The basic volcanic rocks in the Mangnai area, Altun belong to the tholeiitic ser ies. Their REE patterns show a slight enrichment in LREE; other element ratios are Ti/V=37～62, Th/Ta≈1 (greater than 1.5 for a few samples ), Nb/Th=7.7～16.8, characteristics of EMORB or OIB. The εNd(t) values, rangeing f rom 3.95～4.12, indicate that the basic volcanic rocks are derived from depleted upper mantle and mixed with materials from enriched mantle. The trace element characteristics of the basic v olcanic rocks in the Mangnai area are similar to those of Alcedo basalts from Ga lapagos A rchipelago, Iceland, suggesting that the ophiolite in the Mangnai area, Altun, i s formed in the setting of an oceanic ridge with source materials derived from depleted u pper mantle and mixed with materials from a plumerelated enriched mantle.