Geology,Geochemistry of Langao Mafic Volcanic Rocks in South Qinling Orogenic Belt and Its Tectonic Implications
To study the petrogenesis and tectonic setting of Langao mafic volcanic rocks in South Qinling orogenic belt,basalts and volcanic breccias are sampled from Langao County for petrography,major elements,trace elements and REE analysis.The results show that they are classified to alkaline basalts,and characterized by strong differentiation between LREE and HREE,Rb,K depletion relative to Th,Hf and no Nb,Ta,Zr,Ti negative anomalies.All of the samples display highly consistent REE patterns and trace elements spider diagrams with OIB.Petrogenenis studies reveal that they were derived from a congenetic primitive magma which was the product of low-degree partial melting of the spinel-garnet lherzolites and garnet lherzolites.These rocks mainly underwent fractional crystallization of clinopyroxene with minor plagioclases and were not accompanied by contamination of continent crustal materials during their magmatic evolution processes.Tectonic discrimination for mafic volcanics,using trace elements,and regional geological analysis demonstrate that Langao mafic volcanic rocks were most likely formed in the ocean basin plateau or ocean island setting.