The basic lavas in Jigen area of southwest Tianshan Mountains occur as lenses or slices within the Pa-leozoic metasediments, with their source magma belonging to the tholeiitic series. The chondrite-normalized REE patterns indicate that they are slightly enriched in LREE. On the other hand, the trace element patterns normalized by MORB or primitive mantle show the enrichment of Th, Nb, Ta, Zr and other HFS elements with the values of Zr/Nb being 10 or so, similar to characteristics of P-type mid-ocean ridge basalts (P-MORB).A Sm-Nd isochron age of 392±15 Ma obtained for the basic lavas is interpreted as their crystallization age. The basic lavas show high positiveεNd(t) values in the range of +4.24 to +6.79, suggesting that the basic lavas were derived from mid-ocean ridge ocean setting. The study of Sr-Nd isotopes show that the source of basic lavas was derived from the N-MORB-like mantle and was affected by the enrichment of the mantle source during their formation processes. Other ophiolite members associated with basic lavas such as gabbro slices and altered ultra-basic rock slices were also found in Jigen area, which represent the remnants of fragmented oceanic crust of the Paleo-South Tianshan ocean. According to the age and sites, it is proposed that the ophiolitic remnants can be correlated with the Late Paleozoic ophiolite belt in south Tianshan Mountains which serves as the boundary b-tween the Tarim Plate and the South Tianshan tectonic belt.