Role of pre-kinematic fluid-rock interactions on phase mixing, quartz recrystallization and strain localization in low-temperature granitic shear zones
Introduction
Differences in mechanisms of strain accommodation are often observed in crustal shear zones of granitic composition formed at different depths. The rheological behavior of feldspars and quartz - the major components of granitic rocks - strongly controls the overall rock strength and varies with syn-tectonic temperature, fluid availability and variations of differential stress and/or strain rate (e.g. Stipp et al., 2002; Piazolo et al., 2002 and references therein). It was shown that, for example at greenschist-facies conditions, deformation of quartz in metapelites may shift from brittle to plastic within ∼50 °C (T ∼ 250–300 °C, Stöckhert et al., 1999) and its dynamic recrystallization regime may change from bulging (regime I) at T < 450 °C to subgrain rotation (regime II) at 450 °C < T < 500 °C (at constant strain rate, Stipp et al., 2002). However, what is more difficult to explain is that different microstructures and microtextures of quartz can be observed in shear zones that formed at very similar crustal depths, as in the case of brittle-plastic transition under greenschist-facies conditions (e.g. Spruzeniece and Piazolo, 2015). At these conditions, fluid availability, which is well documented in shear zones (O'Hara, 1988; Rolland et al., 2003) promotes quartz recrystallization (e.g. Spruzeniece and Piazolo, 2015; Palazzin et al., 2018) and favors textural re-organization at the macro- and micro-metric scale as well as mineral reactions (e.g., McCaig, 1984; Fitz Gerald and Stünitz, 1993; Wintsch et al., 1995; Wibberley, 1999; Goncalves et al., 2016). Mineral reactions often result in phase mixing that may cause rock weakening by preventing grain growth due to pinning of grain boundaries (e.g., Etheridge and Wilkie, 1979; Herwegh et al., 2011). Specifically, the growth of phyllosilicates also lowers the overall rock strength (e.g. Gueydan et al., 2003), and inhibits recrystallization and development of crystal preferred orientation (CPO) of quartz (Song and Ree, 2007; Hunter et al., 2016). At larger scale, phyllosilicate abundance controls the structural style (location of shear zones) and the strain intensity of the bulk rock (Wehrens et al., 2016; Airaghi et al., 2020a; Graziani et al., 2020).
The timing of phyllosilicate growth in deformed granite strongly relates to (i) fluid influx, (ii) the mechanisms of fluid percolation and (iii) the composition of the fluid phase (Rossi et al., 2005; Cenki-Tok et al., 2014; Rossi and Rolland, 2014). While some greenschist-facies shear zones are almost isochemical and evolve as closed system (Kerrich et al., 1980; Hippertt, 1998), others show open-system behavior with significant mass transfer, either at an early stage of the shear zone development (Rolland and Rossi, 2016, Tursi et al., 2021), or at a later stage (i.e. Glanzer and Bartley, 1991; Rolland and Rossi, 2016; Tursi et al., 2018; Ceccato et al., 2022). As a consequence, no systematic timing of phyllosilicate growth and phase mixing in the development of shear zones can be assessed. Their relative importance over microstructural development and mechanisms of strain accommodation remain therefore difficult to compare to the effect of varying temperature.
The present work investigates structural and microstructural differences in shear zones of two Variscan granitoids of the central Pyrenees (Bielsa and Maladeta). Located at ∼40 km from each other, the two granitoids are altered in greenschist-facies conditions. Both granitoids were deformed during Alpine orogeny (Airaghi et al., 2020a; Bellahsen et al., 2019 and references therein). Two different alteration events were recognized in the Bielsa granitoid: the first during Permian-Triassic time, at the end of Variscan orogeny (230–300 Ma), the second during Alpine compressional stage (40–70 Ma, Airaghi et al., 2020a). Microstructures, mineral abundance, whole-rock chemistry, syn-kinematic temperature conditions and quartz deformation mechanisms across shear zones of the two massifs have been studied and compared. Our results highlight the key role of pre-kinematic (pre-Alpine shearing) alteration on strain localization, leading to drastically different responses to deformation at similar temperatures.
