冀北郭家屯地区中生代火山岩年代学和地球化学特征研究

Geochronology and geochemistry of the Mesozoic volcanic-subvolcanic rocks in west Longhua area, northern Hebei Province

单颗粒锆石U-Pb测年结果表明,冀北郭家屯地区中生代火山活动有两个时代:中—晚侏罗世(约143～164Ma)和早白垩世(约126Ma)。中—晚侏罗世火山岩以流纹岩和粗面质流纹岩为主,夹石英粗面岩,下部有少量安山岩;早白垩世火山岩以粗安岩和粗面岩为主,夹少量玄武质粗安岩和流纹质粗面岩。微量元素数据显示粗面质火山岩、正长斑岩具高Al2O3(≥15%)、Na2O/K2O>1、高Sr(≥400μg/g)、高Sr/Y(≥20～40)和La/Yb≥10;低的MgO(<3%)、Y(≤18μg/g)和Yb(≤1.9μg/g);Mg#高(38.47～57.78,平均51.18);Eu异常不明显等特征,与Adakite岩的地球化学特征有某些相似性。而流纹质火山岩具有SiO2、K2O、Y、Yb含量相对较高;Al2O3、Sr、Ba含量相对较低;轻稀土元素富集、重稀土元素平坦、Eu负异常明显等地球化学特征。两类岩石在时空上密切共生,具有一定的演化关系。地球化学总体特征显示本区火山岩具有弧岩浆作用的特征,如岩石系列高钾钙碱性属性、高场强元素(HFSE)和大离子亲石性元素(LILE)解偶,微量元素的俯冲带特征(与Adakite岩相似)等。研究表明本区中—晚侏罗世和早白垩世火山岩总体形成在地壳加厚、岩石圈拆沉、底侵作用发育的构造背景下,地幔脱气形成的CO2-H2O流体在岩浆形成过程中起了很重要的作用。

The single zircon U-Pb ages of the Mesozoic volcanic-subvolcanic rocks distributed in the Guojiatun area, west Longhua, northern Hebei Province are precisely determined. The results show that there are two periods of the Mesozoic volcanic-subvolcanic rocks： Middle Jurassic to Late Jurassic （ca. 143-164 Ma） and Early Cretaceous （ca. 126 Ma）. The rocks of the early period are mainly rhyolite and trachyrhyolite, with some quartz trachyte and trachyandesite in the interlayer and minor andesite in the bottom. The rocks of the late period are chiefly trachyandesite, basaltic trachyandesite and trachyte. Geochemical data suggest that the rocks are potassium-rich, metaluminous, and are predominantly high-K calc-alkaline and subordinately shoshonitic. Advanced study of geochemistry shows that the trachytic rocks are characteristic of high Al2O3 （≥15% ）, Sr （≥400μg/g） and low Y （≤18μg/g）, Yb （≤1.9μg/g） contents, high NaEO/K20 （〉1）, Sr/Y （≥20） and La/Yb （≥10） ratios, relative high Mg^＋（38.47-57.78）, as well as weak negative Eu anomalies. All these characteristics are similar to adakite. Compared with the high Ba-Sr type, the rhyolites are characterized by relatively high SiO2, K2O, Y, Yb, and low Al2O3, Sr, Ba, as well as marked negative Eu anomalies. The two types of rocks associate each other in space and time. Detailed major, trace, rare earth element geochemistry suggests that the majority of the Mesozoic volcanic rocks are of the characteristics of arc magmatism, such as the calc-alkaline affinity, the presence of the “subduction signature” （similar to adakite）, and the decoupled HFSE and LILE. However, these geochemical characteristics are not always associated with contemporaneous subduction setting. It is thought that the Mesozoic volcanisms were developed under the condition of thickened lower crust, thinned lithospheric mantle, and the tectonic setting of underplating. Meanwhile, mantle-derived CO2-H2O possibly played important roles in the magma formation.