Southward subduction of the Mongo-Okhotsk Ocean Middle Triassic magmatic records of the Luomahu Group in northwest of Lesser Khingan Mountains

Luomahu Group outcropped in the northwest of Lesser Xing'an Range is considered to be part of the Neoproterozoic metamorphic basement of the Xing'an Massif.Based on zircon U-Pb dating of the biotite-hornblende-plagioclase gneiss from the Luomahu Group outcropped near Woduhe Township, the results show that the age of protolith is 246±1 Ma.Combined with the related regional research data, it is suggested that the Luomahu Group is a set of tectonic complex containing geological bodies of different ages.The petrography and geochemistry of the biotite-hornblende-plagioclase gneiss indicate that the protolith is a basic magmatic rock, rich in MgO, TFeO and Mg^# value, relatively rich in Na₂O, low in K₂O, and mediate contents of Cr, Ni and Co, similar to the average composition of the lower crust.In addition, large ion lithophile elements Rb, Ba, Sr, Th are enriched, and high field strength elements Nb, Ta, Ti, P, Hf are depleted.The in-situ ε_Hf (t) values of zircons range from + 9.55 to + 14.43, and the Hf isotope single-stage model ages (t_DM1) and two-stage model ages(t_DM2) are 319~520 Ma and 350~666 Ma, respectively.The Hf isotopic composition is similar to the Phanerozoic igneous rocks of Xing'an island arc in Central Asia Orogenic Belt.It is suggested that the magma was originated from the partial melting of the "hydrated" Paleozoic hyperplasia lithospheric mantle and was mixed with continental crust materials in the Duobaoshan island arc zone, related to the southward subduction of the Mongolia-Okhotsk Ocean plate and controlled by the NNE trending intracontinental transform tectonic belt.