张家口地区张家口组火山岩元素地球化学特征及成因

ELEMENT GEOCHEMISTRY AND PETROGENESIS OF THE VOLCANIC ROCKS OF ZHANGJIAKOU FORMATION IN THE ZHANGJIAKOU AREA,NORTH CHINA

针对张家口该地区张家口组火山岩的主量元素和微量元素高精度分析测试,结果显示:该套流纹岩和粗面岩具有富铁,高钾和全碱,富集REE,Rb,Th,U,Zr,Ga等不相容元素,Nb,Ta相对LREE亏损的特点;低分异样品不具有明显负Eu异常。张家口地区张家口组长英质火山岩的元素地球化学特征与还原型A型花岗岩相似,是华北克拉通拉斑质基性下地壳在高温、低氧逸度和低水逸度条件下的部分熔融产物,形成于碰撞后构造背景。

The Zhangjiakou Formation of Early Cretaceous exposed in Zhangjiakou area, North China,is mainly composed of rhyolite and trachyte. Previous researches were mainly focused on the stratigraphy correlation and geochronology, but paid no attention to the geochemistry of these volcanic rocks,especially did not touched the data of high-precise trace elements. For understand- ing the volcanic series, petrogenetic regime and regional tectonic setting of Zhangjiakou volcanic rocks,the high-precise analysis of major and trace elements of these rocks were firstly presented in this study. XRF method was used to analyze major elements, and ICP-MS and XRF methods used to analyze trace elements in the volcanic rocks. It was showed that, these felsic volcanic rocks were iron enriched and reduced, and bad high contents of potassium, total alkali, and incompatible elements of rare earth elements （REE）,Rb,Th,U,Zr,Ga and so on,with Nb,Ta depletion rela- tive to the light rare earth elements （LREE） and no significant negative anomaly of Eu for those less-fractionated samples. The element geochemistry of Zhangjiakou volcanic rocks in Zhangjiakou area is similar to that of reduced A-type granitoid. It was deduced that these felsic volcanic rocks were originated by partial melting of the tholeiitic lower crust of North China craton under conditions of high temperature,low H2 O fugacity,and low O2 fugacity. These volcanic rocks showed a tectonic setting of post collisional environment.