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

ABSTRACT

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 ε_Hf(t) values whereas those from the Late Triassic gabbro dominantly by negative ε_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.

KEYWORDS:

• Geochemistry
• Geochronology
• Hf isotopes
• Post-rift
• Neo-Tethys
• Subduction
• Geodynamic process

Acknowledgments

We are grateful to Robert Stern for kind and timely editorial handling, and to the anonymous reviewers for the constructive and thoughtful review comments, which significantly improved the manuscript.

Highlights

• Gabbros and diorite Gangdese were dated at ~255 Ma, ~214 Ma and ~203 Ma, respectively.

• The 255-Ma gabbro is geochemically characterized by intraplate OIB affinity.

• The 214 Ma and 203 Ma gabbro and diorite are geochemically of distinct arc affinity.

• Intraplate post-rift switched to magmatic arc between 255 Ma and 214 Ma.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This study was financially supported by the Second Tibetan Plateau Scientific Expedition and Research (STEP) Program (grant 2019QZKK0703), the National Natural Science Foundation of China (grants 41902099, 41372228 and 41472209), and the President Fund for Science and Education Fusion of University of Chinese Academy of Sciences (grant Y552011Y00), and China Geological Survey (grants DD2016008003, DD20160015, 12120113033004, 1212011086065). The support provided by the China Scholarship Council during a one-year visit of QHL to the University of Queensland is also acknowledged.