Late Triassic Continental Eclogite in the Central Tibetan Plateau Reveals Paleo-Tethys Continental Subduction

Academic Background

The Tibetan Plateau is one of the youngest and highest plateaus on Earth, and its formation is closely related to the closure of the Tethys Ocean and the collision of the Indian-Eurasian plates. The Paleo-Tethys Ocean was a significant oceanic basin during the Mesozoic, and its closure process has had a profound impact on the geological evolution of the Tibetan Plateau. For a long time, the eclogites in the Tibetan Plateau were considered to be products of the subduction of the Paleo-Tethys oceanic crust. However, recent studies suggest that some eclogites may have a continental subduction origin. This discovery is of great significance for understanding the geological evolution of the Tibetan Plateau.

This study aims to reveal the origin of the Late Triassic continental eclogites in the central Tibetan Plateau and their implications for the closure process of the Paleo-Tethys Ocean. Through detailed petrological, geochemical, and geochronological studies, the authors seek to answer the following questions: What is the protolith of these eclogites? How did they form? What do these eclogites reveal about the closure process of the Paleo-Tethys Ocean?

Source of the Paper

This paper was co-authored by Wang Xu, Lishuang Liu, Matthew J. Kohn, Pinghua Liu, and Jia Cai. The authors are affiliated with the Institute of Geology, Chinese Academy of Geological Sciences; the School of National Safety and Emergency Management, Beijing Normal University; and the Department of Geosciences, Boise State University. The paper was published online on October 18, 2024, in the journal Geology, with the DOI 10.1130/G52796.1.

Research Process

1. Sample Collection and Preliminary Analysis

The research team collected samples from four different areas (A-D) within the Pianshishan eclogite belt in the central Tibetan Plateau. These samples are mainly fine-grained, granoblastic eclogites composed of garnet and clinopyroxene, with varying amounts of amphibole and minor phengite, quartz, and rutile. The team conducted detailed petrological and mineralogical analyses to determine the mineral assemblages and metamorphic conditions of these samples.

2. Phase Equilibrium Modeling

To determine the metamorphic conditions of the eclogites, the research team conducted phase equilibrium modeling. Using H2O-saturated isochemical phase diagrams, the team predicted the mineral assemblages and pressure-temperature conditions at the peak metamorphic stage. The results show that the peak metamorphic conditions for samples 23xw13-9 and 23xw14-8 are 2.08-2.12 GPa/528-537°C and 2.07-2.13 GPa/524-537°C, respectively. These results indicate that the Pianshishan eclogites underwent high-pressure eclogite-facies metamorphism but did not reach ultrahigh-pressure conditions.

3. Zircon U-Pb Geochronology

The research team conducted laser ablation–multicollector–inductively coupled plasma–mass spectrometry (LA-(MC)-ICP-MS) U-Pb dating on zircon grains from six eclogite samples. The results show that the zircon cores from samples 23xw14-10b, 23xw14-20, and 23xw14-25 have ages of 240±1 Ma, 241±1 Ma, and 239±1 Ma, respectively, while the zircon rims from sample 23xw14-20 have an age of 235±1 Ma. Additionally, the zircon cores from samples 23xw13-10, 23xw14-9, and 23xw14-10a have ages of 1847±10 Ma, 1862±6 Ma, and 1856±7 Ma, respectively. These results indicate that the protolith of the Pianshishan eclogites formed around 240 Ma and underwent eclogite-facies metamorphism around 235 Ma.

4. Geochemical Analysis

The research team conducted whole-rock geochemical analysis on the Pianshishan eclogites. Based on geochemical characteristics, the eclogites can be divided into two groups: Group 1 samples have low SiO2 (45.0-47.4 wt%) and Al2O3 (11.5-13.2 wt%) but high TiO2 (4.0-5.8 wt%), resembling ocean-island basalts (OIB) and within-plate basalts; Group 2 samples have higher SiO2 (47.9-49.0 wt%) and Al2O3 (14.4-15.7 wt%) but lower TiO2 (1.1-2.6 wt%), resembling mid-ocean ridge basalts (MORB) and lower continental crust. These results suggest that the protolith of the Pianshishan eclogites may have experienced crustal contamination.

Key Findings

1. Metamorphic Conditions of the Eclogites: Phase equilibrium modeling results show that the Pianshishan eclogites underwent high-pressure eclogite-facies metamorphism, with peak conditions of approximately 2.1 GPa/530°C. These results indicate that the protolith of the eclogites was subducted to a depth of at least 75 km.

2. Geochronology of the Eclogites: Zircon U-Pb dating results show that the protolith of the Pianshishan eclogites formed around 240 Ma and underwent eclogite-facies metamorphism around 235 Ma. Additionally, the ancient ages of the zircon cores (ca. 1850 Ma) suggest that the protolith of the eclogites may have experienced crustal contamination.

3. Geochemical Characteristics of the Eclogites: Geochemical analysis results show that the Pianshishan eclogites can be divided into two groups, resembling ocean-island basalts and lower continental crust, respectively. These results suggest that the protolith of the eclogites may have experienced crustal contamination.

Conclusions and Significance

This study is the first to identify eclogites formed by Paleo-Tethys continental subduction in the central Tibetan Plateau. The protolith of these eclogites formed around 240 Ma and underwent high-pressure eclogite-facies metamorphism around 235 Ma. Combined with the Qianmai eclogites in southwest China, the research team proposes that continental material was subducted to mantle depths during the closure of the Paleo-Tethys Ocean, forming a 2500-km-long continental subduction zone. This discovery not only sheds light on the closure process of the Paleo-Tethys Ocean but also provides new insights into global climate change during the Late Triassic.

Research Highlights

1. First Discovery of Eclogites Formed by Paleo-Tethys Continental Subduction: This study is the first to identify eclogites formed by Paleo-Tethys continental subduction in the central Tibetan Plateau, providing new evidence for understanding the geological evolution of the Tibetan Plateau.

2. Revealing the Closure Process of the Paleo-Tethys Ocean: Through detailed petrological, geochemical, and geochronological studies, the research team revealed the closure process of the Paleo-Tethys Ocean and proposed a 2500-km-long continental subduction zone.

3. New Perspectives on Late Triassic Global Climate Change: The research team suggests that the high-elevation mountain range formed during the closure of the Paleo-Tethys Ocean may have influenced global climate during the Late Triassic, particularly the strong monsoonal climate of the Carnian Pluvial Episode.

Other Valuable Information

The research team also proposes that the high-elevation mountain range formed during the closure of the Paleo-Tethys Ocean may have had a significant impact on global climate during the Late Triassic. Future climate modeling studies could further explore the influence of this high-elevation mountain range on Late Triassic climate.