Abstract:

The volcanic rocks of  Tamulangou Formation in southern Manchuria are composed mainly of olivine basalt, pyroxene basalt and basaltic andesite. LA-ICP-MS zircon U-Pb dating results show that the volcanic rocks formed at the end of Middle Jurassic, ranged from 164 Ma to 161 Ma. Geochemical data show that the volcanic rocks belong to high-K calc-alkaline series, have low contents of MgO(1.64%-9.59%) and small Mg^# (0.26-0.66); The chondrite-normalized rare earth element (REE) patterns indicate that the volcanic rocks are enriched in light rare earth elements(LREE) with significant fractionation of HREE and LREE ((La/Yb)_N=6.86-22.29) and weak Eu anoalies(δEu=0.70-0.93); The trace element geochemistry are characterized by enrichment in large ionic lithophile elements(e.g. Rb, Ba, K) and LREEs, depletion in high field strength elements(e.g. Nb, Ta, P, Ti) and compatible elements (Cr, Co, Ni); The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios of the magmatic zircons from volcanic rocks vary from 0.282 691 to 0.282 740, and the corresponding ε_Hf(t) values range from 0.67 to 2.30. The geochemical data suggest that the magma could be derived from partial melting of lithosphere mantle which had been metasomatized by subduction fluids, and probably experienced fractional crystallization of olivine and clinopyroxene,and insignificant crustal contamination. Combined with the regional research results, we suggest that the metasomatic fluids could be originated from the oceanic crust of Mongol-Okhotsk Ocean, and these volcanic rocks formed under a lithospheric extension after the closure of Mongol-Okhotsk Ocean.

Key words: Tamulangou Formation, chronology, geochemistry, petrogenesis, Manchuria