Pb dating of monazite in fibrous veins: Direct dating of veins and deformation in the shallow upper crust of the Mexican Orogen

Journal of Structural Geology 124 (2019) 136–142

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In situ Th/Pb dating of monazite in fibrous veins: Direct dating of veins and deformation in the shallow upper crust of the Mexican Orogen

T

Elisa Fitz-Diaza,∗, John M. Cottleb, Maria Isabel Vidal-Reyesc, Ben van der Pluijmd a

Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, CDMX, Mexico UC-Santa Barbara, Santa Barbara, CA 93106, USA c Posgrado en Ciencias de la Tierra, Universidad Nacional Autónoma de México, Ciudad Universitaria, CDMX, Mexico d Department of Earth & Environmental Sciences, University of Michigan, 1100 North University Ave., Ann Arbor, MI, 48109-1005, USA b

A B S T R A C T

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (∼10 μm diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 °C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapán Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 ± 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.

1. Introduction Radiometric dating of mineral phases grown during deformation constrains the timing of tectonic events, and, with additional constraints (e.g. P, T, and fluid(s) composition), enables a comprehensive understanding of the nature and conditions of tectonic processes. In the shallow upper crust, there are relatively few mineral chronometers that grow during deformation. The most common datable phases in the shallow upper crust include: illite crystals in fault gouge and in bed parallel shear zones on fold limbs and dated by the encapsulated Ar-Ar illite method (van der Pluijm et al., 2001; Fitz-Díaz and van der Pluijm, 2013) or K-Ar (e.g. Surace et al., 2011; Garduño-Martínez et al., 2015); calcite, occurring as slickenfibers and vein fillings, and analyzed by either U-Th (Causse et al., 2004; Nuriel et al., 2012; Pickering, 2017) or U-Pb methods (e.g., Hansman et al., 2018; Parrish et al., 2018); hematite found as patinas on fault planes, and dated with (U-Th)/He to determine the age of fault slip related heating (Ault et al., 2015); and, monazite found in low-grade shale and sandstone (Rasmussen and Muhling, 2007, 2009) and in brittle structures highly influenced by hydrothermalism (e.g., the “Alpine” clefts; Janots et al., 2012; Berger et al., 2013; Gnos et al., 2015), and dated by U-Th/Pb. Monazite is a light rare-earth element (LREE)-bearing phosphate

typically hosted in igneous and medium-to high-grade metamorphic rocks (e.g., Parrish, 1990). Because it is a robust mineral that incorporates Th and U into its structure and little common Pb (SeydouxGuillaume et al., 2002), monazite has been used to constrain the age of magmatic, metamorphic and hydrothermal events (Pyle et al., 2001; Spear and Pyle, 2002; Aguado et al., 2005; Williams et al., 2007; Cottle et al., 2009). However, the use of monazite to define the timing of deformation at very low-grade metamorphic conditions has been underexplored (e.g., Rasmussen and Muhling, 2007, 2009). In part, this is due to the small size of monazite (less than tens of microns), and that, like zircon, monazite commonly preserves multiple age domains in single crystals (Harrison et al., 2002). This is in part due to the fact that monazite is physically robust, but not necessarily chemically robust in the presence of brines at low temperature (Seydoux-Guillaume et al., 2012). Therefore, to utilize monazite in dating low-temperature processes, it is necessary to describe its microstructures in host vein and rock, as well as characterize the internal structure of the monazite by electron beam methods and by high spatial resolution in situ dating (e.g., Williams and Jercinovic, 2002). By doing a careful structural characterization of monazite bearing veins and hydrothermal alterations, it has been possible to obtain not only meaningful geological ages of hydrothermal monazite (e. g., Schandl and Gorton, 2004) but also

∗

Corresponding author. E-mail addresses: [email protected] (E. Fitz-Diaz), [email protected] (J.M. Cottle), [email protected] (M.I. Vidal-Reyes), [email protected] (B. van der Pluijm). https://doi.org/10.1016/j.jsg.2019.04.004 Received 12 December 2018; Received in revised form 1 April 2019; Accepted 5 April 2019 Available online 13 April 2019 0191-8141/ © 2019 Published by Elsevier Ltd.

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Fig. 1. Location of the study area in the Mexican Orogen. The cross-section (A-A′) shows major fold in the Zimapan basin as well as the relative position of sampling sites in them. Modified from Fitz-Díaz et al. (2018).

