冀北沽源盆地中生代火山岩的同位素年龄及成因

根据同位素年龄测定,冀北地区张家口组三段酸性火山岩的形成年龄为138Ma,属于上侏罗统顶部;花吉营组中基性岩石的形成年龄为133Ma,应划归下白垩统底部为宜。根据火山岩全岩Pb—Pb等时线年龄、残留锆石的年龄及基底岩石的变质时代等信息,结合火山岩中稀土元素的分布特征等推断:张家口组火山岩主要是由基底岩石(硅铝壳)重熔形成,而花吉营组岩石可能为壳幔混染成因。

ISOTOPE AGE AND GENESIS ON MESOZOIC VOLCANIC ROCKS IN GUYUAN BASIN,NORTHERN HEBEI

Guyuan volcanic basin is situated in area where the Mesozoic volcanic activity strongly occured. In this area a suite of acidic alkali-lime volcanic rocks called Zhangjiakou group is well developed Intermediate basic volcanic rocks called Huajiying group are locally distributed. Isotope age determination results show that the age of formation is 138 Ma for the 3-d member of the Zhangjiakou group volcanic rocks and is 133 Ma for the Huajiying group rocks. The basement rocks in this area formed in the late Archaean era, and isotope age of zircon of 1860 Ma from biotite-plagioclase granulite represents the time of metamor-phism of the basement rocks. The age of Pb-Pb whole-rock isochron of 2412 Ma from the Zhangjiakou group volcanic rocks indicates that source rock from which have derived volcanic rocks formed in the late Archaean era. As a result of strong alteration the sub rhyolite-porphyry intruding into the Zhangjiakou group volcanic rocks give a young age of 126 Ma. However the upper intercept age of 2004 Ma on concordia diagram for zircon data from sub-rhyolite-porphyry closes to time of metamorphism of the basement rocks.On the basis of age determination results and geological analysis, is it considered that the 3-d member of the Zhangjiakou group belongs to the top of upper Jurassic series, while the Huajiying group belongs suitably to the bottom of Cretaceous series. According to the form of remnant zircon, isotope age determination and distribution pattern of rare earth element, it is suggested that the Zhangjiakou group volcanic rocks have from-ed mainly from the basement rocks (silica alumina crust) by remelting, but the Huajiying group rocks have formed possibly by mixed material from both crust and mantle.