Evolution of the subcontinental lithospheric mantle beneath accretionary orogens: Implications for the stabilization of cratons

Abstract

Thick lithospheric roots characterized by low density and high viscosity are crucial for the long-term stability of cratons. Examining the thick lithospheric roots currently developing in young continental terranes provides valuable insights into the processes driving cratonic stabilization. This study presents a comprehensive elemental and Re-Os isotopic investigation of 103 mantle peridotite xenoliths from the western Xing'an Mongolia Orogenic Belt (XMOB), a component of the Central Asian Orogenic Belt. Most peridotites experienced low to moderate degrees of polybaric fractional melt extraction (4–25%), while hydrous melting (∼30 %) was superimposed on a few refractory residues with low densities similar to cratonic counterparts. Geothermobarometry and geophysical observations indicate that the lithospheric root beneath the western XMOB is up to 60 km thicker than at its initial stage. The peridotites with Mesoproterozoic to Phanerozoic Re-depletion Os model ages match temporally with overlying crust, while those with Archean model ages represent mantle fragments derived from either heterogeneous asthenosphere or adjacent Archean cratonic roots. During the Mesoproterozoic to Neoproterozoic, this mixture of ancient fragments and ambient mantle underwent decompressional melting at spreading centers. With further hydrous re-melting and thickening, these newly formed blocks were assembled during a major Phanerozoic accretionary orogeny. This model is interpreted to indicate that since the onset of plate tectonics, accretionary orogenesis has contributed to both lithospheric thickening and enhancement of lithospheric mantle compositional buoyancy and viscosity, thereby laying a structural foundation for the stabilization of potential cratons.