Late Devonian to early Carboniferous magmatism in the western Songliao-Xilinhot block, Northeast China: Implications for eastward subduction of the Nenjiang oceanic lithosphere

The timing and direction of narrowing and closure of the Nenjiang Ocean (NJO) by plate subduction remain poorly constrained. This paper presents geochemical, isotopic, and geochronological results for Late Devonian to early Carboniferous intrusive rocks in the western Songliao-Xilinhot block to constrain the evolution of subduction in the NJO region. The Late Devonian syenogranites and monzogranites formed at 366-360 Ma and are classified as A-type granites. The wide range of epsilon(Hf)(t) values (-17.62 to +8.13) and T-DM2 ages (789-2,208 Ma) of the syenogranites indicate that the primary magma was derived from partial melting of multiple sources, including both young accreted continental crust and old lower crust rocks. The early Carboniferous granitic mylonites formed at 342 Ma and are I-type granites. Their high SiO2 and K2O contents, epsilon(Hf)(t) values of +6.61 to +9.86, and young T-DM2 ages of 675 to 857 Ma suggest a preexisting juvenile crustal material source region. The early Carboniferous hornblende gabbros formed at 334 Ma and are part of an appinite suite. The gabbros have low silica and high MgO contents, E-MORB-like trace element features, epsilon(Hf)(t) values of +1.34 to +8.97, and young T-DM1 ages of 595 to 902 Ma, which together suggest a primitive magmatic source including juvenile MORB and/or enriched lithospheric mantle. Magmatism in the western Songliao-Xilinhot block formed a late Palaeozoic magmatic arc stretching along the eastern Hegenshan-Heihe suture. A new model of the evolution of subduction in the NJO region and associated coeval magmatic events is proposed that includes westward subduction of NJO oceanic crust during the Late Devonian at 366-360 Ma, bidirectional subduction during the early Carboniferous at similar to 342 Ma, and break-off/delamination of the eastward subducting slab at similar to 334 Ma.