幕阜山复式花岗岩体锆石年代与微量元素对伟晶岩矿床成因的限定

Genesis of Mufushan Pegmatite Deposits Constrained by U-Pb Ages and Trace Elements of Zircon from Complex Granitic Batholith

江南成矿带晚侏罗世-早白垩世幕阜山复式花岗岩体内部及周缘发育多个早白垩世伟晶岩稀有金属矿床,成矿伟晶岩是否源自幕阜山复式岩体演化花岗岩浆高度分异还存在争议.幕阜山麦市等地发育含电气石、石榴石及白云母二长花岗岩,LA-ICP-MS锆石U-Pb年龄介于130~135 Ma,在误差范围内与区内大规模成矿伟晶岩年龄相当.与早期斑状黑云母二长花岗岩和白云母二长花岗岩(151~143 Ma)相比,晚期含电气石、石榴石及白云母二长花岗岩锆石具有较高的Hf、Ta、Nb、Th、U含量和较低的Th/U和Eu/Eu^(*)比值,体现较高的演化程度,与岩石矿物组合及锆石结晶温度相一致.锆石年代与微量元素说明,幕阜山地区成矿伟晶岩可能是幕阜山复式岩体中早白垩世演化花岗岩浆进一步分异的产物.

Several Early Cretaceous rare metal pegmatite deposits have been discovered within and around the Late Jurassic-Early Cretaceous Mufushan complex granitic batholith in the Jiangnan metallogenic belt. It is still controversial whether the pegmatites were originated from the highly evolved Mufushan granite in this region. Tourmaline-, garnet-, and muscovite-bearing monzogranites in the Maishi and other areas have zircon LA-ICP-MS U-Pb ages between 130 and 135 Ma, similar to ages of the pegmatites within the errors. Compared with the early porphyritic biotite monzogranite and muscovite monzogranite(151-143 Ma), the late tourmaline-, garnet-, and muscovite-bearing monzogranites have zircon with high Hf, Ta, Nb, Th, U contents and low Th/U and Eu/Eu^(*) ratios, indicating a highly evolved nature. This is also consistent with mineral assemblages and zircon crystallization temperatures of the granites. Ages and trace elements from zircon indicate that rare metal pegmatites may be the fractionated product of Early Cretaceous evolved granitic magmas in the Mufushan batholith.