The discovery of Carboniferous high-Mg diorites and adakites in the Jalaid Banner area, the central Great Xing'an Range and their implications for the subduction of the Nenjiang ocean

The Late Paleozoic magmatism in the Jalaid Banner area is of great constraint significance in revealing the subduction process and closure mechanism of the Nenjiang ocean. Through field geological survey, petrology, rock geochemistry and zircon U−Pb dating, early Late Carboniferous high−Mg diorites (weighted average age 320.5±1.2 Ma) and O−type adakites (weighted average age 317.9±3.3 Ma) have been discovered in Yuanbaoshan and Jiajiatun, Jalaid Banner. The Yuanbaoshan high−Mg diorite is characterized by low SiO₂, high MgO, high Mg ^#, rich Na₂O, and high Ni and Cr contents. It is rich in light rare earth elements (LREE) and large ion lithophilic elements (LILE), loss of high field strength elements (HFSE). It is considered that the partial melting of subduction plates results in the formation of Si−rich fluids, which displace mantle rocks and cause them to form after partial melting. The Jiajiatun quartz diorite is characterized by high SiO₂, Na₂O, Sr and low Yb, Y, K₂O/Na₂O, strong REE differentiation, enrichment of LREE and LILE, loss of HFSE, no negative Eu anomaly, poor Mg, Cr, Ni. It is an O−type adakite which formed by partial melting of subtractive plates and crystallization differentiation. The newly discovered adakite−Sanukite rock assemblage in the early Late Carboniferous is consistent with the formation time of the Carboniferous granite in the study area (333~304 Ma), and the high−Mg diorite is associated with gabbro (325.2~317.3 Ma). It is considered that the Jalaid Banner area was located in the active continental margin arc at the western edge of the Songnen massif during this period. At least since the early Late Carboniferous, the Nenjing ocean has been subducted to the southeast beneath the Songnen massif.