Late Paleozoic magmatic records from northern Beishan orogenic belt and their constraints on the evolution of the Baihuishan Ocean

The nature of mantle source and magmatic evolution during accretionary orogeny are the key issues in the study of orogenic belt evolution. Baiheshan Ocean is an important branch ocean of the Paleo-Asian Ocean in the Beishan region. Its evolution formed the Baiheshan ophiolitic melange belt in the northern Beishan orogenic belt and has retained a clear magmatic record. This work has identified a new set of sillite in the Hongliuxia area in the southern ophiolitic melange belt. The rock shows ophitic texture, porphyritic texture and was intruded at 320±3 Ma, which is the product during the major subduction of the Baiheshan Ocean. The sillite shows high-K calc-alkaline geochemical characteristic and was enriched in large-ion lithophile elements like Rb, Th, U, Sr and light rare earth elements and depleted in high field strength elements like Nb, Ta, Ti, showing characteristics of arc magmatic rocks. The values of ε_Nd(t) and ε_Hf(t) of the sillite vary from 3.09 to 4.29 and 5.8 to 12.3 respectively, implying magma originated from depleted mantle. It may be the product of partial melting of garnet dipyroperidotite restrained by Yb, La and Sm elements. The primary magma underwent limited fractional crystallization and various degrees of crustal contamination. The Early Permian magmatism represents the youngest tectono-magmatic event at the end of the subduction of Baiheshan Ocean. In this work, the porphyritic monzogranite was intruded at 275.6±1.4 Ma, belonging to high-K calc-alkaline I-type grantie. They are enriched in large-ion lithophile elements like Rb, Th, U and light rare earth elements and depleted in high field strength elements like Nb, Ti. The values of ε_Hf(t) and T_DM²of the porphyritic monzogranite vary from 9.1 to 14.6 and 368 to 719 respectively, implying magma originated from partial melting of the juvenile crust from Neoproterozoic to Pzaeozoic. Combined with regional geological material, the overall trend of magmatic evolution during the subduction of Baiheshan Ocean is rich in potassium, alkali and depleted in aluminum. It is suggested that the end of the accretionary orogeny is early to middle Permian in the northern Beishan orogenic belt.