Geochronology of the Dong Tso Ophiolite and the Tectonic Environment

Abstract

AbstractThe wedge shaped Dong Tso ophiolitic block is distributed near the transition point from the western to the middle sub–belt of the Bangong–Nujiang suture zone. The ophiolite is characterized by well–developed cumulate rocks that are mainly composed of cumulate and massive gabbros. In the cumulate gabbros, the adcumulate amphiboles are distributed extensively around the plagioclase and residual pyroxene grains; hence, the rocks are named adcumulate amphibole–gabbro. In this study, the formation age of the ophiolite has been estimated to be 166 ± 4 million years (Ma) by the sensitive high–resolution ion microprobe (SHRIMP) II U–Pb isotopic analysis of the zircons from the adcumulate amphibole–gabbro; the 40Ar/39Ar plateau age was estimated to be 148.19 ± 1.53 Ma, which should represent the emplacement time of the ophiolite, by isotopic dating of the pure amphibole mineral from the amphibole–schist. Two different suits of volcanic lavas have been recognized in this work. The purple colored pillow basalts have high TiO2 and P2O5 contents, and are rich in light rare earth elements (LREEs), large–ion lithospheric elements (LILEs) and high–field–strength elements (HFSEs), the characteristics that are the typical of the oceanic island basalt (OIB). On the other hand, other massive basaltic andesites of celadon color are poor in MgO; rich in Fe2O3, LREEs, LILEs, and HFSEs; and especially characterized by negative Nb and Ta anomalies, the properties that establish the andesites as continental arc volcanic rocks. It is concluded that hotspots had developed in the old Dong Tso basin, the oceanic basin that had been developing from middle Jurassic (166 Ma) or even before and emplaced northward in late Jurassic (about 148 Ma).