冀北滦平地区中生代火山岩中单颗粒锆石的稀土元素特征及启示

Characteristics of Rare Earth Elements of Zircons from Mesozoic Volcanic Rocks in Luanpin North Hebei and Its Application

通过对滦平地区中生代火山岩中锆石的稀土元素分析,表明髫髻山组顶部火山岩样品中锆石δEu的均值为0.49,(Lu/Gd)N的均值为21.87;然而,土城子组顶部层位、底部层位和张家口组底部层位火山岩样品中锆石的δEu均值分别为0.15,0.06和0.09,(Lu/Gd)N的均值分别为14.51,15.66和16.25。反映了土城子组中锆石的稀土元素特征与张家口组中锆石特征的一致性,而与髫髻山组中的锆石有较大的区别,表明土城子组与张家口组中的火山岩有明显的亲缘性和一致性。结合同位素测年结果和锆石的阴极发光特征,可以认为:该地区的髫髻山组与土城子组之间不仅时间间隔长,而且火山岩的源区也有较大的区别;土城子组与张家口组之间不仅在时间上基本连续,而且火山岩的源区也有明显的一致性;该分析结果暗示了滦平地区的土城子组与张家口组之间不存在时间的间断和大的构造界面,是相同(或相近)地质背景下形成的,即二者之间应不存在构造转换界面和J3-K1界面。研究表明,在壳源岩浆锆石的稀土元素分析中,(Lu/Gd)N,δEu是相对稳定的、有指示意义的两个重要参数;壳源岩浆锆石的∑LREE的值变化范围较大,尤其是La的丰度变化范围较大,所以在锆石的稀土元素分析中,与La相关的各种比值的指示意义并不明显。

Rare earth elements of the zircons from the mesozoic volcanic rocks in Luanping area, Hebei, were analyzed： The results show that the average values of δEu and （Lu/Gd）N are 0.49 and 21.87 respectively in the zircons from the top part of Tiaojishan Formation; but the average values of δEu and （Lu/ Gd）N are 0.15, 0.06, 0.09 and 14.51, 15.66, 16.25 respectively in the zircons from the lower, top part of the Tuchengzi Formation and the bottom bed of the Zhangjiakou Formation. The results illuminate that the characteristics of the zircons from the Tuchengzi Formation are coincident with those of the zircons from the Zhangjiakou Formation, but are different from those of the zircons from the Tiaojishan Formation, and imply that the Tuchengzi Formation has close relation with the Zhangjiakou Formation. Combining the results above with the former isotopic dating results of the volcanic rocks, the authors draw the conclusions as follows： The Tuchengzi Formation not only has a long interval period with the Tiaojishan Formation, but also is very different from the Tiaojishan Formation in zircon characteristics; The Tuchengzi Formation not only is nearly continuous with the Zhangjiakou Formation in time, but also is coincident with the Zhangjiakou Formation in geochemistry of zircons. The resuhs imply that the Tuchengzi Formation and the Zhangjiakou Formation were formed in the same geological background, that is, there are not the boundary of the J3K, and the interface of the transition of tectonic framework between the Tuchengzi Formation and the Zhangjiakou Formation in the Luanping area. The research shows that the （Lu/Gd）N,δEu are two important parameters which are relatively stable in the analysis of zircons from Crust-source ; but the values of ∑ LREE of zircons from Crust-source change greatly, especially the abundance of La element, so some ratios of rare earth elements related with La （or ∑ LREE） are not usable in determining the characteristics of zircons.