Abstract: The volcanic stratum of the Early Cretaceous Duohemao Formation (K1d), developed in the TengbuRizhou area of Xiahe County, Gansu Province, is a typical sedimentary association of intra-continental rift. The rock assemblage is composed mainly of basalts, with occasional occurrence of polymictic conglomerates at the bottom of the assemblage. Based on petrographic and geochemical analyses, we classified TengbuRizhou basalts as sodium alkaline olive basalt. They are enriched in LREE and Nb, Ta, La and Zr, depleted in Rb and K. They have similar chondrite-normalized rare earth and primitive mantle-normalized trace elemental patterns as that of typical global ocean island basalt (OIB). However, we found great difference in the ratios of Th to Nb (0.0550.060, averaging 0.057 vs. 0.08 for OIB), Zr to Nb (6.98, averaging 7.25 vs. 5.83 for OIB) and Th to La (0.07 vs. 0.11 for OIB). This suggests that the basalt magma may have derived from the asthenospheric mantle and is contaminated with crustal materials. The zircon U-Pb age is 106.27±1.3 Ma. Given that the pollen assemblage, mainly Classopollis-Osmundacidites, is produced in the Duohemao Formation of the Maixiu Mountain area, we propose that TengbuRizhou basalts are the product of the late Mesozoic continental rift magmatism in West Qinling, while rifting likely ended in the early stage of the lithospheric extension, since we found no signs of typical bimodal volcanic rocks except red bed formation in the Duohemao Formation and molasse-like formation in the upper Wanxiu Formation.

Key words: West Qinling, Xiahe, Early Cretaceous, basalts, tectonic significance