Microstructural vs compositional preservation and pseudomorphic replacement of muscovite in deformed metapelites from the Longmen Shan (Sichuan, China)
Introduction
Rocks that have experienced a multi-phased deformation history frequently exhibit complex meso and microstructures. In metapelites it is common to observe microstructures related to three to five deformation phases (i.e. Bell and Hickey, 1999, Cosgrove, 1976, Goswami et al., 2009, Huang et al., 2016, Worley et al., 1997) as it is the case in the internal domains of the Longmen Shan (Sichuan, China), at the eastern border of Tibetan plateau (Fig. 1 and Fig. 2a). In some favorable case a different metamorphic assemblage can be preserved in each microstructure. Pressure (P) and temperature (T) changes can be then quantified (e.g. Lanari et al., 2013, Lanari et al., 2014b, Scheffer et al., 2016), and linked to deformation (ε) to derive P–T–ε paths (Parra et al., 2002, Vidal et al., 2006). This provides constraints on the large-scale geodynamic processes that drive metamorphism (Putnis and Timm, 2010). However, in the case of metapelites that have experienced a low to medium grade metamorphism (up to the amphibolite facies conditions) the prograde record in the matrix is often overprinted near the temperature peak. Consequently the majority of the studies focused on the P–T conditions of the peak assemblage (Boyle and Westhead, 1992, Huang et al., 2016) characterized by index minerals such as garnet, biotite, staurolite etc. Less attention has been paid to the evolution of the phyllosilicate-rich matrix and to its chemistry, while it does record the long-lasting deformation history.
Recently, high-resolution quantitative compositional mapping has provided a picture of the complexity of the compositional zoning observed in the matrix minerals such as white mica (Scheffer et al., 2016). A similar complexity is observed in metapelites from the Longmen Shan (Sichuan, China) (Fig. 2b). Here metamorphism reaches amphibolite facies conditions (6–10 kbar, 550–600 °C, Dirks et al., 1994, Worley and Wilson, 1996) and evidences of a multi-phased deformation history are well preserved both in field (Harrowfield and Wilson, 2005) and microstructures (Worley and Wilson, 1996). Compositional maps reveal a significant zoning in composition within the matrix K-white mica (Fig. 2b).
Such observations call into question the validity of two major assumptions on which the construction of P–T paths often relies: (1) the continuous re-equilibration of the matrix minerals during metamorphism and (2) the link between the microstructural and the metamorphic record. It is therefore critical to understand which composition of white mica is stable for specific P–T conditions. Incomplete re-equilibration as suggested by Fig. 2b has important consequences on the interpretation of 40Ar/39Ar ages on mica (Villa, 2010, Lanari et al., 2014c) and the 87Rb/86Sr ages of mica and garnet, these last potentially affected by incomplete Rb and Sr re-equilibration among matrix minerals (Sousa et al., 2013). Although some petrological and geochronological studies have dealt with the problem of partial equilibration of the mineral assemblage (Dempster, 1992, Konrad-Schmolke et al., 2011) and the consequences on the age record (Mulch and Cosca, 2004, Mulch et al., 2002), this issue is commonly overseen.
In this study we investigated the relative timing and mechanisms K-white mica (Kwm) re-equilibration in a series of metapelites from the Longmen Shan (Sichuan, China) by coupling a detailed microstructural study with high-resolution quantitative compositional mapping and phase equilibrium modeling. P–T conditions of the successive metamorphic assemblages were reconstructed using empirical and semi-empirical thermobarometry, as well as multi-equilibrium and forward equilibrium models. Raman spectroscopy on carbonaceous material (RSCM) was used to retrieve the maximum temperature experienced by the organic-rich schists and thus to characterize the metamorphic peak. The combination of all methods provides a good framework to study the low to medium grade metamorphic area of the Longmen Shan and its regional tectonic implications.
