Decompression Melting Of Asthenospheric Mantle Research Articles

Early Paleozoic tectonic transition from oceanic to continental subduction in the North Qaidam tectonic belt: Constraints from geochronology and geochemistry of syncollisional magmatic rocks

The North Qaidam tectonic belt (NQTB) records the tectonic evolution of the South Qilian Ocean from subduction to closure followed by continental subduction. However, the timing of closure of the South Qilian Ocean and the nature of the tectonic transition from oceanic to continental subduction are poorly constrained. In this contribution, an integrated study of petrology, geochemistry, geochronology and Sr–Nd–Hf isotopes is performed on two groups of intermediate–mafic rocks from the Chahanhe intrusive complex in the NQTB. Group 1 intermediate–mafic rocks (441–428 Ma) have arc-like geochemical compositions, high (87Sr/86Sr)i ratios (0.711171 to 0.713181), negative εNd(t) values (−7.22 to −5.49) and low zircon εHf(t) values (−6.3 to +0.5), suggesting that they are derived from partial melting of enriched subcontinental lithospheric mantle. The enriched lithospheric mantle was produced by interaction of the subcontinental mantle wedge peridotite with subducting slab-derived melt and fluid during previous subduction of the South Qilian Ocean. Group 2 mafic rocks (441–439 Ma) exhibit E-MORB-like geochemical compositions with positive εNd(t) values (+2.44 to +3.31) and high zircon εHf(t) values (+1.3 to +6.9), but with high (87Sr/86Sr)i ratios (0.706775 to 0.708661); these features indicate derivation from partial melting of asthenospheric mantle with the involvement of aqueous fluid and minor melt that were probably derived from subducted oceanic crust. We propose that rollback of subducted South Qilian oceanic slab during the initial stage of continental subduction triggered decompression melting of asthenospheric mantle to produce E-MORB-like mafic rocks; convecting and upwelling asthenosphere also provided heat that induced partial melting of the pre-existing enriched lithospheric mantle and generated mafic magmas with arc-like geochemical signatures. Our results improve the understanding of the tectonic transition process from oceanic subduction to continental subduction in the NQTB, the closure of the Proto-Tethyan Ocean, and provide new insights into the importance of crustal growth during initial continental collision.

Mantle Response to Slab Breakoff in the North Qaidam Tectonic Belt: Geochemical Constraints from Syn‐subduction Mafic Igneous Rocks

AbstractSlab 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.

Petrogenesis of Triassic Mafic Complexes with MORB/OIB Affinities from the Western Garzě‐Litang Ophiolitic Mélange, Central Tibetan Plateau

There is a general consensus that most ophiolites formed above subduction zones (Pearce, 2003), particularly during forearc extension at subduction initiation (Shervais, 2001; Stern, 2004; Whattam and Stern, 2011). “Supra‐Subduction zone” (SSZ) ophiolites such as the well‐studied Tethyan ophiolites, generally display a characteristic sequential evolution from mid‐oceanic ridge basalts (MORBs) to island arc tholeiities (IATs) or bonites (BONs) (Pearce, 2003; Dilek and Furnes, 2009, 2011), which were generated in sequence from the decompression melting of asthenospheric mantle and partial melting of subduction‐metasomatized depleted mantle (Stern and Bloomer, 1992; Dilek and Furnes, 2009; Whattam and Stern, 2011). However, ophiolites with MORB and/or oceanic‐island basalt (OIB) affinities are rare, and their origin and tectonic nature are poorly understood (Boedo et al., 2013; Saccani et al., 2013). It is interesting that the composition of these ophiolites from the central Tibetan Plateau (CTP) is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs, which is inconsistent with most ophiolites worldwide (Robinson and Zhou, 2008; Zhang et al., 2008). But the generation and tectonic nature of these ophiolites are still controversial.In this study, we present new geochronological, mineralogical and Sr‐Nd isotopic data for the Chayong and Xiewu mafic complexes in the western Garzě‐Litang suture zone (GLS), a typical Paleo‐Tethyan suture crossing the CTP (Fig. 1). The Triassic ophiolite in the western GLS has been described by Li et al. (2009), who found that it mainly consists of gabbros, diabases, pillow basalts and a few metamorphic peridotites. The ophiolite has been tectonically dismembered and crops out in Triassic clastic rocks and limestones as tectonic blocks. The Chayong and Xiewu mafic complexes are generally regarded as important fragments of the Triassic ophiolites (e.g., Jin, 2006; Li et al., 2009). Zircon LA‐ICP‐MS U‐Pb ages of 234±3 Ma and 236±2 Ma can be interpreted as formation times of the Chayong and Xiewu mafic complexes, respectively. The basalts and gabbros of the Chayong complex exhibit enriched MORB (E‐MORB) compositional affinities except for a weak depletion of Nb, Ta and Ti relative to the primitive mantle, whereas the basalts and gabbros of the Xiewu complex display distinct E‐MORB and OIB affinities. The geochemical features suggest a probable fractionation of olivine ± clinopyroxene ± plagioclase as well as insignificant crustal contamination.The geochemical and Sr‐Nd isotopic data reveal that the Chayong mafic rocks may have been derived from depleted MORB‐type mantle metasomatized by crustal components and Xiewu mafic rocks from enriched lithospheric mantle metasomatized by OIB‐like components. The ratios of Zn/Fet, La/Yb and Sm/Yb indicate that these mafic melts were produced by the partial melting of garnet + minor spinel‐bearing peridotite or spinel ± minor garnet‐bearing peridotite. We propose that back‐arc basin spreading associated with OIB/seamount recycling had occurred in the western GLS at least since the Middle Triassic times, and the decompression melting of the depleted MORB‐type asthenosphere mantle and partial melting of sub‐continental lithosphere were metasomatized by plume‐related melts, such as OIBs, which led to the generation of the Chayong and Xiewu mafic melts.

A Binary Source Model for Extension-Related Magmatism in the Great Basin, Western North America

Models for extension-related magmatism based on decompression melting of asthenospheric mantle poorly simulate fluxes and bulk compositions of magmas produced during early stages of continental extension. For the Great Basin of western North America, it is proposed that magmatism proceeded in two stages, the first involving melting of lithospheric mantle sources between 40 and approximately 5 million years ago (Ma), followed (since approximately 5 Ma) by melting of upwelling asthenospheric mantle in areas where extension has exceeded about 100 percent. This transition in magma sources is diachronous, depending on initial variations in lithosphere thickness and on rates of lithospheric thinning.