shear zones
The MBM belongs to a suite of Variscan external crystalline massifs of the western Alps (Fig. 1). It is composed of gneisses and a granitic batholith that crystallised at 300 AE 3 Ma (Bussy and von Raumer, 1994). The general deformation pattern of the MBM consists of sub-vertical narrow (1-50 m) shear zones, arranged in a fan-like geometry, separated by low strain domains (100-500 m). Deformation, strain localisation and associated upper greenschist facies metamorphism in the MBM have been considered Alpine in age (Rolland et al., 2008).
The majority of shear zones are transpressive and form a complex network of anastomosing NNE-SSW (N40-60°E) and N-S (N160-20°E) components with sub-vertical stretching lineations (Fig. 1). These two groups of shear zones have a dextral and sinistral component, respectively, resulting in a NW-SE compression regime (Rossi et al., 2005). In addition, domains of distinct mineral assemblages within shear zones can be recognised with an NW to SE zoning ( Fig. 1). In the NW part, the dominant assemblage is epidote, quartz and muscovite, in the central part, phlogopite, chlorite and quartz are present, and in the SE part, phengite dominates.

Microstructure and petrology of the sample
This study focuses on a greenschist facies shear zone in the chlorite-and phlogopite-bearing domain (central domain of Fig. 1). This shear zone may be observed along-strike inside the Mont Blanc tunnel, emphasising its regional two-dimensional exposure. Lineations are sub-vertical and shear sense indicators reflect exclusively pure shear, i.e. horizontal shortening. The main mylonitic foliation is marked by chlorite, elongated K-feldspars and recrystallised quartz. Locally, phlogopite is present as larger crystals, and albite crystals occur in low strain parts of the sections of the shear zone. Thermodynamic phase equilibria indicate that shear zone recrystallisation occurred at 0.51 AE 0.05 GPa and 400 AE 25°C (Rolland et al., 2003).
The contact between high-strain domains and low-strain granite pods is anastomosing and finger-like. In addition, a low Fe-content of fabric minerals is associated with high bulkrock Mg/(Mg + Fe) values compared to the unaltered, undeformed granite.   These evidence suggest a localised fluid alteration front associated with greenschist facies metamorphism. These are Mg-rich fluids percolating upwards in the core of the MBM, with high fluid/rock ratios (Rossi et al., 2005).
Small (<100 μm), newly crystallised REE-rich phases are parallel to, or occasionally overgrow, the main greenschist facies mylonitic foliation (Fig. 2). New aeschynite and elongate allanite crystals are idioblastic and contain chlorite, albite and uraninite inclusions. In addition, there are clear differences in zoning, texture, composition and REE patterns between allanite in the host granite and in the shear zone (Rolland et al., 2003), with the shear zone grains being homogeneous in major and REE elements composition (Fig. 3). These observations indicate that, texturally, allanite is not inherited, is in equilibrium with the mylonitic assemblage and therefore synkinematic.

Allanite dating methods
Laser ablation (LA)-ICP-MS U-Th-Pb isotope analyses were undertaken at the University of Portsmouth, using a New Wave 213 nm Nd:YAG laser coupled with an Agilent 7500cs ICP-MS. Analytical protocols and instrument conditions are described in detail by Darling et al. (2012a). Key points of the methodology are: (i) line-raster ablation, in order to minimise time-dependent elemental fractionation; and (ii) external normalisation to the zircon standard Ple sovice (Slama et al., 2008); (iii) the use of measured 204 Pb to correct for inherited common-Pb. Accuracy was monitored via analyses of the allanite reference materials Tara, Mucrone, BONA and SISS (von Blanckenburg, 1992;Gregory et al., 2007;Cenki-Tok et al., 2011), for each of which mean common Pb-corrected 208 Pb/ 232 Th ages are within uncertainty of reference values, with uncertainties of 0.5-1.5% (2r; Table 1).
