Geochronology and petrogenesis of the early Silurian Zeluo mafic-ultramafic intrusion, eastern Tibet: implications for the tectonic setting and evolution of the eastern Proto-Tethys Ocean

The Tibetan Plateau is a key region to understand the evolution of the Tethys Oceans. To better constrain the tectonic evolution of the Proto-Tethys Ocean on the western margin of the Yangtze plate, we present an integrated petrography, geochemistry, and zircon U-Pb-Lu-Hf isotope study on newly recognized early Silurian gabbro and serpentinite rocks from the eastern Yidun terrane of the Tibetan Plateau. Zircon U-Pb dating of the gabbro yields an Early Silurian age of 438.2 ± 2.8 Ma. Zircon ε_Hf(t) values of 5.4 to 8.5 suggest a single-stage model age (T_DM1) ranging from 729 to 858 Ma. The gabbros exhibit low total rare earth element abundances but are moderately enriched in the light rare earth elements and the large-ion lithophile elements (e.g. Rb, Ba, and Sr), and display representative negative high-field strength elemental anomalies for Nb, Ta, Zr, and Hf on spidergrams. The gabbro and serpentinites were derived from a depleted mantle-like source made of garnet-spinel lherzolite composition, from a sub-arc mantle wedge that was metasomatized by slab dehydration. Thus, the gabbro and serpentinites record an Early Silurian subduction event of the Proto-Tethys Ocean under the Yangtze plate. Furthermore, this study confirms that the Yidun terrane on the western margin of the Yangtze plate is underlined by a Precambrian crystalline basement.