Paleoceanographic and Tectonic Reconstructions of the Eastern Meso-Tethys through Sr Isotopes and Trace Elements of Middle Jurassic Limestones

In the Middle Jurassic, whether the Bangong-Nujiang Tethys Ocean extended eastward from the central Tibetan Plateau into western Yunnan, China, thereby separating the Tengchong and Baoshan blocks, remains unresolved. Middle Jurassic carbonate rocks of the Liuwan Formation of the Baoshan Block provide essential insights. This study examines the petrography, rare earth element and yttrium (REE+Y) geochemistry, and strontium isotopic composition of the Liuwan Formation limestones to determine their depositional environment and tectonic setting. Results indicate that these carbonate rocks show no significant recrystallization or dissolution. REE+Y patterns show light-REE enrichment, slightly positive La and Ce anomalies, a minimal Eu anomaly, and a Y anomaly, all indicating open marine conditions. The 87Sr/86Sr ratios range from 0.707287 to 0.707554 (averaging 0.707365) and are slightly higher than those of coeval seawater. These geochemical signatures suggest a depositional environment between open marine and restricted settings, influenced by freshwater influx and volcanic activity, indicating an ocean separating the Baoshan and Tengchong blocks. Tectonic proxies suggest a stable passive margin for the Baoshan Block and a subduction-related tectonic/depositional setting for the Tengchong Block. Subduction of the Bangong-Nujiang Ocean beneath the Tengchong Block explains these distinct tectonic settings. Correlations among 87Sr/86Sr values and REE+Y geochemistry of the Liuwan Formation reveal paleoceanographic characteristics of the ancient subducted ocean and the contributions from multiple sources. These findings support the existence of the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks during the Middle Jurassic, enhancing our understanding of eastern Tethys paleogeography and evolution.