Pb-isotope measurements of albite and chlorite were undertaken in a polished block of the studied sample at the University of Bristol, using a New Wave Research 193 nm ArF Excimer laser coupled with a Ther-moFinnigan Neptune multi-collector (MC)-ICP-MS. Analytical procedures followed those of Foster and Vance (2006) and Darling et al. (2012b

Allanite U-Th-Pb systematics
The results of allanite analyses are detailed in Table 2, and key points are summarised in Fig. 4. The measured grains typically have high common-Pb contents, with measured 206 Pb/ 204 Pb and 208 Pb/ 204 Pb varying from 32-184 to 60-474 respectively. A Tera-Wasserburg type concordia plot (Fig. 4A) highlights the dominance of common-Pb. The regression has a poorly defined lower intercept of 21 AE 18 Ma (all uncertainties 2r, unless otherwise stated), but interestingly has a very low y-intercept (0.41 AE 0.02), which reflects the 207 Pb/ 206 Pb composition of the common-Pb component (Tera and Wasserburg, 1972). This value is far removed from model terrestrial Pbisotope evolution curves (total 207 Pb/ 206 Pb range 0.84-1.11; Stacey and Kramers, 1975).
The isotopic composition of common-Pb in the metamorphic assemblage associated with allanite was   (Fig. 4B). These ratios were age corrected to 28.2 AE 2.6 Ma (the allanite Th-Pb isochron age; Fig. 4C) using measured 238 U/ 204 Pb and 232 Th/ 204 Pb, although the magnitude of this correction is less than analytical uncertainty in albites due to low 238 U/ 204 Pb (<5.1) and 232 Th/ 204 Pb (<0.11). The single measurement of chlorite has 206 Pb/ 204 Pb i , 207 Pb/ 204 Pb i and 208 Pb/ 204 Pb i values within uncertainty of the albite mean. Two further analyses of chlorite were rejected due to ablation through Pbrich inclusions and very low Pb concentration in one case. Importantly, these measured albite and chlorite values are within uncertainty of the initial 208 Pb/ 204 Pb and 206 Pb/ 204 Pb values provided by the Th-Pb and U-Pb isochrons (Fig. 4C-D). As suggested by textural and geochemical evidence, this indicates that allanite is in equilibrium with the mylonitic assemblage and offers new opportunities for accurate common-Pb correction.
Previous studies have advocated the use of assumed common-Pb compositions taken from model terrestrial Pb-isotope evolution curves (Stacey and Kramers, 1975), in order to correct Pb-isotope signals for common-Pb in magmatic allanite (e.g. Gregory et al., 2007). However, as shown in Figure 4E, Th/Pb ages corrected using a Stacey and Kramers (1975) composition are scattered and do not define a single age population. In contrast, when the measured Pb-isotope composition is used for correction, the data define a single age population, with a weighted mean of 29.44 AE 0.95 Ma (MSWD = 0.85; n = 30; Fig. 4F). This value is within uncertainty of the Th-Pb isochron age for these data (28.2 AE 2.6 Ma; Fig. 4C), which is independent of common-Pb correction. A necessary pre-requisite for this approach is that metamorphic allanite, chlorite and albite have     Terra Nova,Vol 26,No. 1,[29][30][31][32][33][34][35][36][37] Pb/ 238 U ages are highly variable, and significantly older than Th-Pb ages. The slope of the U-Pb isochron (Fig. 4D) also provides a significantly older age (122 AE 32 Ma). Two lines of evidence suggest that the 238 U-206 Pb system is compromised by excess 206 Pb, either from 230 Th disequilibrium (e.g. Sch€ arer, 1984;von Blanckenburg, 1992), inherited radiogenic Pb from a U rich precursor (Romer and Siegesmund, 2003), labile 206 Pb from another source, or a combination of these factors: (i) the initial 206 Pb/ 204 Pb provided by the U-Pb regression is within uncertainty of the composition of albite and chlorite, suggesting variable levels of 206 Pb incorporation into different grains of allanite; and (ii) the low 207 Pb/ 206 Pb intercept of the Tera-Wasserburg regression (0.41 AE 0.02) compared to the albite-chlorite initial value (0.540 AE 0.014). Furthermore, there is a positive correlation between the Th/U ratios and 206 Pb/ 238 U ages of the allanite population, which suggests that U-Th fractionation during crystallisation of allanite, causing initial 230 Th disequilibrium (t 1/2 = 75 kyr; e.g. Sch€ arer, 1984;von Blanckenburg, 1992), may be the dominant control on excess 206 Pb.

