内蒙古那仁乌拉早白垩世高分异花岗岩年代学及其成因

内蒙古那仁乌拉地区具有较大的稀有金属找矿潜力,但与成矿有关的那仁乌拉花岗岩体在年代学和成因方面仍存在较多争论。为厘定该花岗岩体成岩年龄及成因类型,对其进行了LA-ICP-MS锆石U-Pb定年和岩石地球化学分析。测年结果表明,中粗粒黑云母二长花岗岩和似斑状二长花岗岩成岩年龄分别为(142.1±1.0)Ma和(142.3±1.2)Ma,指示那仁乌拉花岗岩体为早白垩世岩浆活动的产物,与区域上晚侏罗世—早白垩世花岗岩大规模侵入时代基本吻合。岩石地球化学分析显示,岩体富硅(SiO₂含量为73.69%~76.67%)、富碱(Na₂O+K₂O含量为8.40%~8.80%)和贫铁、镁、钙、磷等;Zr、Hf、Rb、Th等元素明显富集,Ba、Sr、P、Ti等元素强烈亏损,Nb元素相对亏损;稀土元素总量较低,显示较强烈的Eu负异常(δEu=0.20~0.36),轻、重稀土分馏明显(LREE/HREE=6.23~14.28),富集轻稀土;属于弱过铝质高钾钙碱性花岗岩。综合矿物学、岩石学和地球化学特征,判断那仁乌拉花岗岩成因类型应归为高分异I型花岗岩。结合区域构造和地质背景,认为位于华北板块北缘的那仁乌拉花岗岩体形成于早白垩世的板内伸展环境。

Geochronology and Petrogenesis of the Highly-Fractionated Early Cretaceous Narenwula Granite,Inner Mongolia,China

A number of rare metal deposits have been discovered in Narenwula area of Inner Mongolia, with high metallogenic potential. However, there are still many controversies about the chronology and genesis of Narenwula granite. LA-ICP-MS zircon U-Pb dating and geochemical analysis have been carried out to determine the age and genetic type of the Narenwula granite. The U-Pb dating results show that the medium-coarse biotite monzogranite and porphyritic monzogranite were emplaced in the Early Cretaceous, at (142.1±1.0)Ma and(142.3±1.2)Ma, respectively. Its diagenetic age is basically consistent with the ages of the extensive regional granite emlacement in the Late Jurassic-Early Cretaceous period. Whole-rock geochemistry shows that the granite is characterized by high silica(SiO₂=73.69%-76.67%)and alkali(Na₂O+K₂O=8.40%-8.80%), but low P₂O₅, MgO, FeO^T, CaO, and REE. The rocks have high LREE/HREE ratios(LREE/HREE=6.23-14.28) and marked negative Eu anomalies (δEu=0.20-0.36). They are obviously enriched in Zr, Hf, Rb, and Th, but strongly depleted in Ba, Sr, P, and Ti, and slightly depleted in Nb. The granite is high-K calc-alkaline and weakly peraluminous. Based on the characteristics of mineralogy, petrology and geochemistry, we concluded that the granite belongs to highly-fractionated I-type. Considering the structural features and regional geological background, we inferred that the Narenwula granite in the northern margin of the North China Plate was formed in the Early Cretaceous within-plate extensional setting.