Elsevier

Lithos

Volumes 282–283, June 2017, Pages 262-280
Lithos

Microstructural vs compositional preservation and pseudomorphic replacement of muscovite in deformed metapelites from the Longmen Shan (Sichuan, China)

https://doi.org/10.1016/j.lithos.2017.03.013Get rights and content

Highlights

  • Deformed garnet-biotite metapelites of the Longmen Shan reached 11 kbar, 530 °C.

  • Matrix muscovite in different microstructures preserves a different chemistry.

  • Muscovite in prograde microstructures re-equilibrated at the P and T peaks.

  • Samples with similar microstructures show a different degree of muscovite re-equilibration.

  • The extent of muscovite re-equilibration mainly depends on the fluid availability.

Abstract

Pressure-temperature-deformation (P-T-ε) paths for metamorphic rocks that experienced a complex deformation history commonly rely on the link between successive metamorphic assemblages and the different microstructures preserved at the sample scale. However, with changing P-T conditions, metamorphic minerals in a specific microstructure can chemically re-equilibrate. The direct link between deformation phases and mineral compositions for thermobarometry purposes can therefore be distorted. This study focuses on a series of garnet-biotite metapelites from the Longmen Shan (Sichuan, China) that preserve muscovite of different chemistry in distinct microstructures. To quantify the degree of re-equilibration of muscovite, a microstructural study was coupled with high-resolution chemical mapping. The chemical evolution of muscovite was modeled along a P-T loop previously constrained with phase equilibria calculations, semi-empirical and empirical thermobarometry. Results show that metapelites experienced a three-stage metamorphic history: (1) heating and burial up to 11 ± 2 kbar, 530 ± 20 °C, (2) minor decompression and heating up to 6.5 ± 1 kbar, 575 ± 10 °C and (3) decompression and cooling down to 4 ± 1 kbar, 380–450 °C. Muscovite partially re-equilibrated by pseudomorphic replacement during the three metamorphic stages, although it is mainly observed in prograde microstructures preceding the pressure peak. The main factors controlling the re-equilibration are the intensity of the deformation and the fluid availability during metamorphism. The P-T conditions of metamorphic assemblages thus reflect pulses of fluids release that enhanced mineral resorption and local replacement. Such micro-chemical behavior is probably relatively common in metapelites and questions the reliability of the P-T predictions based on relict phase chemistry, apparently preserved in microstructures that might have been affected by later re-equilibration.

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

References (90)

  • G. Huang et al.

    P-T-t constraints of the Barrovian-type metamorphic series in the Khondalite belt of the North China craton: evidence from phase equilibria modeling and zircon U-Pb geochronology

    Precambrian Research

    (2016)
  • B. Jamtveit et al.

    Reaction enhanced permeability during retrogressive metamorphism

    Earth and Planetary Science Letters

    (2008)
  • M.J. Kohn

    Geochemical zoning in metamorphic minerals A2 – Holland, Heinrich D

  • M. Konrad-Schmolke et al.

    Combined thermodynamic and rare earth element modelling of garnet growth during subduction: examples from ultrahigh-pressure eclogite of the Western Gneiss Region, Norway

    EPSL

    (2008)
  • M. Konrad-Schmolke et al.

    Fluid migration above a subducted slab—thermodynamic and trace element modelling of fluid–rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)

    Earth and Planetary Science Letters

    (2011)
  • P. Lanari et al.

    Diachronous evolution of the alpine continental subduction wedge: evidence from P-T estimates in the Brianonnais Zone houillère (France - Western Alps)

    Journal of Geodynamics

    (2012)
  • P. Lanari et al.

    XMapTools: a MATLAB-based program for electron microprobe X-ray image processing and geothermobarometry

    Computers & Geosciences

    (2014)
  • A. Robert et al.

    Structural and thermal characters of the Longmen Shan (Sichuan, China)

    Tectonophysics

    (2010)
  • F. Roger et al.

    The tectonic evolution of the Songpan-Garze (North Tibet) and adjacent areas from Proterozoic to Present: a synthesis

    Journal of Asia Earth Sciences

    (2010)
  • F. Roger et al.

    Timing of granite emplacement and cooling in the Songpan-Garzê fold belt (eastern Tibetan plateau) with tectonic implications

    Journal of Asian Earth Sciences

    (2004)
  • G. Sanchez et al.

    Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

    Lithos

    (2011)
  • C. Scheffer et al.

    Syn to post-orogenic exhumation of high-grade nappes: structure and thermobarometry of the western Attic-Cycladic metamorphic complex (Lavrion, Greece)

    Journal of Geodynamics

    (2016)
  • B. Worley et al.

    Crenulation cleavage formation: evolving diffusion, deformation and equilibration mechanisms with increasing metamorphic grade

    Journal of Structural Geology

    (1997)
  • B.A. Worley et al.

    Deformation partitioning and foliation reactivation during transpressional orogenesis, an example from the Central Longmen Shan, China

    Journal of Structural Geology

    (1996)
  • B. Yardley et al.

    Origin of retrograde fluids in metamorphic rocks

    Journal of Geochemical Exploration

    (2000)
  • L. Airaghi et al.

    Total vertical offset of the Beichuan Fault (Longmen Shan, China) deduced from metamorphic minerals

    (2012)
  • M.P. Atherton

    The variation in garnet, biotite and chlorite composition in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to the zonation in garnet

    Contributions to Mineralogy and Petrology

    (1968)
  • E.F. Baxter et al.

    Garnet growth as a proxy for progressive subduction zone dehydration

    Geology

    (2013)
  • T.H. Bell et al.

    Complex microstructures preserved in rocks with a simple matrix: significance for deformation and metamorphic processes

    Journal of Metamorphic Geology

    (1999)
  • R.G. Berman

    Internally consistent thermodynamic data for minerals in the system Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-SiO2-TiO2-H2O-CO2

    Journal of Petrology

    (1988)
  • R.G. Berman

    Mixing properties of Ca-Mg-Fe-Mn garnets

    American Mineralogist

    (1990)
  • R.G. Berman

    Thermobarometry using multi-equilibrium calculations: a new technique, with petrological applications

    The Canadian Mineralogist

    (1991)
  • O. Beyssac et al.

    Graphitization in a high-pressure, low-temperature metamorphic gradient: a Raman microspectroscopy and HRTEM study

    Contributions to Mineralogy and Petrology

    (2002)
  • A.P. Boyle et al.

    Metamorphic peak geothermobarometry in the Furulund Group, Sulitjelma, Scandinavian Caledonides: implications for uplift

    Journal of Metamorphic Geology

    (1992)
  • I. Cantarero et al.

    Long-term fluid circulation in extensional faults in the central Catalan Coastal Ranges: P-T constraints from neoformed chlorite and K-white mica

    International Journal of Earth Sciences

    (2014)
  • A.D. Chapman et al.

    Metamorphic evolution, partial melting and rapid exhumation above an ancient flat slab: insights from the San Emigdio Schist, southern California

    Journal of Metamorphic Geology

    (2011)
  • S.F. Chen et al.

    Tectonic transition from the Songpan-Garzê Fold Belt to the Sichuan Basin, southwestern China

    Basin Research

    (1995)
  • J.W. Cosgrove

    The formation of slaty cleavage

    Journal of the Geological Society of London

    (1976)
  • C. de Capitani et al.

    The computation of equilibrium assemblage diagrams with Theriak/Domino software

    American Mineralogist

    (2010)
  • T.J. Dempster

    Zoning and recrystallization of phengitic micas: implications for metamorphic equilibration

    Contributions to Mineralogy and Petrology

    (1992)
  • J.F.A. Diener et al.

    Influence of ferric iron on the stability of mineral assemblages

    Journal of Metamorphic Geology

    (2010)
  • B. Dubacq et al.

    Dehydration of dioctahedral aluminous phyllosilicates: thermodynamic modelling and implications for thermobarometric estimates

    Contributions to Mineralogy and Petrology

    (2010)
  • T.P. Evans

    A method for calculating effective bulk composition modification due to crystal fractionation in garnet-bearing schist: implications for isopleth thermobarometry

    Journal of Metamorphic Geology

    (2004)
  • M.L. Fuhrman et al.

    Ternary feldspar modeling and thermobarometry

    American Mineralogist

    (1988)
  • F. Gaidies et al.

    THERIA_G : a software program to numerically model prograde garnet growth

    Contributions to Mineralogy and Petrology

    (2008)
  • Cited by (0)

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