Discussion and conclusion
Significant improvement in understanding the petrology and geochemistry of allanite was made in the past decade (e.g. Janots et al., 2008), but the effects of deformation and recrystallisation on allanite U-Th-Pb systematics have as yet been little studied. A first attempt to test whether allanite may be used to infer the age of mylonitisation revealed that the mineral can be remarkably resistant to deformation in relatively dry conditions at eclogite facies due to mechanical shielding that prevents chemical equilibration (Cenki-Tok et al., 2011). This study shows that new crystallisation of allanite may occur in shear zones associated with fluid flux, which can efficiently reset U-Th-Pb isotopic ratios. In the studied shear zone, allanite texturally belongs to the greenschist facies assemblage. As shown by recent studies (e.g. Janots et al., 2008), allanite is expected to be stable at these PT conditions (ca. 0.5 GPa and 400°C, Rolland et al., 2003). The U-Th-Pb closure temperature of allanite is estimated to be above 700°C (Heaman and Parrish, 1991), because: (i) allanite has been shown to remain close to Pb loss and retain trace element and Sr-Nd isotope zonation during prolonged magmatic conditions (Oberli et al., 2004;Gregory et al., 2009);and (ii) in general zoning patterns developed in allanites during prograde metamorphic growth may be retained through peak conditions (Janots et al., 2008). Due to the high closure temperature of allanite, the age of this study is interpreted as a crystallisation age that records shear zone activation under greenschist facies conditions.
The allanite U-Th-Pb in situ isotope data from this study also highlight the importance of using measured Pb-isotope compositions for common-Pb, particularly in metamorphic rocks in which several processes may fractionate Th/Pb, U/Pb or Th/U. Measured albite and chlorite indicate that fluids associated with the metasomatic event (Rossi et al., 2005) had highly radiogenic Pb-isotope compositions. Similar fluid compositions have already been recognised in this zone of the MBM (Marshall et al., 1998). These were interpreted as being related to the emplacement of the Penninic Front that tapped fluid from deep crustal and mantle sources (Rossi et al., 2005). Indeed, metasomatised rocks with similar d 13 C calcite ratios may also be found at the Penninic Front itself (Rossi et al., 2005).
In summary, the mean Th-Pb age of 29.4 AE 1.0 Ma is taken as the crystallisation age of allanites in the studied shear zone, and hence the age of deformation and fluid percolation. In the SE domain of the MBM (Fig. 1), shear zones yielded younger 40 Ar-39 Ar crystallisation ages (16 Ma; Rolland et al., 2008). The diachroneity revealed by these two studies highlights a succession of previously unrecognised events in the MBM including: (i) ductile deformation and fluid percolation at c. 29 Ma ascribed to activation of the Penninic Front, which is also recognised further to the South in the Pelvoux Massif (Simon-Labric et al., 2009); and (ii) reactivation of the shear zones at 16-14 Ma is ascribed to the onset of exhumation in relation with the rotation of Apulia (Rolland et al., 2012).
More generally, allanite may be found in a wide variety of lithologies (felsic, pelitic and mafic). It is a petrologically important mineral in greenschist to amphibolite grade rocks typical of the upper-mid crust, particularly when found together with monazite. As its closure temperature is well above these moderate mid-crustal temperatures (ca. 700°C; Oberli et al., 2004), allanite ages in these environments are likely to record crystallisation rather than cooling. Allanite therefore helps us understand the timing and rates of low to medium temperature processes, which are known to be difficult to date.