SHRIMP zircon dating of gabbro from the ophiolite melange in the northern Altyn Tagh and its geological implications.

The Qingyazi ophiolitic melange in the Akesai area, located in the northern margin of the Altyn Tagh, is composed of massive faulted-block rocks of serpentinite and gabbro. Analysis shows that the serpentinites are mostly characterized by high MgO contents, low Al2O3, CaO, TiO2 and Sigma REE contents, which are similar to the metamorphic peridotite of the world's typical ophiolite. The gabbrao has 49.06% similar to 50.70% SiO2, high Fe2O3 (6.36% similar to 12.85%) and MgO (7.16% similar to 10.01%) and low to moderate TiO2 (0.31% similar to 1.02%) contents, with equal of Na2O and K2O (the average is 0.73% and 0.75%, respectively). It is similar to the E-type MORB and/(or) OIB that the gabbro has lower contents of Sigma REE with low fractionation of LREE/HREE ((La/Yb)(N) = 1.07 similar to 5.19) and no or slightly Eu anomaly. The major and trace elements characteristics of the gabrro indicated that it may not only retained the MORB origin signatures, but also records the results of the transformation of SSZ environment. All mentioned above show that some of the ophiolitic units in the melange may have originated in different tectonic settings, indicating that the Qingyazi ophiolitic melange represents a complex plate suture. The SHRIMP U-Pb age of zircons from gabbro of the Qingyazi ophiolitic melange is 521 +/- 12Ma (MSWD = 2.1), which stands for the formation time of the ocean basin. The result of the zircons SHRIMP U-Pb age shows that the gabbro has a metamorphic age of 472 +/- 10.0Ma(MSWD = 0.20), possibly representing the minimum age of the closure of Paleozoic ocean basin.