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Gneiss domes of the Danba Metamorphic Complex, Songpan Ganze, eastern Tibet

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Highlights

  • Danba Metamorphic Complex (DMC) exhumation due to extrusion.

  • Correspondence between isograds and isotherms shown by field data and thermometry.

  • Isograds and isotherms are folded during the extrusion deformation event.

  • Extrusion of deep hot material induces heat advection in the DMC surrounding rocks.

  • Presence of three kind of gneiss domes explained by long term structural inheritance.

Abstract

In this paper we address the formation and exhumation of the Danba Metamorphic Complex (DMC) that represents the deepest structural level of the Songpan Ganze terrane situated along the eastern margin of the Tibetan plateau. The DMC comprises a variety of gneiss domes and offers a unique opportunity to decipher their development during orogenic evolution. For that purpose, PTtD paths of metamorphic rocks sampled at different structural levels have been reconstructed. The DMC is composed of Triassic metaturbidites of the Xikang group, Paleozoic metasedimentary cover and basement of the Yangtze craton. The DMC is structurally marked by transposition of the upright S1 foliation of the Triassic metaturbidites into a NW-SE trending S2 composite foliation dipping to the NE. Transposition is associated with a localized top-to-the-northeast shear zone along the northeastern edge of the DMC and with pervasive top-to-the-southwest shearing from the core to the border of the complex. These structures are consistent with extrusion of the core of the DMC relative to the lower grade Triassic metaturbidites. The position of the biotite isograd overlapping the structural boundary of the DMC suggests that the Triassic metaturbidites have been affected by an increase in temperature as a result of extrusion. Within the DMC, the position of the metamorphic index minerals relative to the composite S2 foliation reveals that biotite, garnet, staurolite and kyanite grew before the transposition into S2, in contrast with sillimanite which crystallizes in the hinge of F2 folds and along the axial planar S2 schistosity. The sillimanite isograd delineates regional-scale overturned F2 folds and cross-cuts the staurolite and kyanite isograds consistent with an increase in temperature during D2. The melt-in isograd characterizes the deepest structural level of the DMC. PT conditions for these metamorphic rocks, determined using pseudosections and conventional thermometry, indicate a temperature increase from 400 °C to more than 600 °C from the edge to the core of the DMC for a relatively homogeneous pressure ranging from 5 to 6.5 kbar suggesting that isograds and isotherms represent the syn-D2 thermal structure of the orogenic crust. Migmatites exposed in the deepest structural level of the DMC yield a pressure significantly lower than the surrounding metamorphic rock suggesting that they crystallized after D2 and after some exhumation of their hosts. Three different types of gneiss domes are distinguished on the basis of their position relative to the isograds, their structural characteristics, and their position relative to the margin of the Yangtze craton. Close to the craton and at the highest structural level, the Gezong dome represents a basement-rooted tectonic slice, in an intermediate position, the Gongcai dome corresponds to a basement-cored nappe, and further away and at the deepest structural level, the Bawang, Cunuchan and Qingaling domes are migmatite-cored domes. The presence at the current-day surface of this variety of gneiss domes reflects the difference in burial of the margin during the Mesozoic Indosinian orogeny.

Introduction

Orogenic belts typically expose metamorphic rocks opening the question of the conditions required for their formation and of the mechanisms leading to their exhumation. Among these metamorphic rocks, gneisses call for special attention as the identification of their protolith is not straightforward. This paper is interested in integrating the formation of metamorphic complexes and gneiss domes in the geodynamic evolution of orogenic belts. The exhumation of metamorphic complexes is invoked either during crustal thickening, attributing a predominant role to erosion and tectonics (Armstrong, 1982, Thompson et al., 1997, Malavieille and Konstantinoskaya, 2010), or during crustal thinning, recognizing the impact of low angle detachments characterizing the so-called metamorphic core complexes (Armstrong, 1972, Wernicke, 1981). Gneiss domes, typically associated with metamorphic complexes, are characterized by a core of crystalline rocks surrounded by metasediments and might correspond to exhumed basement rocks or to migmatites (Eskola, 1949, Whitney et al., 2004, Yin, 2004). Basement rocks might be exposed as basement-rooted tectonic slices or as basement-cored metamorphic nappes. Basement-rooted tectonic slices are marked by localized deformation preserving a textural-structural and magmatic-metamorphic heritage from previous magmatic-tectonic history, as exemplified by the external crystalline massifs exposed in the Alpine belt (e.g. Schimd et al., 2004, Choukroune, 1992) or in the Longmen Shan belt (e.g. Burchfiel et al., 2008, Robert et al., 2010). In the case of basement-cored nappes, deformation and metamorphism are more pervasive and the texture-structure and magmatic-metamorphic assemblage from previous magmatic-tectonic history are transposed during nappe emplacement as evidenced by the penninic nappes exposed in the internal zone of the Alps (e.g. Schimd et al., 2004) or the Tso Morari dome in the Himalayas (e.g. de Sigoyer et al., 2004). In the case of partial melting, the synmigmatitic structure and mineral assemblage are representative of deformation and crystallization of the partially molten rocks, and the inherited structure and metamorphic assemblage of the protoliths are only locally preserved (Brown, 1973, Sawyer et al., 1999, Vanderhaeghe, 2001). Such migmatitic gneisses are exposed in channels as the High Himalayan Crystalline (Le Fort, 1975, Grasemann et al., 1999) or in domes such as the Montagne Noire and the Velay dome in the Variscan French Massif Central (Van Den Driessche and Brun, 1992, Burg and Vanderhaeghe, 1993, Ledru et al., 2001, Rey et al., 2012) or in the northern part and in the syntaxes of the Himalaya (Le Fort et al., 1987, Burg and Chen, 1984, Rolland et al., 2001, Maheo et al., 2002). The driving forces invoked for the development of gneiss domes encompass tectonic, isostatic, and/or buoyancy forces (Yin, 2004). Tectonic forces control the formation of basement-rooted tectonic slices and basement-cored nappe or might lead to the formation of domal structures as a result of the superposition of folds with perpendicular shortening directions (Myers and Watkins, 1985). Isostasy is involved in the case of flow of a low-viscosity layer induced by a pressure gradient caused by heterogeneous deformation as is invoked for the development of metamorphic core complexes (Brun et al., 1994, Rey et al., 2012). The buoyancy force is appealed when buoyant and low-viscosity partially molten rocks and magmas are generated during orogenic evolution leading to the development of gravitational instabilities (Vanderhaeghe, 2009, Eskola, 1949, Ramberg, 1980, Ramberg, 1981, Brun et al., 1981, Choukroune, 1992).

