Geochemistry and Petrogenesis of Late Mesozoic Volcanic Rocks at DayushanIslands of Northern Geochemistry and Petrogenesis of Late Mesozoic Volcanic Rocks at DayushanIslands of Northern Fujian

The Late Mesozoic volcanic rocks on the offshore islands of northern Fujian are dominated by tuff and rhyolite, occasionally with mid-basic igneous rocks. The dating results of zircon suggest that the volcanic rocks in the northwest of the Dayushan Island are (100.2±1.7) Ma in age, corresponding to the upper volcanic rock series of Huangkeng Formation in east Fujian, and the western and eastern volcanic rocks are (116.7±1.5) Ma and (111.4±2.0) Ma respectively, which can be correlated to the Xiaoxi Formation of the lower volcanic series in eastern Fujian. The geochemical characteristics of the major elements show that the volcanic rocks of the area are mostly intermediate-acid (SiO₂ > 65%), belonging to high potassium calcium alkaline-potassium rock series. The normalized pattern diagram of REE suggests a right-leaning pattern or light rare earth enrichment, whereas the diagram of trace elements shows the enrichment of LILE such as Rb, K, Pb, loss of HFS such as Ta, Nb, Ti, and the negative anomaly of Eu, with obvious shell source characteristics. T_2DM=1 401~1 327 Ma, suggesting that the magma may have originated from the crustal material of the Mesoproterozoic.Pb isotope shows that they are related with EM Ⅱ, which indicates that the volcanic magma in the study area may be mainly originated from the continental crustal material related to subduction.The (⁸⁷Sr/⁸⁶Sr) of the Xiaoxi Formation and Huangkeng Formation are 0.710 13~0.711 10 and 0.722 82, ¹⁴³Nd/¹⁴⁴Nd are 0.512 208~0.512 214 and 0.512 267, ξNd(t) are -6.1~-6.0 and -5.2, suggesting that there may be mantle-derived mixing of the two groups. According to the regional tectonic setting, the Late Mesozoic volcanic rocks in the study area were formed in a plate extension environment of an active continental margin, with the characteristics of intraplate and volcanic arc.