Magmatic evidence for Late Carboniferous-Early Permian slab breakoff and extension of the southern Mongolia collage system in Central Asia

<p>Oceanic subduction and its last underthrusted part can both triggers arc-like magmatism. As the existence of multi-subduction zones in the Central Asian Orogenic Belt, controversy still surrounds on when and especially how the subduction of the (Paleo-Asian Ocean) PAO terminated. We present geochronological, geochemical, and Lu-Hf isotopic data for a suite of basalt-andesites, dacite-rhyolites and later trachyandesite-mugearitic dykes from the Khan-Bogd area in the Gobi Tianshan Zone (GTZ) of the southern Mongolia. U-Pb dating of zircons indicate the basalt-andesites and dacite-rhyolites were formed at ~334-338 Ma, and the dykes at ~300 Ma. These Early Carboniferous volcanic rocks display high U/Th, Ba/Th, low La/Sm and variable Zr/Nb ratios, implying the involvement of subduction fluids or sediment melt. They display arc geochemical features such as calc-alkaline and metaluminous nature and positive Ba and U and negative Nb, Ta and Ti anomalies. Moreover, their continental geochemical signals (e.g. positive Pb, K anomalies) and some old captured zircons implying a continental arc setting. Comparatively, the ~300 Ma dykes are characterized by high alkaline contents, which are common for coeval (~320-290 Ma) and widespread post-subductional granites there. Given a mainly crust-derived magma source for those granites, these dykes likely reflect a mantle disturbance due to: (1) their relative low SiO<sub>2 </sub>(51.71-55.85 wt. %) and high Mg# (40.3-67.3) values, and (2) positive zircon Ɛ<sub>Hf</sub>(t) (most > 12). Considering a slab rollback model during the Carboniferous and Triassic, the mantle disturbance was possibly induced by the oceanic slab breakoff. Combined with previous work, this ~320-290 Ma slab breakoff-induced extension marks the closure of a wide secondary ocean (North Tianshan-Hegenshan ocean) north of the main ocean basin of the PAO. This research was financially supported by NSFC Projects (41730213, 42072264, 41902229, 41972237) and Hong Kong RGC GRF (17307918).</p>

A role for subducted albite in the water cycle and alkalinity of subduction fluids

Albite is one of the major constituents in the crust. We report here that albite, when subjected to hydrous cold subduction conditions, undergoes hitherto unknown breakdown into hydrated smectite, moganite, and corundum, above 2.9 GPa and 290 °C or about 90 km depth conditions, followed by subsequent breakdown of smectite into jadeite above 4.3 GPa and 435 °C or near 135 km depth. Upon the hydration into smectite, the fluid volume of the system decreases by ~14 %, whereas it increases by ~8 % upon its dehydration into jadeite. Both the hydration and dehydration depths are correlated to increases in seismicity by 93 % and 104 %, respectively, along the South Mariana trench over the past 5 years. Moreover, the formation of smectite is accompanied by the release of OH− species, which would explain the formation of moganite and expected alkalinity of the subducting fluid. Thus, we shed new insights into the mechanism of water transport and related geochemical and geophysical activities in the contemporary global subduction system.

Genesis of ca. 850-835 Ma high-Mg# diorites in the western Yangtze Block, South China: Implications for mantle metasomatism under the subduction process

<p>High-Mg<sup>#</sup> (molar 100 × Mg/(Mg + Fe)) diorites can provide significant insights on the mantle metasomatism under the subduction zone. Here we investigate the genesis of Neoproterozoic high-Mg<sup>#</sup> diorites in the western Yangtze Block to constrain mantle metasomatism during the subduction process. Zircon U-Pb dating results display new weighted mean <sup>206</sup>Pb/<sup>238</sup>U ages of 850.1 ± 1.7 Ma, 840.9 ± 2.4 Ma, and 836.6 ± 1.9 Ma for these high-Mg<sup>#</sup> diorites. They are metaluminous and calc-alkaline rocks, and characterized by moderate SiO<sub>2 </sub>(57.08–61.12 wt.%), high MgO (3.36–4.30 wt.%) and Mg<sup>#</sup> values (56–60). The relatively low initial <sup>87</sup>Sr/<sup>86</sup>Sr ratios (0.703406 to 0.704157), highly positive whole-rock εNd(t) (+3.26 to +4.26) and zircon εHf(t) values (+8.43 to +13.6) imply that they were predominantly sourced from depleted lithospheric mantle. These high-Mg<sup>#</sup> diorites also show the enrichment of large ion lithophile elements (LILEs, e.g., Rb, Ba, K, and Sr) and depletion of high field strength elements (HFSEs, e.g., Nb, Ta, Zr, and Hf), resembling typical arc magma affinity. The highly variable Rb/Y, Th/Ce, Th/Sm, and Th/Yb ratios indicate the significant incorporation of subduction-related fluids and sediment-derived melts into primary mantle source. We therefore propose that the ca. 850–835 Ma high-Mg<sup>#</sup> diorites in this study were formed by the partial melting of metasomatized mantle source influenced by subduction fluids and sediment melts. Our new data, in conjunction with numerous studies of metasomatized mantle magmatism from the western Yangtze Block, suggest that Neoproterozoic mantle sources were progressively metasomatized by the subduction-related compositions from slab fluids, sediment melts, to oceanic slab melts during persistent subduction process.</p>

The Fate of Subduction Fluids above the Subduction Interface: Implications for Mantle Wedge Deformation Processes

<p>The physical and mechanical processes rooted in the hydrated, serpentinized mantle above subduction zones (the “cold nose”) remain debated and poorly understood, despite fundamental consequences on the elastic loading of the seismogenic interface. The fluids crossing this interface are expected to generate nests of seismicity and at the same time weaken the interface hanging wall through serpentinization and metasomatic processes. Ultramafic and jadeitite samples from two natural laboratories where such fossil settings are now visible at the Earth’s surface are used here to document multi-scale deformation mechanisms and fluid-rock interaction processes. Field relationships enable tracking the pathways followed by the fluids during HP metamorphism. Petrographic, geochemical, geochronological and microstructural observations demonstrate the complex interplay between brittle and plastic deformation processes throughout the gradual hydration of the cold nose mantle over millions of years. Changes in bulk rock geochemical and paragenetic sequence also reveal the evolution of the composition of the fluid source through time. These results shed light on the geometry of the cold nose above the interface, with implications for volatile budget estimates, rheology of the plate interface (including the various types of seismicity) as well as the interpretation of Vp/Vs ratios from active subduction settings worldwide.</p>

Earthquakes track subduction fluids from slab source to mantle wedge sink

Subducting plates release fluids as they plunge into Earth’s mantle and occasionally rupture to produce intraslab earthquakes. It is debated whether fluids and earthquakes are directly related. By combining seismic observations and geodynamic models from western Greece, and comparing across other subduction zones, we find that earthquakes effectively track the flow of fluids from their slab source at >80 km depth to their sink at shallow (<40 km) depth. Between source and sink, the fluids flow updip under a sealed plate interface, facilitating intraslab earthquakes. In some locations, the seal breaks and fluids escape through vents into the mantle wedge, thereby reducing the fluid supply and seismicity updip in the slab. The vents themselves may represent nucleation sites for larger damaging earthquakes.