Abstract: The basic and acidic volcanic rocks with SiO2 contents of 41.89%-52.75% and 74.79%-78.51%, respectively, are closely
associated in time and space in the area of Jiamante, western Tianshan Mountains. These volcanic rock associations show distinct
“Daly gap”, thus can be ascribed to the typical bimodal volcanic assemblage. The basic units can be grouped into alkaline series,
whereas the acidic units are calc-alkaline series. LA-ICP-MS zircon U-Pb dating and Hf isotopic analysis for rhyolites show that their
formation age is (294.2±0.8) Ma, indicating that they were formed in Early Permian. The volcanic rocks have positive εHf(t) values (8.0-
17.1), and young two-stage model ages with an average of 468 Ma. By integration of the geochemical characteristics and genetic types, it
is shown that the bimodal volcanic rocks in the Jiamante area can not be produced by differentiation from the same magma chamber.
Rather, they were likely originated from different sources. Among them, the basalt was originated from partial melting of the enriched
garnet peridotite induced by the phase transition from spinel peridotite to garnet peridotite, and rhyolite is generated by partial melting
of juvenile crust resulted from underplating of depleted mantle. Both the basic and acidic volcanic rocks were formed in an intraplate
extension environment. In combination with analysis of the regional tectonic evolution, it is suggested that the western Tianshan
Mountains were entered an extensional tectonic setting since the Early Permian.

Key words: western Tianshan Mountains, bimodal volcanic rocks, rhyolite, basalt