浙东南地区晚中生代"祝村组"的解体: 相关火山岩形成时代及岩石成因

The once designated Zhucun Formation in Xiaoxiong basin is investigated to be geochronologically earlier (128Ma) not only than the overlying upper volcanic series ( 110 ~ 88Ma) , but also than the underlying Jiuliping Formation ( 122 ~ 120Ma) , but coeval to the Xishantou Formation ( 130 ~128Ma). Zircon Hf isotopic compositions of the studied Zhucun Formation rhyolitic tuff show enriched characteristics with Paleoproterozoic crustal model ages ( ϵ_HF(t) = - 16. 4 ~ - 13. 6, t_Dm2 = 2. 01 ~ 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.