华北克拉通重力梯度带两侧晚中生代火山岩地球化学特征对比研究及其对岩石圈减薄的时空制约

摘要

The Yuanjiazhuang and Bainaobao volcanic rock suites are of late Mesozoic age located respectively in the east and west of the Daxingˊanling-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 lowεNd(t) andεHf(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.%对华北克拉通重力梯度带东西两侧的河北承德袁家庄和内蒙古四子王旗白脑包出露的晚中生代火山岩进行了地球化学对比研究。全岩主量元素、微量元素和Sr-Nd-Hf同位素数据表明，袁家庄火山岩和白脑包火山岩具有相似的地球化学特征。这些火山岩轻、重稀土强烈分异，富集大离子亲石元素(如Rb，Ba，Th，U，K等)，亏损高场强元素(如Nb，Ta，Zr， Hf，Ti等)；其同位素具有高(87Sr/86Sr)i，低εNd(t)和低εHf(t)的特征，因此认为袁家庄和白脑包火山岩具有相似的源区，即均来源于被交代的古老岩石圈地幔的部分熔融。结合文献资料，作者认为，这些晚中生代火山岩是华北克拉通岩石圈减薄的直接产物，中生代时期华北克拉通岩石圈的减薄不仅仅局限于重力梯度带以东地区，以西的部分地区也有发生。