Section snippets
A two end-member scenario for matrix re-equilibration in metapelites and modelling strategy
A two end-member scenario of matrix re-equilibration is presented in Fig. 3a and b to illustrate the consequences of replacement or preservation of Kwm in a typical garnet-biotite metapelite affected by several phases of metamorphism and deformation (Fig. 3c). An early S1 sub-vertical cleavage composed of muscovite (msA, red in Fig. 3a), chlorite and quartz grains is preserved in the matrix. S1 is folded and a new S2 crenulation cleavage develops parallel to microfold axial planes. The S2
Geologic setting
The Longmen Shan thrust belt (Sichuan, China) is situated at the eastern border of the Tibetan plateau at the boundary between the South China craton to the East and the Songpan-Ganze terrane to the West (Fig. 1a). Thermochronological data show that the belt underwent a phased of rapid exhumation since 30 Ma related to the India-Asia collision (Godard et al., 2009, Kirby et al., 2002, Wang et al., 2012). Metamorphic studies carried out in the Danba (Huang et al., 2003a, Huang et al., 2003b) and
Sampling and sample description
A total of 14 samples were collected for this study along a NW-SE oriented cross section in the Cambrian and Silurian meta-sedimentary units (see Fig. 1 for sample location and Table 1 and Table 5 for sample geographic coordinates). Nine samples are organic-rich schists showing a high content of carbonaceous material, phyllosilicates in variable quantities and quartz grains. The carbonaceous material consists of flakes of more than 100 μm. Samples to13-4 and to13-7 are garnet-biotite-mica
Analytical method
High-resolution chemical analyses were performed with an electron probe micro-analyzer (EPMA) JEOL JXA-8230 at the Institut des Sciences de la Terre (ISTerre) of the University of Grenoble Alpes. Point analyses were acquired using 15 keV accelerating voltage and 12 nA beam current with a beam size of 2 μm for all studied samples. In order to investigate the link between compositional variability of metamorphic minerals and their microstructural position, compositional X-ray maps were also acquired
Thermobarometry
In order to constrain which chemical composition of muscovite (msA, msB and msC) has been stable during the successive P–T–ε stages and to compare them with the observed compositions, it is firstly crucial to precisely constrain the P–T path followed by rock. In the following section, phase equilibria modelling is used to reconstruct the P–T path. The results are tested against the predictions from empirical and semi-empirical thermobarometers.
Raman spectroscopy on carbonaceous material
The maximum temperature (Tmax) experienced by the organic-rich sediments was estimated with the Raman Spectroscopy on Carbonaceous Material (RSCM) thermometer of Beyssac et al. (2002) with a relative accuracy of ± 15 °C (Beyssac et al., 2002). RSCM was applied on polished thin sections following the procedure of Beyssac et al. (2002). Spectra were obtained using a Renishaw InVia Reflex microspectrometer at the Geosciences laboratory of Ecole Normale Supérieure (Paris, France) with laser power of
A multi-method approach: comparison between thermobarometric methods
Results from the multi-equilibrium approach of Dubacq et al. (2010) are in line with the data obtained by phase equilibria modelling (Fig. 9). For a given temperature, a relative uncertainty of ± 2 kbar was estimated using Monte-Carlo simulations, following the technique described in Cantarero et al. (2014) for chlorite. Fig. 9 shows that the pressures predicted at temperature > 450 °C are slightly higher than those obtained with the phase diagrams. Dubacq et al. (2010) observed a similar increase
Conclusion
Our petrological observations show that typical garnet-biotite metapelites, collected in the internal domains of the Longmen Shan thrust belt (Sichuan, China) record microstructural evidence of several deformation phases that can be related to different metamorphic assemblages. High-resolution X-ray compositional mapping and a multi-method thermobarometry approach based on the combination of empirical thermometers, phase equilibria and multi-equilibrium modelling successfully enable to
Acknowledgments
The project was made possible by the financial support of Agence Nationale de la Recherche (ANR) AA-PJCJC SIMI5-6 LONGRIBA and ANR-13-BS06-012-01 DSP-Tibet, the INSU-CNRS and LabEx “OSUG@2020” (ANR10 LABX56). We also thank Tan Xibin (China Earthquake Administration), Professor Xu Xiwei (China Earthquake Administration), Professor Li Yong (Chengdu University of Technology) and students from both universities, for their logistical support in the field and scientific discussions. We also
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