Abstract:

The pillow lava, which crops out in Zuohai of Xiahe County at the conjunction of QinlingQilianKunlun orogens, overlies midPermian limestones of the Daguanshan Group disconformably. These rocks are porphyritic with abundant olivine, minor plagioclase and clinopyroxene phenocrysts. The matrix is composed of tabular plagioclase microlites, granular pyroxene, olivine, and minor ilmenite. The olivine phenocrysts have altered into iddingsite, talcum and chlorine, and some of them have totally converted into iddingsite. Their geochemical characteristics are very similar to the OIBtype alkali basalts from Hawaii and NW Turkey, with high TiO2 content (1.99%2.24%) and Mg# values (5864). The Fo values of olivine phenocrysts range from 82.73 to 84.57, with CaO contents higher than 0.1%. The plagioclase phenocrysts (An=64.7270.39) have lower contents of Si, Fe, Na, K and Ti and higher Al and Ca contents than those plagioclase (An=58.8861.53) in groundmass. These suggest that the magma has undergone crystallization differentiation of olivine and plagioclase. SHRIMP zircon UPb dating yields a mean age of (220.5±4.2) Ma, indicating that these pillow lava should form in the Norian. Regional data also indicate that the PaleoTethys Ocean died out and continentcontinent collision took place in the late Triassic within the conjunction of the QinlingQilianKunlun orogens. Then, the delamination of lithosphere probably gave rise to a thinner lithosphere that resulted in volcanism with OIB geochemical affinities.

Key words:  OIBtype basalt, Late Triassic, tectonic setting, conjunction of the QinlingQilianKunlun orogens