Rapid formation of eclogites during a nearly closed ocean: Revisiting the Pianshishan eclogite in Qiangtang, central Tibetan Plateau

The Pianshishan eclogites discovered in central Qiangtang are a key piece of evidence constraining the main suture of the Paleo-Tethys Ocean in Tibet. Based on the eclogite-facies metamorphic inclusions found in them, a previous study interpreted the zircon grains as metamorphic in origin, although their catholuminescence images were similar to zircons of igneous origin. Distinguishing metamorphic from igneous zircons can be difficult, however, especially in high-pressure metamorphic rocks. In this study, we report new SIMS zircon U-Pb dates along with zircon O isotope and trace element analyses, to constrain the origin of these zircons and the interpretation of zircon dates. Twenty-five of 26 zircons analyzed from three samples yielded zircon U-Pb dates ranging from 239 to 236 Ma with an average of 238 ± 1 Ma. The zircon rare earth element (REE) patterns are characterized by positive Ce and negative Eu anomalies, strongly indicating an igneous origin. This interpretation is also supported by the high Ti-in-zircon temperatures (> 700 °C) and depleted mantle-like zircon O isotope ratios (5.15 ± 0.57‰, 2SD). Thus, we conclude that the protoliths of the Pianshishan eclogites were generated at ~ 238 Ma. Their geochemistry is similar to the oceanic island basalts (OIB), and they were probably generated by small extents partial melting of enriched oceanic asthenospheric mantle within the garnet stability field and emplaced or erupted as a seamount in the Paleo-Tethys Ocean. We infer the age of eclogite-facies metamorphism to be ~ 233 Ma based on previously reported Lu-Hf isochron ages, and that this metamorphism marks the collision between the Southern and Northern Qiangtang terranes. Thus, metamorphism of the eclogites occurred within 5 My of protolith generation and exhumation occurred only 13 to 19 My later, based on the 40Ar/39Ar dates of 220-214 Ma. The time between protolith generation and exhumation was thus < 24 My. This rare rapid evolution may be due to the protoliths having formed as a “petit-spot” seamount in response to outer rise flexure as the Paleo-Tethys Ocean entered the Longmu Co-Shuanghu subduction zone.