关键词: 晚中生代火山岩；华北克拉通；岩石圈减薄/破坏；重力梯度带；袁家庄；四子王旗；Sr-Nd-Hf同位素

Abstract:

The Yuanjiazhuang and Bainaobao volcanic rock suites are of late Mesozoic age located respectively in the east and west of
the Daxing'anling-Taihangshan gravity gradient zone. These volcanic rocks display similar characteristics in incompatible trace
elements and radiogenic isotopes. They are highly fractionated in rare earth elements (REE) with a high LREEs/HREEs ratio, enriched
in large ion lithophile elements (such as Rb, Ba, Th, U, K), relatively depleted in high field strength elements (HFSE: Nb, Ta, Zr, Hf,
Ti), high (87Sr/86Sr)i and low εNd(t) and εHf(t). All these data suggest that the sources for the two volcanic rock suites are similar and their
parental melts must have derived from partial melting of ancient metasomatized mantle lithosphere. Combined with the data in the
literature, we propose that the lithospheric thinning beneath the North China Craton happened not only in the east but also to the west
of the gradient zone in the late Mesozoic.