The LA-ICP-MS U-Pb zircon dating for gabbro of the Mashan Complex from Southeastern Guangxi yields a weighted mean 206Pb/238U age of 157.8±2.4Ma, the early Yanshanian of late Jurassic age. The extremely low contents of SiO2 for two samples are indicative of accumulate origin. Along with the lack of depletion in Nb and Ta, it is suggested that the role of crustal contamination during their crystallization is insignificant. Thus their d(158Ma) values of +4.46 and +4.24 are likely to indicate a mantle source with moderate depletion, a maximum estimation for the depletion degree of the regional mantle source. The OIB-like elemental patterns, different degrees of depletion in Nb and Ta as well as the variation of Sr-Nd isotopes of mode-basic samples indicate the existence of crustal contamination. Relative to the typical contemperate granites, the granitic samples of the Mashan Complex have smaller d(t) values and younger T2DM ages, which are indicative of involvement of mantle materials. Therefore, it is an extensional setting of Southeastern Guangxi in the early Yanshanian. The Mashan Complex is formed within an extensional setting through a magmatism derived by partial melting of moderately depleted mantle and regional lower crust.