THE EVIDENCE OF SM-ND ISOCHRON AGE FOR THE EARLY PALEOZOIC OPHIOLITE IN MANGYA AREA, ALTUN MOUNTAINS

The REE patterns of the basic volcanic rocks in Mangya area, Altun, ar e slight rich in LREE with (La/Ya)(N)=1,69-3.20, (La/Sm)(N) =1.37-1.87 , other trace element ratios of the rocks are Th/Ta approximate to l ( for a few samples greater than 1.5), Nb/ Y=0.34-0.62, Ti/Y=310-443 (on the average: 381), Ti/V=37-62, Zr/Nb=9.4-12.4,Sr/Rb=12-80 (on the ave rage: 37), and Nb/Th=7.7-16.8. These features are similar to that of E -MORE or GIB. The epsilon(Nd)( t) value, being 3. 95-4.12, shows that the source of the volcanic rocks is derived from depleted asthenospher e mantle mixed with materials from enriched mantle. These, together wi th the information of geological setting and rock assemblages, indicat e that the basic volcanic rocks are of ophiolite. The Sm-Nd isotope ag es for the eight basic volcanic rock samples construct a straight line with good correlation, and the calculated isochron age is (481.3+/-53 ) Ma. Besides, the eight calculated epsilon(Nd)(t) and model ages are close to each other, which suggests that they are homologous, so the i sochron is not a mixed line. In the meantime, the isochron age ((481.3 +/-53) Ma) is lower than the model ages (T-DM=1104-1534 Ma) of the sam ples, suggesting that the isochron age represents the formation age of the basic volcanic rocks and the ophiolite belt in Mangya area, Altun is formed in the early Paleozoic (Cambrian-Ordovician). In spite of t he greater uncertainty of the age, it is still reliable because it is consistent with the age constrained by the regional strata.