82 ± 1 Ma and a second, less intense shortening event occurred 77 ± 1 Ma ago (Fitz-Díaz et al., 2014a). Due to excellent exposure, the inverted Zimapán Basin has served as a natural laboratory for a variety of studies including: development of the 39Ar-40∗Ar encapsulated illite dating method to constrain the age of folding to 84-74 Ma (Fitz-Díaz et al., 2014a); comparative stable isotope studies (δ2H from illite and fluid inclusion trapped in calcite a quartz from fold-related veins), showed that illite not only records the age of deformation but also preserves the composition of fluids active during deformation and those fluids are mostly formational (Fitz-Díaz et al., 2014b); paleomagnetic studies that indicated remagnetization was caused by intense interaction between aqueous fluids and rocks during folding of limestone layers and further, that such folding in the basin occurred in the Late Cretaceous (Nemkin et al., 2015). In addition to these constraints, the temperature of deformation in the basin has been constrained at 200–300 °C using illite crystallinity and microthermometry of fluid inclusions in fold-related veins (Gray et al., 2001; Fitz-Díaz et al., 2016). Together, these data make rocks of the Zimapán basin well-suited to study the new growth of monazite during deformation under anchizonal conditions. Because the age of folding in the basin is established independently, it is possible to validate the new monazite results. Rather than focusing on minerals formed within host-rock (Rasmussen et al., 2005; Rasmussen and Muhling, 2007, 2009), in this study we focus our analysis on minerals transferred via aqueous solution to syntectonic veins that are discrete markers of episodes of deformation.

significantly improving the constrains of orogenic processes associated to them (Rasmussen et al., 2005; Janots et al., 2008, 2012). In this study, we report new data from monazite in fold-related, fibrous, syntectonic quartz-calcite veins that were emplaced under anchizonal conditions (200–250 °C) in volcaniclastic, siliciclastic and calcareous sandstone layers in the Zimapán Basin, Central Mexico (Fig. 1). These conditions are the lowest at which monazite growth has been reported, so we provide new insights into the conditions, mineralogical composition and fluid-rock interaction during folding of basinal sediments. We demonstrate the potential of in situ Th/Pb dating of monazite in fold-related veins as a new approach to constrain the age of deformation in the shallow upper Crust.

2. Geological setting The Zimapán Basin is a Late Jurassic-Early Cretaceous deep water marine basin that developed between two shallow water carbonate platforms (El Doctor and Valles-San Luís Potosí platforms) in central Mexico (Fig. 1; Suter, 1987). Three lithological units characterize this basin: a thick Late Jurassic-Barremian succession of calcareous and volcaniclastic and siliciclastic sandstone interbedded with shale, also known as the Trancas Formation (Carrillo-Martínez, 1990; OrtegaFlores et al., 2014), overlain by a thick Aptian-Cenomanian succession of thinly-bedded limestone interbedded with shale (the Tamaulipas Formation), followed by a thick package of Late Cretaceous calcareous synorogenic turbidites, the Soyatal Formation (Fig. 1). These formations underwent strong internal deformation between two more rigid carbonate platforms on the sides, all of which are part of a larger orogenic wedge, the Mexican Orogen (Fitz-Díaz et al., 2018, Fig. 1) more locally the Mexican Fold-Thrust Belt (Fitz-Diaz, 2010). Tectonic shortening in the basin was dominantly accommodated by pervasive buckle folding and flattening, resulting in 60–70% horizontal shortening (Fitz-Díaz et al., 2012). The shortening is inferred to be the result of the superposition of two almost co-axial phases of deformation, locally evident by the superposition of shear zones into folds, and the presence of a crenulation cleavage superimposed on the axial plane cleavage in fine-grained rocks (Fitz-Díaz et al., 2012; Vázques-Serrano et al., 2018). Ar/Ar encapsulated illite geochronology indicates that the main event of folding in the basin occurred in the Late Cretaceous at

3. Methods A four steps method was followed in this study (Fig. 2). In the first step, we carried on structural analysis, which allowed selecting veins emplaced during folding on the basis of kinematic consistency among veins, fold-axial plane cleavage and bed-parallel shear during flexural folding. The second step was sampling of 1–3 cm thick, well preserved veins. Then, a careful texture analysis of vein minerals, with SEM imaging and elemental mapping with EMPA was carried out in selected veins. Finally, U-Th/Pb ages were obtained with LA-MC-ICP-MS in monazite and U-Pb ages we obtained with LA- ICP-MS in detrital zircons of the host rock. Such steps of analysis are described with more 137

