The Geochemistry Of Late Mesozoic Volcanic Rocks From The North China Craton

The Yuanjiazhuang, Chengde, Hebei located in the east of the Daxing’anling-Taihangshan gravity gradient zone and the Bainaobao, Siziwangqi, Inner Mongolia in the west of the Daxing’anling-Taihangshan gravity gradient zone. These two areas are Mesozoic volcanic rocks exposed. We study the late Mesozoic volcanic rocks of Yuanjiazhuang and Bainaobao. Yuanjiazhuang volcanic rocks display geochemical characteristics with highly fractionated LREEs and HREEs([La/Yb]N=12.7-49.6), enriched Rb、Ba、Th、U、K large ion lithophile elements, depleted Nb, Ta, Hf, Ti high field strength elements. These volcanic rocks have high (87Sr/86Sr)i and low εNd(t), (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i and εHf(t)-Bainaobao volcanic rocks are similar to Yuanjiazhuang volcanic rocks in trace element and isotope characteristics. Namely, Bainaobao volcanic rocks display geochemical characteristics with highly fractionated LREEs and HREEs([La/Yb]N=39.8-40.1), enriched large ion lithophile elements (such as Rb、Ba、Th、U、 K), depleted in high field strength elements (Nb, Ta, Hf, Ti). Correspondingly, these volcanic rocks are high in (87Sr/86Sr); and low εNd(t), (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i and εHf(t). We propose that these volcanic rocks originated from partial melting of ancient metasomatized lithospheric mantle. Combined with geophysical research data, we propose that the North China Craton lithospheric thinning not only happened in the east of gradient belt, but also west region had occurred in the Mesozoic.1) The basalts and andesite of Yuanjiazhuang originated from the same source region.2) The volcanic rocks of Yuanjiazhuang and Bainaobao had experienced a certain degree of crystallization differentiation and the degree of crustal contamination is not obvious.3) The trace elements, istopic characteristics of Yuanjiahzuang and Bainaobao are similar. Yuanjiahzunag and Bainaobao volcanic rocks display geochemical characteristics with highly fractionated LREEs and HREEs ([La/Yb]N=12.7-49.6), enriched Rb、Ba、Th、U、K large ion lithophile elements, depleted Nb, Ta, Hf, Ti high field strength elements. These volcanic rocks have high (87Sr/86Sr)i and low εNd(t), (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i and εHf(t). These characteristics show that the source areas of them are the same and they come from the old lithospheric mantle.4) The volcanics of Mesozoic are the product of lithospheric thinning. Yuanjiazhuang and Bainaobao volcanic rocks show similar geochemical characteristics in trace elements and isotope. These indicate that during the period of the Mesozoic, the lithosphere of the part of thewest North craton has been thinned.5) The North China craton lithospheric thinning and the late Mesozoic magmatic activity are the result of the pacific plate subduction and the botton weakenly of the lithosphere.