Geochemistry And Geochronology Of Dabeigou Volcanic Rocks In Chengde Basin, Hebei Province And Constrains On Lithospheric Mantle Thinning Of North China Craton

Considerable magmatic activities and lithospheric mantle change were triggered by lithospheric mantle thinning in the North China Craton (NCC) which is generally thought to have occured mainly in Meso-Cenozoic. The Chengde basin is located in the middle Yanshan orogeny of the eastern part of NCC. It is documented by the emplacement of voluminous Mesozoic volcanic rocks and sedimentary rocks which reached a peak in the late Jurassic-early Cretaceous. The stratas in late Mesozoic are developed well and from the bottom upward, they are Jiulongshan Formation, Tiaojishan Formation, Zhangjiakou Formation, Dabeigou Formation and Xiguayuan Formation, respectively. Previous geochemistry and geochronologic studies in the Chengde basin are mainly about the volcanics in Tiaojishan and Zhangjiakou Formations, and rare about Dabeigou Formation, which overlies the Zhangjiagou Formation. Volcanics of Dabeigou Formation mainly cropped out near the Yuanjia village of the Chengde basin and is about 600m thick, which conformably overlies the amaranthine andesite of the Zhangjiakou Formation. The lithologies of this suit of volcanics are obviously different from the bottom upward: there is black andesite with highly developed vesicular and amygdaloidal structures about 60m at the bottom; and basalt with columnar joint and occurrence of olivine pyroxene phenocrysts, about 60m in thickness, overlies the andesite, which underlies the huge amaranthine and grey-white andesite with pyroxene and peldspar phenocrysts. Volcanic rocks of the Dabeigou Formation are conformably overlied by mudrocks and siltstones of the Xiguayuan Formation.82 samples were collected systematically from the Toudaogou-Tieluqiao profile and Yuanjiazhuang-Dabeigou profile and 32 samples from the Dabeigou Formation were collected and studied. Geochemical data of analyzed samples classify the Dabeigou Formation into three parts: high Ti andesite at the bottom, basalt in the middle part and andesite at the top. Integrated detailed petrological, geochemical and geochronological studies of Dabeigou rocks lead to the following conclusions: 1. Following 3 age groups of zircon are recognized by precise in-situ zircon dating of andesite sample DBG-06, which overlies the basalt: 131Ma, 122Ma, 114Ma. The age of 114Ma is thought as the age of formation of the volcanic rocks, while the other two age groups might be related to the underplating event in 140～120Ma in NCC.2. Abnormal characteristics of Hihg Ti andesite( high TiO₂(>2.1%), P₂O₅(>1.3%), low MgO(0.99%～1.65%) and Mg# (10.02～29.27), enriched REE(ΣREE=396～439μg/g) et al) should be related to the asnospheric mantle and mantle fluid.3. Andesite at the top may have been related to the asthenospheric mantle upwelling leading to partial metling of the basaltic underplates which intruded at between 120 to 140Ma.4. Available data on Nd isotopes and Nb/U ratios of Mesozoic and Cenozoic basalts from the NCC suggest that the lithospheric mantle might have been thinned at 114Ma. Simple three-component (enriched lithospheric mantle, depleted asthenospheric mantle and basaltic lower crust) mixing calculations can interpret the apparent decoupling between Nb/U ratios and Nd isotopic compositions.