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Fig. 2. Sketch that synthesizes the four step method followed in this research.

study follow those outlined in Cottle et al. (2012). Monazite was ablated using a 7 μm diameter spot at 3 Hz repetition rate for 80 shots at a laser fluence of 1.75 J/cm2, resulting in craters that are ∼3 μm deep. Data reduction, including corrections for baseline, instrumental drift, mass bias, down-hole fractionation as well as age calculations were carried out using Iolite v. 2.3 (Paton et al., 2010). Background intensities and changes in instrumental bias were interpolated using a smoothed cubic spline while down-hole interelement fractionation was modeled using an exponential function. Statistics for baselines, on peak intensities and isotopic ratios were calculated using the mean with a 2.S.D. outlier rejection. Concordia and weighted mean date plots were calculated in Isoplot v.3 (Ludwig, 2003) using the 238U and 235U decay constants of Jaffey et al. (1971) and the 232 Th day constant of Amelin and Zaitsev (2002). All uncertainties are quoted at 2σ and include contributions from the external reproducibility of the primary reference material (Appendix 1) for the 206Pb/238U ratios and 208Pb/232Th ratios. The full dataset is presented in Online supplement Appendix 2. Monazite U-Th/Pb data was normalized to ‘44069’ (424 Ma 207Pb/235U IDTIMS age, Aleinikoff et al., 2006) was employed to monitor and correct for mass bias as well as Pb/U and Pb/Th down-hole fractionation. To monitor data accuracy, two a reference monazite FC-1 (55.7 Ma 206Pb/238U ID-TIMS age, Horstwood et al., 2003) was analyzed concurrently (once every ∼7 unknowns) and mass biasand fractionation-corrected based on measured isotopic ratios of the primary reference material. During the analytical period, 11 analyses of FC-1 gave a weighted mean 206Pb/238U date of 55.4 ± 0.6 Ma, MSWD = 1.4, and a weighted mean 208Pb/232Th date of 54.0 ± 0.6 Ma, MSWD = 1.6 (2σ) (Appendix 1). Because some grains are smaller than the laser beam diameter, we also analyzed numerous areas of the host mineral (calcite or quartz) and determined that the U, Th and Pb concentrations in these minerals are all below the detection limit of our system. Detrital U-Pb zircón dating was carried on in the Laboratorio de Estudios Isotópicos (LEI), Centro de Geociencias UNAM, following the method described in Solari et al. (2010). The zircon U-Pb ages were obtained with LA-ICP-MS on an ablated spot of 23 μm in diameter and a laser fluence of 6 J/cm2. In sample V 130 were analyzed, while for sample T only 100 zircon grains were used. Both, standard Z91500 and Plesovice (Sláma et al., 2008) were used during the analysis. We glass used NIST610 glass to calculate trace elements. The method proposed by Dickinson and Gehrels (2009), was used to calculate

detail in the following paragraphs: 1) The first step includes an initial structural analysis of folded rocks near the base of Trancas Fm. (Fig. 2-1). This analysis includes the observation of structures, measurement and kinematic analysis of the folds on the basis of geometry, as well as history of deformation of fold-related veins. Veins were emplaced early (poorly oriented with respect to and affected by axial plane cleavage and bed-parallel shear caused by flexural folding), contemporaneous (accommodate extension perpendicular to axial plane cleavage) and late in folding (are almost perpendicular but cut axial plane cleavage and previous veins). 2) Following structural analysis, a suite of ten syn-folding vein samples were collected at two localities called V and T (Fig. 1); we especially sampled veins within volcaniclastic sandstone layers and with a thickness less than 3 cm, so that they could be fully analyzed in a petrographic thin-section (Figs. 2-2). Additionally, two samples of the host rock at both localities were collected for U-Pb detrital zircon geochronology. As these units are volcaniclastic, ages constrain their maximum time of depositional. 3) The collected vein samples were cut, polished and observed using a JEOL JSM-7800FLV field-emission scanning electron microscope (SEM) at the EESD of the University of Michigan to locate and identify monazite using Energy-dispersive X-ray spectroscopy (EDS). Grains larger than ≥10 μm were then mapped for U, Th, Pb and Y using wavelength-dispersive spectrometers (WDS) on a JEOL JXA8530FPlus Electron Probe Microanalyzer (EPMA), at the EESD of the University of Minnesota, with a beam current of 50 nA, an accelerated voltage of 15 kV, a beam diameter of 5 μm, dwell time of 50 ms and pixel size of 0.5 μm (Figs. 2–3). This permitted characterization of compositional variations within the grains and correlation to monazite grain shape (e.g. irregular or smooth boundaries), position (e.g. center of the vein, vein-wall interface, host rock, calcite-quartz fibers interface, or association with rock inclusions within the vein). 4) Selected monazite from four samples (V1, V5, T1 and T2) were analyzed directly in thin section in-situ by laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MCICPMS) at the University of California Santa Barbara (Figs. 2–4). Instrumentation consists of a Photon Machines 193 nm ArF Excimer laser and ‘HelEx’ ablation cell coupled to a Nu Instruments HR Plasma high-resolution multi-collector MC-ICP-MS. Methods in this 138

