Geochemistry Characteristics And Its Genesis Of The Late Mesozoic Volcanic Rocks From Mengyin Basin

In this paper, LA-ICP MS zircon U-Pb geochronological, geochemlcal and Sr-Nd-Pb isotopic analyses have been carried out on the Late Mesozoic Qingshan Formation volcanic rocks from Mengyin Basin beneath the southeastern North China Craton. With these study, this paper demarcated formation time of these volcanic rocks, summarized the nature of their petrology and geochemistry, discussed the deep processes of the magmatism, compositions of magma sources, and magmatic evolution features. Based on regional comparsions, this paper also analyzed the patterns of the enriched characteristics in Mengyin area.Qingshan Formation volcanic rocks from Mengyin Basin were erupted during the Early Cretaceous, the eruption ages varied from 118.2Ma to 122.4Ma that anastomosed with primary study in this area. These rocks belong to meta-alkaline series locating in TAS diagram with the SiO2 contents of 53.71%～69.08%and the total alkaline contents of 5.85%～9.35%. Most of the rocks are trachydacite and trachyandesite except a few showing andesite affinity. These volcanic rocks belong to high-K cal-alkaline, metaluminous and silicon-saturation series. And the volcanic rocks are notably rich in LILE and LREE, depleted in HREE and HFSE, detailed showing significant positive Rb, Ba, Pb, and Sr anomalies and negative Nb, Ta and Ti anomalies and negative Eu anomalies weakly. The initial magma of the volcanic rocks has been little contaminated by curstal materials during the magmas ascending based on geochemlcal, isotopic and geochronological evidences. Therefore, the isotopic characteristics of the volcanic rocks can be used to approximate those of their mantle source. The volcanic rocks are rich in Sr-Nd-Pb isotope component, showing the range of the initial isotopic ratios as follows: 87Sr/86Sr(t)=0.7051～0.7109, 143Nd/144Nd(t)=0.5117～0.5123,εNd(t)=-3.9～-15.3, and the radiogenic Pb isotopic ratios: 206Pb/204Pb(t)=16.88～18.39, 207Pb/204Pb(t)=15.31～15.58, 208Pb/204Pb(t)=36.63～38.06. In addition, some geological and geochemical features suggest that the magma sources of Mengyin Basin are different with Jinan, Zouping and lower crust of North China, but similar to Fangcheng and Jiaolai Basin, showing EMⅠand EMⅡlike enriched isotopic characteristics. It is inferred that the mantle source of the volcanic rocks from Mengyin maybe a mixture of the mantle and the crust .Combining with the primary study of geochronological, geochemical and Sr-Nd-Pb isotopic characteristics of volcanic rocks from nearly area in eastern North China Craton during the same period, we consider that the Late Mesozoic Qingshan Formation volcanic rocks from Mengyin Basin originated from the enriched Mesozoic lithospheric mantle, which evolved from its Paleozoic counterpart through extensive interaction with a crust-derived melt. We propose that this melt was generated from the melting of the subducted lower crust of the Yangtze Craton. This geodynamic model is proposed to illustrate such a transform in secular lithosphere evolution and the lower crust was thinned together with lithosphere mantle in eastern North China Block at Mesozoic.