THE RELATION OF ALKALINE ROCKS IN THE QINLINGDABA MOUNTAINS REGION AND THE TECTONIC EVOLUTION OF THE OROGEN AND THEIR FEATURES

There are a great vaiiety of alkaline rocks in the Qinling-Daba Mountains region, including not only alkaline and peralkaline syenite-nepheline syenite but also alkaline gabbro and carbonatite. According to the Rb-Sr and C and O isotope data, they originated in the terminal Proterozoic （Jinningian Stage） -Mesozoic （Yanshanian Stage）. They are characterized by long-continued activity and show mantle-derived reatures. The southern and northern alkaline rock belts are distributed along the Qinling orogen and the boundary fault between the northern margin of the Yangtze platform and the southern margin of the North China platform, respectively, which implies that the formation of alkaline rocks was controlled by the fault zone. The petrochemical features of alkaline rocks and their relation to the tectonic evolution of the Qinling orogen indicate that they are not typical alkaline rocks of continental rift nature but a special transitional type. The opening and closing of the lithosphere not only gave （?）ise to the framework of the nearly EW-trending fault tectonics of the Qinling orogen but also furnished heat and materials for partial melting of the upper mantle and formation of alkaline composition.