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Fig. 3. Structural analysis of folds. On the left, the sketch shows the orientation of fold axial plane, syn-folding veins and bed-parallel shear associated to flexural folding. In the center, major circle traces of measured fold axial planes and veins, notice that these two structures have about the same strike. To the right, the poles to fold axial planes and to extensional veins are found on the same plane and are almost perpendicular to each other.

Fig. 4. Textural analysis of monazite grains. a) Backscattered image of a synfolding, stretched calcite and quartz vein, in which the location of close-up backscattered of monazite grains in the host rocks, as well as smaller monazite grains analyzed with the EMPA, is shown. b) Monazite grain found in an interstitial space of an altered detrital feldspar grain. c) Anhedral monazite grain hosted in the matrix among quartz grains, in association with chlorite. d) Grain of monazite at the wall-vein interface. Notice that fluid typical fluidalteration minerals, such as barite and chlorite are also found at this interface. e) Monazite grains immerse in calcite and chlorite within the vein, notice the irregular shape and the slightly patchy elemental maps. f) Monazite grain at the vein wall, which shows a patchy texture. g) Neoformed polygonal monazite grain immerse in calcite. Notice its homogenous elemental composition. h) Monazite grain attached to a wall rock inclusion, notice its heterogeneous composition.

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Appendix 3), which are taken as the maximum age of deposition of folded host rocks.

máximum deposition ages.

4. Results 5. Discussion Stereographical analyses of fold axial planes (S1) and synfolding veins are shown in Fig. 3. Statistical analysis of 22 S1 measurements and of 20 syn-folding veins (V), shows that both structures have about the same NW-SE strike (Fig. 3). The average orientation of the axial planar cleavage is 145-44 SW, and 309-69 NE for synfolding veins. The poles to S1 and V are almost perpendicular to each other on the same NE-SW oriented plane. This indicates kinematic consistency with the vein opening direction, oriented with the maximum stretching axis of a finite strain ellipse, whose minimum stretching axis is normal to the axial plane of folds (Fig. 3). The kinematic consistency supports the contention that both structures formed contemporaneously, under the same stress/strain regime. However, despite the fact that veins show roughly the same strike, they do change dip around the folds. This suggests that they were formed at slightly different stages during folding. Only four of the ten syn-folding vein samples contain monazite grains (31 grains in total) that are big enough to be analyzed with LAMC-ICPMS. The studied monazite grains vary in size from few microns to 40 μm and typically occur at the calcite-quartz fiber interface, enclosed in calcite or in contact to host rock inclusions typically in the vicinity of the vein wall. The largest grains precipitated near the interface between calcite and quartz fibers, and tend to have smoother boundaries and a relatively regular shape compared to those attached to solid inclusions or to the vein wall. Elemental maps of Pb, U, Th and Y (Fig. 4) indicate that the largest and smooth-wall grains have a more homogeneous composition. Based on Pb and Th content total of 23 reliable Th/Pb ages from about 40 analyzed spots, are obtained in the selected grains of monazite. In all analyzed monazite, U concentration is low (< 40 ppm) compared to typical monazite (∼1000's of ppm U) such that, because of their young age, it is not possible to calculate a reliable Pb/U age. We therefore rely on the 208Pb/232Th system, which yields a range of apparent dates between 154 and 68 Ma (Fig. 5a, and Appendix 2). The majority of analyses cluster toward the younger end of the range, with the youngest 11 analyses (excluding the youngest one, as it is noticeable younger than the rest) defining a weighted mean age of 76.8 ± 0.8 Ma, MSWD = 1.4 (Fig. 5b). Detrital zircon U/Pb dating of two samples of volcaniclastic samples collected in localities V and T, yields ages between 114 and 127 Ma (Appendix 3). The mean age of the youngest cluster of zircons is 125 ± 1 Ma for sample V and 120 ± 7 Ma for sample T (Fig. 5 c and d,

