The conditions for Oligocene diapiric melting of the subducted mélange in the NE Asia

Subduction zones are the main sites of surficial material transfer from subducted slab into the mantle wedge. Increasing numbers of studies have proposed a material-transport models that subducted mélanges detach as solid-state diapirs from the slab-top and then partially melt at higher temperatures as they ascend through the mantle wedge (Nielsen and Marschall, 2017). While the ability to diapiric melting of subducted mélanges was previously constrained in experimental and numerical models, the conditions for its formation were poorly investigated in actual subduction zones.

Here, we report major- and trace-element, and Sr-Nd-Mg-Zn isotopic results for the Oligocene syenites in NE Asia, inferring their affinity with diapiric melting of subducted mélanges as well as mantle dynamics. Furthermore, we investigate the partial melting behaviors of natural mélanges at estimated P-T conditions at which mélange melting begins. These syenites exhibit Hf-Nd fractionation but little variation in Nd isotopes (Nielsen and Marschall, 2017). Moreover, these syenites have heavy Mg isotopic compositions (δ²⁶Mg=−0.02‰~+0.57‰), consistent with the inferred residual components of mélange after dehydration, jointly supporting the mélange-diapir melting model. Our results and the tectonic setting indicate that melting of mélange diapirs occurred pref­erentially during tectonic transitions, such as the formation of a back-arc basin triggered by trench-perpendicular mantle flow. The low-viscosity mantle with an incompressible stress field triggered melting of the mélange diapirs. We roughly constrain the P-T conditions at which mélange melting begins. These syenites have higher LREEs and HFSEs contents than the experimental melts of subducted mélange, which is consistent with the addition of the carbonated silicate melts derived from the carbonated peridotites. The Zn-Sr-Nd isotopic compositions of syenites exhibit trends toward carbonated peridotites, jointly indicating the interaction between molten subducted mélange and carbonated peridotites. Generation of carbonated silicate melts occurs at ≤6 GPa. Moreover, magnesite was involved in the magmatic processes of carbonated peridotites, as recorded by relatively heavy Zn isotopic compositions with depleted Sr and Nd isotopic compositions. Magnesite is stable at pressures of ≥4.5 GPa. Therefore, the Oligocene mélange diapiric melting possibly occurred at the asthenospheric depths assumed by the seismic tomography (Tamura et al., 2002; Hong et al., 2024).

We further investigate the partial melting behaviors on natural sediment-dominated mélange materials from the NE Asian Margin. We performed a series of three melting experiments using large-volume press at estimated P-T conditions (4-6 GPa, 1300-1400 ℃). Partial melts produced in our experiments have trace-element abundance patterns that are typical of alkaline arc lavas, such as enrichment in LILEs and depletion in Nb and Ta. The major- and trace-element compositions of experimental melts are consistent with the Oligocene syenites in NE Asia. These findings confirmed that mélange diapiric melting more possibly occurred in asthenosphere, which is deeper than the depth inferred in previous studies.

This work was financially supported by the National Natural Science Foundation of China (Grant 42372065 and 424B2017).

 

References:

Hong, et al., 2024, Geology, v. 52, p. 539-544.

Nielsen, and Marschall, 2017, Science Advances, v. 3.

Tamura, et al., 2002, Earth and Planetary Science Letters, v. 197, p. 105-116.