Abstract

Tibet, which is characterized by collisional orogens, has undergone the process of delamination or convective removal. The lower crust and mantle lithosphere appear to have been removed through delamination during orogenic development. Numerical and analog experiments demonstrate that the metamorphic eclogitized oceanic subduction slab or lower crust may promote gravitational instability due to its increased density. The eclogitized oceanic subduction slab or crustal root is believed to be denser than the underlying mantle and tends to sink. However, the density of eclogite under high-pressure and high-temperature conditions and density differences from the surrounding mantle is not preciously constrained. Here, we offer new insights into the derivation of eclogite density with a single experiment to constrain delamination in Tibet. Using in situ synchrotron X-ray diffraction combined with diamond anvil cell, experiments focused on minerals (garnet, omphacite, and epidote) of eclogite are conducted under simultaneous high-pressure and high-temperature conditions, which avoids systematic errors. Fitting the pressure-temperature-volume data with the third-order Birch-Murnaghan equation of state, the thermal equation of state (EoS) parameters, including the bulk modulus (KT0), its pressure derivative (KT0′), the temperature derivative ((KT/T)P), and the thermal expansion coefficient (α0), are derived. The densities of rock-forming minerals and eclogite are modeled along with the geotherms of two types of delamination. The delamination processes of subduction slab breakoff and the removal of the eclogitized lower crust in Tibet are discussed. The Tibetan eclogite which containing 40–60 vol. % garnet and 37–64 % degrees of eclogitization can promote the delamination of slab break-off in Tibet. Our results indicate that eclogite is a major controlling factor in the initiation of delamination. A high abundance of garnet, a high Fe-content, and a high degree of eclogitization are more conducive to instigating the delamination.

Highlights

  • The evolution of orogenesis is characterized by lithospheric removal during rapid surface uplift, mantle upwelling, and postcollisional magmatism, in the Central Andes (e.g. Ehlers and Poulsen, 2009; Schurr et al, 2006), Himalayas (e.g. Jiménez-Munt et al, 2008; Singh andKumar, 2009), and Dabie orogen (e.g. He et al, 2011; Zhang et al, 2010).It is widely accepted that delamination is the most important mechanism of lithospheric removal

  • The first is the conventional definition of delamination proposed by Bird (1978, 1979), which was used to interpret the geodynamic evolution of the Colorado Plateau

  • The eclogitization of the subducted slab and lower crust plays a vital role in the process of delamination due to the high density of eclogite (Anderson, 2005; Lee et al, 2011), which makes the formation denser than the surrounding mantle lithosphere and provides critical negative buoyancy (Göğüş and Ueda, 2018; Krystopowicz and Currie, 2013)

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Summary

Introduction

The evolution of orogenesis is characterized by lithospheric removal during rapid surface uplift, mantle upwelling, and postcollisional magmatism, in the Central Andes (e.g. Ehlers and Poulsen, 2009; Schurr et al, 2006), Himalayas The first is the conventional definition of delamination proposed by Bird (1978, 1979), which was used to interpret the geodynamic evolution of the Colorado Plateau In this scenario, mantle lithosphere peels back from the overlying upper crust and is removed entirely, with the rising hot mantle filling the lithospheric removal zone (e.g. Göğüş and Ueda, 2018; Krystopowicz and Currie, 2013; Schott and Schmeling, 1998; Sobolev and Babeyko, 2005). In addition to conventional delamination, an alternative delamination mechanism is convective removal based on the Rayleigh-Taylor-type instability model (Houseman et al, 1981), namely, viscous “dripping” This model postulates that there is sufficient perturbation in the lithospheric mantle, which is ascribed to the strong temperature-dependence of typical mantle rheology, without regard to a specific weak layer (e.g. Conrad and Molnar, 1999; Gorczyk et al, 2012; Houseman and McKenzie, 1982; Schott and Schmeling, 1998). Using a simplistic calculation setup, in this study, this density evolution model will shed light on the possibility of delamination during the orogen process

Geological background
Starting material
Synchrotron X-ray diffraction
EoS of main minerals for eclogite
Garnet
Omphacite
Epidote
The density of main minerals for eclogite along the geothermal profile in Tibet
The density of eclogite in Tibet
Influence of the degree of eclogitization on the density of the subducted slab
Delamination in Tibet
Subducted slab breakoff
Removal of the eclogitized lower crust
Findings
Conclusion
Full Text
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