Textural observations show that monazite grains found in the host rock can be detrital, as they are found in interstitial spaces of K-feldspars in volcanic fragments, and authigenic, as they are commonly found on the rims of sulfides and oxides, where they show a subhedral shape (Schandl and Gorton, 2004). This suggests that fluid-rock interaction not only permitted neoformation of monazite in the vein but also within the host rock and at the wall-vein interface (Fig. 4a–d). Elemental maps of monazite within the vein (Fig. 4e–h) in general show that the larger and polygonal shaped monazite grains immerse in calcite and with homogeneous elemental distributions yield younger ages, compared to grains attached to wall rock, which have irregular shapes and patchy elemental distribution. From these observations, we conclude that monazite within the veins could be partly inherited solid inclusions (detrital monazite) from the host rock trapped during the vein opening as they are commonly associated with solid inclusion bands presumably formed during crack-seal, neoformed minerals transferred in solution to the veins, or multi-domain grains combining those two end-member ages (Fig. 6). The Th/Pb monazite ages in general range between the TithonianBarremian age of host rock deposition (Carrillo-Martínez, 1990; OrtegaFlores et al., 2014) and the Late Cretaceous age of folding of the Zimapán Basin rocks (Fitz-Díaz et al., 2014a). Our results show that neoformed monazite preserves meaningful ages in terms of deformation, and that it might be possible to identify target grains prior to ablation with textural and compositional analysis. Moreover, the statistical analysis of eleven youngest ages (almost half of the obtained dates) cluster around a mean age of 76.8 ± 0.8 Ma, which overlaps with the ages of folding (75.5–82.5 Ma) obtained by Ar-Ar illite dating (Fitz-Díaz et al., 2014a), with syn-folding remagnetization ages (∼77 Ma, Nemkin et al., 2015), and with the timing of sedimentation of syn-orogenic turbidite deposits in the area (Kiyokawa, 1981; Hernández-Jáuregui, 1997; Juárez-Arriaga et al., 2016). Despite the uncertainties on individual spot date determinations, the mean age of the youngest neoformed monazite grains has the potential to improve the resolution of the timing of deformation in the Zimapan Basin (Fig. 6). The success of Th/Pb dating of monazite in veins is likely due to the fact that they are hosted in fibrous fold-related veins, which, according to numerous observations occur in particular arrays (Hudleston, 1989; Fig. 5. Geochronology on monazite and zircon. a). In situ 208Pb/232Th ages of vein-hosted monazite, obtained with LA-MS grains host-rock detrital zircon 206 Pb/238U weighted average plots are shown. Monazite ages vary from 68 to 154 Ma. b) The mean age of youngest monazite ages is 76.8 ± 0.8 Ma. c) Minimum detrital zircon age of volcaniclastic rocks from locality V is 125.5 ± 1.2 Ma. d) Minimum detrital zircon age of volcaniclastic rocks from locality T is 120.3 ± 6.5 Ma.

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Fig. 6. Monazite ages interpretation. (a) Kinematics of studied folds, in which shortening induces fluid-rock interaction by pressure-solution on axial plane cleavage and transfer of calcite, quartz and monazite (NFM) in solution to lower pressure regions, particularly veins. Some monazite grains are transferred as solid inclusions (detrital) from host rocks (DTM) and others are partly detrital and partly neoformed in veins (MDM). (b) Comparison of the monazite ages with the age of folding and the age of deposition of volcaniclastic host rock. Neoformed monazite grain ages coincide with the age of folding, while ages have large error bars as they get older, which corresponds to mixing between host rock deposition and folding. 1-Fitz-Díaz et al. (2014a), 2-Carrillo- Martínez, 1990, 3-Ortega-Flores et al., 2014.

