Abstract: The Marnianshan Group in the middle part of Fujian Province is developed along the Zhenghe-Dapu deep fault zone which is an important geological boundary in Fujian and even in South China．Dating and study on genesis of the Mamianshan Group are important approach to characterize the Zheng he-Dapu deep fault zone，and to better understand the formation and evolution of the Cathaysia block．Samples collected from three sections yield three isochrons with consistent ages within error ranging from 1.73～1.77 Ga and with high initial ENd(t)values．This suggests that the Mamianshan metamorphic basic volcanic rocks were formed in the Mesoproterozoic rather than in the Caledonian period．This also suggests that the Zhenghe—Dapu deep fault zone was active as early as in the Mesoproterozoic when it was a premature rift Zone and that the Mamianshan meta-volcanic rocks were products of the rifting in the Cathaysia．The high initial ENd(t)values (+7.6～+10.9)of the Mamianshan meta-volcanic rocks suggest that they were derived from highly depleted mantle．This implies that this mantle produced large amount of crustal matters be fore the Mesoproterozoic probably in the Paleoproterozoic，which supports the existence of Cathaysia with main Paleoproterozoic crust most of which was later destroyed or reworked