Determining the driving forces and the tectonic context of the development of metamorphic complexes and gneiss domes requires a multimethod approach combining structural, petrological and geochronological analyses leading to the reconstruction of PTtD paths of rocks exhumed from different structural levels (Yin, 2004, Whitney et al., 2004, Vanderhaeghe, 2009, Vanderhaeghe, 2012). The Danba Metamorphic Complex (DMC), where the deepest structural level of the Songpan Ganze terrane in eastern Tibet outcrops, exposes rocks characterized by a complex superposition of structures and metamorphic parageneses and comprises several gneiss domes (Fig. 1). This metamorphic complex represents a perfect target to decipher the dynamical evolution recorded by high-grade terranes forming the exhumed root of an orogenic belt. In this work, we reassess the structural and petrologic record of the DMC based on new field investigations and analytical data integrated with the published ones (Huang et al., 2003a, Huang et al., 2003b, Wallis et al., 2003, Harrowfield and Wilson, 2005, Itaya et al., 2009, Weller et al., 2013, Jolivet et al., 2015) . Due to the complex superposition of structures, an unequivocal kinematic analysis is precluded. In order to circumvent this difficulty, a wide set of structures at various scales has been considered and particular attention has been devoted to the identification of the position of index metamorphic minerals relative to microstructures established by the analysis under the microscope of 112 thin sections. This work allows to reconstruct the geometry of isograds relative to the main structures and to the gneiss domes. The metamorphic history of the DMC is further constrained by thermobarometric study of 11 thin sections from samples representative of metapelites of the different metamorphic zones. At last, we present a new U-Pb age determination on zircon from a migmatite coring the Qingaling dome. We then propose a model for the formation and exhumation of the DMC during the Indosinian orogeny integrating the structural inheritance from the Paleozoic passive margin and the development of a variety of gneiss domes.

Section snippets

Plate tectonics setting

During the Paleozoic, the Paleotethys oceanic domain separated a northern continental margin comprising the Kunlun-Qaidam and the North China continental blocks from the Qiangtang southern continental block (Yin and Harrison, 2000, Pullen et al., 2008, Roger et al., 2008, Roger et al., 2010). Formation of the Songpan Ganze accretionary-orogenic wedge resulted from closure of the Paleotethys Ocean along two main subduction zones active from the Lower Permian to the Upper Triassic (Pullen et al.,

The structural organisation of the DMC

At first sight, the DMC may be defined by the Trias-Permian lithologic boundary delineating a half antiform with an axis plunging toward the NW as described in the geological map of the BGMRSP (1991) reproduced in all subsequent papers (Fig. 3, Huang et al., 2003a, Huang et al., 2003b, Wallis et al., 2003, Harrowfield and Wilson, 2005, Roger et al., 2008, Suhua et al., 2009, Weller et al., 2013, Jolivet et al., 2015). Within this half antiform, lithologic contacts, attributed to stratigraphy,

Tectonic interpretation of the structural and metamorphic record of the Danba Metamorphic Complex

The data presented in this paper, integrated with previously published data, document the structural and petrologic record of the Danba Metamorphic Complex. Following Huang et al., 2003a, Huang et al., 2003b, Harrowfield and Wilson, 2005, we define the boundary of the DMC by the presence of the S2 foliation, transposing the upright folds of the Triassic metaturbidites (Fig. 4, Fig. 5, Table 1).

The structural analysis presented in this paper (Fig. 5, Fig. 6, Fig. 7) concurs to the

Conclusions

The DMC, in the eastern part of the Songpan Ganze terrane along the Yangtze craton, is structurally defined by the transposition of the S1 foliation of the Triassic metaturbidites of the Xikang group into a NW-SE trending composite S2 foliation steeply dipping toward the NE affecting also metasedimentary rocks with a protolith of Paleozoic age. This transposition is associated with kinematic criteria consistent with a top-to-the-northeast sense of shear along the northern boundary of the DMC

Acknowledgements

This work is part of the PhD thesis of Audrey Billerot (G2R, Nancy University, France) funded by the French Ministry of Superior Education and Research. We thank the French National Research Agency (ANR) AA-PJCJC SIMI5-6 LONGRIBA and INSU-CNRS for financial support. We also thank all the students of Zhu Jieshou and Li Yong (from the Chengdu University of Technology at Chengdu) for their helpful contribution to our field work. Alexandre Flammang and Cédric Demeurie are thanked for the confection

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