Acknowledgements

Jessell et al., 1994; Fischer et al., 2009), are formed at particular stages of folding and are sensitive to episodic pore fluid pressure increases (Hilgers et al., 2006; Evans and Fischer, 2012; Bons et al., 2012), during which pressure-solution and solution-transfer occur that provided the means for neocrystallization of monazite. Evidence for extensive fluid-rock interaction during folding in the Zimapan basin is also recorded by δ2H isotope analyses of water in fluid inclusions in syn-folding veins and in neoformed illite in bentonitic shale (Fitz-Díaz et al., 2014b). These data suggest that the veins formed in confined conditions that permitted the dissolution and transfer of quartz, calcite and monazite to the vein filling. Combining fluid inclusion microthermometry and comparative stable isotope analyses Δ18Ocal-qz constrains the P-T conditions of vein formation as shown in the Alps (e.g., Kirschner et al., 1995). Additionally, microthermometry of fluid inclusions and stable isotopic analyses in vein minerals and aqueous fluid inclusions preserve the source of fluid involved in deformation. Thus, our study shows that integration of isotopic analysis of different generations of veins, fluid inclusion analysis, and dating have the potential to establish P-T-t paths from veins that formed during folding of upper crustal rocks.

This work would not be possible without the funding by the Consejo Nacional de Ciencia y Tecnología (México) grant 240662, granted to Elisa Fitz-Díaz, and to the Geological Society of America (United States) for the 2018 Grant in Aid 9249906 given to Maria Isabel Vidal Reyes. We express our gratitude to Andrew Kylander-Clarkand for support with dating, to Ellery Frahm for support of EPMA monazite characterization and to Augusto Rodriguez for his support to take SEM pictures. We also thank to Prof. Alfons Berger and anonymous reviewer 2 for their comments and suggestions to improve this manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jsg.2019.04.004. References Aguado, B.V., Azevedo, M.R., Schaltegger, U., Catalán, J.M., Nolan, J., 2005. U–Pb zircon and monazite geochronology of Variscan magmatism related to syn-convergence extension in Central Northern Portugal. Lithos 82 (1-2), 169–184. Aleinikoff, J.N., Schenck, W.S., Plank, M.O., Srogi, L., Fanning, C.M., Kamo, S.L., Bosbyshell, H., 2006. Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite. Geol. Soc. Am. Bull. 118 (1-2), 39–64. Amelin, Y., Zaitsev, A.N., 2002. Precise geochronology of phoscorites and carbonatites: The critical role of U-series disequilibrium in age interpretations. Geochim. Cosmochim. Acta 66 (13), 2399–2419. Ault, A.K., Reiners, P.W., Evans, J.P., Thomson, S.N., 2015. Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA. Geology 43 (9), 771–774. Berger, A., Gnos, E., Janots, E., Whitehouse, M., Soom, M., Frei, R., Waight, T.E., 2013. Dating brittle tectonic movements with cleft monazite: fluid-rock interaction and formation of REE minerals. Tectonics 32, 1176–1189. Bons, P.D., Elburg, M.A., Gomez-Rivas, E., 2012. A review of the formation of tectonic veins and their microstructures. J. Struct. Geol. 43, 33–62. Carrillo-Martínez, M., 1990. Hoja Zimapán 14Q-e(7). Carta Geológica de México, serie de 1:100,000. Instituto de Geología. UNAM, pp. 32. Causse, C., Moretti, I., Eschard, R., Micarelli, L., Ghaleb, B., Frank, N., 2004. Kinematics of the Corinth Gulf inferred from calcite dating and syntectonic sedimentary characteristics. Compt. Rendus Geosci. 336 (4–5), 281–290. Cottle, J.M., Kylander-Clark, A.R., Vrijmoed, J.C., 2012. U–Th/Pb geochronology of detrital zircon and monazite by single shot laser ablation inductively coupled plasma mass spectrometry (SS-LA-ICPMS). Chem. Geol. 332, 136–147.

6. Conclusions Using a multiscale analytical method, a total of 23 Th/Pb monazite dates were obtained from four veins formed during folding of the Trancas Formation in the Zimapán Basin in Central Mexico. The comparison of age and texture (shape and smooth vs. patchy elemental zonation) among microscopic monazite, indicates that some of the grains are inherited (solid inclusions from the host rock), while the majority are neoformed during vein formation. The eleven youngest, neoformed grains, have a mean age of 76.8 ± 0.8 Ma that coincides with the age of folding of the basin from other dating methods, including Ar-Ar illite dating. Despite the complexity of small monazite precipitated in syn-tectonic veins emplaced in the shallow upper crust, our approach opens new avenues for application toward P-T-t-f paths of folding. If systematically applied, it has the potential to resolve the spatio-temporal history of deformation and fluid-rock interaction at different crustal levels and at different times during the evolution of fold-thrust belts. 141

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