Abstract: The fanjinshan region lies in a Proterozoic island arc, in which occurs slightly metamorphosed Fanjinshan group of great thickness, with its lower part marked by a suite of ophiolite consisting of bedded ultrabasic rocks and basic lava. According to the differential sequence, the rocks of this group may be divided into two rock associations: spilite-ultrabasic and diabase-ultrabasic. In their middle-lower part exist ultrabasic rocks including pyroxene-peridotite, olivene pyroxenite, pyroxe-nite and feldspar-bearing pyroxenite. Their chemical composition is characterized by being rich in aluminium and poor in calc-alkaline, so they may be regarded as fer-romagnesian ultrabasic rocks. By analyzing practical data it is suggested that the ultrabasic rocks belong to the products of differentiation in situ of submarine basic lava.