Igneous records of transformation from ocean to continent of Mongolia–Okhotsk Ocean: Evidence from granitoids of Woduhe area, Heilongjiang Province, China

The Woduhe area of Heilongjiang Province is located in the eastern side of the Greater Xing'an Range. This area has experienced the evolution of the Palaeo-Asian Ocean, the Palaeo-Pacific, and the Mongolia-Okhotsk Ocean, and it can be used to discuss the transformation process of these palaeo-oceanic plates to continental plates. However, controversies exist over its time period and the process of transformation. The 1:250000 Woduhe Geological Map shows that Early Cretaceous granitic batholith developed near the Dajinshan area. However, the LA-ICP-MS zircon U-Pb dating presented in this paper shows that the trondhjemites from the southern part of the Dajinshan area were formed at 156.9 +/- 2.2 Ma, syenogranites were formed at 131.7 +/- 1.6 Ma, and diorite porphyrite dikes were formed at 126.0 +/- 3.1 Ma. Therefore, Early Cretaceous batholiths should be reclassified as different, separate units. The Late Jurassic intrusive rocks belong to TTGG assemblages ( tonalite-trondhjemite-granodiorite-granite). Trace element spider diagrams show that the rocks are enriched in K, Pb, Zr, and Hf; depleted in Nb, Ta, and Eu; with higher Sr; and lower Y content. The rare earth element pattern is right-sloping ( La/Yb > 12). The Early Cretaceous intrusive rocks belong to G assemblages (granodiorite). Combined with the tectonic settings, we determined that the Late Jurassic and the Early Cretaceous granites were formed in a magmatic arc setting related to subduction. Late Jurassic intrusive magma mainly originated from the partial melting of the thickened lower crust under the Xing'an arc, and was likely contaminated by mantle material. Early Cretaceous intrusive magma originated from the partial melting of the Xing'an sub-arc crust. Based on the distribution of the Late Jurassic TTG (N and NE) and G (S and SW) assemblages, intrusive records of the southward and southwestward subduction of the Mongolia-Okhotsk Ocean can be observed. In addition, the grade of maturity of the Xing'an arc is increasing during Late Jurassic to Early Cretaceous.