北秦岭蟒岭岩体的锆石U-Pb年龄、地球化学及其演化

蟒岭岩体位于北秦岭构造带北部，岩石类型主要为似斑状黑云母二长花岗岩、中粗粒黑云母二长花岗岩、中细粒二长花岗岩、含辉石黑云角闪闪长岩和黑云母钾长花岗岩。依据LA-ICPMS锆石U-Pb定年结果，结合前人测试的年龄，将蟒岭岩体的岩浆演化划分为晚侏罗世早期、晚侏罗世晚期—早白垩世早期和早白垩世中期3期。第一期为含辉石黑云角闪闪长岩，其LA-ICPMS锆石U-Pb年龄为（157±1） Ma，该期岩石SiO₂质量分数较低，富碱，属于准铝质，钾玄岩-高钾钙碱性系列；第二期二长花岗岩，侵位年龄为（148±1） Ma~（144±1） Ma，具有高硅、富碱的特征，属于准铝质-弱过铝质，钾玄岩-高钾钙碱性I-A过渡型花岗岩；第三期黑云母钾长花岗岩，侵位年龄为（124±2） Ma，具高硅、富碱、低镁、铝饱和指数偏高的特征，属过铝质，高钾钙碱性I-A过渡型花岗岩。组成蟒岭岩体的花岗岩从早到晚，SiO₂质量分数逐渐升高，而Al₂O₃、TiO₂、MgO、CaO、P₂O₅、TFe₂O₃质量分数逐渐降低；稀土元素总量具有由高到低的变化趋势，第一期和第二期岩石的稀土元素配分曲线为轻稀土元素相对富集的右倾型，而第三期的稀土元素配分曲线呈两边高中间低的不对称弧形，整体上负铕异常不明显或呈微弱正铕异常；微量元素上，这3期岩石均富集K、Rb、Ba、Sr等大离子亲石元素，而相对亏损P、Nb、Ta、Ti等高场强元素。与区域上同时代成矿花岗岩体对比，两者均具有高硅、富碱的特征，稀土元素球粒陨石标准化曲线呈轻稀土元素富集的右倾斜型，但蟒岭岩体中二长花岗岩没有明显的Eu异常，且Ba、P、Ti亏损及Ta、Nb富集没有含矿花岗岩明显。

Zircon U-Pb ages, geochemistry and evolution of Mangling pluton in North Qinling Mountains

Located in the North Qinling belt, the Mangling pluton consists of porphyritic biotite monzogranites, mid-coarse-grained biotite monzogranites, mid-fine-grained monzogranites, pyroxene-bearing biotite-hornblende diorites and biotite K-feldspar granites. The results of LA-ICPMS zircon U-Pb dating, combined with previous researches, suggest that the Mangling pluton can be divided into three stages, i.e., Early Jurassic, Late Jurassic-Early Cretaceous and Middle Cretaceous. The first stage pyroxene-bearing biotite hornblende diorites yielded a zircon U-Pb age of (157±1) Ma, and they are characterized by low silica and high alkali, suggesting metaluminous and shoshonitic to high K cal-alkaline series. The second monzogranites with ages from (148±1) Ma to (144±1) Ma are rich in silica and alkali, being metaluminous to weak peraluminous and shoshonitic to high K cal-alkaline series with I-A type transitional features. The last stage biotite K-feldspar granites gave an age of (124±2) Ma, and are rich in silica, alkali and poor in magnesium with high aluminum saturation index, indicating that they are peraluminous and high K cal-alkaline series and I-A transitional type granites. From Late Jurassic to Early Cretaceous, the SiO₂ content of these granitoids increased, but Al₂O₃, TiO₂, MgO, CaO, P₂O₅ and TFe₂O₃ decreased. The total REE content of granitoids is characterized by the variation from high value to low value, the granites of the first two stages are characterized by enrichment of LREE, but the REE patterns of the biotite K-feldspar granite assume the shape of asymmetric arc. All the granitoids have slightly negative to positive anomalies of Eu. The Mangling granites are enriched in large ion lithophile elements (K, Rb, Ba, Sr), and depleted in high-field strength elements (P, Nb, Ta, Ti). A comparison with the ore-bearing granites in the same area shows that both of them are rich in silicon and alkali as well as LREE; nevertheless, the monzogranites of Mangling pluton have no obvious anomalies of Eu, and their depletion of Ba, P, Ti and enrichment of Ta, Nb are not so obvious as the ore-bearing granites.