Time Sequence And Petrogenesis Of Late Mesozoic Volcanism In The Area Of Zhejiang-Fujian Boundary

Late Mesozoic volcanism was intense in the area of Zhejiang-Fujian boundary, which constitutes an important part of China southeast costal volcanic rock belt. However, due to its geographical inconveniencies, geological investigations and scientific researches in this area have long been in a low degree, particularly systematic studies of chronology, stratigraphy and petrogenesis. Therefore, understanding of late Mesozoic volcanism and geological evolution of southeast coast is seriously hampered. The thesis chooses typical late Mesozoic volcanic formations in this area for field investigations and observations of route geological profiles. It carries out systematic studies of zircon U-Pb chronology, lithology, petrography and geochemistry, accordingly analyses geological process of volcano-sedimentary, and discusses the relationship between petrogenesis of volcanic rocks and crust-mantle interaction.Based on the characteristics of rock composition, isotopic ages, fossil assemblages, stratigraphic sequence and contact relation between strata, late Mesozoic volcano-sedimentary strata in the area of Zhejiang-Fujian boundary could be divided into upper and lower volcanic rock series. From bottom to top, stratigraphic sequence inside Zhejiang Proince are Gaowu Formation and Xishantou Formation of lower volcanic rock series, Guantou Formation, Chaochuang Formation, Xiaopingtian Formation and Taishun Formation of upper volcanic rock series; while for Fujian province, it is respectively Nanyuan Formation and Xiaoxi Formation of lower volcanic rock series, Huangkeng Formation and Zhaixia Formation of upper volcanic rock series.Using zircon LA-ICPMS U-Pb chronology, the paper gets different ages for the two series in the study area. Respectively, the bottom and top ages of Gaowu Formation of lower volcanic rock series are136.0Ma,133.2Ma; the bottom and top ages of Xishantou Formation131.9Ma,127.9Ma; the bottom and top ages of Nanyuan Formation143.1Ma,140.1Ma; the top age of Xiaoxi Formation126.2Ma. While for the upper volcanic rock series, the top age of Xiaopingtian Formation is114.9Ma; the bottom and top ages of Taishun Formation112.7Ma,111.2Ma; the age of outside Beishan miarolite intruded into Taishun Formation96.2Ma. The isotopic chronology and route geological profiles show that, volcanic activities in the study area took place during143～111Ma (with a peak between143～126Ma), i.e., Early Cretaceous, in which occurred a long intermission in126～116Ma and an end of magmatic activity in ca.96Ma. The volcano sedimentation process can be divided into five stages, which is conducive to understand temporal and spatial evolutions of Early Cretaceous volcanism in coastal Southeast China.The lithology for the lower volcanic rock series are mainly rhyolitic ignimbrite for Gaowu Formation; rhyolitic ignimbrite, trachyandesite bedded with sedimentary rocks for Xishantou Formation; rhyolitic ignimbrite and thick layer of porphyroclastic lava for the bottom and top of Nanyuan Formation respectively; rhyolitic ignimbrite interbedded with sand and shale for Xiaoxi Formation. For the upper volcanic rock series, Guantou Formation and Chaochuang Formation are respectively sallow tuffaceous sandstone and red mud; Xiaotian Formation mainly rhyolitic vitric tuff and rhyolitic ignimbrite bedded with red mud; Taishun Formation, which is the most advanced Late Mesozoic volcano-sedimentary strata in this area, mainly rhyolitic crystal vitric ignimbrite interbedded with Dark grey mudstone. Geochemical analyses imply that the lower volcanic rock series are widely-spread felsic volcanic rocks, weakly peraluminous high-k calc-alkaline, enriched in large ion lithophile elements (such as Cs、Rb、Ba、Th、K、Pb) and LREE, without evident Eu anomaly, short of high field-strength elements Nb、Ta、Ti、P, low (176Hf/177Hf), value (0.282098～0.282577) and low εHf.(t) value (-20.43～4.51), Zircon Hf isotopic model age of stage two TDMt concentrating between1.8and2.2Ga. For the first time, low Sr trachyandesite rarely seen in southeast coast is found in Xishantou Formation. It belongs to Shoshonite series, enriched in Sr, Ti, Co, Ni, with low εHf (t) value, which clearly differentiate it from other felsic volcano rocks. It is probably formed by the fractional crystallization of basalt magma. The upper volcanic rock series are felsic volcano rocks restricted in graben basin, high K calc-alkaline metaluminous, obvious negative Eu anomaly, higher εHf(t) value than the lower volcanic rock series. According to the age and composition of the pre-Cambrian metamorphosed basement of the study area, the paper holds that the cretaceous upper and lower volcanic rock series all developed in active continental margin environment. The underplating mantle-derived magma caused partial melting of Proterozoic basement, and underplating magma could evolve into low Sr Rhyolitic magma, the effusion of which formed trachyandesite of Xishantou Formation. Comparatively, the development of upper volcanic rock series underwent more intense crust-mantle-derived magmatic mixing.Through comparison of zicron εHf(t) values, this thesis discovers that the formation of the two volcanic rock series in the study area is closely related to crust-mantle interaction, which has evident spatio-temporal differences:temporally, from the early period of early Cretaceous to the late period, the crust-mantle interaction is obviously more intense in the formation of upper volcanic rock series; spatially, from south to north, the crust-mantle interaction weakens gradually, probably in accordance with the tendency of the subduction of the ancient Pacific plate.