Contrasting latest Permian intracontinental gabbro and Late Triassic arc gabbro-diorite in the Gangdese constrain the subduction initiation of the Neo-Tethys

When the Neo-Tethys subduction initiated has been a subject of debate, perhaps due to the lack of reliable evidence. This paper reports geochronological, geochemical, and Hf isotopic data for recently identified gabbro as well as diorite exposed in the Chanv area of the Gangdese in the southernmost margin of the western Lhasa block, western China. Zircon U-Pb dating indicates the Chanv magmatic rocks formed in two stages: latest Permian (gabbro, 254.8 +/- 1.5 Ma) and Late Triassic (gabbro, 214.2 +/- 2.0 Ma; diorite, 202.8 +/- 1.1 Ma). Geochemical and Hf isotopic analyses reveal that the Chanv magmatic rocks of both stages have contrasting characters. The latest Permian gabbro has apparently higher TiO2 concentrations (> 3 wt.%) than the Late Triassic gabbro (0.86-1.24 wt.%). Although they all are enriched in light rare-earth elements (LREEs), the latest Permian gabbro displays no depletions in Nb and Ta, while the Late Triassic gabbro and diorite are apparently depleted in these two elements. In addition, the zircons from the latest Permian gabbro are all characterized by positive epsilon(Hf)(t) values whereas those from the Late Triassic gabbro dominantly by negative epsilon(Hf)(t) values. Conventional plots using multi-elements or elemental ratios all indicate the latest Permian gabbro has inherent resemblance to intraplate oceanic-island basalt (OIB) whereas the Late Triassic gabbro and diorite are of distinct arc affinity. Therefore, The Chanv latest Permian-Late Triassic magmatic rocks testify a switch of the tectonic environment in the southernmost Eurasian margin from latest Permian post-break-up to Late Triassic arc-related setting and thus constrain the initiation of the Neo-Tethys subduction between 255 Ma and 214 Ma.