Geochronological And Geochemical Study On The Mesozoic Volcanic Rocks In Beihuaiyang Belt

The Beihuaiyang belt in the northern part of Dabie orogenic belt is characterized by the outcrop of a great amount of Mesozoic volcanic rocks. Compared with the neighboring area, such as the granite outcropping area of Dabie Orogen and volcanic rocks outcropping area in the middle and lower reaches of the Yangtze River, relatively less work has been done to the Mesozoic volcanic rocks in the Beihuaiyang belt. This in turn has greatly constrained the interpretation of the Dabie orogen’s post-collision evolution process and the thinning mechanism of the lithosphere in the east part of China. After considering and colligating the pre-existing divisiory scenario, this thesis studied mainly these four formations of the Beihuaiyang volcanic rocks which are (from west to east), the maotanchang Formation, the Xianghongdian Formation, the Jingangtai Formation and the Chenpeng Formation. Through a combined study of petrology, geochronology and geochemistry, we get mainly these several new findings:(1). The pre-Mesozoic rock-groups before the volcanism, which experienced low grade metamorphism, can be divided into three parts after comparison. The Dingyuan Formation of the Sujiahe Group and the Xiaoxihe Formation of the Luzhenguan Group is a set of meta-magmatic and sedimentary rocks which recorded the Nanhua-rift-magmatism in the Neoproterozoic; The Nanwan Formation of the Xinyang Group and upper Member of the Foziling Group is a set of pre-subduction flysch; The Carboniferous strata of part of the Meishan Group and Yangshan Group is a set of the relic of sea-facies molasse formation.(2). The Maotanchang Formation in the eastern part of Beihuaiyang and Jingangtai Formation in the western part of Beihuaiyang make up of the main part of the volcanic rocks in the area. Both of them are a set of explosion-effusive facies volcanic rocks and they resemble each other in aspects of rock assemblage, geochemical characteristics and the diagenetic ages of the volcanic rocks. The similarities of the characteristics of the two volcanic-rock formations suggest that the main body of the volcanic rocks of the Beihuaiyang belt was formed in one episode of magmatism.(3). LA-ICPMS zircon U-Pb dating results of the Mesozoic volcanic rocks in the Beihuaiyang belt show that the andesitoid of the Maodanchao Formation is formed at about128.6±0.6Ma. The midalkalite of the Xianghongdian Formation is formed at about135.1±0.7Ma. The flood tuff of the Jingangtai Formation is formed at about127.9±0.8Ma, the necrolite is formed at about127.6±0.5Ma. The rhyolite in the upper part of Chenpeng Formation is formed at about125.1±0.7Ma. Based on these dating results and the results obtained by other researchers, we divided the magmatism into two stages. The first-stage eruption happened mainly in the east and west part of the area at127-131Ma, which resulted in the formation of the main part of the Mesozoic volcanic rocks:the maotanchang Formation, the Jingangtai Formation and the Chenpeng Formation. The second-stage eruption happened in the middle part of the area at about122Ma, which resulted in the formation of ultrapotassic volcanic rocks.(4). Geochemical study shows that the Mesozoic volcanic rocks in the area are high in K, LREE and LILE, strong LREE and HREE fractionation, low in HFSE. Trace element spider diagram shows typical low curve in Nb and Ta. They are high in ISr ratio, and low in sNd(t). Palaeo-and Meso-proterozoic Nd depleted mantle model age ranges from1.5to2.6Ga. Also, they are low in206Pb/204Pb ratio. Isotopic study further constrains that the source of the Beihuaiyang volcanic rocks is enriched lithosphere mantle which experienced the adding of ancient crustal materials. In other words, the source is the lithosphere mantle of the north China with the mixture of the Kongling Group of the Lower crust of the Yangtze block.(5). The ultrapotassic rocks formed during the second eruption stage in the area have similar geochemical characteristics with that of rocks formed during the first stage. They are also high in K and LREE. But they show less influence by crustal materials. This characteristic suggests that during the late stage of the volcanism, due to the great consumption of the crustal materials in the mantle source, magma formed by partial melting inherited geochemical characteristics more from the mantle materials. After carefully summarizing the existing diagenesis model, we present in this dissertation a model that the source of the Beihuaiyang volcanic rocks is the lithosphere mantle of the north with the mixture of the relic of crustal materials of the Yangtze block, which is added to the mantle by the subduction of Yangtze toward North China Craton.(6). The comparison of the volcanic rocks in Beihuaiyang and the mid-and lower reach of the Yangtze River shows that the volcanic rocks in both these two areas are the result of the thinning of lithosphere in the eastern part of China in early Cretaceous. The differences in rock assemblage and geochemistry of the two area show that the thinning of lithosphere in different tectonic backgrounds has different mechanism. In Beihuaiyang belt, the aesthenosphere heated the thickened lithosphere and resulted the extensive partial melting. In the mid-and lower reach of the Yangtze River, the aesthenosphere actively intruded into the lithosphere and this resulted in the generation of mixed magma in the area.