CONSTRAINTS ON THE ROLLBACK OF THE NEO-TETHYAN OCEANIC PLATE: GEOCHRONOLOGY AND GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE DIANZHONG FORMATION, WESTERN GANGDESE BELT (TIBETAN PLATEAU)

In this contribution, we report new U-Pb SHRIMP zircon ages and geochemical data for volcanic rocks from the Dianzhong Formation. The unit repre-sents the lower part of the Linzizong volcanic succession located in the Shiquanhe area, western part of the Gangdese belt. Zircon U-Pb dating of three trachytes yields Pb-206/U-238 crystallization ages of 71.5 +/- 0.6 Ma, 70.8 +/- 1.0 Ma and 68.9 +/- 1.0 Ma. Whole rock major and trace element analyses indicate a main trend of calc-alkalic to high-potassic calc-alkalic, as well as an enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), with negative Nb, Ta and Ti anomalies. In addition, these volcanic rocks are enriched in LREE and depleted in HREE without a clear Eu anomaly. These geochemical features are similar to those of active continental margin volcanic rocks. In combination with previously published zircon U-Pb ages and geochemical data of the Dianzhong Formation, we suggest that the Linzizong volcanism started as early as the late Cretaceous (similar to 71 Ma) and the vol-canic rocks of the Dianzhong Formation in the Shiquanhe area are the product of rollback of the Neo-Tethyan oceanic plate.