Neoproterozoic assembly of the Yangtze and Cathaysia blocks: Evidence from the Cangshuipu Group and associated rocks along the Central Jiangnan Orogen, South China

Abstract

The early Neoproterozoic amalgamation process of the Yangtze with Cathaysia blocks in South China is an important geoscience issue and its collisional time has attracted extensive attention during the past decades. The Linjiawan conglomerate and Yinzhuba agglomerate of the Cangshuipu Group and associated rocks along the Central Jiangnan Orogen preserved key records for better constraining the assembly process of the Yangtze with Cathaysia blocks. This study presents a set of new geochronological and geochemical data of the volcanic gravels and the associated rocks from the Lengjiaxi, Cangshuipu and Banxi groups. Our data show that the dacitic and rhyolitic gravels in the Linjiawan conglomerate give zircon U–Pb ages of 834±2Ma and 835±6Ma, respectively, significantly older than that (814–824Ma) of the Yinzhuba agglomerate. The volcanic gravels in the Linjiawan conglomerate are characterized by subalkaline dacite and rhyolite, geochemically distinct from the high-mg andesite of the Yinzhuba agglomerate. Such characteristics, along with our field observations, suggest that the Yinzhuba agglomerate is conformably underlain by the Linjiawan conglomerate. The detrital zircons from the sandstones in the Lengjiaxi Group and the lower and upper parts of the Banxi Group yield the predominant early Neoproterozoic age-peaks at 863Ma, 810Ma and 795Ma, respectively. The sandy gravel from the Hengluchong conglomerate at the basal Banxi Group give the youngest age-peak of 810Ma. These data suggest that the Linjiawan and Hengluchong conglomerates represent two angular unconformities, in turn separating the Cangshuipu from Lengjiaxi (∼860–835Ma) and Banxi (∼810–720Ma) Groups. The unconformities were deposited at ∼830Ma and ∼810Ma, respectively, reflecting the initial collision and final assemblage of the Yangtze with Cathaysia blocks along the Central Jiangnan Orogen. The synthesis of all the available data indicates that the assemblage process of the Yangtze with Cathaysia blocks might be episodic with the collisional time of the Eastern Jiangnan Orogen being significantly earlier than that of the Central Jiangnan Orogen. An early Neoproterozoic arc-continent assembly model is herein proposed for the Jiangnan Orogen.