青龙矿田兰旗组火山岩岩石学、地球化学特征及成因探讨

Petrology, Geochemistry Characteristics and Genesis of the Mid Jurassic Volcanic Rocks of the Lanqi Formation, Qinglong Orefield

摘要: 青龙铀矿田中侏罗统兰旗组为一套较厚中性火山熔岩–火山碎屑岩，是含矿层上覆最重要保护层。基于火山岩相学特征将兰旗组划分为3段：下段安山集块岩、安山角砾岩、及碎斑安山岩、安山岩；中段紫红色凝灰质砂岩、砾岩；上段气孔状安山岩夹薄层黑云母晶屑凝灰岩。对各层位的火山熔岩进行了岩石地球化学分析，结果显示总体具有低镁低钛高铝高钾特征，属准铝质系列和高钾钙碱性系列；稀土元素、相容元素、不相容元素等显示下地壳岩浆混入幔源特征，并显示了各阶段混染程度存在强–弱–强演化趋势；岩浆成因并非大规模结晶分异，可能主要为部分熔融；其形成构造背景为板内造山环境，处于地壳缩短加厚晚期向减薄转换阶段。

Abstract: The Lanqi volcanic rocks at mid Jurassic are composed of a series of intermediate volcanic rocks and volcanic-clasitic rocks. They are the most important protecting thick cover on the orebody. This formation could be divided into three layers based on the volcanic petrography feature: the lower—andesitic agglomerate, breccia, porphyroclast and andesite; the middle—the purple Red tuffaceous sandstone and conglomerate; the upper—vesicular andesite with thin layer black mica crystal tuff. The Lanqi volcanic rocks are characterized by low MgO value, low TiO2 value, high Al2O3 value and high K2O value, belonging to quasi-aluminous, high-k calc-alkaline series. The REE, compatible elements and incompatible elements suggest that lower crust magma is mixed with some mantle magma and shows a trend of strong-weak-strong of contamination levels in different stages. The formation of magma is not due to the Large-scale crystallization differentiation, but probably mainly due to the partial melting. The tectonic setting is at intraplate orogeny, on the stage of transformation from the earthcrust shorting thickening to thining.