Petrology, geochemistry and P–T–t path of lawsonite‐bearing retrograded eclogites in the Changning–Menglian orogenic belt, southeast Tibetan Plateau

Abstract

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau, is considered as the main suture zone marking the closure of the Palaeo‐Tethys Ocean between the Indochina and Sibumasu blocks. Here, we investigate the recently discovered retrograded eclogites from this suture zone in terms of their petrological, geochemical and geochronological features, with the aim of constraining the metamorphic evolution and protolith signature. Two types of metabasites are identified: retrograded eclogites and mafic schists. The igneous precursors of the retrograded eclogites exhibit rare earth element distribution patterns and trace element abundance similar to those of ocean island basalts, and are inferred to have been derived from a basaltic seamount in an intra‐oceanic tectonic setting. In contrast, the mafic schists show geochemical affinity to arc‐related volcanics with the enrichment of Rb, Th and U, and depletion of Nb, Ta, Zr, Hf and Ti, and their protoliths possibly formed at an active continental margin tectonic setting. Retrograded eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, white mica, lawsonite and rutile, and underwent five‐stage metamorphic evolution, including pre‐peak prograde stage (M1) at 18–19 kbar and 400–420°C, peak lawsonite‐eclogite facies (M2) at 24–26 kbar and 520–530°C, post‐peak epidote–eclogite facies decompression stage (M3) at 13–18 kbar and 530–560°C, subsequent amphibolite facies retrogressive stage (M4) at 8–10 kbar and 530–600°C, and late greenschist facies cooling stage (M5) at 5–8 kbar and 480–490°C. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) U–Pb spot analyses of zircon show two distinct age groups. The magmatic zircon from both the retrograded eclogite and mafic schist yielded protolith ages of 451 ± 3 Ma, which is consistent with the ages of Early Palaeozoic ophiolitic complexes and ocean island sequences in the CMOB reported in previous studies. In contrast, metamorphic zircon from the retrograded eclogite samples yielded consistent Triassic metamorphic ages of 246 ± 2 and 245 ± 2 Ma, which can be interpreted as the timing of closure of the Palaeo‐Tethys Ocean. The compatible peak metamorphic mineral assemblages, P–T–t paths and metamorphic ages, as well as the similar protolith signatures for the eclogites in the CMOB and Longmu Co–Shuanghu suture (LCSS) suggest that the two belts formed part of a cold oceanic subduction system in the Triassic. The main suture zone of the Palaeo‐Tethyan domain extends at least 1,500 km in length from the CMOB to the LCSS in the Tibetan Plateau. The identification of lawsonite‐bearing retrograded eclogites in the CMOB provides important insights into the tectonic framework and complex geological evolution of the Palaeo‐Tethys.

Detrital zircons from Late Paleozoic to Triassic sedimentary rocks of the Gongshan-Baoshan Block, SE Tibet: Implications for episodic crustal growth of Eastern Gondwana

“…3). The 223 Ma age is consistent with ones of the HP metamorphic terranes along the Central Qiangtang-Lancangjiang suture (Zhai et al, 2011;Wang et al, 2019aWang et al, , 2019b, and the 273 Ma age is nearly synchronous with the SSZ ophiolite in Lancangjiang suture zone (270-264 Ma;Jian et al, 2009). Our new detrital zircon ages indicate that subduction of the Paleo-Tethys Ocean most likely initiated at ~273 Ma and ended at ~223 Ma.…”

“…1A). It is generally considered to be a region of tectonic escape during the Cenozoic India-Asian continental collision (e.g., Tapponnier et al, 1986;Holt et al, 1991), and it has experienced the long-term and complex tectonic evolution of branches of the Tethyan oceans (Metcalfe, 1996(Metcalfe, , 2013Jian et al, 2009;Metcalfe et al, 2010;Zhao et al, 2017;Wang et al, 2019aWang et al, , 2019b. The northern Indochina continent has been intensively modified by deformation since the Cenozoic (e.g., Yin, 2010;Song et al, 2010;van Hinsbergen et al, 2011).…”

“…The suture zone is connected with the Inthanon suture zone in Thailand (Sone and Metcalfe, 2008), and contains dismembered ophiolitic mélanges, volcanic rocks, deep-water sedimentary rocks and shallow-marine carbonates. Highpressure rocks including lawsonite-bearing eclogite and blueschist with metamorphic ages of ~245-220 Ma have been recently reported (Wang et al, 2019a(Wang et al, , 2019b), suggesting a cold oceanic subduction during the northeastern convergence of the Sibumasu Terrane.…”

“…Based on ages of the SSZ ophiolite (270-264 Ma), Jian et al (2009) suggested the subduction of the main Paleo-Tethys Ocean started in the Permian. High-pressure (HP) metamorphic terranes along the Central Qiangtang-Lancangjiang suture zone have uniform ages of 245-220 Ma (Zhai et al, 2011;Wang et al, 2019aWang et al, , 2019b, indicating that the Paleo-Tethys Ocean closed at or before 220 Ma.…”

Tectonic Evolution and Paleoposition of the Baoshan and Lincang Blocks of West Yunnan During the Paleozoic

“…The arc-liked igneous rocks (a-d) and mantle-plume-related basalts (d) along the East Gondwana margin are summarized in Tables S3-S5. 10.1029/2019TC006028 Tectonics 2. The~445 and~260 Ma ocean island basalts had undergone high-pressure metamorphism at the same time of~243 Ma (Fan et al, 2015;Wang, Liu, Schertl, et al, 2019;Wang, Liu, Li, et al, 2019) and were cropped out as eclogite and blueschist lenses entrapped in the Lancang Group within the CMSZ. In addition, the Silurian collision-related high-pressure metamorphic rocks akin to those in the Longmu Co-Shuang Suture Zone (Zhang et al, 2014) and the angular unconformity between the Devonian and the Ordovician-Silurian strata in the BSB and SMB (BGMRYP, 1980(BGMRYP, , 1981(BGMRYP, , 1990) are absent.…”