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, are 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≈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 average: 371, and Nb/Th= 7.7–16.8. These features are similar to that of E-MORE or OIB. TheεED (t) value, beiig 3.95–4.12, shows that the source of the volcanic rocks is derived from depleted asthenosphere mantle mixed with materials from enriched mantle. These, together with the information of geological setting and rock assemblages, indicate that the basic volcanic rocks arc of ophiolite. The Sm-Nd isotope ages for the eight basic volcanic rock samples construct a suaight line with good correlation, and the calculated i.whmn age is (481.3 ± 53) Ma. Besides, the eight calculated εNd(t) and model ages are close to each other, which suggests that they are homologous, so the isochmn is not a mixed line. In the meantime, the isachron age ((481.3 ± 53) Ma) is lower than the model ages (T DM = 1 004–1 534 Ma) of thc samples. suggeting that the isochron age represents the formation age of the basic volcanic rocks and the ophiolite belt in Mangya area, Altun is foxmed in the early Paleozoic (Cambrian-Ordovician). In spite of the greater uncertainty of the age, it is still reliable because it is consistent with the age constrained by the regional strata.