四川龙门山北段唐王寨-仰天窝向斜麻柳村辉绿岩斜锆石U-Pb同位素定年及其地质意义

为确定龙门山北段唐王寨-仰天窝向斜内发育的NE向辉绿岩脉的侵位时代、构造背景,探讨其是否与峨眉山地幔柱活动相关,对向斜内麻柳村处的辉绿岩样品进行了斜锆石LA-ICP-MS U-Pb同位素测年、主量和微量元素分析。结果表明,辉绿岩斜锆石U-Pb同位素年龄为261.1±1.8Ma;属于高钛型拉斑玄武岩,高铝(13.39%~14.02%)、高铁(13.88%~14.67%)、低镁(4.43%~4.56%),富集大离子亲石元素和轻稀土元素,Nb/La=0.84~0.85、Th/Ta=2.40~2.45、Ta/Hf0.25,具有与洋岛玄武岩相似的地球化学特征,Nb-Ta略微负异常,暗示其可能源于富集型地幔并受到少量陆壳物质混染,形成于大陆板内环境。与峨眉山玄武岩质火成岩对比发现,两者具有高度一致的形成时代及相似的地球化学特征。据此认为,该区辉绿岩为峨眉山地幔柱活动的产物,峨眉山玄武质岩浆的活动已经波及至龙门山北段及川西北地区,可能影响了该区的生物环境及油气成藏。

Baddeleyite U-Pb geochronology of Maliucun diabase from Tangwangzhai-Yangtianwo syncline in the northern segment of Longmen Mountain, Sichuan, and its geological signifi-cance

To confirm the emplacement age and tectonic setting of the NE-striking diabase dykes developed in the TangwangzhaiYangtianwo syncline of northern Longmen Mountain as well as to reveal whether or not the diabases were associated with the Emeis-han mantle plume activity, the authors investigated U-Pb baddeleyite age, major element content and trace element abundances of the diabase samples collected from the Maliucun area within the syncline. The results showed that the diabases yielded a baddeleyite concordant U-Pb age of 261.1±1.8Ma; they belong to the high-Ti (Ti/Y>500) tholeiitic series, and are characterized by high Al (13.39%~14.02%), high Fe (13.88%~14.02%), and low Mg (4.43%~4.56%) values as will as enrichment of the large iron lithophile ele-ments and light rare earth elements; coupled with the data Nb/La (0.84~0.85), Th/Ta (2.40~2.45), and Ta/Hf>0.25, the features of the diabases are similar to the geochemical characteristics of ocean island basalt; the slightly negative anomaly of Nb-Ta indicates that they were probably derived from the enriched mantle with minor contamination by crust materials. A comparison with the Emeishan basaltic rocks shows that they are consistent highly with each other in ages and geochemical characteristics. It is considered that the di-abases were formed by Emeishan mantle plume activity and the activity of Emeishan basaltic magma was spread to the northern sec-tion of Longmen Mountain and northwestern Sichuan, which had an impact on the biotic environment and hydrocarbon accumula-tion of this area.