Geochemical Characteristics And Genesis Of The Volcanic Rocks Of Meifeng Formation In Wuyiling Area, Yichun City Of Heilongjiang Province

The Mesozoic volcanic rocks are distributed in the Wuyiling area, Yichun city of Heilongjiang province. In this thesis, the research of petrology, geochemistry and zircon SHRIMP U-Pb dating have been carried out on the volcanic rocks. Based on these studies, the lithologic features, the classification of the rock series, the formation age are determined. The volcanic strata was designated the Meifeng Formation and the geochemistry of the volcanic rocks are determined and the magma sources and its properties and the deep processes of the magmatism are analyzed. At the same time we also discussed the petrogenesis of the volcanic rocks in Wuyiling area.The zircon SHRIMP U-Pb dating of the volcanic rocks in Meifeng Formation is 105±1.5Ma. Thus the age of Meifeng Formation can be determined to be late Early Cretaceous. The SiO2 contents of rocks are between 51.27%-70.13%, the total alkaline contents are 3.9%-9.6%. The rocks have higher contents of K2O(w (Na2O)/w (K2O)=0.3-2.5), lower contents of Ti2O (w (Ti2O)=0.04%-1.9%). The Al2O3 contents are high but large-scale(w (Al2O3)=14.05%-18.24%). Combining with the TAS diagram and the relative contents of Na2O and K2O (Na2O-2.0≤K2O), we found that the volcanic rocks are typical shoshonite series firstly. The volcanic rocks are mainly composed of potassic trachybasalt, shoshonite, latite and trachyte. All the samples of the volcanic rocks are characterized by the enrichment of LREE, depletion of HREE and negative Eu anomalies inconspicuously. The volcanic rocks are notably rich in LILE like Cs, Rb, Ba, Th relatively, depleted in HFSE like Nb, Ta relatively, especially depleted in Ti. The mantle-derived magma had been modified by oceanic crust and marine sediments and had undergone the assimilation-contamination of the granite from ancient Asia structural domain. The magma experiences fractional crystallization of olivine, calcium-rich pyroxene, plagioclase during magma evolvement.Combining with the primary study of petrology and geochemical, we conclude the formation of volcanic rocks of Meifeng Formation is as follows:Because of the west Pacific plate subduction beneath the Northeastern Asia of China at a smaller angle, the dehygration of deep-subducted slab formed the melt or fluid with the participation of marine sediments. The melt or fluid moved up and triggered the partial melting of the mantle, which are rich in LILE and LREE. The magma had been modified by the crustal materials' contamination visibly. And fractional crystallization is an evolutionary type during the rising process of magma. Then the magma was erupted to the surface, which belong to the shoshonite magma of silicon-rich and K-rich.