青龙建昌地区中生代火山岩地球化学特征及岩石成因

摘要

The Mesozoic volcano rock in Qinglong Jianchang area is mainly occurred in Tiaojishan and Yixian formation.Trachyandesite of Tiaojishan formation is rich in silicon and sodium,poor in magnesi-um,not high in total rare earth elements [∑REE=(1 10.93~1 69.79)×10 -6 ],significant in the frac-tionation of HREE and LREE [(La/Yb)N =13.52~24.46],with weak negative or positive Eu anomaly (δEu= 0.96 ~ 1.1 5 ),enriched in LILE (Ba,K,Sr) and depleted HSFE (Nb,Ta,P,Ti),high ratios of Sr/Y (49 ~ 74) and Sr,low Y [(9.1 ~ 1 7.9 )× 10 -6 ] and Yb [(0.79 ~ 1.74)× 10 -6 ], which display the geochemical characteristics of adakite of partial melting of the lower crust.Porphyritic trachyandesite of Yixian formation is similar to the trachyandesite of Tiaojishan formation in REE and trace element distribution,but with low Sr,high Y and Yb,and does not show the geochemical charac-teristics of adakite.The acidic volcano rock of Yixian formation is rich in silicon and alkalis,poor in calci-um,iron and magnesium,strongly depleted in Sr,P,Eu,and the results of fractionation crystalliza-tion of plagioclase and apatite.Comparison to the uranium producing volcanics in adjacent Zhangjiakou formation in rock assemblage and geochemistry characteristics suggest that the study area has a good prospect for uranium mineralization.%青龙建昌地区的中生代火山岩地层主要有髫髻山组(兰旗组)和义县组。髫髻山组粗安岩富碱高钠、低镁含量；稀土元素总量不高[∑REE=(110.93~169.79)×10-6],轻重稀土分馏明显[(LaN/YbN )=13.52~24.46],具有弱的负铕异常或正异常(δEu=0.96~1.15)；富集大离子亲石元素 Ba、K、Sr,亏损高场强元素 Nb、Ta、P、Ti；高 Sr 含量[(682~1065)×10-6]和 Sr/Y 值(49~74),低 Y[(9.1~17.9)×10-6]和 Yb 含量[(0.79~1.74)×10-6],显示为埃达克岩的地球化学特征。义县组粗安玢岩与髫髻山组粗安岩具有相似的稀土和微量元素特征,但低 Sr、高 Y 和 Yb 含量,为非埃达克质岩石。义县组酸性火山岩高硅富碱、低钙铁镁含量；强烈亏损 Sr、P、Eu,为斜长石和磷灰石结晶分异的结果。通过与邻区(沽源红山子)张家口组产铀火山岩的岩石组合、地球化学等特征进行对比分析,认为研究区具有良好的铀成矿前景。