Duration and final closure of the Changning–Menglian Paleo-Tethys Ocean in the Southeastern Tibetan Plateau

Abstract

The timescales for the duration and final closure of the Paleo-Tethys Ocean (PTO) remain controversial. To address these issues, this study reports the geochronological and geochemical data of two newly identified retrograded eclogites in the southeastern Tibetan Plateau. These eclogites outcropped in the Genhen and Mangna regions within the Changning–Menglian suture that represents the extinct site of the PTO. Geochemically, the protoliths of the Genhen and Mangna samples are arc-related and oceanic crust-related mafic rocks, respectively. Zircon cores that exhibit oscillatory zoning, high Th/U ratios, and enriched heavy rare earth element (HREE) patterns yield UPb ages of 250 ± 1 Ma and 429 ± 3 Ma, which are interpreted as the formation ages of their protoliths. Combined with other early Paleozoic magmatic and metamorphic records in this suture, we propose that the PTO may have existed for over 200 Myr in the southeastern Tibetan Plateau. By contrast, zircon rims characterized by low Th/U ratios and flat HREE patterns yield UPb ages of 230 ± 4 Ma and 231 ± 2 Ma, which are consistent with the ⁴⁰Ar/³⁹Ar plateau age (232.6 ± 2.2 Ma) of phengites from the country-rock schist. In addition, given that phengite inclusions have been found in zircon rims, we propose that these eclogites with varying protolith ages simultaneously experienced retrograde metamorphism at ∼230 Ma, which might be associated with slab break off and exhumation after continental collision. Our data reconstruct the timelines for the development of the PTO in the southeastern Tibetan Plateau, including its persistence from the early to late Paleozoic and final closure prior to the high-pressure metamorphism at ∼230 Ma but not earlier than the youngest protolith ages (∼250 Ma) of retrograded eclogites. These approaches and results have general implications for understanding the tectonic evolution of the Paleo-Tethys Ocean.