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
Delamination is induced and accompanied by two major requisites: (a) the density difference caused by the negative buoyancy of the delaminated lithosphere; and (b) the presence of a weak lower crust that exists between the strong upper crust and lithospheric mantle
The pressure-volume-temperature (P-V-T) data in this study are fitted by the high-temperature third-order Birch-Murnaghan-equation of state (EoS) (HT-BM3-EoS) (Birch, 1947) to obtain the thermal EoS
Summary
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 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). The Neo-Tethyan oceanic subduction, India-Asia collision, and Indian continental subduction could be further considered responsible for the abnormal thinning of the mantle lithosphere under Tibet (Chung et al, 2005; DeCelles et al, 2011; Li et al, 2019; Ma et al., 2017; Xu et al, 2008; Zhao et al, 2020). Slab breakoff and convective lithospheric removal under Tibet are adopted as the background in this study to discuss the possibility of the delamination process
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