Yandang World Geopark Volcanic Research

The Mt. Yandangshan World Geopark is situated at southeastern Zhejiang Province, consists of a Cretaceous caldera, covering an area of over 294.6 km~2. It belongs to the marginal Pacific tectonic domain within the southern circum-Pacific Asian continental margin. In Mesozoic, due to oblique subduction of Pacific plate toward Asian continent plate at a low angle, the tectonic patterns of Eastern China changed from E-W direction to NE and NNE. This tectonic movement is called the Yanshanian Movement in China. The Cretaceous Mt, Yandangshan caldera in southeastern China coast was formed within this tectonic background which has a global significance, composed the representative rhyolitic geologic landscape, and thus is of great typical and scientific significances for paleo-volcanology, volcanic tectono-petrofacies, tectonics, Geotourism, eomorphology and Landscape Architecture.This thesis studies the volcanic tectono-petrofacies, petrology, mineralogy, geochemistry, isotopic chronology, stratum and high-precision SHRIMP zircon U-Pb datings of the Mt.Yandangshan volcano-intrusive complex, and concludes that: Yandangshan volcanic rock units belong to high-K calc-alkaline acid rock associations, mainly ryholitic and trachytic dacitic in composition. They have A/CNK values of 0.9-1.2 (i.e., aluminium saturation and supersaturation), enriched LREE and LILEs and depleted HFSE. Their 8Eu values are 0.11-0.32, indicating a evident fractional evolution of plagioclase crystallization. Their ∑REE varied very limitedly, ((87)~Sr/(86)Sr)i ratios and eNd (t) values are 0.7086～0.7095 and -10.20～-9.96 for volcanic rocks, and 0.7087~0.7095 and -7.6～6.92 for the central intrusive facies syenite, which means that all the igneous rocks from Mt. Yandangshan are evolutional products of magmas with the same source, and that they derived mainly from crust, but mixed with mantle components. Their tectonic background was not both typical island arc and Andean-type active continental margin, and belonged to a setting of active continental margin close to the inner continent.According to our SHRIMP U-Pb ziron datings, the Mt.Yandangshan volcanic rocks are 97.2～105.6Ma in age, i.e., late stage of early Cretaceous and correspond to the Xiaopintian Formation of Yongkang Group in eastern Zhejiang. It not only ends the dispution about the time of the Mt. Yandangshan World Geopark volcanic rocks, but also ofifers chronologic limits for both stratigraphic divisions of Cretaceous volcanic rocks in eastern Zhejiang and the YanshanianMovement in Southeast China.It is also proposed that the Yandangshan Caldera is an integrated resurgent caldera. It underwent evolutional 8 phases as following: initial explosion of pyroclastic flow —>caldera collapse and local hydromagmatic explosion—?resurgent uplifting of caldera—? explosion of pyroclastic flow again —>Last explosion of pyroclastic flow —>the second collapse—>magma intrusion—?uplifting and denuding of the caldera—+modern physiognomy sight forming phase of the Mt. Yandangshan, it is said that crater has twice collapse and volcano has twice explosion.