Geological characteristics and origin of the felsic porphyroclastic lavas in the Zhedaikou area in Suichang, Zhejiang Province

This study carried out a detailed analysis of porphyroclastic lavas to understand its geological characteristics and origin based on field survey and comprehensive research. Through study of microscopic lithofacies, lithogeochemistry, isotopic chronology and zircon saturation temperature, porphyroclastic lava facies belt was defined at macro level, and spatial-temporal distribution of porphyroclastic lavas and its contact relationship with the surrounding rock was ascertained. The porphyroclastic lavas in the study area are controlled by a revived caldera in the large-scale NE-trending volcanic structural depression in Zhedaikou-Hushan and characterized by extrusion and effusion, suggesting a composite dome. Magma evolution of the porphyroclastic lavas shows a certain continuation spatially and temporally, indicating a magmatic differentiation process from effusion to extrusion to intrusion. Porphyroclastic lavas with typical porphyroclastic structure and pearl-edge texture are enriched in HFSEs(Nb, Ta, Zr, Hf)and LILEs(Rb, Sr, Ba, Th), and also rich in alkali(K₂O+Na₂O=8.54%～9.25%), A/CNK=0.88～1.04, suggesting the lavas belong to a metaluminous shoshonite series. The diagenetic temperatures of the lavas range from 803 ℃ to 884 ℃,indicative of high temperature hypabyssal. A Rb-Sr isochron age of 123 Ma suggests that the lavas were the result of Early Cretaceous magmatic activity. Out study shows that the porphyroclastic lavas and associated granite are co-magmatic I-type rocks derived from crust-mantle mixing source, and generated in an active continental margin close to intraplate.