Disintegration of Late Mesozoic Zhucun Formation in southeastern Zhejiang: Geochronology and petrogenesis of the related volcanic rocks

The once designated Zhucun Formation in Xiaoxiong basin is investigated to be geochronologically earlier (128Ma) not only than the overlying upper volcanic series (110 similar to 88Ma), but also than the underlying Jiuliping Formation (122 similar to 120Ma), but coeval to the Xishantou Formation (130 similar to 128Ma). Zircon Hf isotopic compositions of the studied Zhucun Formation rhyolitic tuff show enriched characteristics with Paleoproterozoic crustal model ages (epsilon(Hf)(t) =-16.4 similar to-13. 6, t(DM2) = 2. 01 similar to 2. 09Ga), also similar to the Lower Moshishan Group, such as the Xishantou Formation. Note these and the pyroclastic-rhyolitic volcanic rock assemblages and other regional geological features, the foundation basis of the Zhucun Formation is considered to be insufficient, and the Formation should be disintegrated. On the other hand, the enriched zircon Hf isotopic compositions of the studied Zhucun Formation rhyolitic tuff show that it was derived mainly from partial melting of the Paleoproterozoic crustal basement, further indicating that the Lower Moshishan Group volcanic rocks inland were derived with depleted-mantle derived material contribution, which was barely noticed in previous studies. The increased depleted-mantle derived material contribution in the volcanic petrogenesis and the pyroclastic-rhyolitic-bimodal volcanic assemblages both imply increased lithospheric extension, plausibly caused by rollback of the subducted paleo-Pacific slab.