Zircon U/Pb dating and Hf-O isotopes of the Zhouan ultramafic intrusion in the northern margin of the Yangtze Block, SW China: Constraints on the nature of mantle source and timing of the supercontinent Rodinia breakup

The Zhouan ultramafic intrusion in the northern margin of the Yangtze Block is mainly composed of lherzolite. Zircon grains selected from lherzolite are irregular in shape with distinct oscillatory and sector zoning and have Th/U ratios ranging from 0.8 to 10.6, indicating a magmatic origin. The weighted average ²⁰⁶Pb/²³⁸U age is 637±4 Ma (2σ, n=15), which can be considered as the crystallization age of the Zhouan intrusion. Zircon grains have δ ¹⁸O values ranging from 5.2‰ to 7.0‰, with an averaged value of 5.8±0.4‰ (1σ, n=33), similar to the mantle δ ¹⁸O value of zircon. Their ¹⁷⁶Hf/¹⁷⁷Hf_(t) ratios range from 0.282410 to 0.282594 with ? _Hf(t) values ranging from 1.3 to 7.6, lower than the corresponding value of the depleted mantle (～15), indicating an enriched mantle source. The enriched mantle source may have generated from a metasomatized lithospheric mantle with subducted slab. A number of ～635 Ma mafic-ultramafic intrusions in the Suizao basin are associated with coeval bimodal volcanics of the Yaolinghe Formation, indicating a continental rift setting. The ～635 Ma magmatic event in this region may represent the product of the last breakup of the Rodinia supercontinent in the northern margin of the Yangtze Block at Neoproterozoic.