Section snippets
Geological setting
The Axial Zone of the central Pyrenees (Fig. 1a) consists of a series of basement slices forming a South-verging fold-and-thrust system (Fig. 1b-c, Roure et al., 1989; Muñoz, 1992). The basement of the Axial zone is mainly composed of granitic plutons (e.g., Maladeta, Bielsa, Néouvielle and Eaux-Chaudes) emplaced during Variscan orogeny (c. 300 Ma, Román-Berdiel et al., 2004). Alpine shortening was accommodated on thrusts active from 70 Ma to Oligocene times (Wayne and McCaig, 1998;
Sampling
In Bielsa, seventeen samples were collected east of the Urdiceto lake (Fig. 1d), across two brittle-ductile shear zones, along N-S oriented transects of 10 to 15 m long, at the boundary with the Triassic sediments (transect B1 of Fig. 2a and transect B2 of Fig. 2b). Transect B1 (samples from ZAL18-14A to ZAL18-14I) includes undeformed granitoid in the southern part and a mylonitic corridor of c. 5 m width in the central part (Fig. 2a and 3a-c). In transect B2 (samples from ZAL18-17A to
Methods
Mineral abundance was estimated by identifying mineral phases in >500 points on thin-section scans with the software JMicroVision v1.2.7 (https://www.jmicrovision.com). Backscattered electron images have been acquired using a scanning electron microscope (SEM) Zeiss Supra 55VP at ISTeP (Sorbonne Université, Paris). Quartz deformation textures were investigated by electron backscatter diffraction (EBSD) using an EDAX PEGASUS system (OIM DC 6.4 software) connected to a TESCAN SEM (MIRA3) located
Petrography and microstructures
Petrographic observations are presented from the less to the most altered and deformed sample of each transect (Fig. 3), where the degree of alteration and deformation is qualitatively estimated from the development of cleavage, grain size reduction, phyllosilicate abundance and interconnectivity. For the Bielsa granitoid, detailed petrographic and mineralogical descriptions of complementary samples can be found in Airaghi et al. (2020b); only the main petrographic features are presented below
Electron backscatter diffraction of quartz
Intragranular misorientations, kernel average misorientation, grain size and CPO have been quantified for quartz using EBSD in high strain samples (Bielsa B1: ZAL18-14B; Bielsa B2: ZAL18-17C; Maladeta: ZAL18-20E) and in two medium strain samples (Bielsa: ZAL18-17B and Maladeta: ZAL18-20C). CPO are constructed using one-point-per-grain in equal-area lower-hemisphere pole figures (Fig. 9, Fig. 10, Fig. 11). Quartz-rich areas were selected for EBSD analyses to be representative of quartz
Whole-rock chemistry
Whole-rock compositions are reported in Table 2 and Fig. S5. Samples of Bielsa exhibit similar composition, typical of granodiorite, except for samples ZAL18-14A (dioritic composition) and ZAL18-17A and ZAL18-17B that include Qz-rich veins. Following the plutonic total alkali silica classification, Maladeta samples are categorized as granites and quartz-monzonites (Fig. S5). The major element composition was plotted against the composition of undeformed samples for each transect (black dot in
Chlorite thermometry
Crystallization temperature estimated with the thermometer of Bourdelle and Cathelineau (2015) for chlorite in Maladeta samples are in the range of 250–380 °C (Fig. 13a), hence largely overlapping with the majority of the temperature values estimated with the multi-equilibria approach of Vidal et al. (2006) comprised between 250 and 330 °C (Fig. 13b). The estimated Fe3+ proportion is comprised between 20 and 30% of the total iron. Temperature values were statistically tested for 3000 random
Fluid-rock interactions and element mobility in Bielsa and Maladeta
Geochemical variations in samples from a same transect appear primarily due to fluid-rock interactions, rather than to lithological variations of the protolith (with the exceptions specified above). This is motivated by the similar composition among protoliths, and the correlation of compositional variations with strain and with mineralogy, in particular sericitization. In Bielsa, the lack of correlation between alkalis and other major elements, together with the general loss of Na and gain of
Conclusions
Greenschist-facies shear zones of the Bielsa and Maladeta granitoids (Axial zone, Pyrenees) show different structural styles (distributed vs. localized shear zones) and microstructures, despite having similar lithologies, tectonic and syn-kinematic histories. Based on microstructural analysis and geochemical results, we explain this discrepancy by reconstructing the post-Variscan geological history of the two massifs as follows:
- 1.
The shear zones of Bielsa evolved under fluid-abundant conditions
CRediT authorship contribution statement
Khadija Alaoui: Conceptualization, Writing – original draft, Investigation, Formal analysis, Visualization. Laura Airaghi: Conceptualization, Investigation, Formal analysis, Writing – review & editing. Benoît Dubacq: Conceptualization, Formal analysis, Writing – review & editing. Claudio L. Rosenberg: Conceptualization, Supervision, Validation. Nicolas Bellahsen: Supervision, Validation, Funding acquisition. Jacques Précigout: Formal analysis, Validation, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This study was founded by the CNRS-BRGM-TOTAL ‘OROGEN’ project and partly by SYSTER-TELLUS grant from the INSU-CNRS. We thank Michel Fialin, Nicolas Rividi and Omar Boudouma for their help with electron microscopy and electron microanalyser of ISTeP. We acknowledge Benoit Caron and Benoît Villemant for their support with ICP-OES and ICP-MS analytical techniques. We are grateful to Samuel Angiboust for his careful editorial handling. Alberto Ceccato, Yann Rolland and Fabrizio Tursi are thanked
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