Mantle Response to Slab Breakoff in the North Qaidam Tectonic Belt: Geochemical Constraints from Syn‐subduction Mafic Igneous Rocks
Authors: LI Xiucai , NIU Manlan , CAI Qianru , WU Qi , SUN Yi , YUAN Xiaoyu , LI Chen 
Published: November 2020

Slab breakoff originally denotes the detachment of dense subducted oceanic slab from the light subducted continental slab, which is driven by opposing buoyancy forces during continental collision (Davies and von Blanckenburg, 1995; von Blanckenburg and Davies, 1995). The breakoff of subducted oceanic slab can induce the upwelling of sub‐slab asthenosphere through the slab window, and then heat the overriding lithospheric mantle resulting in the melting of its fertile layer within the metasomatic mantle wedge. The decompression partial melting of uprising asthenospheric mantle commonly produce mafic magma with depleted MORB‐like geochemical signatures (Davies and von Blanckenburg, 1995; Cole et al., 2006; Wang et al., 2018), whereas the partial melting of enriched lithospheric mantle will produce mafic magma with alkaline, calc‐alkaline or ultrapotassic features (von Blanckenburg and Davies, 1995). These mafic magmas rise into overlying lower crust and trigger crustal melting to generate the granitic magma. The North Qaidam tectonic belt (NQTB) records the evolutionary process of the South Qilian Ocean from subduction to closure. The subduction of oceanic and continental lithosphere to mantle depths is proven by the identification of oceanic‐type and continental‐type eclogites enclosed in crustal metapelite and gneiss from the North Qaidam tectonic belt (Song et al., 2006; Zhang et al., 2008; Zhang et al., 2010; Zhang et al., 2017). However, details of this process are not exactly constrained, in particularly, the closure timing of South Qilian Ocean. The study of characteristic mafic magmatism, combined with the previous studies of ultra‐high pressure metamorphism, give us an excellent opportunity to trace the detailed processes associated with the transition from oceanic subduction to continental subduction, and assess the feasibility of slab breakoff in the North Qaidam tectonic belt.

In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr‐Nd‐Hf isotopes is performed on the mafic igneous rocks from Chahanhe area in the North Wulan gneiss complex. These mafic igneous rocks can be divided into two groups, namely, arc‐like type and E‐MORB type based on their trace element patterns. Arc‐like mafic rocks (441–428 Ma) were characterized by enrichment of light rare earth elements (LREEs), large ion lithophile elements (LILEs) and depletion of heavy rare earth elements (HREEs), high field strength elements (HFSEs). Combined with variable zircon ∊Hf(t) values of −6.17 to +1.58, it is suggested that arc‐like mafic rocks are predominantly derived from the partial melting of the enriched lithospheric mantle, and minor juvenile materials have contributed to their sources. The E‐MORB mafic rocks (440 Ma) exhibit relatively flatted REE patterns and positive ∊Nd(t) values of +1.63 to +4.28, but high (87Sr/86Sr)i ratios of 0.706825 to 0.708979, indicting a derivation from partial melting of asthenospheric mantle, with involvement of enriched components probably derived from ambient lithospheric mantle or stagnant subducted oceanic crust. Collectively, it is proposed that the break‐off of the subducted South Qilian oceanic slab triggered the decompression melting of asthenospheric mantle, and the upwelling of asthenosphere provided heat and induced partial melting of the enriched lithospheric mantle and pre‐existing crust, resulting in generation of arc‐like mafic rocks and widespread granites.