Subvolcanics of the Yanyaozhai Formation in the southern margin of the North China Craton has a zircon SHRIMP U-Pb age of 1781 ±12 Ma, on which the upper limit age of the Xioang'er Group volcanic rock is restricted later than 1781Ma. The subvolcanics displays complicated compositons from alkaline trachyte-toscanite, dacite to rhyolite. Its SiO2content of 59. 30%-72. 33% indicates that the rock belongs to intermediate to acidic rock with over-saturated aluminum. It is characterized by of enriched alkali and high K, poor Ca and Mg, negative Eu anomaly, low Sr and high Rb/Sr, and high contens of Ga, Zr, Nb, Ce and Y, which all exhibit the geochemical features of the A-type granite. Geochemcial analysis shows that the subvolcanics is deplete in HREE, with distinct fractionation of HREE and LREE, and weak negative Eu anomaly, except several samples showing weak positive Eu anomalies. As for trace element features, the rock are characterized by enrichment of large-ion lithophile elements and relative depletion of the high field strength elements. The 176Hf/177 Hf ratios are between 0. 281415 and 0. 281557, and εHf(t) values are -11.264%--6. 684% (averaging -8. 652%); the initial ratios of 143Nd/'144Nd are 0. 5095~0.5099, with an average εNd(t) value of -11. 37%. All these results suggest that magma source of the rocks was from deep-seateed magma, related to enriched mantle and contamined by crustal materials. Therefore, the rock of this kind probably formed in a post-collisional extensional enviorment.%研究表明华北克拉通南缘熊耳群眼窑寨组次火山岩的形成年龄为1781±12Ma.依此将熊耳群火山岩的上限年龄限定在1781Ma之后.眼窑寨组次火山岩岩石类型较为复杂,主要由碱性粗面-粗面英安岩、英安岩与流纹岩组成,其SiO2含量变化范围为59.30%～72.33%,为铝过饱和的中酸性岩石.岩石富碱高钾、贫钙镁、负铕异常、低Sr和高Rb/Sr比值,并高Ga、Zr、Nb、Ce、Y等元素含量,显示A型次火山岩的地球化学特征.岩石的轻、重稀土分馏作用比较明显,亏损重稀土元素,以微弱的负铕异常为主,个别样品呈现出微弱的正铕异常.其176 Hf/177 Hf比值分布于0.281415～0.281557之间,εHf(t)值为-11.264‰～-6.684‰,平均为—8.652‰；143 Nd/144 Nd初始值为0.5095～0.5099,εNd (t)平均值为-11.37‰.由此推测,岩石岩浆源区较深,岩浆来源与富集地幔有关,并受到地壳物质的混染,可能形成于碰撞后的拉张环境.
展开▼
